Stephen Gobel

An HRP study of the central projections of primary trigeminal neurons which innervate tooth pulps in the cat

Trigeminal ganglia and brain stem of adult cats were studied following HRP injections into tooth pulps or after exposure of the cut end of the inferior alveolar nerve to HRP. Ipsilateral ganglion cells within a wide range of sizes were labeled in both experimental situations, whereas no labeled cells were observed in the contralateral ganglion in any animal. Labeled central branches of tooth pulp and inferior alveolar neurons were observed in all subdivisions of the ipsilateral trigeminal sensory complex. Terminal labeling in the tooth pulp experiments was confined to the dorsomedial parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was restricted to the medial halves of laminae I, IIa and V of the medullary dorsal horn. In the inferior alveolar nerve experiments dense terminal labeling was observed in the dorsal parts of the main sensory nucleus and subnuclei oralis and interpolaris. Caudal to the obex terminal labeling was located throughout laminae I to V in contrast to the tooth pulp experiments. Neither of the two experimental situations offers any evidence for a bilateral or contralateral brain stem projection of primary trigeminal neurons.

Synaptic organization of the substantia gelatinosa glomeruli in the spinal trigeminal nucleus of the adult cat

SummaryIn the spinal trigeminal nucleus of the adult cat, the neuropil of the substantia gelatinosa (SG) layer is made up of axonal endings of V nerve axons, dendrites and axons of SG neurons and dendrites of marginal and magnocellular neurons. A typical SG glomerulus consists of a centrally located V nerve ending which contains large synaptic vesicles and forms asymmetrical axodendritic synapses on two kinds of dendritic spines and on dendritic shafts. The dendritic processes of the SG glomerulus are linked by two kinds of dendrodendritic synapses: type 2 dendritic spines, containing large synaptic vesicles, form slightly asymmetrical (intermediate) synapses on type I dendritic spines. Dendritic shafts, containing clusters of small synaptic vesicles, form symmetrical dendrodendritic synapses amongst themselves and on type I dendritic spines. Small axonal endings, containing small synaptic vesicles form intermediate axodendritic synapses on type I dendritic spines and on dendritic shafts. These small axons also form symmetrical axoaxonic synapses on the V nerve ending. The V nerve endings in the SG layer are dark and differ morphologically from the pale V nerve endings in other parts of the trigeminal nuclei.The data are interpreted based on considerations that some marginal and magnocellular neurons are projection neurons and that some SG neurons are interneurons which form inhibitory synapses. The following circuits in the SG glomeruli are suggested: V nerve endings form excitatory synapses on the dendrites of projection neurons (marginal and magnocellular neurons) and on the dendrites of inhibitory interneurons (SG neurons). The interneurons inhibit transmission through the V nerve-projection neuron circuit by means of their axoaxonic synapses on the V nerve endings. Interneurons are linked together through inhibitory dendrodendritic synapses along their dendritic shafts. Two interpretations of dendrodendritic circuits between projection neurons and interneurons are considered. The morphologically distinct V nerve innervation of the SG layer and the internal circuitry of the SG glomeruli appear to be essential for the perception of thermal and painful stimuli.

Ultrastructural characterization of axonal endings in the substantia gelatinosa which take up [3H]serotonin

Serotonergic axonal endings in layers I and II of the dorsal horn of the medulla were identified by autoradiography. In adult cats, pretreated with a monoamine oxidase inhibitor, tritiated serotonin ([3H]5-HT) was topically applied onto the surface of the caudal medulla. Light autoradiographs from 1 micrometer sections demonstrated silver grains in both layers I and II. In EM autoradiographs, two categories of axonal endings were labeled by [3H]5-HT uptake: dome-shaped endings which form a single synapse and scalloped endings which form multiple synapses. Each category was further divided into several types based on morphological criteria. The [3H]5-HT-labeled endings synapse primarily on small caliber dendritic shafts and spines, with the dome-shaped endings forming both symmetrical and asymmetrical synapses and the scalloped endings forming only asymmetrical synapses. Dome-shaped endings were most common and two types were found in layers I and II while a third type was found only in layer II. Layer I contained a single type of scalloped ending while layer II contained three types of scalloped endings. In a series of experiments designed to provide another approach to identifying serotonergic endings, 5,6-dihydroxytryptamine, a serotonin neurotoxin, was either topically applied onto the caudal medulla or injected into the fourth ventricle. Following treatment with the neurotoxin, blackened degenerating dome-shaped and scalloped endings similar to those labeled in the [3H]5-HT uptake experiments were found in layers I and II. The presence of serotonergic endings in layer I suggests that some of these endings synapse on the dendrites of layer I projection neurons where they may inhibit the output of the projection neuron directly. Serotonergic endings in layer II may modulate the activity of layer II interneurons by synapsing directly on these interneurons. The interneurons in layer II may function by mediating the transfer of inputs from primary endings in these layers to layer I projection neurons.

Enkephalin immunoreactive stalked cells and lamina IIb islet cells in cat substantia gelatinosa

Neurons in lamina II of the lumbar spinal cords of colchicine-pretreated cats were stained immunocytochemically for enkephalin. Two morphological types were found. The most common type had the light microscopic characteristics of stalked cell. The other type was found in the deep part of the lamina and had the light and electron microscopic characteristics of the lamina IIb islet cell.

Anatomical studies of the organization of the spinal V nucleus: The deep bundles and the spinal V tract

Abstract Many neurons of the spinal V nucleus send their axons into an extensive longitudinal axonal plexus made up of interconnected small bundles of myelinated and unmyelinated axons (the deep bundles). These bundles run through the entire spinal V nucleus and contain ascending and descending axons. The axons of the bundles emit widely spaced simple collaterals which effectively link up different levels of the spinal V nucleus. The bundles, tightly packed in nucleus oralis, spread out in a looser arrangement in nucleus interpolaris and in the magnocellular layer of nucleus caudalis. They form a distinct band beneath the spinal V tract in the substantia gelatinosa layer of nucleus caudalis. At the level of nucleus interpolaris individual 40 μm diameter bundles contain approximately 1000 axons with myelinated and unmyelinated axons present in an approximate 1:1 relationship. Of the myelinated axons 80–90% are between 0.3 and 1.5 μm in diameter. The bundles are an integral part of the spinal V tract. In nucleus oralis many bundles merge with the spinal V tract so that it is actually thicker than at more rostral levels. Throughout nuclei interpolaris and caudalis, the deep bundles stream out of the spinal V tract and are the major factor responsible for the thinning of the tract in its descent through the medulla. As the spinal V tract descends through nuclei oralis and interpolaris, it harbors an irregular mosaic-like network of gray matter which contains scattered small neurons. The neurons of this network comprise the interstitial nucleus of the spinal V tract originally described in Cajals Golgi studies. The neuropil of the interstitial nucleus resembles the substantia gelatinosa layer of nucleus caudalis structurally.

An anatomical demonstration of projections to the medullary dorsal horn (trigeminal nucleus caudalis) from rostral trigeminal nuclei and the contralateral caudal medulla.

This study demonstrates that the medullary dorsal horn (MDH), the most caudal subdivision of the spinal trigeminal nucleus, receives input from neurons located in the trigeminal main sensory nucleus, the more rostral subdivisions of the spinal trigeminal nucleus, and the contralateral MDH. Using the retrograde transport of horseradish peroxidase (HRP), we show here that the MDH receives ipsilateral projections from rostral trigeminal nuclei but not from adjacent areas of the retricular formation. The rostral pole of spinal trigeminal nucleus oralis (nucleus oralis, pars beta) contains the highest density of MDH projection neurons. In addition, the MDH on one side receives projections from contralateral MDH neurons located in layers I, III, IV, V, VII and VIII but not from neurons in layers II and VI. We conclude that: (1) specific subdivisions of rostral trigeminal nuclei send projections to the MDH that could modulate the activity of MDH neurons; (2) projections from trigeminal nuclei to layers V and VI of the MDH, but not from adjacent areas of the reticular formation, provide further evidence that these deeper layers are related functionally to the MDH and trigeminal sensory processes; and (3) several populations of MDH neurons send axons across the midline into the contralateral MDH and may mediate contralateral inhibitory effects.

Primary neurons maintain their central axonal arbors in the spinal dorsal horn following peripheral nerve injury: an anatomical analysis using transganglionic transport of horseradish peroxidase.

Abstract This study in adult cats demonstrates that primary neurons of all sizes survive following the transection and capping with a polyethylene tube of their peripheral processes in the superficial radial nerve. The central axonal arbors of these injured primary neurons remain intact and maintain their normal topographic position across laminae I–VI of the cervical (C6–C8) dorsal horn. In addition, they maintain their synaptic vesicles, some of their synaptic connections and their ability to transport horseradish peroxidase transganglionically.

Dendritic changes in the spinal dorsal horn following transection of a peripheral nerve

In an attempt to examine the morphologic changes which take place in the spinal dorsal horn as a consequence of peripheral nerve injury, the superficial radial nerve was cut and prevented from regenerating in adult cats. In laminae I-V of the 7th cervical segment of spinal dorsal horn at 2 weeks, 1 and 2 months following transection, small and medium-sized dendritic shafts developed membrane-bounded cavities. These cavities varied in number and size in each dendrite and were sometimes connected to the agranular reticulum. Large cavities often hollowed out a dendrite, leaving little remaining cytoplasm between the cavity membrane and the plasma membrane. Small and large cavities were frequently found open to the intercellular space. Synaptic glomeruli showed a loss of small dendrites with empty scalloped depressions in the central axonal endings of these structures left exposed to enlarged intercellular spaces. In addition, clusters of many enlarged oval or irregular intercellular spaces were found in the neuropil where many cavitated dendrites were observed. At least some of these intercellular spaces were thought to be derived from the loss of dendrites. From these observations we conclude that small and medium-sized dendrites involute through cavitation and eventually disappear from the spinal dorsal horn when primary input is disturbed by transection of peripheral sensory nerves.

Ultrastructural analysis of medial brain stem afferents to the superficial dorsal horn

Axonal projections of medial brain stem areas rich in serotonin-containing neurons were identified in layers I and II of cat medullary dorsal horn using EM autoradiography. Following [3H]amino acid injections into the brain stem, labeled axonal endings were found throughout layers I and II but were most numerous in layer I. Three different morphological types of endings could be distinguished. Each type resembled serotonergic axonal endings identified in previous experiments. The labeled endings formed both symmetrical and asymmetrical synapses on dendritic shafts and spines and occasionally on a neuronal soma suggesting that the major site of action of the descending serotonergic afferents is on the neurons in layers I and II.

Electron microscopical studies of the cerebellar molecular layer

The fine structural characteristics of the Purkinje cell dendrites and the dendrites of the stellate neurons have been described. Granule cell axons, containing synaptic vesicles with an average diameter of 468 A, form type I synapses on principal dendrites of Purkinje cells, the shafts and bulbous enlargements of stellate cell dendrites, as well as on dendritic spines of Purkinje cells and stellate cells. Large-diameter axons, containing somewhat smaller synapses vesicles (367 A), form type II synapses on the shafts of principal and spiny dendrites of Purkinje cells as well as on the shafts and bulbous enlargements of stellate cell dendrites. Type I and type II synapses vary in length and percentage of the apposed axonal surface which they occupied in different locations. In some instances synaptic segments of large-diameter axons from two separate synaptic areas on a single dendritic process whereas granule cell axons always form one synaptic area on any one dendritic process. However, synaptic segments of both granule cell axons and large-diameter axons can synapse simultaneously with two separate dendritic processes. The relationships of the different dendritic and axonal processes to each other and to astrocytic processes have been described.