Light and electron microscopy of motoneurons and neuropile in the amphibian spinal cord

Neural circuits underlying tongue movements for the prey-catching behavior in frog: distribution of primary afferent terminals on motoneurons supplying the tongue

The hypoglossal motor nucleus is one of the efferent components of the neural network underlying the tongue prehension behavior of Ranid frogs. Although the appropriate pattern of the motor activity is determined by motor pattern generators, sensory inputs can modify the ongoing motor execution. Combination of fluorescent tracers were applied to investigate whether there are direct contacts between the afferent fibers of the trigeminal, facial, vestibular, glossopharyngeal-vagal, hypoglossal, second cervical spinal nerves and the hypoglossal motoneurons. Using confocal laser scanning microscope, we detected different number of close contacts from various sensory fibers, which were distributed unequally between the motoneurons innervating the protractor, retractor and inner muscles of the tongue. Based on the highest number of contacts and their closest location to the perikaryon, the glossopharyngeal-vagal nerves can exert the strongest effect on hypoglossal motoneurons and in agreement with earlier physiological results, they influence the protraction of the tongue. The second largest number of close appositions was provided by the hypoglossal and second cervical spinal afferents and they were located mostly on the proximal and middle parts of the dendrites of retractor motoneurons. Due to their small number and distal location, the trigeminal and vestibular terminals seem to have minor effects on direct activation of the hypoglossal motoneurons. We concluded that direct contacts between primary afferent terminals and hypoglossal motoneurons provide one of the possible morphological substrates of very quick feedback and feedforward modulation of the motor program during various stages of prey-catching behavior.

Dendrodendritic connections between the cochlear efferent neurons in guinea pig

The outer hair cells of organ of Corti are innervated by the efferent neurons of medial olivocochlear neurons (MOC) of the brainstem which modify the cochlear auditory processing and sensitivity. Most of the MOC neurons are excited by a dominant ear and only a small portion of them is excited by both ears resulting in a binaural facilitation. The functional role of the feedback system between the organ of Corti and the cochlear efferent neurons is the protection of the ear from acoustic injury. The rapid impulse propagation in the bilateral olivocochlear system is suggestive of an electrotonic interaction between the bilateral olivocochlear neurons. The morphological background of the MOC pathway is not yet completely characterized. Therefore, we have labeled the bilateral cochlear nerves with different neuronal tracers in guinea pigs. In the anesthetized animals the cochlear nerves were exposed in the basal part of the modiolus and labeled simultaneously with different retrograde fluorescent tracers. By using confocal laser scanning microscope we could detect close appositions between the dendrites of the neurons of bilateral MOC. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. These connections might serve as one of the underlying mechanisms of the binaural facilitation mediated by the olivocochlear system.

Crossing dendrites of the hypoglossal motoneurons: possible morphological substrate of coordinated and synchronized tongue movements of the frog, Rana esculenta

Application of different fluorescent tracers to the right and left hypoglossal nerve of the frog revealed the extent of dendrites crossing the midline into the territory of contralateral hypoglossal motoneurons. By using confocal microscopy, a large number of close appositions were detected between hypoglossal motoneurons bilaterally, which formed dendrodendritic and dendrosomatic contacts. The distance between the neighboring profiles suggested close membrane appositions without interposing glial elements. Application of neurobiotin to one hypoglossal nerve resulted in labeling of perikarya exclusively on the ipsilateral side of tracer application, suggesting the absence of dye-coupled connections with contralateral hypoglossal motoneurons. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide electrotonic interactions between the neighboring profiles. We propose that dendrites of hypoglossal motoneurons that cross the midline subserve one of the morphological substrates of co-activation, synchronization and timing of bilateral activity of tongue muscles during prey-catching behavior of the frog.

Dendrodendritic and dendrosomatic contacts between oculomotor and trochlear motoneurons of the frog, Rana esculenta

Gaze fixation requires very fast movements of the eye during body displacement. The morphological and physiological background of the very fine and continuous tuning of gaze fixation is not yet fully understood. In a previous study we have shown that the dendrites of oculomotor neurons form bundles which invade the trochlear nucleus, and vice versa, trochlear dendritic bundles invade the oculomotor nucleus. Earlier physiological observations demonstrating electrotonic coupling between dendrites of spinal motoneurons in the frog suggest a similar mechanism between the oculomotor and trochlear motoneurons. We studied a possible morphological basis of gaze fixation. The experiments were carried out on common water frogs, Rana esculenta. The trochlear and oculomotor nerves were cut, and their proximal stumps were labeled simultaneously with different retrograde fluorescent tracers. Using confocal laser scanning microscope we detected a large number of close contacts in both nuclei, the majority of them were dendrodendritic apposition. The distance between the adjacent profiles suggested close membrane appositions without intercalating glial or neuronal elements. At the ultrastructural level, the dendrodendritic and dendrosomatic contacts did not show any morphological specialization; the long membrane appositions may provide ephaptic interactions between the neighboring profiles. This electrotonic coupling between the oculomotor and trochlear nerve motoneurons may promote the co-activation of the muscles responsible for vertical eye movements.

Anatomical organization of motoneurons and interneurons in the mudpuppy (Necturus maculosus) brachial spinal cord: the neural substrate for central pattern generation

The isolated brachial spinal cord of the mudpuppy is useful for studies of neural networks underlying forelimb locomotion, but information about its anatomy is scarce. We addressed this issue by combining retrograde labeling with fluorescent tracers and confocal microscopy. Remarkably, the central region of gray matter was aneural and contained only a tenuous meshwork of glial fibers and large extracellular spaces. Somata of motoneurons (MNs) and interneurons (INs), labeled retrogradely from ventral roots or axons in the ventro-lateral funiculus, respectively, were confined within a gray neuropil layer abutting the white matter borders, while their dendrites projected widely throughout the white matter. A considerable fraction of labeled INs was found contralaterally with axons crossing beneath a thick layer of ependyma surrounding the central canal. Dorsal roots (DRs) produced dense presynaptic arbors within a restricted dorsal region containing afferent terminations, within which dorsally directed MN and IN dendrites mingled with dense collections of synaptic boutons. Our data suggest that a major fraction of synaptic interactions takes place within the white matter. This study provides a detailed foundation for electrophysiological experiments aimed at elucidating the neural circuits involved in locomotor pattern generation.

Morphology of brachial segments in mudpuppy (Necturus maculosus) spinal cord studied with confocal and electron microscopy

The isolated brachial spinal cord of Necturus maculosus is useful for studies of neural networks underlying forelimb locomotion, but information about its cellular morphology is scarce. We addressed this issue by using confocal and electron microscopy. Remarkably, the central region of gray matter was aneural and consisted exclusively of a tenuous meshwork of glial fibers and large extracellular spaces. Somata of motoneurons (MNs) and interneurons (INs), labeled by retrograde transport of fluorescent tracers from ventral roots and axons in the ventrolateral funiculus, respectively, were confined within a gray neuropil layer abutting the white matter borders, whereas their dendrites projected widely throughout the white matter. About one-third of labeled INs were found contralaterally, with axons crossing ventral to a thick layer of ependyma surrounding the central canal. Lateral MN dendrites proliferated under the pial surface to form a dense, thin (1-2 microm) plexus immediately beneath a thin layer of glial fibrillary acidic protein-positive glia limitans. The latter contained arrays of unusual tubular structures (diameter 200-400 nm, length 3 microm) that resembled mitochondria but lacked double membranes or cristae. Dorsal roots (DRs) produced dense presynaptic arbors within a wedge-shaped afferent termination zone medial to the dorsal root entry, within which dendrites of MNs and INs mingled with dense collections of synaptic boutons. Our data suggest that a major fraction of synaptic interactions takes place within the white matter. This study provides a detailed foundation for designing electrophysiological experiments to study the neural circuits involved in locomotor pattern generation.

Ultrastructure and synaptology of the nucleus ambiguus in the rat: the compact formation

The fine structure of the esophagomotor compact formation of the nucleus ambiguus was studied. Esophageal motoneurons are atypical in that they have extensive direct somato-somatic and somato-dendritic appositions without intervening glial processes. A unique feature is the presence of finger- and leaf-like somatic protrusions which partially wrap longitudinally oriented dendrites and, occasionally, small groups of dendrites and axons. The neuropil contains many longitudinally oriented, small-diameter dendrites of relatively uniform size (1.1 +/- 0.4 S.D. micrograms in diameter). Motoneuronal somatic profiles have 0-5 synapses per profile which represents a synaptic density of 10.6 synapses per soma. Axodendritic synapses measure 0.5 x 0.7 microgram in the transverse plane and are up to 3.0 micrograms long in the sagittal plane. Many axon terminals contact both a soma and dendrite in close apposition. Most axon terminals (> 90%) contain round vesicles and form asymmetric junctions with somata and dendrites. Axon terminal degeneration after electrolytic lesions and labelling after injection of wheat germ agglutinin-horseradish peroxidase in the nucleus of the tractus solitarius show that afferent connections to the compact formation form axodendritic synapses. The ultrastructure and synaptology of esophageal motoneurons is characterized by the close apposition of somata and dendrites (somatic-dendritic bundling), and the longitudinal orientation of dendrites (dendritic bundling), axons and axon terminals in the neuropil. These features may be important morphological substrates for synchronization and coordination of esophageal motoneuronal activity and esophageal peristalsis.

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

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

Cobaltic lysine study of the morphology and distribution of the cranial nerve efferent neurons (motoneurons and preganglionic parasympathetic neurons) and rostral spinal motoneurons in the Japanese toad

The morphology and distribution of the cranial nerve motoneurons (except III, IV, and VI) and rostral spinal motoneurons were systematically studied in the Japanese toad (Bufo japonicus) by retrograde labelling with cobaltic lysine complex. The cobaltic lysine clearly labelled whole neurons, i.e., cell bodies, proximal and distal dendrites, and axons. The branchial motoneurons (V, VII, IX, and X) had similar morphological characteristics and formed a more-or-less continuous cell column through the brainstem. The dendrites could be grouped mainly into the dorsomedial and the ventrolateral dendritic arrays. The dorsomedial dendrites formed a dendritic plexus in the subependymal gray matter, which extended as far peripherally as beneath the ependymal layer. The ventrolateral dendrites formed a broom-like dendritic plexus in the lateral to ventrolateral white matter. They usually extended as far peripherally as the pial surface. The rostrocaudal extent of the dendritic field was also wide and usually exceeded the motor nuclear boundaries. The hypoglossal motoneurons were grouped into the dorsomedial and ventrolateral cell groups, and the latter was considered to be part of the rostral spinal motoneuron column, from their morphology and distribution. The former had well-differentiated dendrites and occupied a more medial position than the branchial motoneurons. Besides the equivalent of the dorsomedial and ventrolateral dendritic arrays of the branchial motoneurons, they had dorsal and commissural dendrites. The accessory motoneurons had morphological characteristics and a distribution pattern similar to those of the rostral spinal motoneurons rather than the branchial motoneurons. The rostral spinal motoneurons had morphological characteristics somewhat different from the branchial motoneurons and the hypoglossal motoneurons (dorsomedial group). Functional implications of the motoneuron morphology are discussed, mainly based on the present results and earlier anatomical and physiological studies of the spinal motoneurons. The present study also revealed the anatomical features of the preganglionic parasympathetic neurons supplying some cranial nerves. These neurons had small somata with less elaborate dendrites and formed an almost continuous cell column that occupied a more dorsal position than the motoneurons of the corresponding nerve. They are thought to be homologous to the salivatory nucleus and the dorsal motor nucleus of the vagus. The basic anatomical organization of the general visceral efferent column seems to be similar throughout vertebrates.

Morphology and distribution of the motor neurons of the accessory nerve (nXI) in the Japanese toad: a cobaltic lysine study

Motoneurons supplying the accessory nerve (nXI) of the Japanese toad were retrogradely labelled by applying the cobaltic lysine to the cut end of the nerve. They had morphological characteristics and a distribution pattern similar to those of the rostral spinal motoneurons rather than the branchial motoneurons. We propose that the anuran nXI is equivalent to the so-called spinal portion of the nXI of other vertebrates, and both should be regarded as part of the rostral spinal nerves rather than the nerve accessory to the vagus nerve.

Regeneration of motoneuron axons into the adult frog spinal cord after ventral-to-dorsal-root anastomosis

Motoneuron axons routed into the adult frog spinal cord via a ventral-to-dorsal-root anastomosis regenerated into the white and the gray matters. The distribution, growth patterns, and arborizations of regenerated ventral root axons were compared to those of regenerated dorsal root axons within the same environment. Within the spinal white matter, regenerating ventral root axons behaved very similarly to regenerating dorsal root axons. Here, the regenerating ventral root axons grew longitudinally beneath the pia and radially toward the spinal gray matter, particularly within the dorsolateral fasciculus. The location of the regenerating axons and the patterns of their growth within the white matter suggest that glial endfeet and radial glial processes play a major role in the determination of these axonal growth patterns. When motor axons entered the gray matter, their arborizations were very similar to those of regenerated dorsal root axons, suggesting that these two very distinct populations of axons respond similarly to local cues within the spinal gray matter. One difference between the arborizations of these two populations of axons was the relative number of varicosities along axonal branches. Regenerated motoneuronal arborizations within the spinal gray matter had fewer en passant varicosities than regenerated dorsal root axonal arborizations. This difference may reflect the synaptogenetic response of the two types of axons to targets within the gray matter. The low number of en passant varicosities associated with the ventral root axonal aborizations suggests that these axons do not synapse with all available targets and that the rules governing synaptic specificity during development may apply during regeneration in the adult frog spinal cord.

The development of the relationship between dorsal root afferents and motoneurons in the larval bullfrog spinal cord

The relationship of dorsal root afferents to motoneuron somata and dendrites was studied by labelling dorsal and ventral roots of the tadpole lumbar enlargement with HRP at different stages of hindlimb development. Procedures were used which allowed for sequential light and electron microscopic analysis to determine whether close appositions between labelled elements represented synaptic contacts. Lateral motor column (LMC) motoneuron dendrites grow first into the lateral funiculus, and later begin arborizing within the spinal gray, concurrent with the arrival of developing dorsal root afferent fibers. Mature-appearing synaptic contacts between dorsal root afferents and motoneuron dendrites are established first on distal dendrites, and are observed on progressively more proximal dendrites as hindlimb development proceeds. Migrating motoneurons were also labelled in some animals. Distinct dorsal and ventral migratory pathways were noted; cells migrating dorsally were contacted by developing dorsal root afferents. Migrating motoneurons were associated with radially oriented processes, and were often closely apposed to other cells. The coincident development of dorsal root projections and the motoneuron dendrites which these fibers innervate in the adult, as well as the interaction between these two systems during cell migration, suggest that these two systems may be interdependent in establishing their normal relationship during development.

Tracing of frog sensory-motor synapses by intracellular injection of horseradish peroxidase

Monosynaptically connected primary afferent fibres and motoneurones of the isolated spinal cord of the frog were injected with horseradish peroxidase (HRP). Six labelled afferent fibre-motoneurone pairs were reconstructed and subjected to detailed analysis. Frog motoneurones possess eight to twelve dendritic arrays displaying some dorso-ventral asymmetry. Dorsal dendrites exhibit a rostro-caudal extent of 1.7-2.6 mm (average 2.2 mm). Primary afferent fibres bifurcate in the dorsal funiculus. First-order collaterals emanate from the main ascending and descending branches, at an average distance of 407 micron. The average number of boutons per collateral is 670. To reach a contacting bouton the presynaptic spike must pass on average five bifurcations and then zero to twelve boutons en passant, attached to a single terminal collateral branch. The structural equivalent of the axon cylinder of the collateral tree roughly preserves cross-sectional area. The branch power ranged between 1.15 and 3.35 (average 2.06). Primary afferent fibres usually form clusters of contacting boutons (contact regions). Connexions between an afferent fibre and a motoneurone comprise from five to twenty-three contact regions (average 12.5). Each contact region contains one to twelve contacting boutons (average 3.3). In two of three experiments contacting boutons were found to be significantly larger than non-contacting boutons. The average diameter of the former was 2.6 micron (range 1.2-4.0). In five out of six cases more than one collateral belonging to the same fibre participated in the connexion with a given motoneurone. The average number of contacting boutons per motoneurone and collateral is 19.1. It was estimated that each collateral could supply not more than thirty-five motoneurones. This would be less than 8.5% of the motoneurones with their dendrites which cross the termination space of a single collateral. The average number of contacting boutons forming one primary motoneurone connexion was 41.5 (range 21-72).

The relationship of dorsal root afferents to motoneuron somata and dendrites in the adult bullfrog: a light and electron microscopic study using horseradish peroxidase

The relationship of lumbar dorsal root afferents to lateral motor column motoneurons was studied using anterograde injury filling of dorsal roots and retrograde injury filling of ventral roots with horseradish peroxidase. At the light microscopic level, horseradish peroxidase labelled dorsal root axons were observed to separate into a medial division of large diameter axons which enter the dorsal funiculus and a lateral division of small diameter axons which form a compact bundle in the dorsolateral funiculus which may be homologous to the mammalian tract of Lissauer. Within the spinal gray, primary afferents terminate in two distinct regions. The more ventral of these terminal fields, which receives collaterals of primary afferent axons in the dorsal funiculus, overlaps the dendritic arborizations of the lateral motor column motoneurons. Some axons leave the ventral terminal field to enter the dorsal lateral motor column. Here they terminate on the primary dendrites and somata of lateral motor column motoneurons. At the electron microscopic level, labelled primary afferent terminals were seen to synapse upon lateral motor column motoneuron dendrites as well as upon the somata of dorsally positioned lateral motor column motoneurons. These terminals contain small spherical vesicles and occasional dense-cored vesicles. The synaptic specializations are characterized by a small amount of postsynaptic material. The lateral motor column may be divided into dorsal and ventral portions on the basis of the primary afferent distribution and this is in accord with functional, physiological and developmental data.

Dye and electrical coupling between frog motoneurons

Coupling between lumbar motoneurons in the isolated frog spinal cord was studied by using intracellular recordings and intracellular injections of Lucifer Yellow CH. Physiological studies revealed intra- and intersegmental, short latency electrical interactions between many motoneurons. Injections of Lucifer Yellow into motoneurons that were found to be coupled electrically revealed intra- and intersegmental dye-coupling.

The dendritic organization of the human spinal cord: the dorsal horn

The human spinal cord was studied with the Golgi method from 26 weeks gestational age onto adult life. Impregnated neurons were analyzed morphometrically by an adaptation of Sholl's concentric circle method in order to specify dendritic geometry, ramification richness and branching pattern. Neurons were classified according to Rexed's laminar scheme, identified on adjacent Nisslstained sections. The following features were found to be characteristic of the dorsal horn laminae. Lamina I is formed of a uniform population of large, poorly ramified neurons. Their main dendritic domaine is disk-shaped in the tangential plane with a mean diameter of about 800 micrometers. Orthogonal, spiny dendrites penetrating into lamina II are numerous even in adult material. Four cell types, all lying in the sagittal plane, are found in lamina II. (1) "Islet cells' (about 30% of impregnated neurons) have a rostrocaudal, cylindrical dendritic domaine with a long axis of +/- 600 micrometers, a few scattered spines and a richly branched axonal network, confined to the dendritic territory. They are more numerous in the central part of lamina II. A few islet cells have two axons. (2) "Filamentous cells' (about 20%), so called because of their multiple filiform, spiny dendrites, are vertico-sagittally oriented. Their soma is located in inner or outer lamina II, their dendritic tree, trapezoïd of about 280 micrometers in height, being dorsal or ventral. The axon emits some collaterals in the vicinity of the dendritic tree, then it penetrates into lamina I or Lissauer's tract. (3) "Curly cells' (about 10%) have a complex, twisted, spine-rich dendritic tree. The dendritic domaine can be schematized by a sagittal disk with a diameter of about 200 micrometers, the cell body being eccentric. Curly cells are mainly found in outer lamina II. Their axon penetrates into lamina I or Lissauer's tract. (4) "Stellate cells' (about 40%) are multipolar neurons preferentially found in inner lamina II. They have straight, spine-poor dendrites, which cover a large (longest diameter ca. 500 micrometers), elliptical territory extending into laminae I and III. Their axon gives longitudinal collaterals to lamina II before penetrating deeply into laminae III and IV. Lamina III contains a mixed population of "antenna-like neurons' with a vertical, cone-shaped dendritic domaine and "radiate cells' characterized by a small, spherical territory. All lamina IV neurons are medium or large sized "antenna-like neurons' whose dorsally oriented, cone-shaped dendritic domaine may have a height of 1000 micrometers. It can be concluded that the dorsal horn of the human spinal cord has several distinct dendroarchitectonic features, different from those reported in animals. The possible functional implication of some dendritic features is examined and a laminar dendroarchitectonic scheme of the human cord is proposed as a morphological tool for future neuroanatomical and neuropathological studies.

Development of axosomatic synapses of the Xenopus spinal cord with special reference to subsurface cisterns and C-type synapses

The relationships of highly flattened subsurface cisterns (SCCs) were investigated electron microscopically in the spinal cord at various developmental stages of tadpoles and adult toads, Xenopus laevis. In medial ventral motor cells (MVCs) of the adult, more than 90% of 156 SSCs examined were situated postsynaptically. Similarly, more than 90% of 540 SSCs in lateral motor column cells (LMCs) were postsynaptic. By contrast, in early developmental stages, the SSCs were initially formed by regional flattening of cisterns of rough-surfaced endoplasmic reticulum just beneath the cell surfaces opposite to glial processes. Then, the glial processes were displaced by nerve endings with an elongated bouton, and thus the C-type synapses were formed. The ratio of postsynaptic SSCs to the total SSCs reached the adult level at around Stage 60. This finding suggests that the SSCs in the MVCs and LMCs draw a certain type of nerve ending to form C-type synapses. Such a mechanism is totally lacking in the dorsal and lateral small nerve cells, since the SSCs in these cells were always situated under the surface opposite to glial processes throughout the developmental stages and in the adult. In mature C-type synapses, an aggregate of synaptic vesicles and a structural specialization of presynaptic membrane occurred only at the region where the postsynaptic membrane was associated with the SSC. The postsynaptic membrane itself of the C-type synapse showed no marked structural specialization at any stage of development or in the adult. The postsynaptic SSC In the mature C-type synapse seems to be involved in some way in the reception of synaptic transmission.

Freeze fracture study on three types of synapses in the Xenopus spinal cord

Three types of synapses (S-, F- and C-types) were identified in the thin-sectioned Xenopus spinal cord and their structure was analyzed with the freeze-fracturing technique. All three types of synapses showed similar specializations of the presynaptic membrane. This finding suggests that the three types of synapses may release their transmitters by a similar mechanism. By contrast, the three types of synapses revealed different specializations of the postsynaptic membrane. The E-face of the S-type postsynaptic membrane was characterized by a dense aggregate of large intramembrane particles, 12 to 15 nm in diameter. An aggregate of small particles, 8 to 9 nm, was evident on the E-face of the postsynaptic membrane of the F-type synapse. In the C-type synapse, there was a striking aggregate of intramembrane particles, 10 to 14 nm in diameter, on the P-face of the postsynaptic membrane. These characteristic features in the distribution of particles in the three types of postsynaptic membranes may reflect differences in the type of transmitter released or transmitter action on the postsynaptic neuron. The overall size of the area of aggregated particles on the P-face of the C-type postsynaptic membrane was coextensive with the underlying subsurface cistern (SSC) which showed partial occlusion of the lumen. This fact supports the view that the SSC is closely related to the C-type synaptic action which might be distinct from the other synaptic types.

Morphology and distribution of the synapses to the spinal motoneuron of the frog

The quantitative distribution of the different structural elements on the surface of the motoneurons (MN) in the spinal cord of the frog was studied in thin sections and freeze-fracture replicas. In particular, the different synaptic types and their distribution on the MN soma and dendrites are described. In thin sections three types of synapses were discerned: the S-type with spherical vesicles, the F-type with flattened vesicles, and finally the C-type synapse with spherical vesicles and a subsynaptic cistern. The synaptic covering at the MN surface as determined by thin sections is about 36%. In freeze-fracture replicas were observed boutons with and without gap junctions, and C-type boutons. When using the latter technique the synaptic covering was found to be 43%. With both techniques special attention was paied to the morphology of the C-type bouton and both the subsynaptic and extrasynaptic cisterns. The soma membrane over the sub- and extrasynaptic cisterns reveals characteristic and very similar morphological features with respect to both the distribution and size of the membrane particles. the possible functions of the two cisternal types are discussed.

On the postsynaptic action of glutamate in frog spinal motoneurons

In the isolated frog spinal cord depolarization of motoneurons (MNs) induced by glutamate (GLUT) was not accompanied by measurable changes of neuronal input resistance when chemical synaptic transmission was blocked by Mn2+ or Mg2+. The GLUT depolarization was, however, paralleled by a considerable increase in K+ in the extracellular space. To clarify, whether the GLUT depolarization was exclusively due to a reduction of the transmembrane K+ gradient or whether ion conductances not detectable by measurements of neuronal input resistance were involved, membrane potential (MP) was plotted semilogarithmically versus extracellular K+ activity (aKe+). During experimental elevation of aKe+ the function delta MP/dec. delta aKe+ was found to agree fairly with the Nernst equation. The slope of this function was much steeper during GLUT superfusion, indicating an influx of positive ions. The elevation of aKe+ during the GLUT action can mimic postsynaptic effects by release of transmitter from presynaptic terminals synapsing with the recorded cell. In vivo preparations do not allow blockade of chemical synaptic transmission. Therefore, it is impossible to decide, whether the recorded cell is depolarized either postsynaptically by GLUT or by K+ release from surrounding GLUT sensitive cells. As an experimental proof of the postsynaptic GLUT action is not feasible in such preparations, the ubiquitous action of GLUT in the CNS may have been overestimated.

Normal dendritic morphology of frog spinal motoneurons: a Golgi study

Normal dendritic morphology of frog (Rana pipiens) lumbar motoneurons was studied using Golgi silver impregnation. Branching characteristics and quantitative measurements of dendrites were obtained using computer-aided serial reconstruction of a typical lumbar motoneuron over seven adjacent 80-micrometer transverse sections. Dendrites were classified based upon site of dendrite origin from the soma and distribution of the dendritic array within the spinal cord. Eight possible sites of dendritic origin from the soma were identified. Two dendrites, D1 and D2, are planar dendrites which arise from the dorsal aspect of the soma. They are moderately complex, reaching branch order 5-6, and are oriented predominantly in the transverse plane. Input to these dendrites is primarily segmental via dorsal root projections. Three dendrites, D3, D4, and D5, arise laterally from the soma and extend through the lateral funiculus toward the subpial region. Two dendrites, D6 and D7, arise ventrally. D6 extends ventrolaterally and is a simple dendrite reaching branch order 3-4. D7 aborizes extensively in the ventral funiculus and in the central gray, reaching a branch order of 8-9. This dendrite extends rostrally and caudally over a distance of at lest 560 micrometer. Another dendrite (D8) arises from the medial aspect of the soma and projects toward the central canal. Four sites (D1, D2, D6, and D7) almost invariably give rise to dendrites. Dendrites arise at D4 in 66% of the cells examined. Dendrites are found at D3, D5, and D8 much less frequently (6-21%). Total dendritic length (12,043 micrometer) and lengths of the individual dendrites, branch length versus branch order, and number of branches at increasing radii were examined, and Sholl analysis was performed.

Interactions among lumbar motoneurons on opposite sides of the frog spinal cord: morphological and electrophysiological studies

Light and electron microscopy have been used to study the projections of dendrites from motoneurons in lumbar segments of the spinal cord of the frog following administration of horseradish peroxidase to cut ventral roots. Processes originating from motoneurons crossed to the opposite side of the spinal cord via the anterior commissure and made contact with dendrites and motoneuronal somata. Typically, in segments 6 to 8 the crossing dendrites showed irregular enlargements in diameter. Electrophysiological recordings were obtained both extracellularly from ventral roots and intracellularly from motoneuronal somata. In Ringer's solution containing 1 mM calcium, stimulation of a lumbar ventral root, elicited population responses with early and late components in the ventral root of the opposite side of the same segment. Only the early, short latency component remained in calcium-deficient Ringer's solution. In calcium-containing Ringer's solution, intracellular recording from an antidromically activated motoneuron showed an action potential with a short latency; this response was followed by excitatory postsynaptic potentials (epsps) from which action potentials could be generated. Contralateral ventral root stimulation also elicited in the same motoneuron a short latency action potential that was rarely followed by epsps. The short latency responses, that were elicited by stimulation of ventral roots of either side persisted in calcium-deficient Ringer's solution, but the epsps were abolished. Contralaterally elicited short latency responses were eliminated by section of the anterior commissure. We believe that electrically mediated crossed interactions among lumbar motoneurons may serve as a means of coordinating muscle groups of opposite sides that are used in movements that require bilateral synchronization, such as jumping and swimming.

Myelinated dendrites in the spinal cord of frogs (Rana esculenta)

Myelinated dendrites--probably of motoneurons--were found in the spinal cord of the frog. It is assumed that the myelin sheath, by increasing the membrane resistance, improves the function of the dendrite as a cable.

Interactions between tectal radial cells in the red-eared turtle, Pseudemys scripta elegans: an analysis of tectal modules

The optic tectum is a major subdivision of the visual system in reptiles. Previous studies have characterized the laminar pattern, the neuronal populations, and the afferent and efferent connections of the optic tectum in a variety of reptiles. However, little is known about the interactions that occur between neurons within the tectum. This study describes two kinds of interactions that occur between one major class of neurons, the radial cells, in the optic tectum of Pseudemys using Nissl, Golgi and electron microscopic preparations. Radial cells have somata which bear long, radially oriented apical dendrites from their upper poles and short, basal dendrites from their lower poles. They are divided into two populations on the basis of the distribution of their somata in the tectum. Deep radial cells have somata densely packed in the stratum griseum periventriculare. Their plasma membranes form casual appositions. Middle radial cells have somata scattered throughout the stratum griseum centrale and stratum fibrosum et griseum superficiale and do not contact each other. The apical dendrites of both populations of radial cells participate in vertically oriented, dendritic bundles. The plasma membranes of the dendrites in these bundles form casual appositions in the deeper tectal layers and chemical, dendrodenritic synapses within the stratum fibrosum et griseum superficiale. The synapses have clear, round synaptic vesicles and slightly asymmetric membrane densities. Thus, radial cells interact via both casual appositions and chemical synapses. These interactions suggest that radial cells may form a basic framework in the tectum. Because both populations of radial cells extend into the stratum fibrosum et griseum superficiale and stratum opticum, they may receive input from some of the same tectal afferent systems. Because the deep radial cells alone have somata and dendrites in the deep tectal layers, they may receive additional inputs that the middle radial cells do not. Neurons in the two populations interact via chemical dendrodentritic synapses, thereby forming vertically oriented modules in the tectum.

A raphe dendrite bundle in the rabbit medulla

A Golgi-Cox, histofluorescence, and electron microscopic examination of the serotonergic raphe nuclei of the rabbit medulla has revealed a large, vertically-oriented midline dendrite bundle extending from the floor of the fourth ventricle to the ventral boundary of nucleus raphe pallidus. The bundle was confined to the medulla, and averaged 150-200 micrometer in width in the adult. This dendrite bundle received contributions from four major sources: (1) Dendrites of midline and paramedian neurons of nucleus raphe obscurus; (2) Dendrites of midline and paramedian neurons of nucleus raphe pallidus; (3) Shafts from tanycytes located on the midline floor of the fourth ventricle; and (4) Dendrites from neurons of the medullary reticular formation. Perikarya and dendrites of serotonergic raphe neurons frequently abutted tanycyte shafts, midline bhood vessels, and perikarya and dendrites of other raphe neurons. The tanycyte shafts extended from the floor of the fourth ventricle into the bundle, and often ran the entire length of the bundle, where they intertwined themselves among neurons and dendrites of the medullary raphe nuclei. This study suggests that neurons of the medullary raphe may be influenced by communication channels including dendro-dendritic contacts within the midline bundle, fourth ventricular cerebrospinal fluid-borne influences through tanycyte shafts, blood-borne influences through the direct neuronal-vascular relationship in the raphe, and traditionally described axonal contacts impinging upon raphe neurons. We suggest that the raphe neurons might act as both neurons and endocrine-neural transducer cells.

Golgi-staining of "primary" and "secondary" motoneurons in the developing spinal cord of an amphibian

The Golgi technique was used to study the morphology of spinal motoneurons at various stages in the early development of swimming behaviour in embryos and larvae of the palmate newt, Triturus helveticus ((Razoumowsky). The earliest motoneurons stained appeared to be associated with the Mauthner-cell system. The overall morphology of these "primary" motoneurons seems to be similar throughout the lower vertebrates and the distinctive characteristics found in earlier descriptions of those from caudate amphibia were probably due to misinterpretation. At about the time of hatching and development of low-frequency swimming behaviour, other motoneurons were found to innervate the axial musculature, cells with a central morphology different from those of the "primary" type. It was found likely that these "secondary" motoneurons innervate a separate muscle system concerned with tonic and "slow phasic" activity, while "fast phasic" acitivity in rapid swimming is supplied by "primary" cells.

Direct excitatory interactions between spinal motoneurones of the cat

1. Ninety-seven spinal motoneurones were identified by their antidromic invasion following stimulation of the muscle nerve and submitted to a series of four tests to reveal a possible direct excitation between motoneurones. 2. Threshold differentiation, refractoriness, hyperpolarization and collision revealed antidromically induced depolarizations in fourteen of the ninety-seven tested motoneurones. 3. The parameters of the antidromically induced depolarizations indicate a short latency, a low amplitude and independence with regard to the membrane polarization. 4. It is concluded that the antidromically induced depolarizations reached the impaled motoneurone via a route other than its own axon. 5. The mechanism may involve either electrotonic interactions between neighbouring motoneurones or excitatory recurrent collaterals between synergist motoneurones.

Cytoarchitectonic analysis of the bullfrog (Rana catesbeiana) spinal cord by means of electron microscopy with special reference to distribution of microneurons

In order to determine cytological nature of the "small cells" found in the dorsal half of the bullfrog spinal cord at the electron microscopic level, the complete mapping of these elements was carried out, using both paraffin and thick sections. As a result, these elements were demonstrated to have four main fine structural characteristics of an unequivocally neuronal nature: (1) synaptic contacts, (2) subsurface cisternae, (3) typical pattern of the rough-surfaced endoplasmic reticulum, and (4) close contacts of astroglial processes with the somal surface. The most prominent features in the bullfrog spinal cord can be summarized as follows: the microneurons occupy the dorsal region as well as the region around the central canal of the gray, and the most dorsal part of the gray in exclusively occupied by these microneurons. They show, furthermore, a tendency to separate grossly into medial and lateral groups. No microneurons are visible in the ventral horn. According to the results obtained from the cytoarchitectonic comparison of the bullfrog spinal cord and the rat spinal cord, it can be said that in the former no differentiated laminar structures can be recognized, while in the latter the gray matter is sufficiently differentiated to permit full percellation.

The connections of the trigeminal and facial motor nuclei in the brain of the carp (Cyprinus carpio L.) as revealed by anterograde and retrograde transport of horseradish peroxidase

The connections of the rostral and caudal parts of the trigeminal and facial motor nuclei in the carp were studied with the horseradish peroxidase technique. Following ionophoretic peroxidase injections in these motor nuclei, retrogradely labeled cells were observed together with anterogradely labeled motor cell processes. Several cellular areas in thalamus, cerebellum and medulla oblongata were shown to project to the V and VII motor nuclei. Labeled cells were found in the inferior lobe and the glomerular complex of the thalamus. In the medulla oblongata, cells in the descending trigeminal nucleus, reticular nuclei and motor nuclei other than those injected were labeled. Besides these conspicuous projections several smaller connections were also found. These findings are discussed on their significance to respiratory function. Anterogradely labeled cellular processes constitute a relatively simple network of fiber connections between the various motor nuclei and the reticular nuclei of the brainstem. This apparently dendritic system of the bulbar motor complex shows a certain degree of similarity to the structure of the motor system in the spinal cord, and might play a role in the coordinated control of the muscular system.

Dendritic responses of frog motoneurons produced by antidromic activation

Responses to ventral root stimulation were studied in spinal cords in situ in unanesthetized frogs. Extracellular as well as intracellular recordings from motoneurons indicated that considerable depolarization of the dendrites occurred in the response to ventral root volley. Active components of this dendritic depolarization could also be observed. Extracellularly, negative field potentials were recorded both in the vicinity of motoneuron cell bodies and in areas occupied mostly by motoneuron dendrites. The refractory period of the negativity recorded at the vicinity of dendrites was longer than in the vicinity of somata. Changes in antidromic excitability were studied by the double volley technique. Augmentation of the field potential to a test volley was observed during the period of dendritic depolarization, followed by a longer-lasting period of depression of the test response. It is concluded that an action potential in frog motoneurons induces depolarization of the dendrites. The depolarized dendrites can generate local action potentials and can produce negative field potentials remotely from the somatic pool. The response of dendrites to stimulation of the ventral root has particular importance in the recurrent facilitation of the frog motoneurons.

Morphological characteristics of dendrite bundles in the lumbar spinal cord of the rat

The finding of motoneuron dendrites organized into small compact bundles in cats, monkeys and pigs suggested that a study of this phenomenon in rats should be undertaken. An analysis was performed with electron microscopy, light microscopy and Golgi methods. An extensive dendrite bundle organization was found in the sixth lumbar segment of the spinal cord. Two discrete bundles were localized bilaterally: a lateral bundle in the ventrolateral gray substance, and a medial bundle in the ventral funiculus. The lateral bundle was found to consist of longitudinally oriented dendrites, neurocytons, glial cells and capillaries. As many as 1678 closely packed dendrites were observed in the lateral bundle. The medial bundle contained dendrites directed across the midline and also longitudinally oriented dendrites. Neurocytons in the medial dendrite bundle were found singly or in clusters, and many radiating bundles of dendrites were observed projecting toward the lateral bundle. Golgi analysis confirmed that neurons in the lateral bundle had most of their dendrites oriented longitudinally. It was possible to trace several dendrites into the lateral bundle from dorsally and medially lying neurons. Electron microscopy substantiated the fact that the bundles were composed of dendrites. It also revealed numerous dendrodendritic and dendrosomatic contacts which were desmosomal in type as well as an abundance of small unidentified processes. Various functions which have been attributed to the dendrite bundles are discussed.

The morphology of motoneurons and dorsal root fibers in the frog's spinal cord

Ventral and dorsal roots of the frog's spinal cord were filled with cobaltous chloride, and the resulting cobaltous sulfide precipitate, following treatment with H2S-buffer solutions, was intensified with physical developers. A ventromedial and a dorsolateral motoneuron group could be discerned in the ventral horn. The ventromedial, motoneurons gave origin to a strong dendrite crossing to the contralateral side. In the dendritic arborization pattern of the dorsolateral motoneurons a dorsomedial, a dorsal and a lateral dendritic array were distinguished. They were regarded as representing three different input channels to the motoneurons. Intramedullary branching of motor axons and recurrent axon collaterals were never observed. The dorsal root could be divided into a medial and lateral division carrying small and large caliber fibers, respectively. The end-branches of the small caliber fibers were seen to terminate in the substantia gelatinosa. Fine collaterals of the large caliber fibers also terminated in the substantia gelatinosa; coarser collaterals penetrated deeper and terminated in a triangular-shaped area in the base of the dorsal horn and in the intermediate gray matter. From this area a tail was followed into the ventral horn and several synapses were seen on the proximal dendrites and on the somata of motoneurons. A few dorsal root fibers could be seen crossing to the contralateral side.

Ultrastructure of a large ventro-lateral dendritic bundle in the rat ventral horn

There is an extensive bundle of dendrites with a rostro-caudal axis in the ventro-lateral lamina of the sixth lumbar segment in each side of the rat spinal cord. Such a bundle has a diameter of about 250 mum and contains over 1400 parallel dendrites, each with a diameter of less than 8 mum, interspersed between neuronal somata. The volume fraction of dendrites in the bundle neuropil is about 55%, the remainder being equally distributed between astrocytes, synaptic boutons, and axons, most of which are unmyelinated. An analysis of the median percentage covering of dendrites by contiguous elements of the neuropil reveals that as the dendrite diameter decreases from 4 to 0.2 mum (mean equals 2 mum), astrocytes increase from 43 to 75%, axons decrease from 21% to zero, boutons decrease from 28% to zero, and dendrites decrease from 10% to zero. There is a mean of 18 synaptic boutons per 100 mum-2 of the overall dendritic surface, but larger boutons tend to be more frequent on larger dendritic profiles. Apposed dendrites and their somata may have either puncta adhaerentia or confronting subsurface cisternae. Synaptic types in the rat are similar to those reported for the cat. The morphological findings are discussed with respect to previously proposed interaction between neural elements.