Postnatal development of cat hind limb motoneurons. I: Changes in length, branching structure, and spatial distribution of dendrites of cat triceps surae motoneurons

Postnatal growth of the lumbosacral spinal segments in cat: Their lengths and positions in relation to vertebrae

Cat is a prominent model for investigating neural networks of the lumbosacral spinal cord that control locomotor and visceral activity. We previously proposed an integral function, establishing the topographical relationship between the spinal cord segments and vertebrae in adult animals. Here, we investigated the dynamic of this topographical relationship through early and middle periods of development in kittens. We calculated the length of each vertebra relative to the total length of the region from 13th thoracic (T) to the 7th lumbar (L) vertebrae (V) as well as the length of each segment relative to the total region from T13 to the three-dimensional sacral (S) segment. As in our previous work, the length and position of VL2 were used to establish relationships between the characteristics of the segments and vertebrae. Cubic regression reliably approximates the lengths of segments relative to VL2 length. As the cat aged, the relative length of VT13 and VL1 decreased while the relative length of VL5 increased. The relative length of the T13 and L3 segments increased while the relative length of the S1-S2 segments decreased. The T13-L2 segments are descended monotonically relative to the VL1-VL2 border. The L3-S1 segments are also descended, though with more complex dynamics. The positions of the S2-S3 segments remained unchanged. To conclude, different spinal segments displayed different developmental dynamics. The revealed relationship between vertebrae and lumbosacral spinal segments may be helpful for clearly defining stimulation regions to invoke particular functions, both in experimental studies on the spinal cord and clinical treatment.

Defining neuroplasticity

Neuroplasticity, i.e., the modifiability of the brain, is different in development and adulthood. The first includes changes in: (i) neurogenesis and control of neuron number; (ii) neuronal migration; (iii) differentiation of the somato-dendritic and axonal phenotypes; (iv) formation of connections; (v) cytoarchitectonic differentiation. These changes are often interrelated and can lead to: (vi) system-wide modifications of brain structure as well as to (vii) acquisition of specific functions such as ocular dominance or language. Myelination appears to be plastic both in development and adulthood, at least, in rodents. Adult neuroplasticity is limited, and is mainly expressed as changes in the strength of excitatory and inhibitory synapses while the attempts to regenerate connections have met with limited success. The outcomes of neuroplasticity are not necessarily adaptive, but can also be the cause of neurological and psychiatric pathologies.

Normal Development and Pathology of Motoneurons: Anatomy, Electrophysiological Properties, Firing Patterns and Circuit Connectivity

This chapter will provide an introduction into motoneuron anatomy, electrophysiological properties, firing patterns focusing on development and also describing several pathological conditions that affect mononeurons. It starts with a historical retrospective describing the early landmark work into motoneurons. The next section lays out the various types of motoneurons (alpha, beta, and gamma) and their subclasses (fast-twitch fatigable, fast-twitch fatigue-resistant, and slow-twitch fatigue resistant), highlighting the functional relevance of this classification scheme. The third section describes the development of motoneurons' passive and active electrophysiological properties. This section also defines the major terms one uses in describing how a neuron functions electrophysiologically. The electrophysiological aspects of a neuron is critical to understanding how it behaves within a circuit and contributes to behavior since the firing of an action potential is how neurons communicate with each other and with muscles. The electrophysiological changes of motoneurons over development underlies how their function changes over the lifetime of an organism. After describing the properties of individual motoneurons, the chapter then turns to revealing how motoneurons interact within complex neural circuits, with other motoneurons as well as sensory neurons, and how these circuits change over development. Finally, this chapter ends with highlighting some recent advances made in motoneuron pathology, focusing on spinal muscular atrophy, amyotrophic lateral sclerosis, and axotomy.

Electrical Properties of Adult Mammalian Motoneurons

Motoneurons are the 'final common path' between the central nervous system (that intends, selects, commands, and organises movement) and muscles (that produce the behaviour). Motoneurons are not passive relays, but rather integrate synaptic activity to appropriately tune output (spike trains) and therefore the production of muscle force. In this chapter, we focus on studies of mammalian motoneurons, describing their heterogeneity whilst providing a brief historical account of motoneuron recording techniques. Next, we describe adult motoneurons in terms of their passive, transition, and active (repetitive firing) properties. We then discuss modulation of these properties by somatic (C-boutons) and dendritic (persistent inward currents) mechanisms. Finally, we briefly describe select studies of human motor unit physiology and relate them to findings from animal preparations discussed earlier in the chapter. This interphyletic approach to the study of motoneuron physiology is crucial to progress understanding of how these diverse neurons translate intention into behaviour.

Electrical and Morphological Properties of Developing Motoneurons in Postnatal Mice and Early Abnormalities in SOD1 Transgenic Mice

In this chapter, we review electrical and morphological properties of lumbar motoneurons during postnatal development in wild-type (WT) and transgenic superoxide dismutase 1 (SOD1) mice, models of amyotrophic lateral sclerosis. First we showed that sensorimotor reflexes do not develop normally in transgenic SOD1^G85R pups. Fictive locomotor activity recorded in in vitro whole brainstem/spinal cord preparations was not induced in these transgenic SOD1^G85R mice using NMDA and 5HT in contrast to WT mice. Further, abnormal electrical properties were detected as early as the second postnatal week in lumbar motoneurons of SOD1 mice while they develop clinical symptoms several months after birth. We compared two different strains of mice (G85R and G93A) at the same postnatal period using intracellular recordings and patch clamp recordings of WT and SOD1 motoneurons. We defined three types of motoneurons according to their discharge firing pattern (transient, sustained and delayed onset firing) when motor units are not yet mature. The delayed-onset firing motoneurons had the higher rheobase compared to the transient and sustained firing groups in the WT mice. We demonstrated hypoexcitability in the delayed onset-firing motoneurons of SOD1 mice. Intracellular staining of motoneurons revealed dendritic overbranching in SOD1 lumbar motoneurons that was more pronounced in the sustained firing motoneurons. We suggested that motoneuronal hypoexcitability is an early pathological sign affecting a subset of lumbar motoneurons in the spinal cord of SOD1 mice.

Postnatal development of inner lamina II interneurons of the rat medullary dorsal horn

ABSTRACT

Pain processing in young mammals is immature. Despite the central role of the medullary dorsal horn (MDH) in processing orofacial sensory information, the maturation of the neurons within the MDH has been largely overlooked. Combining in vitro electrophysiological recordings and 3D morphological analysis over the first postnatal month in rats, we investigated the age-dependent development of the neurons within the inner lamina II (IIi) of the MDH. We show the lamina IIi neuronal population transition into a more hyperpolarized state, with modification of the action potential waveform, and a shift from single spiking, at early postnatal ages, to tonic firing and initial bursting at later stages. These physiological changes are associated with a strong structural remodelling of the neuronal morphology with most of the modifications occurring after the third postnatal week. Among the lamina IIi neuronal population, the subpopulation of interneurons expressing the γ isoform of the protein kinase C (PKCγ+) are key elements for the circuits underlying facial mechanical allodynia. How do they develop from the rest of the lamina IIi constitute an important question that remained to be addressed. Here, we show that PKCγ+ interneurons display electrophysiological changes over time comparable with the PKCγ- population. However, they show a distinctive increase of the soma volume and primary branches length, as opposed to the PKCγ- population. Together, our data demonstrate a novel pattern of late postnatal maturation of lamina IIi interneurons, with a spotlight on PKCγ+ interneurons, that may be relevant for the development of orofacial sensitivity.

Spinal motoneuron firing properties mature from rostral to caudal during postnatal development of the mouse

Key points

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Abstract

Altricial mammals are born with immature nervous systems comprised of circuits that do not yet have the neuronal properties and connectivity required to produce future behaviours. During the critical period of postnatal development, neuronal properties are tuned to participate in functional circuits. In rodents, cervical motoneurons are born prior to lumbar motoneurons, and spinal cord development follows a sequential rostro‐caudal pattern. Here we asked whether birth order is reflected in the postnatal development of electrophysiological properties. We show that motoneurons of both regions have similar properties at birth and follow the same developmental profile, with maximal firing increasing and excitability decreasing into the third postnatal week. However, these maturative processes occur in cervical motoneurons prior to lumbar motoneurons, correlating with the maturation of premotor descending and local spinal systems. These results suggest that motoneuron properties do not mature by cell autonomous mechanisms alone, but also depend on developing premotor circuits.

Many mammals are born with immature motor systems that develop through a critical period of postnatal development.

In rodents, postnatal maturation of movement occurs from rostral to caudal, correlating with maturation of descending supraspinal and local spinal circuits.

We asked whether development of fundamental electrophysiological properties of spinal motoneurons follows the same rostro‐caudal sequence.

We show that in both regions, repetitive firing parameters increase and excitability decreases with development; however, these characteristics mature earlier in cervical motoneurons.

We suggest that in addition to autonomous mechanisms, motoneuron development depends on activity resulting from their circuit milieu.

Trophic effects of brain-derived neurotrophic factor blockade in an androgen-sensitive neuromuscular system

In adult male rats, androgens are necessary for the maintenance of the motoneurons and their target muscles of the sexually dimorphic, steroid-sensitive spinal nucleus of the bulbocavernosus (SNB) neuromuscular system, regulating motoneuron and muscle morphology, function, and expression of trophic factors. Castration of males results in somal, dendritic, and muscle atrophy as well as increases in brain-derived neurotrophic factor (BDNF) in the target musculature. Because BDNF can have either facilitative or inhibitory effects in other systems, we examined SNB neuromuscular morphology after BDNF blockade using a fusion protein (tyrosine kinase receptor type B IgG). Blockade of BDNF in gonadally intact males resulted in hypertrophy of SNB motoneuron dendrites and target musculature, suggesting that normal levels of BDNF are inhibitory in SNB neuromuscular system. BDNF blockade in castrated males prevented SNB motoneuron atrophy and attenuated target muscle weight loss. This is the first demonstration that the highly androgen-sensitive SNB motoneuron dendrites and target muscles can be maintained in the absence of gonadal hormones and, furthermore, that blocking BDNF can have trophic effects on skeletal muscle. These results suggest that whereas BDNF is involved in the signaling cascade mediating the androgenic support of SNB neuromuscular morphology, its action can be inhibitory. Furthermore, the elevations in BDNF after castration may be responsible for the castration-induced atrophy in SNB motoneurons and target muscles, and the trophic effects of androgens may be mediated in part through a suppression of BDNF. These results may have relevance to therapeutic approaches to the treatment of neurodegenerative disease or myopathies.

Changes in somatodendritic morphometry of rat oculomotor nucleus motoneurons during postnatal development

This work investigates the somatodendritic shaping of rat oculomotor nucleus motoneurons (Mns) during postnatal development. The Mns were functionally identified in slice preparation, intracellularly injected with neurobiotin, and three-dimensionally reconstructed. Most of the Mns (approximately 85%) were multipolar and the rest (approximately 15%) bipolar. Forty multipolar Mns were studied and grouped as follows: 1-5, 6-10, 11-15, and 21-30 postnatal days. Two phases were distinguished during postnatal development (P1-P10 and P11-P30). During the first phase, there was a progressive increase in the dendritic complexity; e.g., the number of terminals per neuron increased from 26.3 (P1-P5) to 47.7 (P6-P10) and membrane somatodendritic area from 11,289.9 microm(2) (P1-P5) to 19,235.8 microm(2) (P6-P10). In addition, a few cases of tracer coupling were observed. During the second phase, dendritic elongation took place; e.g., the maximum dendritic length increased from 486.7 microm (P6-P10) to 729.5 microm in adult Mns, with a simplification of dendritic complexity to values near those for the newborn, and a slow, progressive increase in membrane area from 19,235.8 microm(2) (P6-P10) to 24,700.3 microm(2) (P21-P30), while the somatic area remained constant. In conclusion, the electrophysiological changes reported in these Mns with maturation (Carrascal et al. [2006] Neuroscience 140:1223-1237) cannot be fully explained by morphometric variations; the dendritic elongation and increase in dendritic area are features shared with other pools of Mns, whereas changes in dendritic complexity depend on each population; the first phase paralleled the establishment of vestibular circuitry and the second paralleled eyelid opening.

Effects of limb exercise after spinal cord injury on motor neuron dendrite structure

An integration center subserving locomotor leg movements resides in the upper lumbar spinal cord. If this neuronal network is preserved after a spinal cord injury, it is possible to stimulate this circuitry to initiate and promote walking. The several effective approaches (electrical stimulation, pharmacologic agents, physical therapy training programs) may all share a common modus operandi of altering synaptic activity within segmental spinal cord. To understand the neural substrate for the use-dependent behavioral improvement, we studied the dendritic architecture of spinal motor neurons. In the first experiment, we compared three groups of animals: animals with an intact spinal cord, animals that had a complete spinal cord transection (SCT) and animals with SCT who engaged in a daily exercise program of actively moving paralyzed hindlimbs through the motions of walking. When compared with animals with an intact spinal cord, the motor neurons from animals with SCT displayed marked atrophy, with loss of dendritic membrane and elimination of branching throughout the visible tree within transverse tissue slices. None of these regressive changes were found in the motor neurons from SCT animals that underwent exercise. In a second experiment, we inquired whether exercise of animals with an intact spinal cord influenced dendrite structure. Increased exercise had very modest effects on dendrite morphology, indicating an upper limit of use-dependent dendrite growth. Our findings suggest that the dendritic tree of motor neurons deprived of descending influences is rapidly pruned, and this finding is not observed in motor neurons after SCT if hindlimbs are exercised. The functional benefits of exercise after SCT injury may be subserved, in part, by stabilizing or remodeling the dendritic tree of motor neurons below the injury site.

Associations between the morphology and physiology of ventral-horn neurons in the adult turtle

This study compared some morphologic and physiological properties of adult turtle spinal motoneurons (MNs) vs. interneurons (INs). Reconstructions were made of 20 biocytin-stained cells, which had been previously studied physiologically in 2-mm-thick slices of lumbosacral spinal cord. The intracellularly measured physiological properties included resting potential, input resistance (R(N)), threshold (rheobase, I(Rh)), and slope of the stimulus current (I) -spike frequency (f) relation. The seven morphologic properties that were quantified for each cell included three indices of somal size (diameter, area, volume), and four of dendritic size: the number of first- and last-order branches, rostrocaudal extent, and sigma individual lengths. Significant differences were shown between all seven morphologic parameters for MNs vs. INs. Despite the small sample size, significant differences were also shown for five of seven parameters for high-threshold vs. low-threshold MNs, and three of seven for low-threshold MNs vs. INs. These latter three parameters were the number of terminal dendritic branches, their rostrocaudal extent, and the sigma dendritic lengths. Linear associations for the MN + IN and the MN samples were stronger between the four dendritic parameters than between soma-dendritic ones. Exponential associations between morphologic and physiological properties were mostly significant (28 of 30), and their strength was in the order I(Rh) < R(N) < f/I slope for the MN +IN sample and I(Rh) < R(N) = f/I slope for the MN sample. There is discussion of the relevance of the above findings to the provisional classification of turtle ventral-horn neurons on the basis of electrophysiology alone.

Physiological changes accompanying anatomical remodeling of mammalian motoneurons during postnatal development

The development of respiratory motoneurons provides unique data that may be generalized to other mammalian motoneuron populations. Like other motoneurons, respiratory motoneurons undergo developmental changes in the shape of the action potential and their repetitive firing. The unique observations concern the postnatal change in the recruitment pattern of cat phrenic motoneurons that is correlated with a halving of mean input resistance, a stasis of growth in the cell membrane and a reduction in the complexity of the dendritic tree. A similar pattern of change was observed for hypoglossal motoneurons studied in rat brainstem slices. Without an increase in total membrane surface area, the decreased resistance must result from a reduced specific membrane resistance. Two mechanisms are proposed to explain this decrease in resistance: proliferation and redistribution of either synaptic inputs and/or potassium channels. Although there was a significant contribution of synaptic input in determining input resistance throughout postnatal development, it was the density of cesium- or barium-sensitive potassium conductances that differentiated low resistance from high resistance motoneurons. Low resistance motoneurons had more cesium- and barium-sensitive channels than their high resistance counterparts. Based on the variations in the relative changes observed in input resistance versus membrane time constant with these two potassium channel blockers (cesium and barium), it is proposed that the distribution of these potassium channels change with age. Initially, their distribution is skewed toward the dendrites but as development progresses, the distribution becomes more uniform across the motoneuron membrane. During postnatal development, the rapid decrease in input resistance results from a proliferation of potassium channels in the membrane and of synaptic inputs converging onto developing respiratory motoneurons while the membrane is being spatially redistributed but not expanded.

Persistence of somatic and dendritic growth associated processes and induction of dendritic sprouting in motoneurones after neonatal axotomy in the rat

The effect of neonatal axotomy on the maturation of motoneurone somadendritic morphology was studied in identified motoneurones innervating the ankle dorsiflexor muscles tibialis anterior (TA) and extensor digitorum longus (EDL) of the rat by intracellular injection of Lucifer Yellow and confocal microscopy. At birth, the entire somatodendritic surface is covered with fine filopodial growth-associated processes. These are eliminated from the soma and proximal dendrites during the first postnatal week as part of a somatofugal process of dendritic maturation. Following neonatal axotomy, the postnatal elimination of growth associated processes was halted and new, axonal-like processes were seen to sprout from the soma and proximal dendrites in some of the axotomized motoneurones. These results indicate that synaptic interaction with the target muscle during the early postnatal period is essential for the maturation of the somatodendritic receptive surface of the motoneurone.

Membrane properties of deep dorsal horn neurons from neonatal rat spinal cord in vitro

Whole-cell patch-clamp recordings were undertaken to characterize and compare the membrane properties of deep dorsal horn neurons in transverse slices of rat lumbar spinal cord in two age groups, postnatal days (P) 3-6 and 9-16. In both age groups, significant correlations were observed between membrane time constant and cell resistance and between action potential height and its duration at half-maximal amplitude. Cell resistance and action potential half-width values were lower in the P9-16 age group. Neurons were divided into four categories based on their firing properties in response to intracellular current injection: single spike, phasic firing, repetitive firing, and delayed firing. The distribution of neurons within these categories was similar in both age groups which suggests that the firing properties of deep dorsal horn neurons are functionally differentiated at an early postnatal age.

Postnatal development of corticospinal projections from motor cortex to the cervical enlargement in the macaque monkey

The postnatal development of corticospinal projections was investigated in 11 macaques by means of the anterograde transport of wheat germ agglutin-horseradish peroxidase injected into the primary motor cortex hand area. Although the fibers of the corticospinal tract reached all levels of the spinal cord white matter at birth, their penetration into the gray matter was far from complete. At birth, as in the adult, corticospinal projections were distributed to the same regions of the intermediate zone, although they showed marked increases in density during the first 5 months. The unique feature of the primate corticospinal tract, namely direct cortico-motoneuronal projections to the spinal motor nuclei innervating hand muscles, was not present to a significant extent at birth. The density of these cortico-motoneuronal projections increased rapidly during the first 5 months, followed by a protracted period extending into the second year of life. The densest corticospinal terminations occupied only 40% of the hand motor nuclei in the first thoracic segment at 1 month, 73% at 5 months, and 75.5% at 3 years. A caudo-rostral gradient of termination density within the hand motor nuclei was present throughout development and persisted into the adult. As a consequence, the more caudal the segment within the cervical enlargement, the earlier the adult pattern of projection density was reached. No transitory corticospinal projections were found. The continuous postnatal expansion of cortico-motoneuronal projections to hand motor nuclei in primates is in marked contrast to the retraction of exuberant projections that characterizes the development of other sensory and motor pathways in subprimates.

Growth cone dynamics and activity-dependent processes in neuronal network development

Many structural and functional properties of neuronal networks find their origin in the dynamic behavior of growth cones during development. The variation in dendritic morphologies can be traced back to random branching of growth cones. Segment length characteristics arise under random branching and steady growth cone propagation. Delayed outgrowth, as a result of competition between growth cones after splitting, is hypothesized to explain different lengths of paired terminal segments in Purkinje cells. The implications of activity-dependent neurite outgrowth were studied using an outgrowth function based on the theory of Kater et al. (1988, 1990). This theory embodies a homeostatic principle, according to which a neuron adapts its neuritic field so as to maintain a certain level bioelectric activity. It is shown that such homeostasis has many implications for neuromorphogenesis and network formation, as it may underlie phenomena such as overshoot during development, size differences among cells, differentiation between excitatory and inhibitory cells and compensatory sprouting. Finally, function-dependent regulation of development involves physiological as well as morphological variables. For instance, activity dependent regulation of ionic conductances such as to stabilize functional activity can result in a differentiation of certain neurons into, respectively, bursting and regular firing sub-types (Abbot et al., 1993; LeMasson et al., 1993). Similarly, the GABAergic phenotype comes fully to expression in hindbrain (cerebellar) and forebrain (neocortical) networks only if the level of ongoing excitatory activity during development is sufficiently high, whereas chronically intensified activity leads to a compensatory hypertrophy of inhibitory mechanisms (for review, see Corner 1994). Many of these results could only have been obtained by the use of mathematical models which allow rigorous analysis of the consequences of basic assumptions in the dynamics of neurite outgrowth. All in all, the findings further emphasize the role of spontaneous bioelectric activity during early development in neuronal network formation, the importance of which was first established in cultures of developing neural tissue.

A fast 3-dimensional neuronal tree reconstruction system that uses cubic polynomials to estimate dendritic curvature

The main goal of this work was to develop and test the accuracy of our 3DARBOR neuronal tree reconstruction system by comparing it with a very precise but time-consuming method of reconstruction (NEUTRACE). Comparison was performed by reconstructing 18 dendritic trees of frog spinal motoneurons from serial sections with both methods and comparing several morphological summaries of the two reconstructions. In 3DARBOR the planar projection of the dendritic trees was drawn and fed into an IBM-compatible PC through a graphic tablet. Dendritic coordinates along the perpendicular (focus) axis on the plane of drawing were estimated by an interpolation method. The interpolation was based on the lengths of projected dendrites and the coordinates of points where dendrites entered the next section. Focus axis coordinates of these points could automatically be calculated from the serial numbers and thicknesses of sections. 3DARBOR was tested by comparison of the distributions of characteristic points of dendritic trees, segment lengths and branching angles. Product moment analysis on dendritic trees was also performed. It was concluded that 3DARBOR is a fast enough reconstruction system without any systematical error of interpolation that can correctly supply the most morphological parameters and visualize the natural arborizations.

Quantitative and qualitative changes in AMPA receptor expression during spinal cord development

Synaptic activity in early postnatal life is important for the acquisition of mature structural and functional properties of neurons. Previous studies indicate that the mature molecular features of spinal motor neurons emerge during a period of activity-dependent development in early postnatal life. Since glutamatergic synaptic transmission provides the major excitatory drive into motor neurons, glutamate receptors are likely to play a central role in motor neuron activity-dependent development. To gain insight into this process, we have used receptor autoradiography, immunoblotting and immunohistochemistry to determine the distribution, temporal expression and potential subunit composition of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid subtype glutamate receptors in the developing rat spinal cord. Using two different ligands, [3H]-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid and [3H]-6-cyano-7-nitroquinoxaline-2,3-dione, we find that alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites in the adult are largely restricted to the substantia gelatinosa. In marked contrast, during early postnatal life, alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid binding sites are transiently expressed at high levels in the ventral horn. This parallels previous findings on the developmental regulation of N-methyl-D-aspartate receptor expression. Using alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor subunit-specific antibodies we show by immunoblot analysis and immunohistology that, to varying degrees, the expression patterns of glutamate receptor subunit 1 and glutamate receptor subunits 2/3 are significantly developmentally regulated. The most conspicuous change is the downregulation of glutamate receptor 1 expression within motor neurons over the first three weeks of postnatal life. The qualitative and quantitative changes we observe in glutamate receptor expression in early postnatal life are likely to have a major impact on the electrophysiological properties of young motor neurons and thus may contribute to their activity-dependent development.

Regulation of motor neuron dendrite growth by NMDA receptor activation

Spinal motor neurons undergo great changes in morphology, electrophysiology and molecular composition during development. Some of this maturation occurs postnatally when limbs are employed for locomotion, suggesting that neuronal activity may influence motor neuron development. To identify features of motor neurons that might be regulated by activity we first examined the structural development of the rat motor neuron cell body and dendritic tree labeled with cholera toxin-conjugated horseradish peroxidase. The motor neuron cell body and dendrites in the radial and rostrocaudal axes grew progressively over the first month of life. In contrast, the growth of the dendritic arbor/cell and number of dendritic branches was biphasic with overabundant growth followed by regression until the adult pattern was achieved. We next examined the influence of neurotransmission on the development of these motor neuron features. We found that antagonism of the N-methyl-D-aspartate (NMDA) subtype of glutamate receptor inhibited cell body growth and dendritic branching in early postnatal life but had no effect on the maximal extent of dendrite growth in the radial and rostrocaudal axes. The effects of NMDA receptor antagonism on motor neurons and their dendrites was temporally restricted; all of our anatomic measures of dendrite structure were resistant to NMDA receptor antagonism in adults. These results suggest that the establishment of mature motor neuron dendritic architecture results in part from dendrite growth in response to afferent input during a sensitive period in early postnatal life.

Descending propriospinal axons in the hindlimb enlargement of the red-eared turtle: cells of origin and funicular courses

Spinal neurons with descending axons are important components of spinal sensorimotor networks. We used an anatomical tracing technique to study the distribution of descending propriospinal axons and cell bodies in red-eared turtles. We injected horseradish peroxidase into a portion of one funiculus in the middle of the hindlimb enlargement and examined six spinal segments rostral to the injection site (dorsal 3 through dorsal 8) for labeled neuronal cell bodies. Injections into each region of the white matter labeled substantial numbers of descending propriospinal neurons. Each injection labeled cell bodies over most of the six spinal segments examined. Each injection also labeled cell bodies in the ipsilateral dorsal horn, intermediate zone, and ventral horn as well as the contralateral intermediate zone and ventral horn. Injections into each of four regions of the white matter, the dorsal funiculus, the medial part of the lateral funiculus, the lateral part of the lateral funiculus, and the ventral funiculus reliably gave rise to a distinct distribution of labeled cell bodies. These experiments establish that descending propriospinal axons in red-eared turtles are found in all regions of the spinal white matter. This finding contrasts with a popular contemporary view of the organization of descending propriospinal axons in mammals. These experiments also demonstrate that neurons in each region of the gray matter give rise to a different distribution of descending, funicular axons, although these distributions are widely overlapping. Different funicular axon distributions could be associated with different sets of synaptic contacts with the white-matter dendrites of spinal neurons.

Dendritic architecture of rat somatosensory thalamocortical projection neurons

This study examines dendrites from physiologically characterized and intracellularly labelled thalamocortical projection (TCP) neurons from the rat ventrobasal complex (VB) and posterior nucleus (POm). The goals were to provide quantitative descriptions of TCP neuron dendrites, examine underlying design principles of dendritic morphology, and determine correlations between dendritic size parameters. Forty-four dendrites from seven VB neurons and 21 dendrites from three POm TCP neurons that responded to low-threshold mechanical stimuli were reconstructed and quantitatively analyzed at the light microscopic level. The dendritic architecture of the neurons was remarkably similar in most parameters studied, including the percentage of dichotomous branching, contribution of terminal branches to total dendritic length, and branching symmetry. There was a positive correlation between stem dendrite diameter and the length of the entire dendrite arbor, making it possible to estimate the total length of a dendritic arbor by measuring the stem dendrite diameter. The correlations of the VB and POm dendrites had different slopes. The path distance (the distance from the soma to a dendritic end point) of individual dendrites showed only a small variation with large differences in the total dendritic length of an arbor. The constant diameter of distal dendrites shows that dendrite diameter is a poor predictor of synaptic location on the dendritic tree. Although the morphology of neurons and their individual dendrites varied considerably in overall size and qualitative appearance, when examined qualitatively, many aspects of dendritic structure were similar within and between groups. We suggest that the rat somatosensory TCP neurons have a stereotyped dendritic architecture and present data which provide a base for future comparative, developmental, and plasticity studies.

Morphology of developing rat genioglossal motoneurons studied in vitro: changes in length, branching pattern, and spatial distribution of dendrites

The aim of this study is to describe the postnatal change in dendritic morphology of those motoneurons in the hypoglossal nucleus that innervate the genioglossus muscle. Forty genioglossal (GG) motoneurons from four age groups (1-2, 5-6, 13-15, and 19-30 postnatal days) were labeled by intracellular injection of neurobiotin in an in vitro slice preparation of the rat brainstem and were reconstructed in three-dimensional space. The number of primary dendrites per GG motoneuron was approximately 6 and remained unchanged with age. The development of these motoneurons from birth to 13-15 days was characterized by a simplification of the dendritic tree involving a decrease in the number of terminal endings and dendritic branches. Motoneurons lost their 6th-8th order branches, in parallel with an elongation of their terminal dendritic branches maintaining the same combined dendritic length. The elongation of terminal branches was attributed to both longitudinal growth and the apparent lengthening caused by resorption of distal branches. The elimination of dendritic branches tended to increase the symmetry of the tree, as revealed by topological analysis. Later, between 13-15 days and 19-30 days, there was a reelaboration of the dendritic arborization returning to a configuration similar to that found in the newborn. The length of terminal branches was shorter at 19-30 days, while the length of preterminal branches did not change, suggesting that the proliferation of branches at 19-30 days takes place in the intermediate parts of terminal branches. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes (hexants). This analysis revealed that GG motoneurons have major components of their dendritic tree oriented in the lateral, medial, and dorsal hexants. Further two-dimensional polar analysis (consisting of eight sectors) revealed a reconfiguration of the tree from birth up to 5-6 days involving resorption of dendrites in the dorsal, dorsomedial, and medial sectors and growth in the lateral sector. Later in development (between 13-15 days and 19-30 days), there was growth in all sectors, but of a greater magnitude in the dorsomedial, medial, and dorsolateral sectors.

Early postnatal changes in the somatodendritic morphology of ankle flexor motoneurons in the rat

The development of locomotor function in the rat spans the first 3 postnatal weeks. We have studied morphological features of the soma and dendrites of motoneurons innervating the physiological flexor muscles of the ankle, tibialis anterior and extensor digitorum longus, by intracellular injection in vitro between the first and ninth postnatal days. We obtained serial optical sections of 96 adequately filled motoneurons in whole-mounted hemisected spinal cords by confocal microscopy, projected them onto a single plane and analysed them morphometrically. On the day after birth, the somatodendritic surfaces of most such motoneurons were covered in growth-associated spiny, thorny or hair-like appendages. These had disappeared from the soma by the fourth postnatal day and from most proximal dendrites by day 7, but were still common distally on day 9. During this period there was little or no net growth of either the soma (which was still much smaller than in the adult) or the dendritic tree. A dorsal dendritic bias was present and 'sprays' of long, loosely bundled dorsal dendrites were often seen. The mean number of primary dendrites remained constant at about eight, and their combined diameter was already significantly correlated with mean soma diameter, as in the adult cat. Thus, the critical neonatal period during which these ankle flexor motoneurons are known to change their electrophysiological properties and to be particularly sensitive to interference with neuromuscular interaction is characterized by major changes in the neuronal surface, presumably linked to synaptogenesis.

Motoneuron morphology in the dorsolateral nucleus of the rat spinal cord: normal development and androgenic regulation

The rat lumbar spinal cord contains two sexually dimorphic motor nuclei, the spinal nucleus of the bulbocavernosus (SNB), and the dorsolateral nucleus (DLN). These motor nuclei innervate anatomically distinct perineal muscles that are involved in functionally distinct copulatory reflexes. The motoneurons in the SNB and DLN have different dendritic morphologies. The dendrites of motoneurons in the medially positioned SNB have a radial, overlapping arrangement, whereas the dendrites of the laterally positioned DLN have a bipolar and strictly unilateral organization. During development, SNB motoneuron dendrites grow exuberantly and then retract to their mature lengths. In this experiment we determined whether the adult difference in SNB and DLN motoneuron morphology was reflected in different patterns of dendritic growth during normal development. Furthermore, the development of both these nuclei is under androgenic control. In the absence of androgens, SNB dendrites fail to grow; testosterone replacement supports normal dendritic growth. Thus, we also examined the development of DLN dendrites for similar evidence of androgenic regulation. By using cholera toxin-horseradish peroxidase (BHRP) to label motoneurons retrogradely, we measured the morphology of DLN motoneurons in normal males, and in castrates treated with testosterone or oil/blank implants at postnatal day (P) 7, P28, P49, and P70. Our results demonstrate that in contrast to the biphasic pattern of dendritic development in the SNB, dendritic growth in the DLN was monotonic; the dendritic length of motoneurons increased more than 500% between P7 and P70. However, as in the SNB, development of DLN motoneuron morphology is androgen-dependent. In castrates treated with oil/blank implants, DLN somal and dendritic growth were greatly attenuated compared to those of normal or testosterone-treated males. Thus, while androgens are clearly necessary for the growth of motoneurons in both the SNB and DLN, their different developmental patterns suggest that other factors must be involved in regulating this growth.

Quantitative analyses of intracellularly characterized and labeled thalamocortical projection neurons in the ventrobasal complex of primates

This study describes the architecture of neurons and individual dendritic arbors of thirteen intracellularly labeled thalamocortical projection neurons that respond to non-noxious stimuli from the primate (Macaca fascicularis or Macaca mulatta) ventrobasal complex (VB). The neurons compose a homogeneous morphological class with total dendritic lengths from 10,169 microns to 21,711 microns (mean 17,615 microns +/- 3,705). The labeled neurons were remarkably similar in most measured parameters including the number of dendrites (7.5 +/- 1.2), percentage of dichotomous branching (89.8% +/- 3.4), and contribution of terminal branches to total dendritic length (88.4% +/- 2.0). The individual dendrites ranged in total length from 443 microns to 7,657 microns with a mean of 2,346 microns (+/- 137, n = 98). There was a positive correlation between stem dendrite diameter and total dendrite length, making it possible to estimate the total size of an individual dendrite by measuring the stem dendrite diameter. There was only a small increase in mean path distance with increasing dendritic size at the whole neuron and individual dendritic levels, so that for individual dendrites the mean path distance of a dendrite consisting of only two segments was 199 microns, while the mean path distance for a dendrite with eight segments was only 45 microns longer. Analysis of dendrite diameter, segment order, and path distance shows that dendritic diameter is not reliable for determining the location of synaptic contacts viewed by electron microscopy onto dendritic trees. The small variation of measured parameters between these neurons presents a powerful tool for future developmental, plasticity and comparative studies of VB neurons.

Neonatal nerve injury causes long-term changes in growth and distribution of motoneuron dendrites in the rat

Disruption of neuromuscular contact by nerve-crush during the early postnatal period results in the death of a large proportion of affected motoneurons. Increased activity and abnormal reflex responses are evident in those that survive. We have studied the aberrant dendritic morphology of surviving cells and have attempted to correlate the observed alterations in morphology with the above experimental findings. Motoneurons supplying the extensor hallucis longus muscles of the rat were retrogradely labelled with cholera toxin subunit-B conjugated to horseradish peroxidase. The dendritic tree of labelled cells was analysed in adult animals having undergone unilateral sciatic nerve-crush at birth. Unoperated control animals were also examined. Following nerve-crush at birth, total visible dendritic length was more than 30% smaller than control cells in the transverse plane. This decrease was confined largely to the medially directed segments of the dendritic field and appeared to be due to a reduction in dendritic branching combined with a failure to achieve the correct branch length. There was no overall change in total visible dendritic length in the longitudinal plane, but a reorientation of dendrites in favour of rostrodorsal regions was observed. There was no alteration in dendritic length in cells contralateral to the nerve injury. These results show that nerve injury during early postnatal development produces lasting changes in the distribution of motoneuron dendrites. The localized nature of these changes may explain the altered activity and induced death of motoneurons seen after neonatal nerve-crush.

Activity-dependent interactions between motoneurones and muscles: their role in the development of the motor unit

In this review article we have attempted to provide an overview of the various forms of activity-dependent interactions between motoneurones and muscles and its consequences for the development of the motor unit. During early development the components of the motor unit undergo profound changes. Initially the two cell types develop independently of each other. The mechanisms that regulate their characteristic properties and prepare them for their encounter are poorly understood. However, when motor axons reach their target muscles the interaction between these cells profoundly affects their survival and further development. The earliest interactions between motoneurones and muscle fibres generate a form of activity which is in many ways different from that seen at later stages. This difference may be due to the immature types of ion channels and neurotransmitter receptors present in the membranes of both motoneurones and muscle fibres. For example, spontaneous release of acetylcholine may influence the myotube even before any synaptic specialization appears. This initial form of activity-dependent interaction does not necessarily depend on the generation of action potentials in either the motoneurone or the muscle fibre. Nevertheless, the ionic fluxes and electric fields produced by such interactions are likely to activate second messenger systems and influence the cells. An important step for the development of the motor unit in its final form is the initial distribution of synaptic contacts to primary and secondary myotubes and their later reorganization. Mechanisms that determine these events are proposed. It is argued that the initial layout of the motor unit territory depends on the matching of immature muscle fibres (possibly secondary myotubes) to terminals with relatively weak synaptic strength. Such matching can be the consequence of the properties of the muscle fibre at a particular stage of maturation which will accept only nerve terminals that match their developmental stage. Refinements of the motor unit territory follows later. It is achieved by activity-dependent elimination of nerve terminals from endplates that are innervated by more than one motoneurone. In this way the territory of the motor unit is established, but not necessarily the homogeneity of the physiological and biochemical properties of its muscle fibres. These properties develop gradually, largely as a consequence of the activity pattern that is imposed upon the muscle fibres supplied by a given motoneurone. This occurs when the motor system in the CNS completes its development so that specialized activity patterns are transmitted by particular motoneurones to the muscle fibres they supply.(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in dendritic morphology of rat spinal motoneurons during development and after unilateral target deletion

During normal development, motoneuron dendrites in the spinal nucleus of the bulbocavernosus (SNB) grow exuberantly to almost twice their adult length and then retract. In this study, we retrogradely labeled SNB motoneurons with cholera toxin B-conjugated horseradish peroxidase (BHRP) to examine the maturation of SNB dendritic arbors in more detail, particularly with regard to its spatial distribution and reorganization. The number and orientation of SNB motoneuron primary processes did not change over the first ten weeks of life. In contrast, total dendritic length, radial extent and arbor area increased significantly through the first four postnatal weeks and declined thereafter. The declines in length and extent were restricted to particular portions of the arbor, specifically the dorsal, ipsi- and contralateral projections. Estimates of the degree of overlap between the dendritic arbors from both sides of the SNB reflected these changes, with overlap initially increasing and then decreasing as the SNB established its adult dendritic morphology. To determine if dendritic interactions facilitated by this arbor overlap might be involved in regulating the normal retraction of SNB dendrites, we reduced SNB motoneuron numbers unilaterally by target muscle removal on the day of birth. Somal size, number and orientation of primary processes developed normally in unilateral muscle-extirpated animals. The dendritic morphology of surviving SNB motoneurons in unilateral muscle extirpated males was altered, with significant increases in dendritic length, extent and arbor area relative to those of normal males. These results indicate that substantial changes in dendritic organization of SNB motoneurons occur in normal development and may be influenced by interactions between dendrites from the two halves of the SNB.

The electrical geometry, electrical properties and synaptic connections onto rat V motoneurones in vitro

1. We have developed a tissue slice preparation which allows the study of the actions of single presynaptic neurones onto single trigeminal motoneurones in the immature rat. Our aim in this first stage of the work has been to assess the validity of this preparation as a model for responses obtained in vivo from trigeminal motoneurones in adult rats. We have quantified the integrative properties of the motoneurones and also the variability in transmission at synapses of single presynaptic neurones onto the motoneurones. This data has then been compared to similar published data obtained from adult (rat) trigeminal motoneurones in vivo. 2. Quantitative reconstructions were made of the morphology of three motoneurones which had been labelled with biocytin by intracellular injection. The neurones gave off six to nine dendrites, of mean length 522 microns (S.D. = 160; n = 22), which branched on average 10.5 times to produce 11.45 end-terminations per dendrite (S.D. = 8.57; n = 22). The mean surface area of the dendrites was 0.92 x 10(4) microns2 (S.D. = 0.67; n = 22), and, for individual cells, the ratio of the combined dendritic surface area to the total neuronal surface area ranged from 98.3 to 99.2% (n = 3). At dendritic branch points the ratio of the summed diameters of the daughter dendrites to the 3/2 power against the parent dendrite to the 3/2 power was 1.09 (S.D. = 0.21; n = 217), allowing branch points to be collapsed into a single cylinder. The equivalent cylinder diameter of the combined dendritic tree remained approximately constant over the proximal 25-40% of the equivalent electrical length of the dendritic tree and then showed tapering. The tapering could be ascribed to termination of dendrites at different electrical distances from the soma. 3. Electrical properties were determined for a total of eighty-seven motoneurones, all with membrane potentials more negative than 60 mV (mean = 66.0 mV; S.D. = 5.2) and spikes which overshot zero (mean spike amplitude = 77 mV; S.D. = 10.5; n = 87). The spikes were followed by after-hyperpolarizations (AHPs) of mean amplitude 2.2 mV (S.D. = 1.7; n = 47), and mean duration 54.1 ms (S.D. = 9.5; n = 47). The mean input resistance of the neurones was 7.5 M omega (S.D. = 2.5; n = 69), the mean membrane time constant was 3.5 ms (S.D. = 2.2; n = 35), and the mean rheobase was 1.6 nA (S.D. = 1.1; n = 56).(ABSTRACT TRUNCATED AT 400 WORDS)

Intracellular injection of neurobiotin or horseradish peroxidase reveals separate types of preganglionic neurons in the sacral parasympathetic nucleus of the cat

Sacral preganglionic neurons are essential to the neural control of the excretory and sex organs. Previously employed multi-cell tracing methods have certain limitations in the precise morphological analysis of the neural pathways that control these organs. These limitations were overcome by the intracellular injection of neurobiotin or horseradish peroxidase into single preganglionic neurons in the lateral sacral parasympathetic nucleus of the cat. Following light microscopic examination, these neurons, as a group, were found to have an average of five stem dendrites, which divided into 15 dendritic end-branches that were distributed among eight dendritic terminal fields. These dendrites had a major transverse orientation and were quite long, many of them reaching well into the dorsal and ventral horns and into the dorsal gray commissure. These dendrites also exhibited a major longitudinal orientation, extending an average of 869 microns (combined length of rostral and caudal dendrites) within the nucleus. Two groups of cells emerged on the basis of different dendritic patterns. Cells classed as Type I had dendrites in lamina I and in the ventral horn but lacked a significant projection into the lateral funiculus. Cells classed as Type II had major dendritic projections into the lateral funiculus but lacked dendrites in lamina I. The diverse dendritic patterns of these two cell types indicate dissimilar afferent control mechanisms and suggest that these preganglionic neurons may innervate different target organs.

Postnatal maturation of the dendritic fields of motoneuron pools supplying flexor and extensor muscles of the distal forelimb in the rat

In the rat cervical spinal cord the corticospinal projection on motoneurons either direct or indirect (via interneurons) comes about postnatally making it accessible for experimental research. Therefore, the postnatal developmental changes of motoneurons and in particular their dendritic fields were examined. Motoneurons innervating the two antagonistic muscles in the distal forepaw, the m. flexor digitorum profundus and the m. extensor digitorum communis, were retrogradely labelled by intramuscular injections of cholera toxin subunit B conjugated with horseradish peroxidase in rats of various postnatal ages. Following a 48–72 hour survival period the motoneurons and their dendritic fields were studied in the seventh and eighth cervical spinal cord segments. Both the number and the position of motoneurons were found to remain constant throughout postnatal development. Extensor motoneurons were positioned dorsolaterally in the ventral horn at the border of grey and white matter, flexor motoneurons were in general medial to extensor motoneurons. The results on the dendritic field demonstrate firstly, that during postnatal development the extension of the dendrites of both flexor and extensor motoneurons changes from spreading out in all directions at postnatal day 2 to spreading in only a few, specific directions from postnatal day 21 onwards, with the restriction that both motoneuron pools follow a different time scale to achieve this. Secondly, both pools have a temporal dendritic component extending into the white matter of the lateral funiculus.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural changes of the soleus and the tibialis anterior motoneuron pool during development in the rat

The morphological development of motoneuron pools of two hindlimb muscles of the rat, soleus (SOL) and tibialis anterior (TA), was studied in rats ranging in age between 8 and 30 postnatal days (P8-P30). Motoneurons were retrogradely labelled by injecting a cholera toxin B subunit solution directly into the muscles. This resulted in extensive labelling of motoneurons as well as their dendritic trees. The distribution of cross sectional areas of neuronal somata was determined for both muscles at various ages. Somal size increased considerably between P8 and P12, whereas growth was moderate between P12 and P20. The size distribution of SOL motoneurons was bimodal from P20, whereas the size distribution of TA motoneurons remained largely unimodal. The morphological development of the dendritic tree was studied qualitatively. The development of dendritic arborization within the SOL and the TA motoneuron pool showed major differences. The arborization pattern of dendrites of TA motoneurons was basically multipolar at all ages. In contrast, dendrites of SOL neurons tended to line up with the rostro-caudal axis and became organized in longitudinal bundles from P16 onwards. The relatively late appearance of dendrite bundles in the soleus motoneuron pool suggests that they might be related to the fine-tuning of neuronal activity rather than patterning of motor activity. The occurrence of dendrite bundles in SOL and not in TA motoneuron pools suggests that they may be related to the different afferent organization of this postural muscle or to its tonic activation pattern.

Restorative effects of reinnervation on the size and dendritic arborization patterns of axotomized cat spinal alpha-motoneurons

In a preceding paper [Brännström, et al. (1992) J. Comp. Neurol. 318:439-451] a marked reduction in dendritic size was observed in cat spinal motoneurons following permanent axotomy. The aim of the present study was to analyse the possible restorative effects of peripheral reinnervation on the size and dendritic branching patterns of cat spinal motoneurons which had been deprived of neuromuscular contact for an extended period of time. In adult cats the medial gastrocnemius (MG) nerve was transected and ligated. After 6 weeks the nerve was allowed to reinnervate its muscle through a nerve graft. With approximately 6 weeks needed for muscle reinnervation [Foehring, et al. (1986) J. Neurophysiol. 55:947-965], the MG motoneurons were devoid of neuromuscular contact for altogether about 12 weeks. Two years later reinnervated MG alpha-motoneurons were intracellularly labelled with horseradish peroxidase to allow quantitative analyses of the cell bodies and dendritic trees. Comparisons were made with previous data from normal and permanently axotomized MG motoneurons. The reinnervated motoneurons exhibited positive correlations between dendritic stem diameter, on one hand, and combined length, volume, membrane area, and number of end branches of the whole dendrite, on the other. By using the regression equations for these correlations, the total dendritic size of whole reinnervated motoneurons could be estimated. Such calculations showed that in comparison with the reduction in dendritic size found at 12 weeks after permanent axotomy (Brännström et al., see above), peripheral reinnervation caused the dendritic volume and membrane area to return to normal values. However, the values for combined dendritic length and number of dendritic end branches were still reduced by more than 25% as compared to the normal situation. The results indicate that following reinnervation of the target muscle, the axotomized motoneurons did not recover their original number of dendritic branches. The normalization of dendritic membrane area and volume was instead accomplished by two other mechanisms, namely an increase in dendritic diameters and an increased number of dendrites per neuron.

Changes in size and dendritic arborization patterns of adult cat spinal alpha-motoneurons following permanent axotomy

This study was performed to analyse quantitatively the changes in dimensions and dendritic branching patterns of adult cat spinal alpha-motoneurons following permanent axotomy, i.e., in a situation in which the transected motoraxons are prevented from reinnervating their peripheral target muscle. After transection and ligation of the medial gastrocnemius nerve of adult cats, homonymous alpha-motoneurons were intracellularly labelled with horseradish peroxidase and subjected to quantitative light microscopic analyses. The cell bodies and proximal dendrites were studied at 3, 6, and 12 weeks after the axotomy. An initial increase in cell body size at 3 weeks was followed by a gradual return towards normal values. The mean diameter of the stem dendrites was decreased at all time periods studied, and the combined diameter of the stem dendrites was reduced at 12 weeks after the axotomy. Entire dendritic trees were reconstructed at 12 weeks postoperatively, and the regression equations describing the correlations between dendritic stem diameter, on one hand, and the size of the entire dendrite, on the other, were used to calculate the total dendritic length, volume, and membrane area of whole axotomized motoneurons. The dendritic branching patterns were also analysed. In comparison with normal medial gastrocnemius alpha-motoneurons, the dendritic membrane area and volume of the axotomized cells had decreased by 36% and 29%, respectively, at 12 weeks after the axotomy. This reduction in dendritic size was due to a loss of preterminal and terminal dendritic segments. Abnormal dendritic elongations were observed in 2 of 16 completely reconstructed dendrites.

A light and electron microscopic study of intracellularly HRP-labeled lumbar motoneurons after intramedullary axotomy in the adult cat

In contrast to many other neurons in the central nervous system, spinal motoneurons in adult cats have been shown to regenerate their axons after an axotomy accomplished within the CNS compartment. This regenerative capacity may be the result of extrinsic influences, or intrinsic properties of the motoneurons themselves, or interactions between extrinsic and intrinsic factors. As part of the effort to establish circumstances of importance for this central regeneration, a detailed analysis of the morphology of lumbar motoneurons was performed 3-11 weeks following a ventral funiculus axotomy. Fourteen large neurons considered to be intramedullarly axotomized alpha motoneurons were labeled intracellularly with horseradish peroxidase. Twelve out of the fourteen analyzed neurons had an axonlike regenerating process. These twelve neurons could, in turn, be separated into two groups, based on the proximity of the axonal lesion and the proximal morphology of the regenerating process. Thus, after a comparatively proximal axotomy, new axons were produced, originating either from the cell soma or from a distal dendritic branch. After a more distal axotomy, but still intramedullarly, it seemed as if the proximal part of the original axon always persisted and subsequently regenerated. Analysis of the relation between the cell soma diameter and the diameter and number of its stem dendrites revealed that dendrites become thinner and also decrease in number after an intramedullary axotomy. In this way, it may be calculated that the total dendritic surface area of lesioned motoneurons will decrease by approximately half. In four neurons, most dendrites had an abnormal appearance in the light microscope with increasing diameter of distal branches. Ultrastructural analysis revealed that such dendrites were surrounded by myelin sheaths. Small filopodia in close relation to axon terminals were found to emerge from the cell membrane of the lesioned motoneurons. Their function may be to establish contact with presynaptic elements and then retract them to the cell membrane. We interpret the morphological changes of the motoneurons as signs of a large capacity for axonal regeneration, even after axotomy in the central nervous system.

Anatomy of dendrites in motoneurons supplying the intrinsic muscles of the foot sole in the aged cat: evidence for dendritic growth and neo-synaptogenesis

Motoneurons (MNs) supplying the intrinsic muscles of the foot sole (IFS) were studied in the aged cat (greater than 15y). Axon conduction velocity of IFS MNs was 30-40% slower in the aged than in young adult cats. IFS MNs that appeared intact during intracellular recordings and labeling with horseradish peroxidase (HRP) were subjected to anatomical investigation of their dendrites. The results were compared with corresponding data from young adult (less than 3y) cats. The average number of dendrites per IFS MN was twelve in both the aged and young adults. However, the branching was significantly more extensive in the aged cat, thus indicating that proliferation of dendritic branches may occur during the later part of life. Topological analysis revealed a significant difference in the frequency distributions of nodal vertices between young adult and aged cats. In the young adult, the dendritic branching pattern was compatible with trees generated by outgrowth from terminal segments, while in the aged there was a clear indication of collateral outgrowth of branches. The dendritic path distance and the length of terminal branches were similar in young adults and aged. The length of preterminal branches was shorter in the aged, while the combined dendritic length of a dendrite was larger compared to young adults. These data are consistent with the topological data, and add further evidence that the proliferation of branches in the aged cat may also take place from preterminal branches. Light microscopic analysis revealed the presence of "growth cone-like" extensions in the dendrites of the aged cats. Such profiles were not encountered in dendrites from young adults. Electron microscopic observations showed that these "growth cone-like" formations were not artifacts and that they were apposed by numerous axonal boutons, of which a number made synaptic contact. A distinct feature of the extensions was their rich content of mitochondria and membranous elements. It was suggested that these "growth cone-like" formations were sites at which novel synaptic connections are established, and that they may represent the initial stage of an outgrowth of new dendritic branches in the aged cat. Local dendritic branch diameter related closely to the amount of dendritic membrane area located distally in both young adults and aged. Curve fitting disclosed that this relationship was quite similar for both age groups, despite concurrent differences in combined dendritic length and branching degree.

Morphometric analysis of phrenic motoneurons in the cat during postnatal development

The dendritic geometry of 20 phrenic motoneurons from four postnatal ages (2 weeks, 1 and 2 months, and adult) was examined by using intracellular injection of horseradish peroxidase. The number of primary dendrites (approximately 11-12) remained constant throughout postnatal development. In general, postnatal growth of the dendrites resulted from an increase in the branching and in the length and diameter of segments at all orders of the dendritic tree. There was one exception. Between 2 weeks and 1 month, the maximum extent of the dendrites increased in parallel with the growth of the spinal cord; however, there was no increase in either combined dendritic length or total membrane surface area. In addition, there was a significant decrease in the number of dendritic terminals per cell (59.8 +/- 9.3 vs. 46.4 +/- 7.4 for 2 weeks and 1 month, respectively). The distance from the soma, where the peak number of dendritic terminals per cell occurred, ranged from 700-900 microns at 2 weeks and 2 months to 1,300-1,700 microns in the adult. The diameter of dendrites as a function of distance from the soma along the dendritic path increased with age. The process of maturation tended to increase the distance from the soma over which the surface area and dendritic trunk parameter (sigma d1.5/D1.5) remained constant. The three-dimensional distribution of dendrites was analyzed by dividing space into six equal volumes or hexants. This analysis revealed that the postnatal growth in surface area in the rostral and caudal hexants was proportionately larger than that in either the medial, lateral, dorsal, or ventral hexants. Strong linear correlations were found between the diameter of the primary dendrite and the combined length, surface area, volume, and number of terminals of the dendrite at all ages studied.

Postnatal development of cat hind limb motoneurons supplying the intrinsic muscles of the foot sole

The postnatal development of dendrite anatomy in alpha-motoneurons intracellularly labeled with horseradish peroxidase (HRP), innervating the intrinsic muscles of the sole of the foot (IFS MNs) in the cat, was investigated. The number of dendrites per neuron was about 11 and did not change from birth to adult. The number of branches per dendrite decreased during the same period by 20-25%. The net elimination of dendritic branches appeared to occur at distal branching points, as revealed by topological analysis. The dendritic branching pattern tended to be asymmetric at birth and the net decrease in dendritic branching postnatally did not alter this pattern. The length of preterminal branches (PTB) increased by a factor of 2, while terminal branch (TB) length increased by a factor of 3.3 postnatally. The large increase in TB length was attributed to both longitudinal growth and an apparent lengthening caused by resorption of distal branches during development. Dendritic length in the transverse spinal cord plane increased in parallel with the overall growth of the parent spinal cord segment, while dendritic growth along the rostro-caudal axis exceeded, by about one order of magnitude, dendritic growth in the transverse plane. Average branch diameter doubled from birth to adult. The decrease in branch diameter across branching points did not obey satisfactorily to the 'power rule' of Rall. However, the 1.5 power ratio of daughters-to-parents branch dropped from 1.18 to 1.08 between 3 weeks of age and adult. Tapering was evident in both PTBs and TBs. The rate of taper did not change postnatally. From birth onwards, 'local' branch diameter correlated closely with amount of membrane area and combined length of the dendritic branches located distal to the 'supporting' parent branch. These relations were similar in all age groups and are suggested to be properties intrinsic to the IFS MNs. The local branch diameter also co-varied with the number of distal dendritic branches, but in this case there was a systematic shift in the relationship with increasing postnatal age. It appears that the local diameter in IFS MN dendrites is a key indicator of the size of the distal dendritic arborization.

Dendritic morphology of pyramidal neurones of the visual cortex of the rat: II. Parameter correlations

This study concerns the correlations between the various morphometric parameters obtained for the dendrites of neocortical pyramidal cells. The primary aims were to uncover underlying design principles in dendritic morphology, to see if these differed between different types of dendrite, and to see if estimates of parameters such as total dendritic shaft membrane area could be obtained from a limited number of measurements, avoiding the need to measure every dendritic segment. The data were from a sample of 39 pyramidal neurones, from layers 2/3 and 5 of the visual cortex of the rat, that had been injected with horseradish peroxidase, reconstructed, and measured with the light microscope as part of an earlier study (Larkman and Mason, '90: J. Neurosci. 10:1407-1414). Correlations between the somal area or the combined diameters of the stem segments and measures of the overall size of the dendrites were generally weak. For basal dendrites, the size of a tree was correlated with both its number of tips and the diameter of its stem segment, but these correlations were weaker for apical dendrites. Within individual cells, the diameter of any basal segment was closely related to the size of the tree arising from it, and quantitatively similar relations applied to apical oblique trees from the same cell. Terminal arbor trees showed relations that were similar in pattern but differed quantitatively, whereas apical trunk segment diameter correlations were often weak. In all cases, the number of tips in a tree was closely related to its size. Segment lengths, however, were not closely related to the size of the trees arising from them. It appears that at least some aspects of pyramidal dendritic morphology obey simple design rules. There was heterogeneity between trees of different types, although basal and oblique trees were very similar in most respects. It should prove possible to make use of correlations to estimate the sizes of basal, oblique, and terminal arbor trees from a limited number of measurements, but this does not seem to be possible for apical trunks.

Morphometric analysis of developing neuronal geometry in the dorsal cochlear nucleus of the hamster

Since the shape of a cell's dendritic field and the distribution of its input determine the information that a cell receives and transmits, it is important to ascertain how the spatial relations between axonal arbors and dendritic fields develop. This study investigates the development of the dendritic fields of fusiform cells in the dorsal cochlear nucleus. Golgi-impregnated cells from postnatal day 10, 15, 25, 45 and 60+ hamsters were reconstructed. The computerized morphometric system used analyzed cells in the plane of section and also allowed rotation to and analysis in other specified planes, such as that parallel to the terminal arbors of the cochlear nerve fibers. The results suggest that the apical dendritic fields are oriented parallel to the axis of cochleotopic organization and that this orientation develops gradually after birth. Dendritic growth is the result of addition of new dendritic branches. The angles between branches also change and were analyzed by viewing the dendritic fields in different planes. This revealed that some preferential expansion of the apical dendritic field in the plane parallel to the cochlear projection planes is the result of increased angles between branches in that plane. Thus, dendritic growth via the addition of branches and the 'fanning-out' of existing branches underlie the development of oriented dendritic fields.

Postnatal development of peroneal motoneurons in the kitten

In 1- to 72-day-old kittens, motoneurons of the 3 peroneal muscle nuclei were labeled by retrograde axonal transport of horseradish peroxidase from individual muscles. At birth, the locations of peroneal nuclei were similar to those of the adult cat. Counts of motoneurons at different ages indicated that postnatal cell death does not occur in peroneal motor nuclei. Primary dendrites were as numerous in motoneurons of newborn kittens as in adult motoneurons but they were thinner, shorter and poorly ramified. The number of recurrent axon collaterals was higher in the first postnatal week than at later stages. The growth of motoneurons followed similar rates in the 3 peroneal nuclei. Distributions of cell body diameters and volumes were unimodal at birth and became bimodal between 15 and 20 days postnatal. The separation of peroneal motoneurons in two size subgroups, presumably corresponding to alpha and gamma populations, was followed by an increase in growth rate which became faster for alpha than for gamma motoneurons.

Postnatal development of phrenic motoneurons in the cat

The postnatal growth of phrenic motoneurons in the cat was studied using retrograde transport of horseradish peroxidase (HRP). The mean somal surface area of these developing motoneurons increased 2.5 times from day 3 to adult while the mean somal volume increased four-fold. This change in mean somal surface area during postnatal development was found to be correlated with the change in mean axonal conduction velocity measured from phrenic motoneurons.

Morphology of developing motoneurons innervating the medial gastrocnemius of the cat

The morphology of medial gastrocnemius (MG) motoneurons labeled by retrograde transport of horseradish peroxidase was quantified in 5 postnatal ages (3 to 79-86 days) and in adults. A bimodal distribution of somal volumes was evident at birth which permitted separating the motoneurons into alpha and gamma subpopulations for analysis. There was a significant increase in the axial dimensions, surface area and volume calculated for both alpha and gamma cell bodies between each of the age-groups studied. A greater relative growth of the major over minor axis for the gammas produced a significant decrease in the form factor (i.e. greater eccentricity) between the youngest and oldest age-groups. The number of primary dendrites observed remained constant throughout postnatal development. The surface area of alpha somata more than tripled while that of the gammas doubled from 3 days to the adult. The mean somal volume of an alpha motoneuron at birth was only 17% of its adult value while the gamma cell bodies were 33% of their adult volume. A positive correlation was found for both alpha and gamma motoneurons when their somal surface area was plotted against postnatal age and weight. The rate of growth of the MG somal surface area is compared to the changes found in axonal conduction velocity and axonal diameter for MG in the literature.

Postnatal growth of genioglossal motoneurons

The postnatal growth of kitten genioglossal motoneurons were examined in six different age groups (newborn, 2, 4, 8, and 12 weeks and adult) using the technique of retrograde transport of horseradish peroxidase (HRP). The cell bodies of 100-150 motoneurons in each age group were analyzed in a transverse plane of section using standard techniques. Somatic genioglossal motoneuron growth occurred primarily along the major axis, which increased from 25.2 microns to 41.3 microns between birth and 8 weeks of postnatal age, after which time there was no further increase in either major or minor dimension of the cell body. The form factor decreased from 0.94 to 0.80 from birth to adulthood indicating an increased eccentricity of the cell body. The number of primary dendrites visible with this technique remained constant throughout the postnatal period. Calculated somal surface area increased in a linear fashion from birth through 8 weeks of postnatal life. There was no further increase in surface area beyond this age. The rate of increase in somal surface area with age was significantly different from both the rate of increase of animal weight and animal surface area with age. The correlations between the demonstrated immature genioglossal morphology and its cellular electrophysiology or integrated respiratory function remain unknown. The recent demonstration of decreased activation of the genioglossus muscle following airway occlusion in premature infants with apnea suggests that the relationships between developing genioglossal motoneuron structure and function warrant further investigation.

Postnatal development of cat hind limb motoneurons. II: In vivo morphology of dendritic growth cones and the maturation of dendrite morphology

The maturation of dendrite morphology was studied by light and electron microscopy in cat spinal alpha-motoneurons intracellularly labeled with horseradish peroxidase. Alpha-motoneurons supplying the triceps surae (TS) and the intrinsic foot sole (SP) muscles were investigated in kittens from birth to 44-46 days of postnatal (d.p.n.) age. At birth, a large number of dendritic branches displayed growth cones, filopodia, and fusiform processes. The growth cones were of lamellipodial and filopodial types, but intermediate forms also occurred. The growth cones shared several morphological features with the neuritic growth cones studied in vitro. It was suggested that the occurrence of different types of growth cones--even in the same dendrite--may reflect their transformation from one type to the other and the level of growth activity could be inferred from the number and form of the growth cones. About 50-70% of the terminal branches in the dendrites of newborn kittens possessed growth cones, filopodia, and/or fusiform processes. The corresponding figure for preterminal branches was 20-30%, with a clear decrease in incidence when approaching the soma. During the period under study, most of these growth-associated processes disappeared from the dendrites so that at 44-46 d.p.n. of age only about 10% of the terminal and less than 1% of the preterminal branches had growth-associated processes. Analysis of the three-dimensional distribution of dendritic branches with such processes disclosed that they were relatively more frequent in the medial, rostral, and caudal dendritic territories. It was concluded that the pattern of distribution and disappearance of growth cones, filopodia, and fusiform processes coincided with postnatal longitudinal dendritic growth and the development of the adult dendritic territories described in a preceding paper (Ulfhake et al., '88). Dendritic growth, with respect to length and caliber, also occurred in the absence of growth cones and filopodia. It is suggested that the important role of these processes may be to act as a steering device in establishing the adult distribution and synaptology of the dendrites. Comparison of TS and SP alpha-motoneuron dendrite morphology at birth and at 22-24 d.p.n. age showed that the SP neurons lagged in the maturation process. Light and electron microscopic observations indicated that postnatally direct contacts might exist between dendrites and fine blood vessels in the neuropil without any interposing glial sheath. The number of such suspected contacts diminished during the period under study, indicating that the glial ensheathment of the blood vessel takes place, in part, postnatally.

Postnatal development of cat hind limb motoneurons. III: Changes in size of motoneurons supplying the triceps surae muscle

The postnatal changes of neuronal dimensions were studied in cat triceps surae motoneurons intracellularly labeled with horseradish peroxidase. Systematic correlations were observed in the analysis of single dendrites at each studied stage, from birth to 44-46 days post natum (d.p.n.) age, between size parameters intrinsic to the dendrites as the diameter of a 1st-order dendrite, the combined dendritic length, the dendritic membrane area, and the degree of branching. Some variability among samples was evident in each studied age group. The correlations were, however, sufficiently close to permit indirect estimations of both combined dendritic length and dendritic membrane area for larger samples of neurons from data on dendritic stem caliber. The total postnatal increase in dendritic membrane area was, on the average, 400%, i.e., from close to 100 X 10(3) microns2 to about 500 X 10(3) microns2. The corresponding increase in soma area amounted to 100%. Analysis revealed that there was a time lag between the increase in somatic and dendritic size. Thus, adult somatic dimensions were attained at age 44-46 d.p.n.; however, at this stage, the mean total dendritic membrane area was only about half of the adult value. The postnatal increase in size appeared to vary among neurons, yielding a wider neuronal size spectrum in the adult cat than that observed in kittens. The measured increase in size corresponded to a calculated average addition of dendritic membrane area of 3700 microns2/day from birth to 22-24 d.p.n. and from that stage to 44-46 d.p.n. of 2700 microns2 per day. Likewise, the increase in combined dendritic length could initially be as large as 1 mm/day down to 0.4 mm/day between 22-24 and 44-46 d.p.n., with a mean growth during the first 44-46 d.p.n. of 0.5 to 0.6 mm/day. The ratios of daughters to parent branch diameters (sigmadd1.5: dp1.5) and the dendritic trunk parameter (sigma d1.5) recorded along the proximodistal dendritic path distance revealed transient changes that might impact on the electrotonic properties of the dendrites during postnatal development. Computations from the measured changes in dendritic branch lengths and calibers indicated that if membrane and internal resistivity remain unaltered during postnatal development, the dendritic domain is electrotonically more compact in the newborn kitten than in the adult cat.(ABSTRACT TRUNCATED AT 400 WORDS)