Heterogeneity of retinogeniculate axon arbors

The retinogeniculate synapse transmits information from retinal ganglion cells (RGC) in the eye to thalamocortical relay neurons in the visual thalamus, the dorsal lateral geniculate nucleus (dLGN). Studies in mice have identified genetic markers for distinct classes of RGCs encoding different features of the visual space, facilitating the dissection of RGC subtype-specific physiology and anatomy. In this study, we examine the morphological properties of axon arbors of the BD-RGC class of ON-OFF direction selective cells that, by definition, exhibit a stereotypic dendritic arbor and termination pattern in the retina. We find that axon arbors from the same class of RGCs exhibit variations in their structure based on their target region of the dLGN. Our findings suggest that target regions may influence the morphologic and synaptic properties of their afferent inputs.

ELECTRON MICROSCOPIC EXAMINATION OF THE MYELINATED AXONS OF CORPUS CALLOSUM IN PERFUSED YOUNG AND OLD RATS

In this study, the myelinated axons of parts of the corpus callosums of young and old rats were examined under the electron microscope and a grading system was performed for quantitating the ultrastructural pathological changes of these axons. Except the old splenium group, the only ultrastructural pathological change, observed in the myelinated axons was the separation in myelin configuration. In addition to this finding, in the old splenium group, in some of the myelinated axons, an interruption was observed in the myelin configuration. Additionally, these ultrastructural pathological findings were present in the larger sized myelinated axons of the corpus callosum.

Quantification of synaptic density in corticostriatal projections from rat medial agranular cortex

Medial agranular cortex (AGm) has a prominent bilateral projection to the dorsocentral striatum (DCS). We wished to develop a normal baseline by which to assess neuronal plasticity in this corticostriatal system in rats with neglect resulting from a unilateral lesion in AGm, followed by treatment with agents that promote sprouting and functional recovery in other systems. Injections of biotinylated dextran amine were made into AGm in normal rats, and unbiased sampling was used to quantify the density of axons and axonal varicosities present in DCS (the latter represent presynaptic profiles). Labeling density in contralateral DCS is approximately half of that seen in ipsilateral DCS (this ratio is 0.50 for axons, 0.55 for varicosities). The ratio of varicosities is stable over a greater than seven-fold range of absolute densities. There is no consistent relationship between the absolute density of axons and axon varicosities; however, the ratio measures are strongly correlated. We conclude that changes in the contralateral/ipsilateral ratio of axon density after experimental treatments do reflect changes in synaptic density, but axon varicosities are likely to be the most sensitive anatomical parameter by which to assess plasticity at the light microscopic level.

Forebrain connectivity of the prefrontal cortex in the marmoset monkey (Callithrix jacchus): an anterograde and retrograde tract-tracing study

The cortical and subcortical forebrain connections of the marmoset prefrontal cortex (PFC) were examined by injecting the retrograde tracer, choleratoxin, and the anterograde tracer, biotin dextran amine, into four sites within the PFC. Two of the sites, the lateral and orbital regions, had previously been shown to provide functionally dissociable contributions to distinct forms of behavioral flexibility, attentional set-shifting and discrimination reversal learning, respectively. The dysgranular and agranular regions lying on the orbital and medial surfaces of the frontal lobes were most closely connected with limbic structures including cingulate cortex, amygdala, parahippocampal cortex, subiculum, hippocampus, hypothalamus, medial caudate nucleus, and nucleus accumbens as well as the magnocellular division of the mediodorsal nucleus of the thalamus and midline thalamic nuclei, consistent with findings in the rhesus monkey. In contrast, the granular region on the dorsal surface closely resembled area 8Ad in macaques and had connections restricted to posterior parietal cortex primarily associated with visuospatial functions. However, it also had connections with limbic cortex, including retrosplenial and caudal cingulate cortex as well as auditory processing regions in the superior temporal cortex. The granular region on the lateral convexity had the most extensive connections. Based on its architectonics and functionality, it resembled areas 12/45 in macaques. It had connections with high-order visual processing regions in the inferotemporal cortex and posterior parietal cortex, higher-order auditory and polymodal processing regions in the superior temporal cortex. In addition it had extensive connections with limbic regions including the amygdala, parahippocampal cortex, cingulate, and retrosplenial cortex.

Impaired axonal regeneration by isolectin B4-binding dorsal root ganglion neurons in vitro

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, alpha7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding alpha7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both alpha7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

Medullary dorsal horn neurons providing axons to both the parabrachial nucleus and thalamus

It has often been suggested that the trigemino- and spino-thalamic pathways are highly implicated in sensory-discriminative aspects of pain, whereas the trigemino- and spino-parabrachial pathways are strongly implicated in affective/emotional aspects of pain. On the other hand, the superficial laminae of the spinal dorsal horn, where many nociceptive neurons are distributed, have been reported to contain projection neurons innervating both the parabrachial nucleus (PBN) and thalamus by way of axon collaterals (Hylden et al., 1989). For the medullary dorsal horn (caudal subnucleus of spinal trigeminal nucleus: Vc), however, the existence of such neurons has not been reported. Thus, in the present study, we examined whether the Vc might contain projection neurons sending their axons to both the thalamus and PBN. Dual retrograde labeling with fluorescence dyes was attempted. In each rat, tetramethylrhodamine-dextran amine and Fluoro-gold were stereotaxically injected into the PBN and thalamic regions, respectively. The proportion of the dually labeled Vc cells in the total population of all labeled Vc cells was about 20%. More than 90% of the dually labeled neurons were distributed in lamina I (marginal zone), less than 10% of them were located in lamina II (substantia gelatinosa), and only a few (about 1%) were found in lamina III (magnocellular zone). The results indicate that some Vc neurons in the superficial laminae mediate nociceptive information directly to the PBN and thalamus by way of axon collaterals and that the vast majority of them project to the ipsilateral PBN and contralateral thalamus.

Branching of calyceal afferents during postnatal development in the rat auditory brainstem

Cells in the anteroventral cochlear nucleus (aVCN) send out calyceal axons that form large excitatory somatic terminals, the calyces of Held, onto principal cells of the contralateral medial nucleus of the trapezoid body (MNTB). It is unclear which fraction of these axons might form more than one calyx and whether this fraction changes during development. We combined in vitro anterograde tracing, stereological cell counts, analysis of apoptosis, and immunohistochemistry to study the development of calyceal afferents in rats of different postnatal ages. We found that some principal cells were contacted by multiple large axosomatic inputs, but these invariably originated from the same axon. Conversely, at least 18% of traced afferents branched to form multiple calyces, independently of age. Calyces from the same axon generally innervated nearby principal cells, and most of these branch points were <50 microm away from the synaptic terminals. Our results show that the projection from the aVCN to the MNTB is divergent, both when calyces have just been formed and in the adult. Cell counts did not provide evidence for principal cell loss during development, although analysis of apoptosis showed a large increase in nonneuronal cell death around the onset of hearing. Our data suggest that, once a calyceal synapse forms in the MNTB, it stays.

Local axonal arborization patterns of distinct neuronal types in the caudal nucleus of the tractus solitarius

Neurons in the caudal nucleus of the tractus solitarius (cNTS) are quite heterogeneous in cell size (50 to 450 microm(2) in somal area) and other morphologic characteristics. For a more objective classification of cNTS neurons, their morphologic features were analyzed quantitatively based on reconstructed biocytin-filled cells after whole-cell patch-clamp recordings. According to the patterns of axonal branching behaviors, cNTS cells could be classified into two groups: smaller cells (94.1 microm(2) in mean somal area, range 62-120 microm(2), n = 22) and larger cells (245 microm(2) in mean somal area, range 142-411 microm(2), n = 23). Extensive axonal arborization with numerous possible synaptic boutons was specifically associated with smaller neurons, while larger cells possessed no or few axon collaterals, suggesting their distinct roles as local circuit neurons (or interneurons) and projection neurons, respectively. With regard to somatodendritic characteristics, the following correlations with cell size were found: smaller cells had larger form factors than larger cells (P < 0.05). Larger neurons had more extensive dendritic arborization, expressed by total dendritic length (P < 0.01) and number of dendritic branching points (P < 0.01), than smaller cells. It was suggested that small cNTS neurons contribute specifically to an integration of input information generated in the local circuits, while large neurons convey the integrated information to other autonomic brain regions.

Development of layer-specific axonal arborizations in mouse primary somatosensory cortex

In the developing neocortex, pyramidal neurons use molecular cues to form axonal arbors selectively in the correct layers. Despite the utility of mice for molecular and genetic studies, little work has been done on the development of layer-specific axonal arborizations of pyramidal neurons in mice. We intracellularly labeled and reconstructed the axons of layer 2/3 and layer 5 pyramidal neurons in slices of primary somatosensory cortex from C57Bl6 mice on postnatal days 7-21. For all neurons studied, the development of the axonal arborizations in mice follows a pattern similar to that seen in other species; laminar specificity of the earliest axonal branches is similar to that of mature animals. At P7, pyramidal neurons are very simple, having only a main descending axon and few primary branches. Between P7 and P10, there is a large increase in the total number of axonal branches, and axons continue to increase in complexity and total length from P10 to P21. Unlike observations in ferrets, cats, and monkeys, two types of layer 2/3 pyramidal neurons are present in both mature and developing mice; cells in superficial layer 2/3 lack axonal arbors in layer 4, and cells close to the layer 4 border have substantial axonal arbors within layer 4. We also describe axonal and dendritic arborization patterns of three pyramidal cell types in layer 5. The axons of tall-tufted layer 5 pyramidal neurons arborize almost exclusively within deep layers while tall-simple, and short layer 5 pyramidal neurons also project axons to superficial layers.

Axonal bias at a representational border in adult rat somatosensory cortex (S1)

The cortex is a highly organized structure and this organization is integral to cortical function. However, the circuitry underlying cortical organization is only partially understood, thus limiting our understanding of cortical function. Within the somatosensory cortex, organization is manifest as a map of the body surface. At the level of the cortical circuitry the horizontal connections of Layer 2/3 express a physiological bias that reflects discontinuities within the somatosensory map. Both excitation and inhibition are smaller when evoked from across a representational border, as compared to when they are evoked from within the representation. This physiological bias may be due to a bias in either the strength or number of synapses and/or the number of axons that cross this border and the extent of their arborization. In this study we used both an anterograde (Phaseolus vulgaris leucoagglutinin) and a retrograde (cholera toxin B) tracer to examine Layer 2/3 horizontal projections in rat S1. We determined that there is a bias in the amount of horizontal axonal projections that cross the forepaw/lower jaw border as compared to projections remaining within an individual representation. This bias in axonal projection and the correlated bias in excitation and inhibition may underlie the expression of the representational border.

Fructose supports energy metabolism of some, but not all, axons in adult mouse optic nerve

We used transmission electron microscopy (TEM) and electrophysiological techniques to characterize the morphology and stimulus-evoked compound action potential (CAP), respectively, of the adult mouse optic nerve (MON). Electrophysiological recordings demonstrated an identical CAP profile for each MON. An initial peak, smallest in area and presumably composed of the fastest-conducting axons displayed the lowest threshold for activation as expected for large axons. The second peak, the largest, was presumably composed of axons of intermediate diameter and conduction velocity, and the third peak was composed of the slowest and presumably smallest axons. In 10 mM fructose, the first CAP peak area was reduced by 78%, but the second and third peaks were unaffected. Histological analysis revealed a cross-sectional area of 33,346 microm2, containing 24,068 axons per MON. All axons were myelinated and axon diameter ranged from 0.09 to 2.58 microm, although 80 +/- 6% of the axons were <0.75 microm in diameter and only 0.6 +/- 0.3% of the axons were >2 microm in diameter. After bathing in fructose for 2 h 94 +/- 2% of normal appearing axons were <0.75 microm in diameter and none were >1.5 microm-all of the larger axons being grossly abnormal in structure. We conclude that fructose is unable to support function of the larger axons contributing to the first CAP peak, thus enabling us to identify a distinct population of axons that contributes to that peak.

Lighting the chandelier: new vistas for axo-axonic cells

Chandelier or axo-axonic cells are the most selective of all cortical GABAergic interneurons, because they exclusively contact axon initial segments of cortical glutamatergic neurons. Owing to their privileged location on initial segments, axo-axonic cells have often been assumed to have the ultimate control of pyramidal cell output. Recently, key molecules expressed at the initial-segment synapses have been identified, and novel in vitro and in vivo electrophysiological studies have revealed unexpectedly versatile functional effects exerted by axo-axonic cells on their postsynaptic targets. In addition, there is also emerging recognition of the mechanistic involvement of these unique cells in several neurological diseases, including epilepsy and schizophrenia.

The ability of axons to regenerate their growth cones depends on axonal type and age, and is regulated by calcium, cAMP and ERK

The processes activated at the time of axotomy and leading to the formation of a new growth cone are the first step in regeneration, but are still poorly characterized. We investigated this event in an in vitro model of axotomy performed on dorsal root ganglia and retinal explants. We observed that the dorsal root ganglion axons and retinal ganglion cell axons, which had grown out on a poly d-lysine/laminin substrate at the time of culture preparation greatly differed in their regenerative response after a subsequent in vitro lesion made far from the cell body. The majority of axons of adult dorsal root ganglia but only a small percentage of axons of adult retinal ganglion cells regenerated new growth cones within four hours after in vitro axotomy, though both kinds of axons were growing before the lesion. The depletion of extracellular calcium and the inhibition of extracellular-signal regulated kinase 1,2 (ERK) and protein kinase A (PKA) at the time of injury significantly impaired the capacity of dorsal root ganglia axons to re-initiate growth cones without affecting growth cone motility. Pharmacological treatments directed at increasing the level of cAMP promoted growth cone regeneration in adult retinal ganglion cell axons in spite of the low regenerative potential exhibited in normal conditions. Understanding the cellular mechanisms activated at the time of lesion and leading to the formation of a new growth cone is necessary for devising treatments aimed at enhancing the regenerative response of injured axons.

Class I and class II ganglion cells of rabbit retina: a structural basis for X and Y (brisk) cells

Morphological studies of rabbit retina have identified ganglion cells resembling "alpha" cells but none resembling cat "beta" cells. Four distinct types of class I cell, similar to alpha cells, were identified, each narrowly stratified and differing from the other three principally in the level of dendritic branching. These four levels of dendritic branching flank the two starburst/cholinergic amacrine cell substrata that mark the middle of sublaminae a and b. Compared with the other three, class Ia2 cells are the largest in cell body and dendritic field size, are sometimes homotypically dye coupled, and have slightly broader dendritic stratification. Class Ia2 and the slightly smaller class Ib2 cells form a paramorphic pair. Compared with class I cells, class II cells have smaller dendritic fields; a greater tendency to "tufted" dendritic branching, as shown in the companion paper; branching at one of three levels of the IPL; and similarly narrow stratification. Class IIa and class Ia1 cells branch at the same level, as do class IIb1 and class Ib1 cells. Class IIb2 cells branch slightly nearer the ganglion cell layer than class Ib2 cells and costratify with "blue-ON" cone bipolar cells. The class IIa and IIb2 cells form a paramorphic pair, whereas class IIb1 cells appear to be unpaired. The four types of class I cell probably correspond to ON- and OFF-center brisk-transient, fast-movement and slow-movement cells, whereas the three types of class II cell probably correspond to ON- and OFF-center brisk-sustained and color-coded ON-center X cells.

VIP-containing deep short-axon cells of the olfactory bulb innervate interneurons different from granule cells

This study investigates the targets of the population of vasoactive intestinal polypeptide (VIP)-containing deep short-axon cells of the rat olfactory bulb (OB), combining single- and double-immunocytochemical approaches under light and electron microscopy. It has been assumed that deep short-axon cells innervate granule cells in the mammalian OB, but their synaptic connectivity has not been demonstrated to date. Our results indicate that, instead of the accepted scheme of the bulbar circuitry, VIP-containing deep short-axon cells are gamma-aminobutyric acid (GABA)ergic interneurons specialized in the selective innervation of other GABAergic deep short-axon cells. Their axons contact with the perisomatic region and the dendritic portions of subsets of deep short-axon cells that contain VIP, calbindin D-28k and neuropeptide Y. Electron microscopy reveals axo-somatic and axo-dendritic symmetrical synapses from VIP-containing boutons. Taken altogether, our data show that the VIP-containing deep short-axon cells of the rat OB form an interneuronal network that modulates the function of other interneurons different from granule cells. They might be involved indirectly in the inhibition or disinhibition of principal cells or might participate in the generation of oscillatory activity and in the synchronization of populations of interneurons and, then, of principal cells. Present data demonstrate that modulation of the OB by local circuits is more complex than the simple inhibition from periglomerular cells and granule cells, and remark the importance of considering the contribution of other classes of GABAergic interneurons different from periglomerular cells and granule cells to the bulbar circuitry.

Nerve compound action current (NCAC) measurements and morphometric analysis in the proximal segment after nerve transection and repair in a rabbit model

In the evaluation of nerve regeneration using magneto-neurography (MNG), the proximal segment showed a reproducible decrease in peak-peak amplitude of the nerve compound action current's (NCAC) of 60%. To explain these changes, morphometry of myelinated axons in the proximal segment is compared to the MNG signals. A standardised nerve transection and reconstruction was performed in rabbits. NCACs were measured approximately 5 cm proximal to the lesion from operated and control nerves after 12 weeks. Histological samples were taken from the same area of the nerve where the NCACs were obtained. Results showed a decrease of the peak-peak amplitude of the NCAC of 57% compared to the control. Conduction velocity decreased 15% (not significant). Morphometry elicited a decrease in larger (10-15 microm) axons (284 +/- 134 vs 82 +/- 55) and an increase in smaller (2-5 microm) axons (1445 +/- 360 vs 1921 +/- 393). A strong correlation existed between the decrease in amplitude and the decrease in larger axons (0.85). Peak-peak amplitude varies approximately with the square of the diameter axon. Therefore, because peak-peak amplitude is mainly dependent on the larger-diameter axons, the decrease in peak-peak amplitude of the NCACs may be explained by a decrease in numbers of 10-15-microm axons.

Evidence for impaired axonal regeneration in PMP22 duplication: studies in nerve xenografts

Whether axonal regeneration in Charcot-Marie-Tooth (CMT) neuropathies is impaired has not been addressed in detail. Our studies in nude mice harboring xenografts from patients with different primary Schwann cell (SC) genetic defects suggested an intimate association between the onset of myelination and impairment in the growth capacity of nude mice axons engulfed by the mutant SCs. To assess the effects of peripheral myelin protein 22 (PMP22) gene duplication on the regeneration process, we conducted morphometric studies to generate temporal growth profiles of myelinated axons within the xenografts obtained from CMT1A patients and from healthy controls. Axon size distribution histograms in controls at different time intervals revealed that size differentiation of myelinated fibers within the grafts is established as early as 2 weeks, and that the temporal pattern of myelination of different sized axons has striking similarities to myelination during development. In PMP22 duplication grafts, the onset of myelination is delayed and the regeneration capacity of all fiber sizes is impaired. This defect, however, is most pronounced for the large diameter axons. In addition, significant large fiber loss occurred after 12 weeks with a concomitant new cycle of regeneration of small size axons. These studies show that the PMP22 duplication in SCs have profound effects on the regeneration process, which might be a contributing factor to preferential distal axonal loss.

Effect of reduced impulse activity on the development of identified motor terminals in Drosophila larvae

In Drosophila larvae, motoneurons show distinctive differences in the size of their synaptic boutons; that is, axon 1 has type Ib ("big" boutons) terminals and axon 2 has type Is ("small" boutons) terminals on muscle fibers 6 and 7. To determine whether axon 1 develops large boutons due to its high impulse activity, we reduced impulse activity and examined the motor terminals formed by axon 1. The number of functional Na(+) channels was reduced either with the nap(ts) mutation or by adding tetrodotoxin (TTX) to the media (0.1 microg/g). In both cases, the rate of locomotion was decreased by approximately 40%, presumably reflecting a decrease in impulse activity. Locomotor activity was restored to above wild-type (Canton-S) levels when nap(ts) was combined with a duplication of para, the Na(+)-channel gene. Lucifer yellow was injected into the axon 1 motor terminals, and we measured motor terminal area, length, the number of branches, and the number and width of synaptic boutons. Although all parameters were smaller in nap(ts) and TTX-treated larvae compared to wild-type, most of these differences were not significant when the differences in muscle fiber size were factored out. Only bouton width was significantly smaller in both different nap(ts) and TTX-treated larvae: boutons were about 20% smaller in nap(ts) and TTX-treated larvae, and 20% larger in nap(ts); Dp para(+) compared to wild-type. In addition, terminal area was significantly smaller in nap(ts) compared to wild-type. Bouton size at Ib terminals with reduced impulse activity was similar to that normally seen at Is terminals. Thus, differences in impulse activity play a major role in the differentiation of bouton size at Drosophila motor terminals.

Electrophysiological and morphological properties of cell types in the chick neostriatum caudolaterale

The neostriatum caudolaterale, in the chick also referred to as dorsocaudal neostriatal complex, is a polymodal associative area in the forebrain of birds that is involved in sensorimotor integration and memory processes. We have used whole-cell patch-clamp recordings in chick brain slices to characterize the principal cell types of the neostriatum caudolaterale. Electrophysiological properties distinguished four classes of neurons. The morphological characteristics of these classes were examined by intracellular injection of Lucifer Yellow. Type I neurons characteristically fired a brief burst of action potentials. Morphologically, type I neurons had large somata and thick dendrites with many spines. Type II neurons were characterized by a repetitive firing pattern with conspicuous frequency adaptation. Type II neurons also had large somata and thick dendrites with many spines. There was no clear morphological distinction between type I and type II neurons. Type III neurons showed high-frequency firing with little accommodation and a prominent time-dependent inward rectification. They had thin, sparsely spiny dendrites and extensive local axonal arborizations. Electrophysiological and morphological properties indicated them as being interneurons. Type IV neurons had a longer action potential duration, a larger input resistance, and a longer membrane time constant than the other classes. Type IV neurons had small somata and short dendrites with few spines. The long axon collaterals of neurons in all spiny cell classes (types I, II, IV) followed similar patterns, suggesting that neurons from all these types can contribute to the projections of the neostriatum caudolaterale to sensory, limbic and motor areas. The electrophysiological and anatomical characterization of the major classes of neurons in the caudal forebrain of the chick provides a framework for the investigation of sensorimotor integration and learning at the cellular level in birds.

Morphology of horizontal cells in the retina of the capuchin monkey, Cebus apella: how many horizontal cell classes are found in dichromatic primates?

The morphology of horizontal cells was studied in the retina of dichromatic capuchin monkeys, Cebus apella. The cells were labeled with the carbocyanine dye, 1,1',dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate (DiI), and the labeling was then photoconverted to a stable product by using a diaminobenzidine reaction. The sizes of cell body, dendritic field, and axon terminal, as well as the number of dendritic clusters and cone convergence, were measured at increasing distance from the fovea. Three distinct morphological classes of horizontal cells were identified. Their dendritic and axonal morphology resembles those of H1, H2, and H3 cells described in trichromatic primates. The size of the cell bodies, dendritic fields, and axon terminals of all cell classes increases towards retinal periphery. H3 cells have larger dendritic fields and more dendritic clusters than H1 cells. All labeled horizontal cells located in selected patches of retina were further analyzed to quantify the differences between H1 and H3 cells. H1 cells have smaller dendritic field area, smaller total length of primary dendrites, more dendritic branching points, and larger fractal dimension than H3 cells. We have distinguished H1 and H3 cells based solely in morphological criteria. Their physiology should be further analyzed with detail, but their presence in both dichromatic and trichromatic primates suggests that neither of them have a specialized role in the red-green color opponent channel of color vision.

The nigrostriatal pathway in the rat: A single-axon study of the relationship between dorsal and ventral tier nigral neurons and the striosome/matrix striatal compartments

Axons from dorsal/ventral tiers of substantia nigra pars compacta (SNc), ventral tegmental area (VTA), and retrorubral field (RRF) were traced after injecting their cell body with biotinylated dextran amine. Fifty-three single axons were reconstructed from serial sagittal sections with a camera lucida, and mu-opiate receptor immunostaining served to differentiate the striosome/matrix striatal compartments. Most dorsal tier SNc axons terminate within the matrix of the dorsal striatum, but their patterns of arborization vary markedly; some axons innervate one specific matriceal area, whereas others arborize in multiple discontinuous loci. Some dorsal tier SNc axons also project to both striosomes and matrix. Other dorsal tier SNc axons, as well as VTA axons, innervate the ventral striatum and send collaterals to striosomes lying ventrally in the dorsal striatum or to the ventral sector of the subcallosal streak (SS). Ventral tier SNc axons arborize principally in striosomes, but some ramify in both compartments or in striosomes and the SS. Ventral tier neurons that form deep clusters in substantia nigra pars reticulata innervate principally the matrix and the SS. The amygdala and ventral pallidum receive secondary collaterals from striatal axons of dorsal/ventral tier neurons or RRF neurons. The subthalamic nucleus receives collaterals from striatal axons of SNc clustered neurons, whereas the globus pallidus gets collaterals from striatal axons of dorsal/ventral tier SNc neurons. These findings reveal that the nigrostriatal pathway is composed of several neuronal subsystems, each endowed with a widely distributed axonal arborization that allows them to exert a multifaceted influence on striatal and/or extrastriatal structures.

Morphology of local axon collaterals of electrophysiologically characterised neurons in the rat medial septal/ diagonal band complex

Neurons in the medial septal/diagonal band complex (MS/DB) in vivo exhibit rhythmic burst-firing activity that is phase-locked with the hippocampal theta rhythm. The aim was to assess the morphology of local axon collaterals of electrophysiologically identified MS/DB neurons using intracellular recording and biocytin injection in vitro. Cells were classified according to previous criteria into slow-firing, fast-spiking, regular-spiking, and burst-firing neurons; previous work has suggested that the slow-firing neurons are cholinergic and that the other types are GABAergic. A novel finding was the existence of two types of burst-firing neuron. Type I burst-firing neurons had significantly longer duration after hyperpolarisation potentials when held at -60 mV, and at -75 mV, type I neurons exhibited a low-threshold spike with more rapid activation and inactivation kinetics than those of type II neurons. We have, also for the first time, described the main features of the local axon collaterals of the five neuron types. All filled neurons possessed a main axon that gave forth 1-12 local primary axon collaterals. All electrophysiological types, except for the type I burst-firing neuron, had a main axon that coursed toward the fornix. Myelination of the main axon was a prominent feature of all but the slow-firing neurons. Branching of the primary axon collaterals of the fast-spiking and type I burst-firing neurons was more extensive than that of the other cell types, with those of the slow-firing neurons exhibiting the least branching. All cell types possessed axon collaterals of the en passant type, and some in addition had twiglike or basketlike axon terminals. All cell types made synapses on distal dendrites; a proportion of the fast-spiking and burst-firing cells in addition had basketlike terminals that made synaptic contacts on proximal dendrites and on somata. Two morphological types of somata were postsynaptic to the basket cells: large (20-30-microm) oval cells with dark cytoplasm, and large oval cells with paler cytoplasm, often with an apical dendrite. The presence of lamellar bodies in the large dark neurons suggests that they may be cholinergic neurons, because previous work has localised these structures in some neurons that stain for choline acetyltransferase. Our work suggests therefore that there may be GABAergic neurons in the MS/DB that form basket synaptic contacts on at least two types of target cell, possibly cholinergic and GABAergic neurons, which means that the basket cells could play a key role in the generation of rhythmic activity in the MS/DB.

Morphology, axonal projection pattern, and responses to optic nerve stimulation of thalamic neurons in the salamander Plethodon jordani

In the salamander Plethodon jordani, the morphology and axonal projections of thalamic (TH) neurons and their responses to electrical optic nerve stimulation were determined by intracellular recording and biocytin labeling under in vitro, whole-brain conditions. Based on their axonal projections, labeled neurons (n = 76) were divided into the following groups: TH1 neurons, with mostly ipsilateral projections to the striatum; TH2 neurons, with ipsilateral or bilateral projections to the medial amygdala and nucleus accumbens; TH3 neurons, with bilateral projections to the medial and dorsal pallium; TH4 neurons, with mostly ipsilateral projections to the striatum and ipsilateral projections to the tectum opticum, tegmentum, and rostral medulla oblongata; and TH5 neurons, with ipsilateral projections to the tegmentum, medulla oblongata, and rostral spinal cord without (TH5.1) or with (TH5.2) additional projections to the optic tectum. TH1-TH4 neurons are found in the dorsal thalamus and around the sulcus medialis, and TH5 neurons are found in the ventral thalamus. Labeled neurons with ascending projections, i.e., the more dorsally situated TH1-TH4 neurons, are mostly inhibited by electrical stimulation of the optic nerve and have significantly longer latencies (mean +/- S.D., 42.1 +/- 11.6 msec) than neurons with exclusively descending projections, i.e., the ventrally located TH5 neurons (8.5 +/- 6.1 msec), which receive the bulk of retinal afferents and show excitation at electrical optic nerve stimulation. Neurons recorded without labeling in the dorsal thalamus likewise exhibit mostly inhibition and have significantly longer latencies (35.7 +/- 18.9 msec) than those recorded in the ventral thalamus (10.9 +/- 7.7 msec), which mostly show excitation. None of the neurons recorded in the dorsal thalamus followed repetitive stimulation of the optic nerve. Thus, neurons situated in the dorsal thalamus and projecting to pallial or subpallial telencephalic targets are unlikely to receive monosynaptic or oligosynaptic, excitatory retinal input. Accordingly, no retino-thalamo-telencephalic pathway homologous to that found in amniotes appears to exist in salamanders.

A comparison of the linear tuning properties of two classes of axons in the bullfrog lagena

Various vertebrate inner-ear end organs appear to have switched their sensory function between equilibrium sensing and acoustic sensing over the courses of various lines of evolution. It is possible that all that is required to make this transition is to provide an end organ with access to the appropriate stimulus mode and frequency range. If, as we believe, however, the adaptive advantage of an acoustic sensory system lies in its ability to sort the total acoustic input into components that correspond to individual acoustic sources, and the adaptive advantage of an equilibrium sensory system lies in its ability to compute the total orientation and motion of the head without regard to the individual sources contributing to that orientation and motion, then it is easy to argue that the differences between acoustic and equilibrium sensors should be more profound than simply access to the appropriate stimuli. Effective signal-sorting requires high resolution in both time and frequency; to achieve this resolution, a peripheral tuning structure must be one of high dynamic order (i.e., constructed from multiple independent energy storage elements). If the peripheral tuning structure simply converts head acceleration to head displacement, velocity, or jerk (i.e., provides one or two steps of integration or differentiation with respect to time, where one energy storage element per step is required), then high dynamic order is inappropriate. Because the bullfrog lagena possesses both acoustic and equilibrium sensitive regions, it is especially suited for comparing these two sensor types and addressing the question of dynamic order of tuning. In this paper we report observations of the linear tuning properties of bullfrog lagenar primary afferent nerve fibers obtained by stimulating the lagena with random, dorsoventral micromotion over the frequency range from 10 Hz to 1.0 kHz. Tuning curves obtained by reverse correlation analysis and discrete Fourier transformation were used to estimate the dynamic order of each fiber's associated peripheral tuning structure. We found two classes of lagenar afferent axons--those with lowpass amplitude tuning characteristics (44 units) and those with bandpass amplitude tuning characteristics (73 units). Lowpass units were found to originate at the equilibrium region of the macula, and they exhibited low dynamic order--summed low- and high-frequency slopes (absolute values) ranged from 10 dB/decade to 64 dB/decade, implying dynamic orders of less than one to three (the modal value was equal to one). Bandpass units were found to originate at the acoustic region of the macula, and they exhibited higher dynamic order than lowpass units--summed low- and high-frequency slopes (absolute values) ranged from 53 dB/decade to 185 dB/decade, implying dynamic orders of three to nine (the modal value was equal to five). It appears that while lagenar equilibrium and acoustic sensors both possess access to signals in the acoustic frequency range, lagenar acoustic sensors are tuned by means of peripheral structures with markedly greater dynamic order and consequently markedly greater physical complexity. These results suggest that steep-sloped (high-dynamic-order) tuning properties reflect special adaptations in acoustic sensors not found in equilibrium sensors, and that any evolutionary transition between the two sensor types must have involved profound structural changes.

A mouse model of familial amyotrophic lateral sclerosis expressing a mutant superoxide dismutase 1 shows evidence of disordered transport in the vasopressin hypothalamo-neurohypophysial axis

Amyotrophic lateral sclerosis (ALS) is a fatal, paralytic disorder that primarily affects motoneurons. By combining physiological and morphological approaches, we examined the effect of a murine superoxide dismutase 1 (SOD1) mutation (G86R), which induces neurological disorders resembling human familial ALS (FALS), on the arginine vasopressin (AVP) hypothalamo-neurohypophysial axis, an unmyelinated tract poor in neurofilaments. First, we observed that G86R mice progressively consumed more water than wild-type littermates. Furthermore, levels of plasma AVP and neurohypophysial AVP content were decreased in the SOD1 mutant mice, whereas the amount of hypothalamic AVP increased in an age-dependent manner. However, hypothalamic AVP mRNA levels were not significantly modified in these animals. At the ultrastructural level, we found that the neurohypophysis of G86R mice had a decreased number of neurosecretory axons. Conversely, the presence of large axon swellings was more pronounced in the SOD1 mutant mice. In addition, the size of neurosecretory granules was higher in G86R than in wild-type animals. All these findings strongly suggest that the FALS-associated SOD1 mutation injures the hypothalamo-neurohypophysial axis by provoking early, progressive disturbances in the axonal transport of neurosecretory products from neuronal perikarya to nerve terminals. This blockade could ultimately result in degeneration of the tract, as proposed for the myelinated, neurofilament-enriched motor axons affected by ALS.

Heterogeneity of vascular innervation in hamster cheek pouch and retractor muscle

The hamster cheek pouch and its retractor muscle have provided valuable insights into microvascular physiology of an epithelial tissue and striated muscle, respectively. Nevertheless, the innervation of these vascular beds has not been resolved. This study has investigated the nature of autonomic and sensory innervation of these vascular beds and has tested whether it varies within or between tissues. Multiple-labelling immunohistochemistry identified autonomic and peptide-containing sensory nerve fibres. Presumptive sympathetic vasoconstrictor axons with immunoreactivity (IR) for tyrosine hydroxylase (TH) and neuropeptide Y (NPY) innervated feed arteries and arterioles (but not veins or venules) of the retractor and anterior (muscular) cheek pouch; these axons were absent from the posterior (epithelial) region of the cheek pouch, as confirmed by catecholamine fluorescence. Presumptive autonomic vasodilator axons with IR for vasoactive intestinal peptide (VIP) consistently innervated feed arteries and proximal arterioles of the cheek pouch, but generally not those of the retractor muscle nor distal arterioles of either tissue. Sparse presumptive sensory axons with IR for calcitonin gene-related peptide (CGRP) and substance P were found near arterial and venous vessels in all regions of the cheek pouch and retractor muscle; CGRP-IR was also located in motor end plates associated with striated muscle fibres. Such regional differences in vascular innervation by autonomic and sensory neurons may selectively effect local and regional control of blood flow between and within vascular beds.

Axoaxonic synapses on terminals of group II muscle spindle afferent axons in the spinal cord of the cat

The purpose of the present study was to determine if terminals of identified group II muscle spindle afferents participate in axoaxonic synaptic arrangements and, if so, to investigate the transmitter content of presynaptic terminals in these arrangements. Group II muscle afferents supplying the gastrocnemius-soleus or semitendinosus muscles were identified in adult cats and stained intra-axonally with horseradish peroxidase. In total, three group II axons were labelled and processed for combined light and electron microscopy. Group II axons gave rise to collaterals which characteristically descended through the superficial dorsal horn and formed relatively sparse terminal arborizations in the dorsal horn (laminae IV and V) and more profuse arbors in the intermediate grey matter (laminae VI-VII). Forty boutons were examined through series of ultrathin sections and all but four were postsynaptic to other axon terminals. Occasionally, more than one axon was presynaptic to a single group II terminal. Immunogold studies showed that all axons in presynaptic apposition to group II boutons contained gamma-aminobutyric acid (GABA) and also that glycine was colocalized in the majority of these axons. This evidence suggests that transmission from group II muscle afferents is under strong presynaptic inhibitory control and that it is mainly the subgroup of GABAergic interneurons with colocalized glycine which mediate this inhibition. Seventeen group II boutons were components of synaptic triads where the presynaptic axoaxonic bouton formed a synapse with the same dendrite as the group II axon. Therefore, a proportion of the interneurons which form axoaxonic synapses with group II axons are also likely to have postsynaptic inhibitory actions on target neurons of group II afferents.

The visualization of a new class of traumatically injured axons through the use of a modified method of microwave antigen retrieval

Antibodies to the amyloid precursor proteins (APP) have become routine markers for detecting traumatically induced axonal injury (AI) in animals and man. Unfortunately, the techniques used to visualize these proteins are not compatible with routine electron microscopic (EM) analysis. In the current communication, we describe a method for the ultrastructural visualization of antibodies to APP and, using this method, we identify a previously unrecognized population of traumatically injured axons. Rats were subjected to an impact acceleration traumatic brain injury and allowed to survive 30 min to 3h postinjury. The animals were then perfused, their brains sectioned on a vibratome and the sections prepared for immunocytochemistry using a computer-controlled microwave capable of temperature regulation. The use of temperature-controlled microwave energy unmasked APP antigenic epitopes without sacrificing ultrastructural detail. The APP antibody was found in two distinct populations of reactive axons that differed in size, morphology, location, and temporal progression. Comparable to previous descriptions, one population showed traumatically related reactive changes that led to swelling and disconnection. The other population, however, revealed unanticipated changes reflected in nodal and paranodal swelling of small continuous fibers that showed no evidence of disconnection during the time periods assessed. These studies provide new insight into the complexity of the pathobiology of AI, while describing a novel approach for enhancing APP immunoreactivity at the EM level.

Typology, early differentiation, and exuberant growth of a set of cortical axons

The corpus callosum interconnects both corresponding (homotopic) and noncorresponding (heterotopic) cortical sites of the two hemispheres. We have studied the axons that establish heterotopic connections from visual areas 17 and 18 (E axons) by using anterogradely transported biocytin and three-dimensional serial reconstructions in adult cats and in kittens. Their site of termination distinguished four types of axons. Type EI ends near the border between areas 19/21a or 7, and type EII near the PMLS/PLLS border (posteromedial and posterolateral lateral suprasylvian areas). Type EIII and EIV terminate the first near the PMLS/PLLS and PMLS/21a borders, and the second near the PMLS/PLLS and 19/21a or 7 borders. Taking into account the previously studied homotopic axons (O axons; Houzel et al. [1994] Eur. J. Neurosci. 6:898-917), it can be concluded that areas 17 and 18 are interhemispherically connected by at least five types of axons, three of which (O, EI, and EII) terminate near one areal border, the other two (types EIII and EIV), near two areal borders. All types terminate near representations of the vertical meridian of the visual field. The different types of axons can be identified already during the first postnatal week; at this age, unlike in the adult, they originate not only near the 17/18 border, but also, transiently, in area 17. This suggests that the developing cortex contains sets of neurons destined to send their axon to different targets; however, the axons grow beyond their sites of adult termination. Indeed, exuberant growth takes place at the stage of axonal elongation, and at subsequent stages of axonal differentiation, i.e., during subcortical branching, intracortical branching and synaptogenesis. The growth is progressively more constrained in its topographic distribution and the axons are subsequently reshaped by regressive events.

Mesodermal guidance of pioneer axon growth

Pioneer axons in insect legs are experimentally accessible model systems for the molecular identification and cellular localization of guidance cues regulating the path of axon growth. A detailed study of the Fe2 pioneer axons in the legs of the cockroach was performed to examine the diversity of guidance mechanisms. A detailed microscopic analysis of the axons at various points in their trajectory indicates that the Fe2 axons grow on a mesodermal substratum which contains the cues guiding their growth along a stereotyped path. An identified pair of muscle pioneer cells (MPC) are likely to play an important role in enabling the Fe2 growth cones to respond to mesodermal guidance cues. The addition of heparan sulfate, heparitinase, and phosphatidylinositol-specific phospholipase C to the medium perturbs the in situ path of growth of the Fe2 axons and the location of the MPC in cultured embryos. This indicates a role for heparan sulfate proteoglycans and glycosylphosphatidylinositol-anchored proteins in axon guidance. When these results are compared to those of similar experiments performed on the well-characterized Ti1 axons, they indicate significant differences in the mechanisms that are used for axon guidance. The Fe2 neurons are a good model for elucidating the mechanisms used to guide axon growth on nonmuscle mesodermal substrates often encountered in the periphery of vertebrate embryos.

The horizontal cells of artiodactyl retinae: a comparison with Cajal's descriptions

The morphology of horizontal cells in ox, sheep, and pig retinae as observed after Lucifer Yellow injections are described and compared with the descriptions of Golgi-stained cells by Ramón y Cajal (1893). Horizontal cells in the retinae of less domesticated species, wild pig, fallow and sika deer, mouflon, and aurochs were also examined. All these retinae have two types of horizontal cell; their morphologies are in common, although with some familial differences. Their basic appearance is as Cajal described; except in one important respect, a single axon-like process could not be identified on the external horizontal cells. It is concluded that external horizontal cells of artiodactyls correspond to the axonless (A-type) cells of other mammals. Cajal's internal horizontal cells have a single axon which contacts rods. This type corresponds to the B-type cells of other mammalian retinae. Artiodactyl A- and B-type horizontal cells differ from those of many other mammals in that the B-type dendritic tree is robust and the A-type dendritic tree is delicate. Historically, this morphological difference between orders of mammals has led to some confusion. The comparisons presented here suggest that the morphological types of primate horizontal cells can be integrated into a general mammalian classification.

Specific subtypes of cutaneous mechanoreceptors require neurotrophin-3 following peripheral target innervation

Neurotrophin-3 (NT-3) is required for the development of most sensory neurons of the dorsal root ganglia. Using electrophysiological techniques in mice with null mutations of the NT-3 gene, we show that two functionally specific subsets of cutaneous afferents differentially require this factor: D-hair receptors and slowly adapting mechanoreceptors; other cutaneous receptors were unaffected. Merkel cells, which are the end organs of slowly adapting mechanoreceptors, are virtually absent in 14-day-old homozygous mutants and are severely reduced in adult NT-3 heterozygous animals. This loss of Merkel cells, together with their innervation, happens in the first postnatal weeks of life, in contrast to muscle spindles and afferents, which are never formed in the absence of NT-3. Thus, NT-3 is essential for the maintenance of specific cutaneous afferents known to subserve fine tactile discrimination in humans.

Differential expression of two cell surface proteins, neuropilin and plexin, in Xenopus olfactory axon subclasses

Immunohistochemistry by using monoclonal antibodies named A5 and B2, which specifically recognize cell surface proteins the neuropilin and the plexin, respectively, revealed that olfactory axons in Xenopus tadpoles were classified into several subgroups by virtue of the expression levels of these two cell surface molecules. The vomeronasal axons expressed the plexin but not the neuropilin. The plexin-positive and neuropilin-negative vomeronasal axons form a discrete fiber bundle, even after they joined with the principal olfactory axons. However, the principal olfactory axons were divided into at least two subclasses; the neuropilin-predominant axons which expressed high levels of the neuropilin and low levels of the plexin, and the plexin-predominant axons which expressed high levels of the plexin and low levels of the neuropilin. Within the olfactory nerve the pathways for these two principal olfactory axon subclasses were initially intermingled with each other, but were gradually segregated throughout their courses from the nose to the cerebrum. Eventually, the neuropilin-predominant and the plexin-predominant principal olfactory axon subclasses projected to specified glomeruli in topographically related regions within the main olfactory bulb. Neuroanatomical tracings of the olfactory projection also confirmed the gradual segregation of the pathways for the principal olfactory axons. These results allow us to speculate that both the neuropilin and the plexin are involved in axon interactions, and play roles in the organization of the precise patterns of the olfactory pathway and projection.

Distinct morphologic classes of serotonergic axons in primates exhibit differential vulnerability to the psychotropic drug 3,4-methylenedioxymethamphetamine

Immunohistochemical methods were used to analyse the distribution and morphology of serotonergic axons in normal macaque monkeys and in monkeys given (+/-)3,4-methylenedioxymethamphetamine. In untreated monkeys, we observed two morphologic classes of serotonergic axon terminals, which differ in regional and laminar distribution. These two axon types, fine and beaded, correspond to the serotonergic axon types which have been described in the rat. In 3,4-methylenedioxymethamphetamine-treated monkeys, there is a profound loss of serotonergic axon terminals, yet some are consistently spared. The surviving axon terminals are nearly all of the beaded type; in contrast, fine serotonergic axons are markedly reduced in density. There are regional differences in the magnitude of denervation, which reflect differences in the distribution of these two types of serotonergic axons in controls. The present study demonstrates that 3,4-methylenedioxymethamphetamine has differential neurotoxic effects on fine and beaded serotonergic axons. These results indicate that in the primate there are two distinct classes of serotonergic axon terminals, which differ in morphology, distribution, and vulnerability to psychotropic drugs. We hypothesize that in the primate, as demonstrated in the rat, these two classes of serotonergic axon terminals may arise from different raphe nuclei. In both rodent and primate, the dorsal and median raphe nuclei give rise to parallel ascending serotonergic projections, which are likely to have different pharmacologic properties and functions.

Motor nuclei of peroneal muscles in the cat spinal cord

The cat peroneal muscles have been used in numerous investigations dealing with the physiological properties of motor units, muscle spindles, and Golgi tendon organs. This report presents a study of the organization of peroneal motor pools in the cat spinal cord by means of retrograde axonal transport of horseradish peroxidase from individual muscles to the corresponding motoneurons. The motor nuclei of peroneus longus (PL), peroneus brevis (PB), and peroneus tertius (PT) muscles formed thin columns in the lateral part of the ventral horn in spinal segments L6-S1. In the transverse plane, the PT and PL nuclei occupied, respectively, dorsolateral and ventromedial positions, with PB nucleus in an intermediate position overlapping with the other two nuclei. Measurements of cell body diameters allowed identification of alpha and gamma subgroups in peroneal motoneuron populations. The average numbers of motoneurons were about 96 alpha and 60 gamma in PL, 75 alpha and 54 gamma in PB, and 34 alpha and 23 gamma in PT. Comparison with data from electrophysiological studies indicated that whole populations of motoneurons were labeled in each motor nucleus. The proportions of gamma motoneurons were the same, and cell bodies of gamma motoneurons had similar sizes in the three peroneal populations. In contrast, alpha motoneurons were significantly smaller in PB than in the two other pools, in keeping with the fact that PB contains a proportion of slow motor units larger than the two other muscles. In large samples of homonymous motoneurons, the numbers of first-order dendrites correlated linearly with motoneuron sizes.

Properties of somata of spinal dorsal root ganglion cells differ according to peripheral receptor innervated

1. Intracellular recordings were made in the somata of dorsal root ganglion cells in the L7 or S1 DRG in cats anesthetized with alpha-chloralose. The properties of the action potentials (amplitude, duration, peak rate of rise), duration of afterhyperpolarization (AHP), magnitude of inward rectification, and axonal conduction velocity were measured. The adequate stimulus was determined, and the extent to which these properties are correlated was investigated. 2. All cells with receptive fields could be classified as mechanoreceptors. Most cells with A-beta-axons (greater than 36 m/s) could be activated by gentle mechanical stimulation but a small minority with conduction velocity in the low end of the A-beta-range were nociceptors. Cells with A-delta-axons (2-36 m/s) innervated either the very sensitive Down hair follicles (D-hairs) or high-threshold mechanoreceptors (HTMRs). In addition a group of A-delta-fibers was found for which no receptive field could be described. Their spikes, AHPs, and membrane properties were indistinguishable from those of cells supplying HTMRs (see below) and they were lumped together with A-delta-HTMRs. 3. A-beta-neurons exhibited smaller, briefer spikes than A-delta-neurons, even those supplying D-hairs. Peak rate of rise (dV/dt)max and inward rectification were significantly larger in A-beta-cells than in A-delta s, whereas AHP duration and input resistance were smaller. However, the values of these parameters in cells of a given conduction velocity range were generally associated with receptor type. 4. A-delta-HTMRs exhibited spikes of greater amplitude and duration, longer AHP duration, and smaller inward rectification than D-hairs. The long duration of these spikes was due largely to a prominent hump on their descending limb. Input resistance was similar in both groups of cells. 5. A-beta-HTMRs differed from A-beta-cells innervating low threshold receptors in the same general way that A-delta-HTMRs differed from D-hairs. However, A-beta-LTMRs supplying different receptor types (e.g., slowly adapting type I, Pacinian corpuscles, etc.) exhibited no correlation between receptor type and electrophysiology of the soma. 6. These differences in spike parameters occur at the level of the membrane rather than in the degree of somal invasion because the largest amplitude spikes also exhibited the slowest time course (i.e., in HTMRs). Systematic variation in AHP duration and inward rectification also suggest differences in the proportions of ionic channel types among these cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Axon diameter distributions across the monkey's optic nerve

The distribution of axons according to diameter has been examined in the optic nerve of old world monkeys. Axon diameters were measured from electron micrographs, and histograms were constructed at regular intervals across a section through the optic nerve to reveal the local axon diameter distribution. The total axon diameter distribution was also estimated. Fine-calibre optic axons (less than 2.0 micron in diameter) are found at all locations across the optic nerve. They are most frequent centrotemporally, where very few coarse optic axons can be found, but also make up the majority at the optic nerve's periphery. Coarse optic axons (greater than 2.0 microns in diameter) are increasingly common at progressively peripheral positions in the nerve. Around the nerve's circumference, these coarse optic axons are least numerous temporally, and most common dorsonasally. The axon diameter distribution peaks around 1.25 microns at most locations across the optic nerve, but there are more, slightly larger (1.5-2.0 microns), optic axons dorsally than ventrally. The estimated total axon diameter distribution is unimodal, peaking at 1.0-1.25 microns, with an extended tail towards larger diameters. This centroperipheral gradient of increasing axon diameters across the optic nerve is not substantial enough to account for the partial segregation of axons by size in the monkey's optic tract: there, coarse optic axons form a conspicuously greater proportion of the local axon diameter distribution along the tract's superficial (sub-pial) border, and fine optic axons are the only axons present near the tract's deep border. Hence, the fibre distribution in the optic tract cannot be formed by a simple combination of the fibre distributions of the two respective half-nerves, as described in the classic neuro-ophthalmologic literature. Rather, the present results, in conjunction with previous results from the optic tract, demonstrate that there must be a reorganization of axons by size in or near the optic chiasm.

Intraretinal axons of ganglion cells in the Japanese monkey (Macaca fuscata): conduction velocity and diameter distribution

In anesthetized and immobilized Japanese monkeys (Macaca fuscata), intraretinal conduction velocities of the ganglion cell axons were measured. The field potentials elicited by optic chiasm shocks consisted of fast and slow components with estimated conduction velocities of 1.19 and 0.72 m/s in recordings from the optic nerve fiber layer, and 1.65 and 1.00 m/s in recordings from the ganglion cell layer. Single cell recordings verified that the time course of the fast component corresponded to the antidromic spike latencies of Y-like cells, whereas that of the slow component covered the latency range of both X-like and W-like cells. In an electron microscopic study of the cross-sections of the intraretinal optic nerve fiber bundles, the axon diameter histograms of large samples (n = 3000-6000) all showed a unimodal distribution with a sharp peak at 0.3-0.6 micron and a long tail extending to 2-3 micron. The mean diameter was largest in the ventral and nasal bundles, smallest in the papillomacular bundle and intermediate in the dorsal, upper arcuate and lower arcuate bundles. However, diameter histograms of a small number of regional axons (n = 255-300) showed a broad tail distinct from the peak at 0.3-0.6 micron, enabling us to segregate a group of larger axons from the medium-sized to small axons. From such regional axon diameter histograms we estimated the mean relative occurrences of the larger axons (7.1-11.3%) and their mean diameters (0.9-1.3 micron). We further applied this relative frequency to the unimodal distribution of the histograms with larger samples in the upper and lower arcuate bundles and estimated the mean axon diameter of the large axons (1.1 micron) and that of the medium-sized to small axons (slightly below 0.5 micron). Finally, in studying the relation between axon diameter and conduction velocity in the two arcuate fiber bundles, we found it to be somewhat different from that previously reported for the cat retina.

Dissociation of the inhibition of fast axonal transport by chlorimipramine from an effect on axonal microtubules

The effect of in vitro exposure of bullfrog spinal nerves to 0.2 mM chlorimipramine on the density of axonal microtubules was studied in an attempt to clarify the mechanism by which chlorimipramine inhibits fast axonal transport. A 17-h exposure to chlorimipramine reduced the density of microtubules in unmyelinated axons by only 18%; this microtubular loss does not reach the upper limit of the range of microtubule reduction associated with inhibition of fast axonal transport. A 23-h exposure to chlorimipramine, which had decreased microtubular density in unmyelinated axons by 40% in a previous study, did not decrease microtubular density in myelinated axons in the present study. These results rule out microtubular destruction as the mechanism responsible for inhibition of fast orthograde axonal transport by chlorimipramine, and greatly reduce the likelihood that microtubular destruction plays a significant role in the inhibition of fast retrograde transport by chlorimipramine.

Subtypes of neurones in Forel's field H as defined by their axonal projection

Projection of neurones in Forel's field H (FFH) to the mesencephalon, the lower brainstem, and the upper cervical spinal cord (C1) was investigated by threshold mapping for evoking antidromic spikes from these areas. Projections of all FFH neurones tested were ipsilateral. Two main types (Type I and II) and their subtypes were differentiated from the pattern of the trajectories. Type Ia FFH neurones were found to project primarily to the oculomotor nucleus (IIIn) and the periaqueductal gray (PAG) bud did not descend down to the medulla, while Type Ib neurones projected to IIIn, PAG and the nucleus reticularis gigantocellularis (NRG) and Type Ic projected further down to the C1. Type II neurones were characterized by the absence of collaterals to IIIn. Type IIb projected to the cuneiform, subcuneiform and red nuclei in the mesencephalon and to the NRG, while Type IIc projected further down to the spinal cord. Type IIa neurones which terminated rostral to the NRG were found only rarely. These results suggested that Type Ib and Ic neurones are involved in the control of synergic eye and head movements, while Type IIb, IIc and Ia neurones are specified for the independent control of head and eye movement.

Serotoninergic innervation of the cat cerebral cortex

Serotoninergic axons in the cat cerebral cortex were demonstrated immunohistochemically with a monoclonal antibody to serotonin (5-HT). Three types of 5-HT axons are distinguished at the light microscopic level by differences in their morphology. Small varicose axons are fine (less than 0.5 micron) and bear fusiform varicosities that are generally less than 1 micron in diameter. These axons extend throughout the width of the cortex and branch frequently, giving rise to widely spreading collaterals. Nonvaricose axons are smooth, show a relatively large and constant caliber (about 1 micron), travel in straight, horizontal trajectories, and branch infrequently. Large varicose axons are distinguished by large round or oval varicosities (1 micron or more in diameter) borne on fine-caliber fibers. These axons often form basket-like arbors around the somata of single neurons. In the simplest basket-like arbors, several large, round varicosities from a small number of axons contact the soma. In complex baskets intertwining collaterals contact the soma and apparently climb along and outline the cell's major dendrites. The patterns revealed by the climbing axons suggest that a variety of nonpyramidal cell types selectively receive dense 5-HT innervation. Serial reconstructions of the 5-HT axons within the cortex show that the large varicose axons arise as infrequent collaterals from the nonvaricose axons. A single nonvaricose parent axon gives rise to several large varicose axon collaterals that may contribute to different basket-like arbors. Conversely, a single basket-like arbor may be formed by large varicose axon collaterals from more than one nonvaricose parent axon. The small varicose axons do not appear to be related within the cortex to either the nonvaricose or large varicose axon types. The results support the hypothesis that the 5-HT projection to the cortex is organized into two subsystems, one of which may exert widespread influence in the cortex via highly divergent branches, while the other, with a more restricted distribution, acts on specific classes of cortical neurons.

The development of activity evoked by fine diameter cutaneous fibres in the spinal cord of the newborn rat

The cutaneous afferent volley on the dorsal root was recorded in normal rat pups and in pups whose fine diameter afferents had been destroyed with capsaicin at birth. From postnatal day 2 (P2) it was possible to identify a long latency wave in the afferent volley that was reduced or absent in capsaicin treated pups and therefore attributable to A delta and C fibers. Despite this, no difference between the evoked activity of dorsal horn cells to electrical skin stimulation in normal and capsaicin treated rat pups could be detected at P2 or P5. Only at P9 was a clear long latency burst of spikes evoked in dorsal horn cells that was absent in capsaicin treated rats. The results support the idea that the postsynaptic activity evoked by small diameter A and C fibres in the rat dorsal horn in the first postnatal week is very immature.

Propriospinal fibers in the white matter of the cat sacral spinal cord

The propriospinal system, which consists of those neurons completely contained within the spinal cord, is important because it underlies much spinal behavior. To provide quantitative data on this system, the present study determines numbers of axons in the isolated S2 cat spinal cord and compares these figures with the normal. The conclusion is that 60% of the fibers in the spinal cord at this location are propriospinal. Findings of particular interest are that the great majority of unmyelinated propriospinal axons are found in the dorsal part of the lateral funiculus, and that there are large numbers of descending myelinated fibers in the dorsal funiculi. These data will serve as a basis for evaluating axon numbers that follow various experimental regimens purporting to result in neural sprouting.

The morphology and distribution of rat serotoninergic intraspinal neurons: an immunohistochemical study

An immunohistochemically derived morphological description of a diverse population of rat lamina VII and X intraspinal 5HT neurons is provided. These bipolar or multipolar neurons occur most frequently in lamina X, dorsal or dorsolateral to the central canal, in thoracolumbar, sacral, and coccygeal spinal segments. These 5HT intraspinal neurons are found in normal rat spinal cords as well as in spinal cords that have been hemisected or transected 60 days prior to serotonin immunostaining. Therefore, 5HT intraspinal neurons are the probable source of the biochemically detectable 5HT that remains in the spinal cord distal to a spinal transection. In the rat, serotonin intraspinal neurons are most often associated with spinal autonomic nuclei but it is unknown if they are preganglionic in nature.

Morphological and electrophysiological identification of gigantocellular tegmental field neurons with descending projections in the cat: I. Pons

Two different descending projections from the pontine gigantocellular tegmental field (PFTG) were defined by the use of intracellular recording and intracellular horseradish peroxidase (HRP) techniques in the cat. Type I neurons (reticulospinal neurons) had antidromic spike potentials produced by stimulation of the ipsilateral medial longitudinal fasciculus (MLF) and sent axons to the ipsilateral MLF. Most type I neurons had large ellipsoidpolygonal somata (mean, 59.7 microns), thick axons (average diameter, 3.33 microns), and slightly oblate large dendritic fields. The mean anteroposterior extent of the dendritic field was 1,492 microns, the mean mediolateral extent was 1,784 microns, and the mean dorsoventral extent was 1,562 microns. There were no type I neurons with axon collaterals. In contrast, type II neurons (reticuloreticular neurons) had antidromic spike potentials produced by stimulation of the bulbar reticular formation (BRF) and sent axons directly to the BRF. In comparison with type I neurons, most type II neurons had smaller ellipsoidpolygonal somata (mean, 40.2 microns), thinner axons (average diameter, 2.32 microns), and smaller, slightly oblate dendritic fields. The mean anteroposterior extent of the dendritic field was 1,264 microns; the mean mediolateral extent was 1,511 microns; and the mean dorsoventral extent was 1,226 microns. Also in contrast to type I neurons, 36% of type II neurons had axon collaterals.

Axon classes and internodal growth in the ventral spinal root L7 of adult and developing cats

Internodal length, number of myelin sheath lamellae and axon diameter were estimated in samples of nerve fibres reconstructed from transverse sections (EM and LM) of the L7 ventral spinal root of adult cats, kittens and cat fetuses. The axon calibre spectrum was unimodal in the youngest fetuses, became bimodal at 47 days after mating, i.e. about 2 weeks before birth (63 days after mating) and trimodal at 1 week of postnatal age. Alpha-axons received myelin during the period 40-45 to 50-55 days after mating while gamma-axons became myelinated from 60-63 days after mating to 2-3 weeks of postnatal age. The mean length of the first completely myelinated internodes measured 139 microns and 209 microns in the alpha- and gamma-fibre group respectively. The adult mean values were 1390 microns and 640 microns respectively. The average internodal elongation was x 10 and x 3 in the two fibre groups, whereas the longitudinal growth of the whole ventral root was x 5.4 and x 3.5. A combined linlog function could be fitted to describe the regression between internodal length and axon diameter and was established from birth onwards. The separate alpha- and gamma-fibre samples were best described by linear functions, the alpha internodal length being independent of axon diameter while the gamma internodal length increased linearly with axon diameter. The amount of internodal myelin increased linearly with increasing mantle area of the internodal axon. The mean amount of internodal myelin of alpha-fibres showed two periods of intense growth; one from the start of myelination to 2 months postnatally and the other between 4 and 6 months of age. The mean length of the lamellar spiral forming the myelin sheath showed about the same increase per unit length of axon circumference (about 160:1) in both alpha- and gamma-fibre groups as long as longitudinal internodal growth persisted.

Topographic differences in retinal axons in the dorsal lateral geniculate nucleus of the rat: a quantitative reexamination using anterograde transport of horseradish peroxidase

Retinal endings in the dorsal lateral geniculate nucleus (dLGN) of the rat were visualized by anterogradely transported horseradish peroxidase following injections in the optic tract. The morphological findings confirm the two types of retinal axons previously suggested by Golgi investigations. In the caudal third of the dLGN type 2b axons, with small dense clusters of boutons, are the only representatives of retinal fibres seen. In the rostral two thirds they are intermingled with the larger type 2a terminals, but tend to accumulate laterally, adjacent to the optic tract. This study supports the concept that large retinal ganglion cells probably give rise to 2a axons that may represent a Y-like channel, whereas 2b axons are derived from small ganglion cells, and may relay a W-like pathway.

Organization of individual afferent axons in layer IV of striate cortex in a primate

Evidence from a number of anatomical and physiological studies shows that information is transmitted from the retina to visual cortex via physiologically and anatomically distinct populations of neurons in the lateral geniculate nucleus (LGN). In order to gain a better understanding of the functional roles of these parallel channels from the LGN to cortex in primates, individual afferent axons to layer IV of striate cortex of galagos were filled with HRP by bulk injection into the white matter underlying striate cortex. A total of 55 axons and their terminal arbors, from zones representing both the central and peripheral visual fields, were completely reconstructed through serial sections. Based upon the sublaminar distribution and the morphology of these axons, they have been categorized into 2 groups, designated type I and II axons. Evidence from both past work and the present study suggests that type I axons represent the projections from physiologically defined Y-like cells in the magnocellular layers of the LGN, while type II axons represent the projections from X-like cells in the parvocellular LGN layers. Our results show that type I (presumed Y-like) arbors occupy relatively more cortical space within their main terminal sublayer (IV alpha) than is the case for the type II (presumed X-like) arbors which ramify primarily in layer IV beta. In addition, type I arbors have larger parent axons, fewer boutons along a restricted length of axon, and a greater tendency to branch in layer VI than type II arbors. Finally, both axon types are larger in the area of cortex representing central vision than in the area representing peripheral vision. These morphological characteristics suggest that the physiological differences between magnocellular and parvocellular geniculate cells may be amplified in cortex by differences in the organization of their terminal arbors. Further, within each afferent population, the terminal organization of axons reflects their visuotopic relationships in striate cortex. Comparison of these findings with data from cats and monkeys supports the idea that the relationship between the size of the terminal arbors of LGN X-like or parvocellular cells and the size of the cortical spatial subunit varies with differences in visual acuity across species; for LGN Y-like (or magnocellular) cells this relationship remains constant.

The organization and hypothalamic projections of the tuberomammillary nucleus in the rat: an immunohistochemical study of adenosine deaminase-positive neurons and fibers

The intense immunohistochemical reaction for the enzyme adenosine deaminase displayed by neurons in the tuberomammillary nucleus in the rat was used to study the distribution and morphology of cells comprising this nucleus, their fiber fields within the posterior hypothalamus and their projection pathways from the hypothalamus. Neurons immunoreactive for adenosine deaminase were found along ventricular and basal aspects of the hypothalamus from the level of the dorsomedial nucleus to the caudal pole of the mammillary body. Approximately 4500 neurons were seen on each side of the brain. Positive neurons showed a complex distribution, largely avoiding nuclear boundaries within the posterior basal hypothalamus and mammillary body. This distribution is mapped in detail and a nomenclature based on topography is introduced so that different regions of the cell distribution may be discussed more easily. Reactive neurons showed a Golgi-like staining which allowed careful study of their morphology. In general, neurons were large, with major axes of from 22 to 30 micron, and bipolar in shape. A second, smaller cell type, 14-16 micron in diameter was also seen and, although often less intensely stained, it was considered a constituent of tuberomammillary nucleus of the hypothalamus as well. Stained dendritic arbours extended considerable distances from the parent cell bodies and branched regularly. Dendrites showed very sparse spines and had an apparently scalloped surface. Features suggestive of varicose segments of dendrites were also noted. The long, smooth dendrites of positive neurons were often seen to aggregate into bundles which avoided nuclear boundaries and tended to collect adjacent to basal and ventricular surfaces of the posterior hypothalamus. Varicose fibers immunoreactive for adenosine deaminase formed a dense network within the hypothalamus. These fibers were considered to derive from the positive neurons in the tuberomammillary nucleus and were similar to adenosine deaminase-immunoreactive fibers seen throughout much of the rest of the brain. The density of this type of positive fiber was, however, much greater within the hypothalamus. The region of the posterior basal hypothalamus also contained relatively sparse populations of adenosine deaminase-positive fibers, apparently distinct from this network. These consisted of a field of fine fibers in the median division of the medial mammillary nucleus and a few large varicosities in the dorsolateral part of the median eminence.(ABSTRACT TRUNCATED AT 400 WORDS)