Small Arbors Research Articles

Dopaminergic innervation of the pallidum in the normal state, in MPTP‐treated monkeys and in parkinsonian patients

The aim of the present study was to characterize the dopaminergic innervation of the pallidum in primates (humans and Cercopithecus aethiops). Firstly, in monkeys, biotin dextran amine was injected into dopaminergic areas, and the anterogradely labelled axons were reconstructed from serial sections and analysed in the pallidum. Secondly, in parkinsonian patients and MPTP-treated monkeys, the dopaminergic innervation of the pallidum was studied using tyrosine hydroxylase-positive fibre quantification. Our study revealed that dopaminergic areas A8 and A9 innervated the two pallidal segments. Individual axonal arborizations displayed a great heterogeneity. Some dopaminergic axons crossed the pallidum without branching, other axons made small terminal arborizations in a restricted region of one pallidal segment, whereas others developed dense arborizations covering extended areas in the two pallidal segments. This heterogeneous organization suggests that dopamine could directly modulate the pallidum using either a point-to-point or a diffuse projection pattern. A statistically significant loss of dopaminergic fibres in the internal (-43%) and external pallidum (-39.6%) of humans, and in the internal (-54.3%) and external pallidum (-59%) of monkeys was revealed in parkinsonian states. The consequences of this alteration are still unknown but it might participate in the triggering of motor symptoms observed in Parkinson's disease.

Single axon analysis of pulvinocortical connections to several visual areas in the macaque.

The pulvinar nucleus is a major source of input to visual cortical areas, but many important facts are still unknown concerning the organization of pulvinocortical (PC) connections and their possible interactions with other connectional systems. In order to address some of these questions, we labeled PC connections by extracellular injections of biotinylated dextran amine into the lateral pulvinar of two monkeys, and analyzed 25 individual axons in several extrastriate areas by serial section reconstruction. This approach yielded four results: (1) in all extrastriate areas examined (V2, V3, V4, and middle temporal area [MT]/V5), PC axons consistently have 2-6 multiple, spatially distributed arbors; (2) in each area, there is a small number of larger caliber axons, possibly originating from a subpopulation of calbindin-positive giant projection neurons in the pulvinar; (3) as previously reported by others, most terminations in extrastriate areas are concentrated in layer 3, but they can occur in other layers (layers 4,5,6, and, occasionally, layer 1) as collaterals of a single axon; in addition, (4) the size of individual arbors and of the terminal field as a whole varies with cortical area. In areas V2 and V3, there is typically a single principal arbor (0.25-0.50 mm in diameter) and several smaller arbors. In area V4, the principal arbor is larger (2.0- to 2.5-mm-wide), but in area MT/V5, the arbors tend to be smaller (0.15 mm in diameter). Size differences might result from specializations of the target areas, or may be more related to the particular injection site and how this projects to individual cortical areas. Feedforward cortical axons, except in area V2, have multiple arbors, but these do not show any obvious size progression. Thus, in areas V2, V3, and especially V4, PC fields are larger than those of cortical axons, but in MT/V5 they are smaller. Terminal specializations of PC connections tend to be larger than those of corticocortical, but the projection foci are less dense. Further work is necessary to determine the differential interactions within and between systems, and how these might result in the complex patterns of suppression and enhancement, postulated as gating mechanisms in cortical attentional effects, or in different states of arousal.

Passive and Active Membrane Properties Contribute to the Temporal Filtering Properties of Midbrain NeuronsIn Vivo

This study examined the contributions of passive and active membrane properties to the temporal selectivities of electrosensory neurons in vivo. The intracellular responses to time-varying (2-30 Hz) electrosensory stimulation and current injection of 27 neurons in the midbrain of the weakly electric fish Eigenmannia were recorded. Each neuron was filled with biocytin to reveal its anatomy. Neurons were divided into two biophysically distinct groups based on their frequency-dependent responses to sinusoidal current injection over the range 2-30 Hz. Fourteen neurons showed low-pass filtering, with a maximum decline in the amplitude of voltage responses of >2.6 dB (X = 4.30 dB, s = 1.10 dB) to sinusoidal current injection. These neurons also showed low-pass filtering of electrosensory information but with larger maximum declines in postsynaptic potential amplitude (X = 9.53 dB, s = 3.34 dB; n = 10). These neurons had broad dendritic arbors and relatively spiny dendrites. Five neurons showed all-pass filtering, having maximum decline in the amplitude of voltage responses of <2.0 dB (X = 1.16 dB, s = 0.61 dB). For electrosensory stimuli, however, these neurons showed low-, band-, or high-pass filtering. These neurons had small dendritic arbors and few or no spines. Voltage-dependent "active" conductances were revealed in eight neurons by using several levels of current clamp. In four of these neurons, the duration of the voltage-dependent conductances decreased in concert with the period of the electrosensory stimulus, whereas in the other four neurons the duration of the voltage-dependent conductances was relatively short (<30 msec) and nearly constant across sensory stimulation frequencies. These conductances enhanced the temporal filtering properties of neurons.

Brain-Derived Neurotrophic Factor and Neurotrophin-4/5 Stimulate Growth of Axonal Branches from Regenerating Retinal Ganglion Cells

To investigate the influences of growth factors on axonal regeneration in the mammalian CNS, we used intracellular tracers to quantitate the effects of brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-4/5, or NT-3 on individual retinal ganglion cell (RGC) axons in the retinas of adult rats after optic nerve transection. A single injection of BDNF or the prolonged administration of NT-4/5 by mini-pump increased axon branch median lengths by eightfold but had no effect on the number of branches formed by the RGC axons. NT-3 did not significantly influence axonal regrowth. These specific in vivo effects of BDNF and NT-4/5 on axonal regeneration from injured RGCs may be used to promote growth and expand the abnormally small terminal arbors observed when RGCs regrow into their CNS targets.

Intrinsic Differences in Axonal Growth from Crayfish Fast and Slow Motoneurons

The motoneurons innervating the fast and slow flexor muscles in the abdomen of crayfish form morphologically distinct motor terminals. Axons of the fast flexor (FF) motoneurons, which innervate the large (fast) flexor muscle, produce extensive motor terminal arbors with many branches. Axons of the slow flexor (SF) motoneurons, which innervate the thin (slow) flexor muscle, produce small terminal arbors with many fewer branches. To determine whether intrinsic factors contribute to these differences in terminal arbors, we compared regenerating axonal arbors from these two populations of motoneurons. We used an explant of the crayfish nerve cord in which axons from the FF and SF motoneurons regenerate on a homogeneous substrate. We found that regardless of the substrate, FF motor axons produced arbors with a greater total length and a greater number and density of branches than SF motor axons. These differences in regenerated arbors persisted in defined medium and in the absence of impulse activity, indicating that they result from intrinsic, neuron-specific factors. The greater branching of the FF motor axons may be related to differences in growth cones: growth cones of FF axons were significantly larger with more filopodia than growth cones of SF axons.

Mature, growing ganglion cells acquire new synapses in the retina of the goldfish.

The goldfish retina grows throughout the animal's life, primarily by a balloon-like expansion. With this expansion, dendritic arbors of ganglion cells show scaled growth; arbors increase in size from small to large with no change in their architecture (Hitchcock & Easter, 1986; Bloomfield & Hitchcock, 1991). The study reported here showed that ganglion cell arbors acquire new synapses with this growth. Arbors from a single type of ganglion cell in retinas of small, young and large, old fish were intracellularly filled with horseradish peroxidase, examined electron microscopically, and the synapses contacting them counted and compared (small arbors vs. large arbors). The small and large arbors had similar numbers and orders of dendritic branches (i.e. similar architectures), but the large arbors were significantly larger than the small ones. The increase in arbor size was correlated with a 2.7x and 1.9x increase in the number of ribbon and conventional synaptic contacts, respectively. The addition of new synapses is proposed as a mechanism by which the signaling properties of the enlarging ganglion cells can remain constant.

Central-peripheral and rostral-caudal organization of the innervation of the saccule in a cichlid fish.

Saccular eighth nerve arbors were examined in the cichlid fish Astronotus ocellatus to determine if their morphology varies with saccular location. The saccule was divided into four regions: rostral-central, rostral-peripheral, caudal-central, caudal-peripheral. Arbors were filled with cobaltous-lysine. Axon diameter, maximum arbor width, and number of terminal points were taken as quantitative measures. Differences in these measures among the four different saccular regions were evaluated using an analysis of variance. The results indicate two types of organization: central-peripheral and rostral-caudal. The central-peripheral differences involve all three quantitative measures. The central saccule is innervated by arbors with larger axon diameters, larger arbor widths, and more terminal points than the peripheral saccule. The rostral-caudal organization involves only two measures. The rostral saccule is innervated by arbors having larger axon diameters and smaller arbor widths than the caudal saccule. In the oscar, we know of no other parameters that are organized along the rostral-caudal dimension. These findings of a spatial organization of innervation suggest to us that the oscar saccule is not homogeneous in function.

Dendritic arbors of large-field ganglion cells show scaled growth during expansion of the goldfish retina: a study of morphometric and electrotonic properties

The retina of the goldfish grows by a balloon-like expansion and by the addition of new neurons at the margin. It has been proposed that as a consequence of this expansion the dendritic arbors of ganglion cells in central retina grow in a uniform manner without the addition of new branches. In the present study, we have examined this proposal by comparing the geometries of individual dendritic arbors of large-field ganglion cells from the retinas of small/young and large/old fish. These comparisons were based on measurements of several parameters of dendritic morphology, including number of segments and branches, branch angles, changes in diameter at branch points, and proximal versus distal distribution of arbor length. In addition, we used passive, steady-state cable modeling as an independent method of estimating the functional architectures of small and large dendritic arbors. Our morphometric data indicate that, though they are very different in absolute size, dendritic arbors of small and large ganglion cells have remarkably similar architectures. Analysis with steady-state cable equations indicates that the arbors from small and large cells have equivalent electrotonic lengths and show comparable propagation of synaptic currents. These data are consistent with the hypothesis that dendritic arbors of small and large ganglion cells are scaled versions of one another. We conclude that the growth of these cells during the expansion of the retina is the result of the addition of dendritic mass to an arbor whose basic geometry remains unchanged.

Palisade pattern of mormyrid Purkinje cells: A correlated light and electron microscopic study

The present study is devoted to a detailed analysis of the structural and synaptic organization of mormyrid Purkinje cells in order to evaluate the possible functional significance of their dendritic palisade pattern. For this purpose, the properties of Golgi-impregnated as well as unimpregnated Purkinje cells in lobe C1 and C3 of the cerebellum of Gnathonemus petersii were light and electron microscopically analyzed, quantified, reconstructed, and mutually compared. Special attention was paid to the degree of regularity of their dendritic trees, their relations with Bergmann glia, and the distribution and numerical properties of their synaptic connections with parallel fibers, stellate cells, "climbing" fibers, and Purkinje axonal boutons. The highest degree of palisade specialization was encountered in lobe C1, where Purkinje cells have on average 50 palisade dendrites with a very regular distribution in a sagittal plane. Their spine density decreases from superficial to deep (from 14 to 6 per micron dendritic length), a gradient correlated with a decreasing parallel fiber density but an increasing parallel fiber diameter. Each Purkinje cell makes on average 75,000 synaptic contacts with parallel fibers, some of which are rather coarse (0.45 microns), and provided with numerous short collaterals. Climbing fibers do not climb, since their synaptic contacts are restricted to the ganglionic layer (i.e., the layer of Purkinje and eurydendroid projection cells), where they make about 130 synaptic contacts per cell with 2 or 3 clusters of thorns on the proximal dendrites. These clusters contain also a type of "shunting" elements that make desmosome-like junctions with both the climbing fiber boutons and the necks of the thorns. The axons of Purkinje cells in lobe C1 make small terminal arborizations, with about 20 boutons, that may be substantially (up to 500 microns) displaced rostrally or caudally with respect to the soma. Purkinje axonal boutons were observed to make synaptic contacts with eurydendroid projection cells and with the proximal dendritic and somatic receptive surface of Purkinje cells, where about 15 randomly distributed boutons per neuron occur. The organization of Purkinje cells in lobe C3 differs markedly from that in C1 and seems to be less regular and specialized, although the overall palisade pattern is even more regular than in lobe C1 because of the absence of large eurydendroid neurons. However, individual neurons have a less regular dendritic tree, there is no apical-basal gradient in spine density or parallel fiber density and diameter, and there are no "shunting" elements in the climbing fiber glomeruli.(ABSTRACT TRUNCATED AT 400 WORDS)

Sex‐Related Differences in Dendritic Branching of Cells in the Prefrontal Cortex of Rats

Abstract Male and female Sprague-Dawley rats were given either gonadectomies or sham surgery on the day of birth. In adulthood they were sacrificed and prepared for Golgi-Cox staining. Using a camera lucida technique, layer II/III pyramidal neurons were drawn in the anterior cingulate cortex areas 1 and 3 and the agranular insular cortex and the number of dendritic branches on the apical and basilar dendrites were then summarized. The results showed that the dendritic arbor of prefrontal cortical cells varied as a function of sex and neonatal gonadectomy. In cingulate 3, castrated males had a smaller arbor on the left relative to normal males, whereas, in cingulate 1 the reverse was found. In addition, females had less apical arbor than males in the cingulate areas, whereas, in the agranular insular cortex, females had greater apical arbor than the males. Furthermore, in this region, gonadectomized groups had less arbor than their respective control groups. Finally, in the agranular insular cortex, the dendritic arbor was greater in the left hemisphere.

Hair cell tufts and afferent innervation of the bullfrog crista ampullaris

Within the bullfrog semicircular canal crista, hair cell tuft types were defined and mapped with the aid of scanning electron microscopy. Intracellular recording and Lucifer Yellow labeling techniques were used to study afferent responses and arborization patterns. Dye-filled planar afferent axons had mean distal axonal diameters of 1.6–4.9 μm, highly branched arbors, and contacted 11–24 hair cells. Dye-filled isthmus afferent axons had mean distal axonal diameters of 1.8–7.9 μm, with either small or large field arbors contacting 4–9 or 25–31 hair cells. The estimated mean number of contacts per innervated hair cell was 2.2 for planar and 1.3 for isthmus afferent neurons. Data on evoked afferent responses were available only for isthmus units that were observed to respond to our microrotational stimuli (<3°/s peak rotational velocity). Of 21 such afferent neurons, 8 were successfully dye-filled. Within this small sample, high-gain units had large field arbors and lower-gain units had small field arbors. The sensitivity of each afferent neuron was analyzed in terms of noise equivalent input (NEI), the stimulus amplitude for which the afferent response amplitude is just equivalent to the RMS deviation of the instantaneous spike rate. NEI for isthmus units varied from 0.63 to 8.2°/s; the mean was 3.2°/s.

Quantitative analysis of age-related dendritic changes in medium spiny I (MSI) striatal neurons of C57BL/6N mice

Our study used quantitative morphometric analysis and Golgi staining methods to evaluate postnatal changes in the dendritic architecture of MSI neurons of the striatum between 1 and 30 months of age. Morphological changes and chronological age were also correlated with functional testing in order to identify subpopulations of aged mice with dendritic alterations that may be more characteristic of a motor deficit rather than the normal aging process. We found that the overall size of the dendritic arbor of MSI neurons in the rostral striatum remained stable with age, while caudal MSI neurons exhibited a significant elongation of terminal dendritic segments between 25 and 30 months of age. In addition, our correlation analysis of motor performance and chronological age found that neither striatal-motor deficits nor their associated anatomical correlates were inevitable consequences of senescence but were characteristic for a select subpopulation of aged mice with striatal-motor deficits. We found that mice that tested poorly on the balance rod had a significant increase in the number of MSI neurons with small dendritic arbors in various stages of atrophic degeneration. Conversely, 30-month-old mice that had no functional impairment showed no significant change in the number of neurons with atrophic dendrites. These data reinforce the premise that the correlation of structure and function plays an important role in the analysis of an aging population since data may vary based on the number of functionally impaired or unimpaired mice that make up an experimental group.

Activity‐driven sharpening of the regenerating retinotectal projection: Effects of blocking or synchronizing activity on the morphology of individual regenerating arbors

Both blocking activity with intraocular tetrodotoxin (TTX) and synchronizing activity with a xenon strobe light (1 Hz) prevent retinotopic sharpening of regenerating optic projection in goldfish (Meyer, 1983; Schmidt, 1985; Cook and Rankin, 1986). In this study, we tested, in both normal and regenerating projections, the effects of these two treatments on individual optic arbors. Arbors were stained via anterograde transport of HRP, drawn in camera lucida from tectal whole mounts, and analyzed for spatial extent in the plane of the retinotopic map, order of branching, number of branch endings, depth of termination, and the caliber of the parent axon. In normal tectum, fine, medium, and coarse caliber axons gave rise to small, medium, and large arbors, which averaged 127 microns, 211 microns and 275 microns in horizontal extent, and terminated at characteristic depths. All three classes averaged roughly 21 branch endings. Optic arbors that regenerated with normal patterns of activity returned to a roughly normal appearance by 6-11 weeks postcrush: the same three calibers of axons gave rise to the same three sizes of arbors at the same depths, but they were much less stratified and well on average about 16% larger in horizontal extent. At this time point, arbors regenerated under TTX or strobe were on the average 71 and 119% larger, respectively, than the control-regenerated arbors (larger in all classes), although they had approximately the same number of branch endings and were equally poorly stratified. Synapses formed under strobe were also normal in appearance. Thus the only significant effect of both strobe and TTX treatment was to enlarge the spatial extent of arbor branches. Arbors that were not regenerating were very slightly (but significantly) enlarged by TTX block of activity or strobe illumination. As previous staining showed that regenerating axons initially make widespread branches and later retract many of those branches (Schmidt, Turcotte, Buzzard, and Tieman, 1988; Stuermer, 1988), the present findings support the idea that blocking activity or synchronizing activity prevents retinotopic sharpening by interfering with the elimination of some of the errant branches.

Postnatal development of the light and electron microscopic features of basket cells in the hippocampal dentate gyrus of the rat

Light and electron microscopic preparations were used to analyze the postnatal development of the basket cells of the rat dentate gyrus. The basket cells, located at the hilar border, were recognized in 2-day-old rats in Golgi preparations, where they displayed immature dendrites and a small axon arbor in the granule cell layer. At 5 days, the basket cells were found to have a large perikaryal cytoplasm, a round nucleus, an axon that forms symmetric synapses with granule cells, and dendrites and somata that are contacted by other axon terminals. The 10-day basket cells display more mature features, such as Nissl bodies and well-developed Golgi complexes. The basket cells from 16-day-old rats are mature in terms of their ultrastructural features, in that the nuclei are highly indented and display intranuclear rods or sheets, the perikaryal cytoplasm is packed with organelles, and the axon has developed an extensive arborization with the somata and dendrites of granule cells at the border with the molecular layer. This arborization will continue to expand as more granule cells are generated and added to the hilar border. These data correlate well with the immunocytochemical and biochemical development of GABAergic neurons in the dentate gyrus. Furthermore, the maturation of the structure of basket cells appears to precede the appearance of adult-like electrical activity in the hippocampus.

Modality‐specific axonal projections in the CNS of the flies Phormia and Drosophila

There is a rich history of behavioral and physiological studies on the leg sensory systems of flies. Here we examine the anatomy of the sensory axons of two species of fly and demonstrate that the location of the axonal projections in the CNS can be correlated with the modality they encode. We studied receptors associated with proprioceptive, tactile, and multimodal hairs. Proprioceptive hairs occur in clusters, called hair plates, and are situated near joints. The neuron innervating each proprioceptive hair has a large axon and coarse arborization in the intermediate neuropil. Tactile receptors have smaller arbors, which are located in a ventral region of the thoracic neuromere. Finally, the multimodal hairs are each innervated by one tactile and four chemosensory neurons. The single tactile neuron has a central arbor that is indistinguishable from those of the tactile hairs; the four chemosensory neurons project to yet a third region of neuropil near the ventral surface of each neuromere. Thus there is a clear modality-specific segregation of axonal arbors in the CNS. This organization is identical in Phormia and Drosophila and thus apparently highly conserved within the Diptera. We presume that, as in other insect sensory systems, this anatomical specificity is linked to synaptic specificity.

Quantitative electrophysiological studies of regenerating visuotopic maps in goldfish—II. Delayed recovery of sensitivity to small light flashes

The regenerating retinotectal projection of goldfish was mapped with punctate flashes of light produced by red light-emitting diodes. The characteristics of multiunit receptive fields were studied in fish kept at 25°C at different times after unilateral optic nerve crush. From about 20 days, when the first visually-evoked responses to black-on-white stimuli appeared, until about 40 days, no consistent responses to light-emitting diodes could be obtained, although high-contrast, long-duration light-emitting diode stimuli elicited weak off-responding. At around 40 days, responses to light-emitting diodes reappeared as the amplitude of evoked multiunit activity increased sharply. At their emergence, light-emitting diode-sensitive multiunit receptive fields were irregular and only slightly enlarged, but quickly regained normal shape and size. Conformity to a linear and uniform visuotopography recovered more slowly and, in some individuals, incompletely. The results suggest that “on” and “off” optic fiber systems, probably with small terminal arbors, are functionally expressed at a later time in regeneration than the predominantly “off” system manifested earlier. The different time courses of recovery in these systems explain several aspects of the recovery of visual behavior during optic nerve regeneration.

Staining of regenerated optic arbors in goldfish tectum: progressive changes in immature arbors and a comparison of mature regenerated arbors with normal arbors.

Individual optic arbors, normal and regenerated, were stained via anterograde transport of HRP and viewed in tectal whole mounts. Camera lucida drawings were made of 119 normal optic arbors and of 242 regenerated arbors from fish 2 weeks to 14 months postcrush. These arbors were analyzed for axonal trajectory, spatial extent in the horizontal plane, degree of branching, number of branch endings, average depth, and degree of stratification. Normal optic arbors ranged in size from roughly 100 to 400 microns across in a continuous distribution, had an average of 20 branch endings with average of fifth-order branching, and were highly stratified into one of three planes within the major optic lamina (SO-SFGS). Small arbors arising from fine-caliber axons terminated in the most superficial plane of SO-SFGS; large arbors from coarse axons terminated in the superficial and middle planes; and medium arbors from medium-caliber axons terminated in the middle and deep planes of SO-SFGS, as well as deeper in the central gray and deep white layers. Arbors from central tectum tended to be much more tightly stratified than those in the periphery. No other differences between central and peripheral arbors were noted. Mature regenerated arbors (five months or more postcrush) were normal in their number of branch endings, order of branching, and depth of termination. Their branches covered a wider area of tectum, partially because of their early branching and abnormal trajectories of branches. Axonal trajectories were often abnormal with U-turns and tortuos paths. Fine-, medium-, and coarse-caliber axons were again present and gave rise to small, medium, and large arbors at roughly the same depths as in the normals. There was frequently a lack of stratification in the medium and large arbors, which spanned much greater depths than normal. Overall, however, regenerates reestablished nearly normal morphology except for axonal trajectory and stratification. Early in regeneration, the arbors went through a series of changes. At 2 weeks postcrush, regenerated axons had grown branches over a wider-than-normal extent of tectum, though they were sparsely branched and often tipped with growth cones. At 3 weeks, the branches were more numerous and covered a still wider extent (average of five times normal), many covering more than half the tectal length or width. At 4-5 weeks smaller arbors predominated, although a few enlarged arbors were present for up to 8 weeks. Additional small changes occurred beyond 8 weeks as the arbors became progressively more normal in appearance.(ABSTRACT TRUNCATED AT 400 WORDS)

Terminal arbors of axons projecting to the somatosensory cortex of the adult rat. I. The normal morphology of specific thalamocortical afferents

A characteristic feature of the rat somatosensory neocortex is a discrete topographic representation of the facial whiskers. Afferent fibers projecting to this vibrissae representation were "bulk-labeled" by injecting horseradish peroxidase into the white matter. Terminal arbors with the morphological characteristics of Lorente de No's (1949) "specific" thalamocortical afferents were then reconstructed through serial sections. These terminal arbors, characterized by the discrete organization of their dense plexus in layer IV, have a laminar distribution of boutons that parallels the laminar pattern of terminal degeneration resulting from lesions of the ventral posterior nucleus of the thalamus. The regional distribution of different-sized arbors corresponds to the distribution of vibrissae-related clusters of different sizes. Larger arbors were found in the posteromedial region corresponding to the mystacial vibrissae representation, while smaller arbors were found in the anterolateral region corresponding to the representation of the anterior sinus hairs. Terminal arbors were also reconstructed from sections stained simultaneously to demonstrate the pattern of vibrissae-related clusters. The greatest concentration of boutons on these axons occurred within a single vibrissae-related cluster. Furthermore, when 2 fibers terminated within a single cluster, their terminal arbors appeared to be largely coextensive. The morphology, size, and distribution of these terminal arbors support the hypothesis that the layer IV plexus of a single specific thalamocortical afferent tends to fill a vibrissae-related cluster. Thus, the organization of specific thalamocortical afferents may be responsible for clustered organization within the somatotopic map of the rodent neocortex.

Morphological characteristics of low‐threshold primary afferents in the trigeminal subnuclei interpolaris and caudalis (the medullary dorsal horn) of the golden hamster

Intra-axonal recording and horseradish peroxidase (HRP) injection techniques were employed to define the response characteristics of low-threshold, rapidly conducting trigeminal primary afferents and the morphological features of their axon arbors in subnucleus interpolaris and subnucleus caudalis (or the medullary dorsal horn; these last two terms are used synonomously throughout the paper). A total of 61 such afferents were characterized and recovered. Of these, ten gave rapidly adapting (RA) and 17 slowly adapting (SA type I) responses to vibrissa deflection. Twenty were sensitive to guard hair deflection and 14 were responsive to indentation of the hairy skin. The vibrissa-sensitive primary afferents were all quite similar morphologically. Primary collaterals proceeded directly, in a radial fashion, to their zone of termination and gave rise to dense and compact arbors. These tended to be larger in the medullary dorsal horn (MDH) than in interpolaris and they also gave rise to more boutons in the former nucleus. Guard hair afferents generally had smaller arbors and gave rise to fewer boutons than vibrissa-sensitive axons. Like vibrissa afferents, their arbor were generally circumscribed in both interpolaris and MDH, but they were larger in the latter nucleus. Skin-sensitive afferents had arbors that tended to be somewhat larger than those of vibrissa- or guard-hair-related fibers. Unlike the other fiber types, the arbors of skin-sensitive afferents were on average larger in interpolaris than MDH. Quantitative analysis of the morphological data from well-filled examples from each of these four functional types verified our qualitative impressions regarding differences between interpolaris and MDH collaterals of a given fiber-type. Statistical comparison of data from different functional classes indicated trends that supported our qualitative impressions, but none of these was statistically significant. The topography of the trigeminal primary afferent input to interpolaris was organized such that the head was inverted and fibers with caudal receptive fields terminated in the lateral portion of the nucleus. This was true for all of the functional afferent types that we examined. Vibrissa-related fibers differed from nonvibrissa afferents in that they tended to avoid the most rostral portion of interpolaris. In the MDH, the primary afferent representation of the head was also inverted, but fibers with caudal facial receptive fields tended to terminate medially rather than laterally.(ABSTRACT TRUNCATED AT 400 WORDS)

The functional organization of the crayfish lamina ganglionaris. II. Large-field spiking and nonspiking cells.

The functional properties of the multicolumnar interneurons of the crayfish lamina ganglionaris were examined by intracellular recording and the cell structures were revealed with the aid of Lucifer yellow or horseradish peroxidase iontophoresis. The multicolumnar monopolar cell M5 responds to a light pulse with a depolarizing compound EPSP and a burst of action potentials. Both the EPSP amplitude and the spike rate decay toward a lower level plateau in less than 200 ms after light onset. M5 is subject to surround inhibition, which is associated with a compound IPSP and net hyperpolarization of the membrane potential. Direct depolarization of M5 may provide a weak excitatory drive to medullary sustaining fibers (SF). Tangenital-cell type 1 (Tan1) has a broad expanse of neurites in the lamina (covering 10 to 15 cartridges) and a much narrower projection in the medulla (1 to 3 cartridges). The response to a light pulse has a long latency consistent with a polysynaptic receptor to Tan1 pathway. The response consists of a nearly rectangular hyperpolarization. Light 'off' elicits a depolarization and a burst of impulses. The polarity of the 'on' response can be reversed by hyperpolarizing the membrane by 23 mV. The receptive field is broad and the intensity-response function exceeds 4 log units. Direct hyperpolarization of Tan1 provides a strong excitatory signal to medullary SFs both in the dark and in the presence of illumination. We propose that Tan1 provides the principal steady-state excitatory drive to the SFs. Tangential-cell type 2 (Tan2) is distinguished from Tan1 by the extent and shape of the lamina process, which is a vertically oriented neurite spanning most of the lamina in a single plane. Functionally, Tan2 is similar in most respects to Tan1, but the response latency is much shorter, comparable to that of monopolar cells. T-cells may exhibit spontaneous impulse activity in the dark which is inhibited by a short latency hyperpolarizing light response. The receptive field, which is about 2 X larger than that of the columnar monopolar cells, is correlated with a small but multicolumnar dendritic arbor in the lamina. Since T-cells are aminergic, it is possible that the amines are normally released in the dark. A single amacrine cell was fully characterized. It exhibited a short latency hyperpolarizing response to light onset and a strong depolarizing 'off' response.(ABSTRACT TRUNCATED AT 400 WORDS)