Glutamate decarboxylase-like immunoreactive neurons in the rat caudate putamen

GAD-IR neurons were roughly divided into those with medium sized perikarya and large perikarya. The medium-sized GAD-IR neurons accounted for about 85% of the GAD-IR neurons. The medium-sized perikarya were further divided into two, those with a smooth nuclear membrane and those with an indented nucleus. The former were very similar to medium-sized spiny neurons and the latter corresponded to medium-sized aspiny neurons. The GAD-IR large cells that were identified by light microscopy, had nuclear indentations and were divided into two classes based on their ultrastructural features, type 1 large cells received few synaptic inputs and type 2 large cells received many synaptic contacts from non-immunoreactive or immunoreactive boutons. The former resembles Type I large cells and the latter Type II large cells identified recently by Chang and Kitai; the latter are also similar to the second type of projecting neurons identified by Bolam et al.     

Large serotonin-like immunoreactive amacrine cells in the retina of developing Xenopus laevis.

The earliest appearance of serotonin-like immunoreactivity (SLI) in different cell types and the development of large SLI amacrine cells were studied in the retina of Xenopus laevis from stage 33/34 to adult. Intense SLI was first found in the somas of large amacrine cells at stage 39. The somas of small amacrine cells showed weak SLI at stage 41, followed by bipolar cells at stage 43. The number of large SLI amacrine cells in the inner nuclear layer of the retina increased from 57 at stage 40 to 774 in adult. Over the same period, retinal area increased from 0.19 mm2 to 24.57 mm2 with an accompanying decrease of cell density from 301/mm2 to 32/mm2. in adult animals large SLI amacrine cells were non-uniformly distributed. Peak cell density of 50-60/mm2 was located in the center of the ventrotemporal quadrant and a trough of 8-15/mm2 in the dorsal periphery of the retina. Peak cell density region of the adult retina corresponded to part of the retina formed at early developmental stages where the rate of cell generation of large SLI amacrine cells was higher. These observations indicate that (1) SLI is expressed first by large amacrine cells, followed by small amacrine and bipolar cells; (2) large SLI amacrine cells are generated continuously throughout life, (3) the non-uniform retinal distribution of large cells results from a spatio-temporally differential cell generation at the ciliary margin.     

Selective damage to large cells in the cat retinogeniculate pathway by 2,5-hexanedione

The neurotoxic hexacarbon 2,5-hexanedione (2,5-HD), which produces transport abnormalities and swellings in the large diameter fibers of the peripheral nervous system, was administered to cats in an attempt to produce similar selective effects in the optic tract. Anatomical findings indicate damage to one type of retinal ganglion cell, the large (alpha) or Y-cell class, both during dosing and after a long recovery period. This selective involvement of the large ganglion cells during dosing was shown by decreased retrograde transport of HRP in these cells relative to smaller cells. Such selectivity was not apparent in axonal swellings and neurofilament accumulations which were present in fibers of all diameters in the distal optic tract. Visual threshold studies during dosing showed a loss of flicker resolution with preservation of visual acuity, a result consistent with the different physiological properties of alpha and beta ganglion cells. In one cat, which survived dosing for a period of 8 months, there was a dramatic reduction in the number of large cells and a pronounced shrinkage of those that remained, but no observed changes in other cell types. Thus, this intoxication caused (1) axonal swellings which were not selective for fiber size; (2) a selective defect in axonal transport with later neuronal degeneration and shrinkage that were limited to large cells; and (3) a loss of flicker resolution that may reflect dysfunction of large ganglion cells.     

Retinal ganglion cell degeneration following loss of postsynaptic target neurons in the dorsal lateral geniculate nucleus of the adult cat.

Kainic acid was used to produce selective degeneration of neurons in the dorsal lateral geniculate nucleus of the adult cat. This degeneration mimics the rapid loss of geniculate neurons seen after visual cortex ablation in the neonate. Following survivals of 2, 4, or 6 months, the geniculate was injected with horseradish peroxidase and the retinae were examined for the presence of retrogradely labeled cells. Analysis of ganglion cell density in peripheral nasal retina revealed a 58% loss of cells overall at 6 months. The proportion of cells labeled with horseradish peroxidase decreased more rapidly, until none were labeled at 6 months. Separate analysis of small, medium, and large ganglion cell populations revealed that only medium-sized cells were lost at 2 months whereas both medium and large cells were lost at 4 and 6 months. By 6 months, 92% of medium cells and 65% of large cells had degenerated. These results show that mature retinal ganglion cells in the cat maintain a dependence on target integrity for their continued survival. When the appropriate target is lost, the ganglion cells respond first by axon terminal retraction and then by cell death.     

Ultrastructure of cerebellar capillary hemangioblastoma

Summary Concentric lamellar bodies of endoplasmic reticulum (CLB) were found in the stromal cells of all five cases of cerebellar capillary hemangioblastoma studied ultrastructurally. CLB were often present in the stromal cells with voluminous loose cytoplasm, particularly those close to the capillaries. They were rarely seen in small stromal cells with abundant organelles and stromal cells distended by large lipid droplets. Small lipid droplets were usually present in the center or in the vicinity of CLB. Vesiculation and vacuolization of the lamellar arrays of CLB were common. Some vacuolized CLB were transformed into large, varying-shaped, multilocular bodies. Some stromal cells were markedly distended by numerous large vacuoles derived from CLB. Granulo-fibrillary material was frequently present in the vacuolized lamellae. Discharge of vacuoles into the interstitial space was observed. It is suggested that CLB is one of the characteristic ultrastructural features of the stromal cells. They may represent a special type of hyperplasia of the endoplasmic reticulum, but their functional significance is not known.Supported by Henry Ford Hospital research grant A30161     

The ultrastructure of pinealomas

Summary Four pinealomas of the two-cell type (atypical teratomas) were investigated by electron microscopy. They all showed the same unique histological ultrastructure. The lymphocyte-like cells did not differ ultrastructurally from lymphocytes. Many plasma cells with well-developed rough surfaced endoplasmic reticulum were also seen. The small cells and the plasma cells were considered to be derived from blood cells. A variety of the large cells in the process of differentiation were observed. They had a large and ovoid nucleus with uniform granular chromatin and contained one or more prominent nucleoli. The large cells had many dilated cytoplasmic cisternae, numerous glycogen granules of about 250–300 Å in diameter and annulate lamellae. Ultrastructurally, the pinealoma of the two-cell type resembled the seminoma of testis which is of germ cell origin. This suggests that the pinealoma may be of germ cell origin.     

Quantitative analyses of neuronal development in the lateral motor column of mouse spinal cord. III. Generation and settling patterns of large and small neurons

The generation and settling patterns of large and small lateral motor column (LMC) neurons were compared in the spinal cords of three inbred strains of mice by means of tritiated thymidine autoradiography. No significant strain differences were observed for the number of large LMC cells (presumptive alpha motor neurons) that were heavily labeled on each injection day, although there were significant strain variations for this measure with regard to small LMC neurons (presumed gamma motor neurons and interneurons). The generation of both large and small LMC neurons began at the same time, but peak production of large cells preceded that of the small neurons. There were no strain differences observed for this relationship between the large and small cells. These findings indicate that the LMC, from the time of its initial formation contains cells destined to become large and small neurons. The positions of large and small neurons within the adult LMC relative to their times of origin (settling patterns) were analzyed statistically. A significant ventrodorsal sequence for early-to-late generated cells was observed for both large and small LMC neurons. No significant strain difference were found in the analysis of settling patterns. A ventrodorsal settling pattern also has been described for amphibia (Prestige, ′73) and, in conjunction with the proximodistal sequence of limb development describedby other investigators, the ventrodorsal sequence could play a key role in the development of motor neuronal somatotopic organization.     

Morphological and electrophysiological characterization of abnormal cell types in pediatric cortical dysplasia

The mechanisms responsible for seizure generation in cortical dysplasia (CD) are unknown, but morphologically abnormal cells could contribute. We examined the passive and active membrane properties of cells from pediatric CD in vitro. Normal- and abnormal-appearing cells were identified morphologically by using infrared videomicroscopy and biocytin in slices from children with mild to severe CD. Electrophysiological properties were assessed with patch clamp recordings. Four groups of abnormal-appearing cells were observed. The first consisted of large, pyramidal cells probably corresponding to cytomegalic neurons. Under conditions that reduced the contribution of K(+) conductances, these cells generated large Ca(2+) currents and influx when depolarized. When these cells were acutely dissociated, peak Ca(2+) currents and densities were greater in cytomegalic compared with normal-appearing pyramidal neurons. The second group included large, nonpyramidal cells with atypical somatodendritic morphology that could correspond to "balloon" cells. These cells did not display active voltage- or ligand-gated currents and did not appear to receive synaptic inputs. The third group included misoriented and dysmorphic pyramidal neurons, and the fourth group consisted of immature-looking pyramidal neurons. Electrophysiologically, neurons in these latter two groups did not display significant abnormalities when compared with normal-appearing pyramidal neurons. We conclude that there are cells with abnormal intrinsic membrane properties in pediatric CD. Among the four groups of cells, the most abnormal electrophysiological properties were displayed by cytomegalic neurons and large cells with atypical morphology. Cytomegalic neurons could play an important role in the generation of epileptic activity.     

Reduction in numbers of large ganglion cells in cat retina following intravitreous injection of antibodies

Antibodies were prepared against large ganglion cells isolated from bovine retina and injected into the vitreous chamber of 1 eye in 6 adult cats. The other eye of each cat received either a control pre-immune γ-globulin injection or was untreated. After a survival time of 9–86 days, ganglion cell density was assessed from Nissl-stained retinal whole-mounts. In each cat, there were fewer large ganglion cells (α-cells) in the immunoglobulin-injected retina than in the control retina. The reduction in large ganglion cells occurred in patches adjacent to areas of approximately normal large ganglion cell density. Counts of the number of large ganglion cells in both eyes of the 6 cats indicated that the immunoglobulin injected eyes had from 8% to 61% (mean 32%) fewer large ganglion cells than the paired control eyes. This was significantly greater than the difference in the number of large ganglion cells between pairs of normal or control-injected eyes. The magnitude of the effect was not related to the survival time following the immunoglobulin injection.Cell size measures of all ganglion cells in selected areas of retina indicated that the small ganglion cells were unaffected by the antibodies. However, there was a suggestion that the largest of the medium size ganglion cells were affected in addition to the large ganglion cells. Counts of total ganglion cells per unit area in affected regions of retina revealed a reduced overall density, suggesting that the ganglion cells were lost rather than decreased in size.These results indicate that antibodies to the large ganglion cells can be used to reduce the number of large ganglion cells (α-cells) in the cat retina. Since these cells correspond to the Y-cell functional class of ganglion cells in the cat retina, the antibodies may provide a useful tool for studying Y-cell function in the visual pathways.     

Cerebellar hemangioendothelioma

We report a 26-year-old woman with cellular hemangioendothelioma that may be the third case diagnosed as cerebellar hemangioendothelioma. The tumor was composed of numerous vascular channels lined by endothelial cells and aggregates of large round endothelial cells. Transitional cells were also present. The large round cells were positive for CD34 and vimentin in their abundant cytoplasm that contained 1 or some red blood cells, revealing intracytoplasmic lumina. The patient remains free of disease over 5 years after tumor resection.     

Two discrete enkephalinergic neuron systems in the superior cervical ganglion of the guinea pig: an immunoelectron microscopic study

Leucine-enkephalin (L-ENK)-like immunoreactive (L-ENKI) structures in the superior cervical ganglion (SCG) were first examined by using immunoelectron microscopy. L-ENKI neurons formed cell clusters and were small. They were filled with large granular or agranular vesicles and small electron lucent vesicles, and had nuclei that lacked a nucleolus. Since these morphological characteristics are identical to those of the small intensely fluorescent (SIF) cells, L-ENKI cells seemed to be a kind of SIF cell. Two types of L-ENKI fibers were identified, a large type filled with large granular or agranular vesicles and a small type filled with small electron lucent vesicles. The large fibers were located near the L-ENKI perikarya and often could be traced directly to the soma. These fibers remained intact after decentralization of the SCG. These findings indicate that the large L-ENKI fibers are processes of L-ENKI SIF cells. The fibers showed a close apposition to the blood vessels and rarely formed synaptic contact with dendrites of the principal cells. On the other hand, the small L-ENKI fibers were found to originate outside the SCG, because they disappeared after decentralization of the SCG. These L-ENKI fibers frequently formed synaptic contact with the dendrites of the principal cells. Thus, the present study demonstrated the presence of two discrete L-ENKI neuron systems in the SCG of the guinea pig, one an intrinsic SIF system and the other an extrinsic L-ENKI system.     

Enhanced Purkinje cell survival in granuloprival cerebellar cultures

The number of large cortical neurons that survived in cerebellar cultures in which granule cells had been destroyed by exposure to cytosine arabinoside was 3-4 times the number in normal cultures. Transplantation of granuloprival cerebellar cultures with granule cells and glia resulted in a reduction of the large cortical neuron population (predominantly Purkinje cells) to normal, while the number of such neurons remained elevated after transplantation with glia alone. These results indicated that granule cells were critical for the reduction of large cortical neurons. The rescue of large cortical neurons in granuloprival cultures was attributed to an expanded target field for Purkinje cell axon collateral projections.     

Voltage-gated calcium currents in whole-cell patch-clamped bullfrog dorsal root ganglion cells: effects of cell size and intracellular solutions

Acutely dissociated bullfrog dorsal root ganglion (DRG) cells could be divided into two classes by measurement of cell capacitance. A bimodal distribution of cell capacitance was found and a value of 75 pF was used to divide frog DRG cells into ‘small’ and ‘large’ types. Two distinct voltage-activated Ca²⁺ currents were evoked in both classes of cells: a rapidly inactivating, low-voltage-activated current and a slowly-inactivating, high-voltage-activated current. When the recording pipette contained CsCl, greater peak inward current values and densities were seen in large cells compared to small cells. No significant differences were observed in the distribution of low-and high-voltage-activated currents in small and large cells. Replacement of pipette solutions containing CsCl with solutions containing equimolar concentrations of Cs glutamate,l-arginine Cl, orN-methyl-d-glucamine significantly increased both the reversal potential and the maximum amplitude of the Ca²⁺ currents in both small and large DRG cells. These increases indicate that internal substitutions with organic ions suppresses outward currents more effectively than does CsCl. In contrast to findings with CsCl, when organic ions were used in the pipette solution a significantly higher proportion of low-threshold Ca²⁺ channels was observed in small cells compared to large cells. These observations indicate that when organic solutions were used internally, significant differences in the proportion of low-threshold to high-threshold Ca²⁺ channels were observed in small and large cells. The composition of the internal solution is a critical variable when determining the type and amount of inward Ca²⁺ current in different types of neurons.     

Subgroup of alpha ganglion cells in the adult cat retina is immunoreactive for somatostatin.

We have previously shown that two types of cells in the ganglion cell layer of the adult cat retina are immunoreactive for somatostatin (White et al., '90). One of the types was identified by morphological criteria as a wide-field amacrine cell. The other cell type had a large, angular soma that resembled the alpha ganglion cell, but evidence was not available to identify it definitively as a ganglion cell. Both cell types were distributed preferentially in the inferior retina. In this report, we demonstrate that the two types of cell are, indeed, displaced amacrine cells and alpha ganglion cells. First, when retrograde tracers were injected into central visual targets, the immunoreactive large cells but not the displaced amacrine cells were found to be labeled. Second, after unilateral section of the optic nerve, the immunoreactive large cells disappeared from the retina on the lesioned side, but the displaced amacrine cells occurred in the same numbers in both retinae. In the periphery, the large cells ranged in diameter from 33 to 47 microns, comparable only to alpha ganglion cells (Boycott and W?ssle, '74). An antiserum to parvalbumin was used to visualize the dendrites (R?hrenbeck and W?ssle, '88) of somatostatin-immunoreactive large cells. Based on dendritic stratification within the inner plexiform layer (Famiglietti and Kolb, '76), the somatostatin-immunoreactive large cells were found to include both on-center cells and off-center cells, but were predominantly of the off-center type. Within a local region, they were found to be arrayed with greater regularity than the overall population of alpha ganglion cells. These results indicate that alpha ganglion cells of the cat retina can be subdivided on the basis of their immunoreactive staining for somatostatin and suggest that the diversity of ganglion cells in the cat retina may be greater than has been recognized on the basis of morphological criteria alone.     

Different targets of different neurons in nucleus gracilis of the cat.

Efferent projections of neurons in the cat nucleus gracilis to the ventrobasal complex (VB) of the thalamus and the dorsal accessory portion of the inferior olive (IOd) were studied using tracing techniques that exploit neuronal orthograde and retrograde intra-axonal transport processes. These projections were studied in order to determine if the heterogeneity of the morphology, physiology and afferent input of the nucleus gracilis is paralleled by a similar heterogeneity in its efferent projections. In an orthograde study, 3H-leucine and/or 3H-proline were injected into the nucleus gracilis of different subjects in an attempt to label different proportions of large (GREATER THAN 18 MU) AND SMALL CELLS. This procedure permitted study of the efferent targets of the two cell types. The amount of labeling in VB was a constant ratio of the number of large cells in the nucleus gracilis that had incorporated the isotope. This relationship was not observed for the small cells. The amount of labeling in IOd was dependent neither on the number of large nor the number of small cells that had incorporated the isotope. In a retrograde study following extensive injections of horseradish peroxidase (HRP) into the thalamus, only large cells (greater than 18 mu) in the dorsal column nuclei were filled with HRP reaction product. These cells were located predominantly in clusters in the middle zone of the nucleus gracilis as well as rostrally. After injections including (but not confined to) the inferior olive, small cells located in the rostral and middle zones were filled with reaction procduct. A few large cells located ventrally in the middle zone of the nucleus gracilis and rostrally were also filled. Some of these ventrally located large cells may project in a collateral fashion both to the thalamus and to the inferior alive. The results of both the orthograde and retrograde studies are consistent with other evidence on the heterogeneity of the nucleus gracilis. These data strongly support the conclusion that the population of cells in the nucleus gracilis that projects to the thalamus overlaps with but is not identical to the population of cells that projects to the inferior olive.     

Cell types and synaptic organization of the medullary electromotor nucleus in a constant frequency weakly electric fish,Sternarchus albifrons

The medullary electromotor nucleus (EMN) of Sternarchus albifrons was studied at the light and electron microscopic levels. The EMN consists of a dense meshwork of myelinated axons and glial elements with interposed large neurons; it is provided with an abundant supply of capillaries. Two types of essentially adendritic nerve cells were distinguished on the basis of size: giant neurons (approx. 70 μm in diameter) and large neurons (approx. 30 μm in diameter). Their population ratio is 1:4. Only giant cells are labelled following the injection of retrograde tracer into the spinal cord; they are therefore identified with the so-called “relay cells” of other gymnotids. Tracer experiments further suggest that the descending axons of these relay cells give off collateral branches throughout the elongated spinal electromotor nucleus. In contrast, the large cells remain unlabelled and therefore lack spinal projections; they most likely correspond to “pacemaker cells”. The perikaryal surface, including axon hillock and proximal part of initial segment of both types of EMN cells, is contacted by clusters of synaptic terminals and astrocytic processes. Two main varieties of synaptic terminals occur: (1) large endings and (2) ordinary end feet with standard size (S-type) and variable size (Sv-type) clear, spherical vesicles. The junction between large endings and EMN cells is characterized by the combination of gap junctions and surrounding intermediate junctions whose freeze-fracture characteristics were morphometrically analyzed. The large endings were formed by nodes of Ranvier as well as by fiber terminations, and synchronization within the EMN may be achieved by presynaptic fibers. Some of the contacts occur directly on the initial segment, which could allow activity to bypass the soma. It is concluded that the electromotor system of Sternarchus is comprised of a rapid conduction pathway where medullary pacemaker and relay cells as well as spinal electromotor neurons are coupled by synapses with gap junctions. In contrast to the spinal electromotor neurons, the medullary EMN cells receive synapses with morphological characteristics of chemical transmission, and the S-type and Sv-type terminals may possibly correspond to Gray's Type I and Type II synapses, respectively. These synapses may be involved in modulation of the electric organ discharge frequency.     

Input conductance, axonal conduction velocity and cell size among hindlimb motoneurones of the cat

Input conductance and axonal conduction velocity were measured for hindlimb motoneurones that were anatomically labelled by substances injected through the intracellular microelectrode (Procion dyes, horseradish peroxidase). We confirmed that there is a good correlation between the axonal conduction velocity of a hindlimb motoneurone and the size of its cell body. Furthermore, we confirmed that the power relation between neuronal input conductance and axonal conduction velocity has an exponent of about 3–4. If large motoneurones were simply scaled-up versions of the smaller ones, this exponent should be have been between 1.5 and 2.0. We showed that the unexpectedly high input conductance of fast-axoned motoneurones, compared to that of the more slow-axoned ones, was not due to a corresponding disproportion between the axonal conduction velocity and the size of the cell body. Neither could it be explained by differences between large and small cells with respect to the relative sizes and numbers of dendritic stems. The unexpectedly high input conductance of large cells seems likely to be largely caused by a lower average value for the specific membrane resistance among these cells than among the smaller ones. Hitherto unknown differences in dendritic architecture between large and smaller cells might conceivably be of some importance as well. Our results are consistent with the view that, in muscle contractions evoked by the central nervous system, thin-axoned motoneurones might be recruited more easily than more thick-axoned ones even if all the cells were activated by the same density of equipotent synapses.     

A case of cerebellar hamartoma suggesting abnormal cell migration

Summary A rare case of hamartoma of the left cerebellar hemisphere was recognized in an 11-monthold male infant whose mother had a history of unspecified medication in the early gestational period and had a difficult delivery. A notably large head and marked developmental disorders, like hypotonic cerebral palsy, were observed soon after birth. A computed tomogram revealed an iso-minimally enhanced large mass in the left cerebellar hemisphere, which deformed the fourth ventricle and compressed the right cerebellum, as well as moderate cerebral atrophy. Histologically, the border between the cerebellar cortex and this tumor was not apparent. The main tumor, located in the cerebellar white matter, was composed of numerous scattered Purkinje cell-like neurons and glial cells surrounded by abundant GFAP-positive matrix. The small part of the tumor, located near the choroid plexus, was composed of intensely proliferated capillaries such as in capillary hemangioma, and numerous fibrocytes, which were intermingled with several large Purkinje cell-like neurons and some GFAP-positive glial cells. The cerebellar cortex showed a thin molecular layer with some residual external granular cells, a marked decrease of Purkinje cells and a moderate decrease in the internal granular layer, in which large Purkinje cell-like neurons were scattered. Purkinje cells and large Purkinje cell-like neurons scattered in the internal granular layer, cerebellar white matter and choroid plexus showed positive immunoreactivity for anti-Leu-4 monoclonal antibody, which is known to be a marker for Purkinje cells. These findings suggest that this case had the background of abnormal cell migration caused by some kind of disorder during pregnancy.     

Retinal ganglion cells in the crucian carp (Carassius carassius). II. Overlap, shape and tangential orientation of dendritic trees.

Ganglion cells were studied in methylene blue stained flat-mounted retinas. Three categories of cells are described: small (S) and large (L) ganglion cells in the main ganglion cell layer, and large ganglion cells (LD) with somata more or less displaced into the inner plexiform layer. These LD cells have two to four very thick primary dendrites and are identifiable as ganglion cells by their axons. An analysis of published data reveals that the large ganglion cells of the crucian carp (type L and LD) have several striking characteristics in common with the large ganglion cells of the dogfish, the frog and the cat: (1) they are selectively stained by methylene blue; (2) they comprise only 2-5% of all the ganglion cells; (3) the large cells can be divided into two or three subtypes, and within each subtype the dendritic trees usually cover the retinal surface with a two- or threefold overlap. New ganglion cells are formed from neuroblasts at the retinal margin and most dendrites first grow along this neuroblastic zone. Thus the main dendrites of the L and LD cells tend to be oriented parallel to the margin all around the periphery of a crucian carp retina. Independent of the size of the eye this parallel orientation disappears at the same relative distance from the margin (about one-third of the distance from the margin to the optic disc). If all L and LD cells are formed at the retinal margin and first develop oriented dendrites, we have to assume that the more randomly oriented dendritic trees in the central retina have undergone a reorganization.     

Synaptic organization in the pacemaker nucleus of a medium-frequency weakly electric fish,Eigenmannia sp

The medullary command nucleus (MCN) of the medium-frequency weakly electric fish,Eigenmannia sp., contains two types of neurones, namely large and small cells, which are embedded in a neuropile of large and small myelinated fibers. Using serial semi-thin and ultra-thin sectioning, combined with HRP labelling established that both cell types possess rich dendritic arborization and large myelinated axons. Only the axons of the large cells leave the nucleus and these contribute the unique output of the MCN. Axon branching has been observed only in the axons of small cells and their collaterals show an exclusively intranuclear course. Two types of synaptic terminals have been found on large as well as on small cells: (1) large club endings forming both gap (electronic) junctions and polarized chemical synapses, which often appear at the same junction constituting morphologically mixed synapses; and (2) small bouton-like terminals forming exclusively chemical synaptic contacts. No differences between the two neuron types could be detected with respect to the arrangement of the synaptic contacts: club endings and small bouton-like terminals synapse on dendritic processes as well as on perikarya, while the unmyelinated initial segments were always found to be free of synaptic contacts. Large and small cells were found to be simultaneously connected by the same club ending or small bouton-like terminal: in the case of club endings by means of gap junctions and chemical synapses, whereas in the case of boutons by chemical synapses only. Club endings sometimes form gap junctions with each other. The possible role of these unusual synaptic connections in local synchronization is suggested. Club endings originate from the large axons of small cells, while small bouton-like terminals originate from the fine myelinated fibers of extranuclear origin. InEigenmannia, small cells, being connected to large cells as well as to each other by axo-somatic and axodendritic synapses, can be considered as the pacemaker cells of the MCN whereas large cells are relay cells. Small bouton-like terminals may convey exogeneous impulses towards the MCN exerting modulatory effects at both pacemaker and relay cell levels. The greater variety of ultrastructural correlates established in the MCN ofEigenmannia, in comparison withSternarchus⁵ (see also ref. 16), suggests increased modulation possibilities in the former fish's EOD behaviour.