Postnatal development of phrenic motoneurons in the cat

Structural and functional identification of two distinct inspiratory neuronal populations at the level of the phrenic nucleus in the rat cervical spinal cord

The diaphragm is driven by phrenic motoneurons that are located in the cervical spinal cord. Although the anatomical location of the phrenic nucleus and the function of phrenic motoneurons at a single cellular level have been extensively analyzed, the spatiotemporal dynamics of phrenic motoneuron group activity have not been fully elucidated. In the present study, we analyzed the functional and structural characteristics of respiratory neuron population in the cervical spinal cord at the level of the phrenic nucleus by voltage imaging, together with histological analysis of neuronal and astrocytic distribution in the cervical spinal cord. We found spatially distinct two cellular populations that exhibited synchronized inspiratory activity on the transversely cut plane at C4–C5 levels and on the ventral surface of the mid cervical spinal cord in the isolated brainstem–spinal cord preparation of the neonatal rat. Inspiratory activity of one group emerged in the central portion of the ventral horn that corresponded to the central motor column, and the other appeared in the medial portion of the ventral horn that corresponded to the medial motor column. We identified by retrogradely labeling study that the anatomical distributions of phrenic and scalene motoneurons coincided with optically detected central and medial motor regions, respectively. Furthermore, we anatomically demonstrated closely located features of putative motoneurons, interneurons and astrocytes in these regions. Collectively, we report that phrenic and scalene motoneuron populations show synchronized inspiratory activities with distinct anatomical locations in the mid cervical spinal cord.

Breathing: Motor Control of Diaphragm Muscle

Breathing occurs without thought but is controlled by a complex neural network with a final output of phrenic motor neurons activating diaphragm muscle fibers (i.e., motor units). This review considers diaphragm motor unit organization and how they are controlled during breathing as well as during expulsive behaviors.

Control of breathing activity in the fetus and newborn

Breathing movements have been demonstrated in the fetuses of every mammalian species investigated and are a critical component of normal fetal development. The classic sheep preparations instrumented for chronic fetal monitoring determined that fetal breathing movements (FBMs) occur in aggregates interspersed with long periods of quiescence that are strongly associated with neurophysiological state. The fetal sheep model also provided data regarding the neurochemical modulation of behavioral state and FBMs under a variety of in utero conditions. Subsequently, in vitro rodent models have been developed to advance our understanding of cellular, synaptic, network, and more detailed neuropharmacological aspects of perinatal respiratory neural control. This includes the ontogeny of the inspiratory rhythm generating center, the preBötzinger complex (preBötC), and the anatomical and functional development of phrenic motoneurons (PMNs) and diaphragm during the perinatal period. A variety of newborn animal models and studies of human infants have provided insights into age-dependent changes in state-dependent respiratory control, responses to hypoxia/hypercapnia and respiratory pathologies.

Anatomical changes of phrenic motoneurons during development

Although the phrenic motoneurons are relatively well-developed at the time of birth as compared to non-respiratory motoneurons, they show distinct anatomical changes during postnatal development. In the present review we summarize anatomical changes of phrenic motoneurons during pre- and postnatal development. Cell bodies of phrenic motoneurons migrate into the ventromedial region of the ventral horn of C3-C6 by E13-E14 in the rat. During development the sizes and surface areas of phrenic motoneurons are increased with changes in dendritic morphology.

Trophic factor expression in phrenic motor neurons

The function of a motor neuron and the muscle fibers it innervates (i.e., a motor unit) determines neuromotor output. Unlike other skeletal muscles, respiratory muscles (e.g., the diaphragm, DIAm) must function from birth onwards in sustaining ventilation. DIAm motor units are capable of both ventilatory and non-ventilatory behaviors, including expulsive behaviors important for airway clearance. There is significant diversity in motor unit properties across different types of motor units in the DIAm. The mechanisms underlying the development and maintenance of motor unit diversity in respiratory muscles (including the DIAm) are not well understood. Recent studies suggest that trophic factor influences contribute to this diversity. Remarkably little is known about the expression of trophic factors and their receptors in phrenic motor neurons. This review will focus on the contribution of trophic factors to the establishment and maintenance of motor unit diversity in the DIAm, during development and in response to injury or disease.

Effect of weight and age on respiratory complexity in premature neonates

Very low-birth-weight premature infants often suffer from a variety of respiratory problems, including respiratory distress syndrome (RDS), hypopnea and periodic breathing, and apnea. These conditions are likely related to immaturity of the respiratory centers; yet how respiratory rhythms originating from these centers, including their complexity, relate to demographic measures of prematurity remains largely unknown. In 39 neonates with mild RDS (22 males, 28 ± 2 wk gestational age, 1,036 ± 234 g body wt), we derived the univariate association between complexity of two respiratory rhythms [respiratory rate (RR) and tidal volume (Vt)] and postmenstrual age, gestational age, postnatal age, and weight at time of study. RR and Vt rhythm complexities were assessed using approximate entropy, sample entropy, base scale entropy, and forbidden words entropy estimated for 300 consecutive breaths determined from respiratory inductance plethysmography, irrespective of breathing effort rate or amplitude, collected during sleep while the neonates were exposed to nasal continuous positive airway pressure (4–6 cmH2O). RR and Vt exhibited increased complexity with increased maturity, but only in terms of base scale entropy and forbidden words entropy, which are based on pattern matching, rather than approximate entropy and sample entropy, which are based on conditional probabilities. Specifically, RR complexity as measured by forbidden word entropy increased with increasing weight ( r = 0.502), postconceptional age ( r = 0.423), and gestational age ( r = 0.493). As measured by base scale entropy, RR complexity increased with increasing weight ( r = 0.488) and postconceptional age ( r = 0.390). Vt complexity, measured by base scale entropy, was greater with increased postnatal age ( r = 0.428). Our results indicate that respiratory rhythms become more complex with increasing levels of maturity, as indicated by increased weight and several age parameters. This emphasizes the importance of the later weeks of gestation in the maturation of respiratory centers in the brain and suggests a promising use of entropy measures in exploring respiratory maturation in infants.

Key aspects of phrenic motoneuron and diaphragm muscle development during the perinatal period

At the time of birth, respiratory muscles must be activated to sustain ventilation. The perinatal development of respiratory motor units (comprising an individual motoneuron and the muscle fibers it innervates) shows remarkable features that enable mammals to transition from in utero conditions to the air environment in which the remainder of their life will occur. In addition, significant postnatal maturation is necessary to provide for the range of motor behaviors necessary during breathing, swallowing, and speech. As the main inspiratory muscle, the diaphragm muscle (and the phrenic motoneurons that innervate it) plays a key role in accomplishing these behaviors. Considerable diversity exists across diaphragm motor units, but the determinant factors for this diversity are unknown. In recent years, the mechanisms underlying the development of respiratory motor units have received great attention, and this knowledge may provide the opportunity to design appropriate interventions for the treatment of respiratory disease not only in the perinatal period but likely also in the adult.

Influence of the vagus nerve on respiratory patterns during early maturation

Nonlinear dynamical analysis was performed on the phrenic neurogram before and after vagotomy in order to study the influence of the vagus nerve on the complexity of the phrenic neurogram in piglets in three age groups: 3-7 days (n = 7); 11-19 days (n = 6); and 29-34 days (n = 8). The phrenic neurogram, generated by the respiratory neural networks in the medulla, projects on the diaphragm muscles and initiate the respiratory movement. On the other hand, the vagus nerves carry the information from mechanoreceptors, located at the lower airway and lungs, to the medulla. The data was recorded during normal breathing (eupnea) before and after vagotomy while piglets were ventilated with 40% O2 in N2 and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy before and after vagotomy during the first 7 days of the postnatal age were 1.32 +/- 0.1 (standard deviation) and 1.34 +/- 0.07, respectively. These values before and after vagotomy during the 11-19 days age group were 1.15 +/- 0.09 and 1.12 +/- 0.05, respectively. For the 29-34 days age group, they were 1.14 +/- 0.05 before vagotomy and 1.19 +/- 0.08 after vagotomy. These differences in the ApEn (complexity) values of the phrenic neurogram before and after vagotomy are not statistically different at each age group. However, the mean mean approximate entropy (complexity) values between the 3-7 days age group and the other two groups were significantly different both before and after vagotomy (p < 0.05) using an analysis off variance test. These results suggest that the vagus nerve may not be mature during early maturation in piglets.

The Effects of Maturation on Early and Late Phases of Phrenic Neurogram During Eupnea

In this paper, we investigate the effects of maturation on the early and late phases of the phrenic neurogram. We have used the matching pursuit (MP) method to examine the effects of maturation on breathing patterns in both time and frequency domains. The MP was chosen since the wavelet transform method may not represent signals whose Fourier transforms have a narrow high-frequency support. The phrenic neurogram was recorded from 25 piglets (3-35 days) during eupnea (normal breathing) at three postnatal age groups: young (3-7 days (n = 9)), (middle) 10-21 days (n = 6), and old (29-35 days (n = 10)). The energy percentage of atoms representing the nonperiodic neural activities (NPNAs) significantly decreased from young age to middle age groups (p<0.01) and from young age to old age groups (p<0.01), and from middle age to old age groups (p<0.055) in the early phase (the first half) of the phrenic neurogram, but these changes were not statistically significant in the late phase (the second half) of the phrenic neurogram as maturation proceeded. However, the energy percentage of atoms representing the periodic neural activities (PNAs) decreased with maturation, but these changes were not statistically significant in the early phase of the phrenic neurogram. The energy percentage of (PNAs) increased in the late phase of the phrenic neurogram as maturation proceeded although these changes were only significant between young and old age groups (p<0.01). These results suggest that the significant decrease of the NPNAs in the early phase and the increase in the late phase of the phrenic neurogram could be a sign of maturation in piglets.

Growth-related changes in cell body size and succinate dehydrogenase activity of spinal motoneurons innervating the rat soleus muscle

Cell body sizes and oxidative enzyme (succinate dehydrogenase) activities of spinal motoneurons innervating the soleus muscle were determined in rats ranging in postnatal age from 3 to 13 weeks. The soleus motoneurons were labeled by a retrograde neuronal tracer, nuclear yellow. The mean cell body sizes of motoneurons increased from 3 to 7 weeks of age, while the mean succinate dehydrogenase activities of motoneurons decreased from 3 to 7 weeks of age. There were no changes in mean cell body size or mean succinate dehydrogenase activity of motoneurons from 7 to 13 weeks of age. An inverse relationship between cell body size and succinate dehydrogenase activity of motoneurons was observed, irrespective of age. These results indicate that motoneurons innervating the rat soleus muscle show the adult pattern of cell body size and succinate dehydrogenase activity at an earlier stage of postnatal growth, 7 weeks of age.

AGE RELATED ALTERATIONS IN THE COMPLEXITY OF RESPIRATORY PATTERNS

The objective of this study is to investigate the relative contributions of maturation to the dynamic behavior of respiration during ontogeny in the neonate. The phrenic neurogram, an output of the respiratory network, was recorded during eupnea at three postnatal ages (3-7, 12-19, and 26-31 days) in decerebrate piglets. The inter-breath interval (IBI) time series were reconstructed from phrenic neurograms for each piglet and analyzed using the approximate entropy (ApEn) method. The mean values of the approximate entropy were high during the first seven days of the postnatal age [1.02 +/- 0.02 (standard error)] and decreased for the 12-19 days (0.67 +/- 0.008) and increased during subsequent maturation [the 26-31 days age group (0.92 +/- 0.015)]. The mean approximate entropy values for the 3-7 days age group was significantly different from those of the 12-19 days age groups (p < 0.05) using the nonparametric statistical test. The complexity values for the 3-7 days age group were slightly higher than those of the 26-31 days age group, but these differences were not statistically significant (p > 0.05). Therefore, these findings suggest that a decrease in the complexity values is unique to the 12-19 days age groups. This could be due to a reduction in the number of dendritic terminals per cell for the 12-19 days age groups. The results of these preliminary experiments also indicate that the behavior of the respiratory pattern generator in the neonate fluctuates during the early maturation period.

Effects of hypoxia on the complexity of respiratory patterns during maturation

During hypoxic gasping, the hypoxic neurogram has a steeper rate of rise, an augmented amplitude, and a shorter duration than is seen during eupnea. Because hypoxia reduces neural activity, we hypothesized that gasping would be characterized by low complexity (irregularity) values compared with eupnea in piglets. In this study, we define and quantify changes in the complexity of the phrenic neurogram, the output of the respiratory neural network in piglets using the approximate entropy (ApEn) method which provides a model independent measure of the complexity of the phrenic neurogram. The phrenic neurogram in vagotomized, peripherally chemodenervated, decerebrated piglets was recorded from the C5 phrenic nerve during eupnea and gasping at four postnatal ages; 3-6 days of age (n=8), 7-13 days of age (n=3), 15-21 days of age (n=4), 29-35 days of age (n=10). Nonlinear dynamical analysis of the phrenic neurogram was performed using the approximate entropy method. The mean approximate entropy values for a recording of 5 consecutive breaths during eupnea and 6-29 consecutive breaths during gasping for each piglet in each group during eupnea was calculated. Our results suggested that the mean approximate entropy values for the 3-6 days age group were 1.46+/-0.003 during eupnea and 0.85+/-0.001 during hypoxic gasping. For the 7-13 days age group, the mean approximate entropy values were 1.35+/-0.009 during eupnea and 1.00+/-0.001 during hypoxic gasping. For the 15-21 days age group, they were 1.33+/-0.005 during eupnea and 0.94+/-0.001 during hypoxic gasping. Finally, for the 29-35 days age group, they were 1.38+/-0.002 during eupnea and 0.93+/-0.001 during hypoxic gasping. The shift from eupnea to gasping caused a drastic drop in the mean values of the approximate entropy values at each of these four age groups. These differences in the complexity values of the phrenic neurogram between eupnea and gasping are statistically different at each age group (p<0.001). These findings suggest that during hypoxic gasping, regardless of degree of development, the output of the central pattern generator becomes less complex probably because hypoxia reduces the neural activity.

Neuron addition and enlargement in juvenile and adult animals

Locomotion requires bilateral symmetry of neural circuitry in the spinal cord. Although not well understood, the mechanisms responsible for establishing and maintaining this symmetry must balance the numbers, sizes, and connectivity of the neurons on both sides of the spinal cord. Those mechanisms do not cease to function after embryogenesis, since there is substantial evidence that these properties continue to change as juvenile animals grow to adult size. We review the evidence that spinal neuron number and size increase in growing juvenile frogs and mammals. We postulate that these increases are regulated by both local and systemic factors. In addition, we discuss evidence that axotomy of spinal sensory and motor neurons also enlists local and systemic regulatory factors, some of which may also be operative in normal growth and development.

Phrenic motoneuron morphology during rapid diaphragm muscle growth

In the adult rat, there is a general correspondence between the sizes of motoneurons, motor units, and muscle fibers that has particular functional importance in motor control. During early postnatal development, after the establishment of singular innervation, there is rapid growth of diaphragm muscle (Dia(m)) fibers. In the present study, the association between Dia(m) fiber growth and changes in phrenic motoneuron size (both somal and dendritic) was evaluated from postnatal day 21 (D21) to adulthood. Phrenic motoneurons were retrogradely labeled with fluorescent tetramethylrhodamine dextran (3,000 MW), and motoneuron somal volumes and surface areas were measured using three-dimensional confocal microscopy. In separate animals, phrenic motoneurons retrogradely labeled with choleratoxin B-fragment were visualized using immunocytochemistry, and dendritic arborization was analyzed by camera lucida. Between D21 and adulthood, Dia(m) fiber cross-sectional area increased by approximately 164% overall, with the growth of type II fibers being disproportionate to that of type I fibers. There was also substantial growth of phrenic motoneurons ( approximately 360% increase in total surface area), during this same period, that was primarily attributable to an expansion of dendritic surface area. Comparison of the distribution of phrenic motoneuron surface areas between D21 and adults suggests the establishment of a bimodal distribution that may have functional significance for motor unit recruitment in the adult rat.

Postnatal development of rat nucleus tractus solitarius neurons: morphological and electrophysiological evidence

Postnatal development of neurons in the caudal nucleus tractus solitarii of rats was studied using the Golgi-Cox technique and whole-cell recordings. Two cell classes were defined on the basis of somatic and dendritic morphology. Elongated neurons have two thick primary dendrites originating from the long axis of the soma. The primary dendrites, tapering distally, give rise to one to four secondary dendrites. Multipolar neurons have pyramidal somas. Extending from each apex of the cell body was a long primary dendrite, which gave rise to a variable number of secondary dendrites. The relative proportion of the two classes was rather constant from birth to adulthood. During the first two postnatal weeks, dendritic length and area of influence increase, but neuronal geometry is not altered in either class. Dendritic appendages appear by postnatal day 5, reach a peak at postnatal day P12 and then almost disappear in adult neurons. Combined intracellular injection of neurobiotin and whole-cell recordings indicate that morphological alteration of caudal nucleus tractus solitarius neurons occurs in parallel with changes in passive properties and spike characteristics. However, the firing pattern of discharge is not correlated with morphology. The physiological significance of these results is discussed.

Electrophysiological properties of rat phrenic motoneurons during perinatal development

Past studies determined that there is a critical period at approximately embryonic day (E)17 during which phrenic motoneurons (PMNs) undergo a number of pivotal developmental events, including the inception of functional recruitment via synaptic drive from medullary respiratory centers, contact with spinal afferent terminals, the completion of diaphragm innervation, and a major transformation of PMN morphology. The objective of this study was to test the hypothesis that there would be a marked maturation of motoneuron electrophysiological properties occurring in conjunction with these developmental processes. PMN properties were measured via whole cell patch recordings with a cervical slice-phrenic nerve preparation isolated from perinatal rats. From E16 to postnatal day 1, there was a considerable transformation in a number of motoneuron properties, including 1) 10-mV increase in the hyperpolarization of the resting membrane potential, 2) threefold reduction in the input resistance, 3) 12-mV increase in amplitude and 50% decrease duration of action potential, 4) major changes in the shapes of potassium- and calcium-mediated afterpotentials, 5) decline in the prominence of calcium-dependent rebound depolarizations, and 6) increases in rheobase current and steady-state firing rates. Electrical coupling among PMNs was detected in 15-25% of recordings at all ages studied. Collectively, these data and those from parallel studies of PMN-diaphragm ontogeny describe how a multitude of regulatory mechanisms operate in concert during the embryonic development of a single mammalian neuromuscular system.

An overview of phrenic nerve and diaphragm muscle development in the perinatal rat

In this overview, we outline what is known regarding the key developmental stages of phrenic nerve and diaphragm formation in perinatal rats. These developmental events include the following. Cervical axons emerge from the spinal cord during embryonic (E) day 11. At approximately E12.5, phrenic and brachial axons from the cervical segments merge at the brachial plexi. Subsequently, the two populations diverge as phrenic axons continue to grow ventrally toward the diaphragmatic primordium and brachial axons turn laterally to grow into the limb bud. A few pioneer axons extend ahead of the majority of the phrenic axonal population and migrate along a well-defined track toward the primordial diaphragm, which they reach by E13.5. The primordial diaphragmatic muscle arises from the pleuroperitoneal fold, a triangular protrusion of the body wall composed of the fusion of the primordial pleuroperitoneal and pleuropericardial tissues. The phrenic nerve initiates branching within the diaphragm at approximately E14, when myoblasts in the region of contact with the phrenic nerve begin to fuse and form distinct primary myotubes. As the nerve migrates through the various sectors of the diaphragm, myoblasts along the nerve's path begin to fuse and form additional myotubes. The phrenic nerve intramuscular branching and concomitant diaphragmatic myotube formation continue to progress up until E17, at which time the mature pattern of innervation and muscle architecture are approximated. E17 is also the time of the commencement of inspiratory drive transmission to phrenic motoneurons (PMNs) and the arrival of phrenic afferents to the motoneuron pool. During the period spanning from E17 to birth (gestation period of approximately 21 days), there is dramatic change in PMN morphology as the dendritic branching is rearranged into the rostrocaudal bundling characteristic of mature PMNs. This period is also a time of significant changes in PMN passive membrane properties, action-potential characteristics, and firing properties.

Intrinsic and extrinsic factors affecting phrenic motoneuronal excitability in neonatal rats

We examined intrinsic and extrinsic factors affecting phrenic motoneuron (PMN) excitability in neonatal rats. Using an in vitro brainstem-spinal cord en bloc, 127 PMNs were recorded under whole-cell patch-clamp conditions. Inspiratory synaptic drives and passive membrane properties, including whole-cell membrane capacitance (Cm), input resistance (Rn), and time constant (tau), were measured with either voltage- or current-clamp techniques. On the basis of firing behavior during inspiration, two types of PMNs could be distinguished: active (107/127 = 84%) and silent PMNs (20/127 = 16%). Active PMNs always produced multiple spikes during inspiration, while silent PMNs remained silent for most inspiratory cycles. Compared to silent PMNs, active PMNs had significantly higher Rn, inspiratory drive potential, and more depolarized resting membrane potential (RMP). With respect to inspiratory drive current, no significant difference was observed between the two types of PMN. Analysis of action potential waveforms did not show a significant difference between their threshold levels. Our results suggest that in addition to size-related properties, RMP determines the recruitment of PMNs and consequently, of motor units in the diaphragm.

Motor pool organization of the external gastrocnemius muscle in the turtle, Pseudemys (Trachemys) scripta elegans

The spinal cord of the adult turtle, Pseudemys (Trachemys) scripta elegans, is now considered a promising model for the study of the segmental motor system in the generalized tetrapod. To facilitate such studies we have examined the location, soma geometry, soma size, and number of motoneurons innervating the external gastrocnemius (EG) muscle in this species, as this muscle is ideally suited to the study of interrelations between the neuronal and muscular components of the segmental motor system. Motoneurons were retrogradely labeled following application of horseradish peroxidase to the EG muscle nerve. In both horizontal and transverse planes, labeled motoneurons innervating the EG muscle were concentrated in the S1 lumbosacral segment, and extended rostrally and caudally as far as the exists of the D10 and S2 spinal nerves, respectively. In the transverse plane, motoneurons were arranged in a longitudinal column which occupied the dorsolateral quadrant of the ventral horn. EG motoneurons are fusiform in shape and present their largest dimension in the transverse plane with their long axis oriented in the ventromedial to dorsolateral plane. The soma diameters of EG motoneurons were normally distributed, reflecting the absence of separate fusimotor innervation in reptilian species. In individual turtles, there was a two- to threefold range in soma diameter while soma surface area extended over a seven- to tenfold range. Based on cell counts from five animals, the EG motor pool was composed of approximately 75 motoneurons. Taken together, the results of this study provide valuable information for interpreting the results of future studies on the segmental motor system of this species under both normal and pathophysiological conditions.

Postnatal changes in cell body size and oxidative enzyme activity of spinal motoneurons innervating the rat tibialis anterior muscle

The cell body size and succinate dehydrogenase (SDH) activity of spinal motoneurons innervating the superficial and deep regions of the tibialis anterior muscle were studied in rats ranging in postnatal age from 3 to 11 weeks, by retrograde neuronal labeling using fluorescent neuronal tracers. The motoneurons innervating the tibialis anterior muscle were located primarily at the L4 spinal cord segment and those innervating the superficial and deep regions of the muscle were distributed throughout the entire extent of the motoneuron pool. The distribution of the motoneurons during postnatal development was similar to that observed in the adult animal. The mean cell body size of the motoneurons innervating the superficial region of the muscle in rats from 5 to 11 weeks of age was greater than that innervating the deep region at corresponding ages. The mean SDH activity of the motoneurons innervating the deep region of the muscle increased during postnatal development, while there were no changes in the mean SDH activity of those innervating the superficial region during this period. At 11 weeks of age, the motoneurons innervating the deep region of the muscle had a higher mean SDH activity than those innervating the superficial region. An inverse relationship between cell body size and SDH activity of motoneurons innervating both the superficial and deep regions of the muscle was observed, independent of age. These results indicate that motoneurons innervating the superficial and deep regions of the rat tibialis anterior muscle have different developmental patterns with regard to cell body size and SDH activity.

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.

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.

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 small and large dorsal root ganglion neurons in the cat. A study on cervical levels (C5-C6) and on phrenic afferents

During feline postnatal development, the size of phrenic afferent neurons labelled by horseradish peroxidase was evaluated in comparison to that of the bulk of counterstained neurons located in the same cervical dorsal root ganglia (DRG) (C5-C6). From age 1 week to maturity, small and large cell components were individualized from experimental size distributions using a mathematical approach. The analysis of data in adult indicated a close correspondence between small cells and unmyelinated afferents and between large cells and myelinated afferents, respectively. From age 1 week to adulthood, mean increases in cell diameter ranged between 10 microns (small cells from phrenic afferents) and 29.5 microns (large counterstained cells). In each population, the ratio of small/large cells remained constant during growth. In contrast to data in adults, at 1 week, large phrenic neurons were bigger than the counterstained ones. At 19 weeks, the cat DRG cells had not yet reached their adult size.

The postnatal growth of motoneurons at three levels of the cat neuraxis

The postnatal growth of motoneuron cell bodies located in the brainstem, cervical and lumbosacral spinal cord was investigated using retrograde transport of horseradish peroxidase in kittens ages 2, 12, 30, 55, 82 and 114 postnatal days and in an adult. The motoneurons innervating an extrinsic tongue muscle, the genioglossus, reached their adult size by eight weeks after birth. In contrast, the phrenic motoneurons innervating the diaphragm achieved adult size by 12 weeks and the motoneurons innervating the medial gastrocnemius muscle continued to grow beyond the twelfth postnatal week. The sizes of these motoneurons relative to one another remained constant during periods of development.