Subsynaptic Cistern Research Articles

Are there efferent synapses in fish taste buds?

In fish, nerve fibers of taste buds are organized within the bud's nerve fiber plexus. It is located between the sensory epithelium consisting of light and dark elongated cells and the basal cells. It comprises the basal parts and processes of light and dark cells that intermingle with nerve fibers, which are the dendritic endings of the taste sensory neurons belonging to the cranial nerves VII, IX or X. Most of the synapses at the plexus are afferent; they have synaptic vesicles on the light (or dark) cells side, which is presynaptic. In contrast, the presumed efferent synapses may be rich in synaptic vesicles on the nerve fibers (presynaptic) side, whereas the cells (postsynaptic) side may contain a subsynaptic cistern; a flat compartment of the smooth endoplasmic reticulum. This structure is regarded as a prerequisite of a typical efferent synapse, as occurring in cochlear and vestibular hair cells. In fish taste buds, efferent synapses are rare and were found only in a few species that belong to different taxa. The significance of efferent synapses in fish taste buds is not well understood, because efferent connections between the gustatory nuclei of the medulla with taste buds are not yet proved.

Ultrastructural observations of efferent terminals in the crista ampullaris of the toadfish, Opsanus tau.

The present study was conducted to visualize the ultrastructural features of vestibular efferent boutons in the oyster toadfish, Opsanus tau. The crista ampullaris of the horizontal semicircular canal was processed for and examined by routine transmission electron microscopy. The results demonstrate that such boutons vary in size and shape, and contain a heterogeneous population of lucent vesicles with scattered dense core vesicles. Efferent contacts with hair cells are characterized by local vesicle accumulations in the presynaptic terminal and a subsynaptic cistern in the postsynaptic region of the hair cell. Serial efferent to hair cell to afferent synaptic arrangements are common, particularly in the central portion of the crista. However, direct contacts between efferent terminals and afferent neurites were not observed in our specimens. The existence of serial synaptic contacts, often with a row of vesicles in the efferent boutons lining the efferent-afferent membrane apposition, suggests that the efferent influence on the crista may involve both synaptic and nonsynaptic, secretory mechanisms. Further, it is suggested that differences in more subtle aspects of synaptic architecture and/or transmitter and receptor localization and interaction may render the efferent innervation of the peripheral crista less effective in influencing sensory processing.

Ultrastructural observations of efferent terminals in the crista Ampullaris of the toadfish, opsanus tau.

The present study was conducted to visualize the ultrastructural features of vestibular efferent boutons in the oyster toadfish, Opsanus tau. The crista ampullaris of the horizontal semicircular canal was processed for and examined by routine transmission electron microscopy. The results demonstrate that such boutons vary in size and shape, and contain a heterogeneous population of lucent vesicles with scattered dense core vesicles. Efferent contacts with hair cells are characterized by local vesicle accumulations in the presynaptic terminal and a subsynaptic cistern in the postsynaptic region of the hair cell. Serial efferent to hair cell to afferent synaptic arrangements are common, particularly in the central portion of the crista. However, direct contacts between efferent terminals and afferent neurites were not observed in our specimens. The existence of serial synaptic contacts, often with a row of vesicles in the efferent boutons lining the efferent-afferent membrane apposition, suggests that the efferent influence on the crista may involve both synaptic and nonsynaptic, secretory mechanisms. Further, it is suggested that differences in more subtle aspects of synaptic architecture and/or transmitter and receptor localization and interaction may render the efferent innervation of the peripheral crista less effective in influencing sensory processing.

Synaptic dimorphism in Onychophoran cephalic ganglia

The taxonomic location of the Onychophora has been controversial because of their phenotypic and genotypic characteristics, related to both annelids and arthropods. We analyzed the ultrastructure of the neurons and their synapses in the cephalic ganglion of a poorly known invertebrate, the velvet worm Peripatus sedgwicki, from the mountainous region of El Valle, Mérida, Venezuela. Cephalic ganglia were dissected, fixed and processed for transmission electron microscopy. The animal has a high degree of neurobiological development, as evidenced by the presence of asymmetric (excitatory) and symmetric (inhibitory) synapses, as well as the existence of glial cell processes in a wide neuropile zone. The postsynaptic terminals were seen to contain subsynaptic cisterns formed by membranes of smooth endoplasmic reticulum beneath the postsynaptic density, whereas the presynaptic terminal showed numerous electron transparent synaptic vesicles. From the neurophylogenetic perspectives, the ultrastructural characteristics of the central nervous tissue of the Onychophora show important evolutionary acquirements, such as the presence of both excitatory and inhibitory synapses, indicating functional synaptic transmission, and the appearance of mature glial cells.

Alpha-motoneurons of the injured cervical spinal cord of the adult rat can reinnervate the biceps brachii muscle by regenerating axons through peripheral nerve bridges: combined ultrastructural and retrograde axonal tracing study.

Following our previous studies related to brachial plexus injury and repair, the present experimentation was designed to examine the ultrastructural features of those motoneurons of the locally injured cervical spinal cord of adult rats that were seen to regenerate into peripheral nerve (PN) bridges and to reinnervate nearby skeletal muscles. Here, the peripheral connection of the PN bridge was made with the biceps brachii (BB) muscle. Three months postsurgery, the spinal motoneurons labelled by retrograde axonal transport of horseradish peroxidase (HRP), after its injection into the BB, were selected on thick sections, using light microscopy, for the presence of dark amorphous granules of the HRP reaction product. Serial ultrathin sections were then made from the selected material. For the 10 labelled neurons studied, we examined the synaptic boutons present on the membrane of the neuronal soma. For five of them, we could observe three of the six types of synaptic boutons described for the alpha-motoneurons of the cat (S-type with spherical vesicles, F-types with flattened vesicles, and C-type with subsynaptic cistern). The largest boutons (type C) are specific to alpha-motoneurons. In comparison to normal material, we noticed a decrease in the number of boutons and an increase in the number of glial processes. After a transient phase of trophic changes, the reinnervated BB muscles showed a return of their fibers to nearly normal diameters as well as evidence of fiber type grouping. Simultaneous staining with silver and cholinesterase also revealed the presence of new motor endplates frequently contacted by several motoneurons. The present study indicates that, after a local spinal injury, typical alpha-motoneurons can reinnervate a skeletal muscle by regenerating axons into the permissive microenvironment provided by a PN graft. These data offer prospects for clinical reconstruction of the brachial plexus after avulsion of one or several nerve roots.

Postnatal development of efferent synapses in the rat cochlea.

The development of olivocochlear efferent axons and their contacts in the postnatal cochlea was studied after DiI applications to the olivocochlear bundle in the ipsilateral brainstem of rats from 0 to 10 days of age (P0-10). Light microscopic analyses showed that labeled axons reached the vicinity of inner hair cells by P0 and outer hair cells by P2. Electron microscopic analyses demonstrated that labeled immature efferent axons are present among supporting cells of the greater epithelial ridge as well as inner hair cells at P0. The first efferent contacts that contacted inner hair cells contained a few irregularly sized vesicles and, occasionally, mitochondria. Postsynaptic specializations within inner hair cells apposed to labeled efferent axons included subsynaptic cisterns, irregularly sized vesicles, and synaptic bodies. Similar features were present in unlabeled profiles, presumed to be afferents, indicating that immature efferent axons could not be reliably distinguished from afferents without positive labeling. Efferent axons synapsed with outer hair cells by P4 and had synapse-like contacts at the bases of Deiters' cells at P4 and P6. Contacts between afferents and efferents were observed frequently in the inner spiral bundle from P6. As they matured, efferent axon terminals contacting hair cells contained increasing numbers of synaptic vesicles and were typically apposed by well-defined postsynaptic cisterns, thus acquiring distinctive profiles.

Differential expression of nerve terminal protein isoforms in VAChT-containing varicosities of the spinal cord ventral horn

Of the different types of synaptic contacts with the mammalian spinal motoneuron, the synapse made by the cholinergic, so-called C-terminal of unknown origin and function has special morphological characteristics. Thus, in this synapse, there is no postsynaptic density but rather a large subsynaptic cistern in the motoneuron. To see whether this particular arrangement imposes special demands on the transmitter release machinery, we examined the presence of nerve terminal proteins in the C-terminal by using immunohistochemistry. Cholinergic nerve fibers and terminals in the spinal cord ventral horn were identified with an antiserum to the vesicular acetylcholine transporter (VAChT) protein. Immunohistochemistry in combination with confocal laser microscopy showed the presence of synaptosomal-associated protein of 25 kDa (SNAP-25)-, syntaxin-, cysteine string protein (CSP)-, synuclein-, synapsin I-, synapsin I/II-, synaptotagmin I-, synaptotagmin I/II-, synaptophysin-, and synaptobrevin-2-like immunoreactivity (-LI) in VAChT-containing C-terminals. Synaptotagmin III and synaptobrevin 1 could not be demonstrated in this type of terminal. VAChT-containing varicosities in the Renshaw cell area, with a probable origin from motoneuron axon collaterals, exhibited CSP, synapsin I/II, and synaptobrevin-1-LI, but not SNAP-25-, syntaxin-, synuclein-, synapsin I-, synaptotagmin I-, synaptotagmin I/II-, synaptophysin- and synaptobrevin-2-LI. The results suggest a differential content of nerve terminal proteins and their isoforms in cholinergic C-terminals apposing motoneurons and in the Renshaw cell area. It is concluded that C-terminals contain synaptic proteins necessary for fast transmitter release, and their origin should not be the motoneurons themselves.

Ultrastructural anatomy of physiologically identified jaw-muscle spindle afferent terminations onto retrogradely labeled jaw-elevator motoneurons in the rat.

Neuronal microcircuits involving jaw-muscle spindle afferents and jaw-elevator motoneurons were studied via retrograde and intracellular labeling in rats. Initially, trigeminal motoneurons were retrogradely labeled from horseradish peroxidase (HRP) injections into the temporalis and masseter muscles. The intracellular response of jaw-muscle spindle afferent neurons was then characterized during palpation, ramp and hold, and sinusoidal stretching of the jaw-closing muscles. Biotinamide was injected into these neurons, and the tissue was processed for the visualization of HRP and biotinamide. The ultrastructure of 243 intracellularly stained jaw-muscle spindle afferent boutons located within the trigeminal motor nucleus (Vmo) was examined. Eighty-five of these boutons synapsed with motoneurons retrogradely labeled with HRP, and 158 boutons synapsed with unlabeled structures within the Vmo. All spindle afferent boutons contained clear, spherical synaptic vesicles. Although the majority of boutons were S type, a few labeled jaw-muscle spindle afferent boutons possessed a long, narrow cleft, with a subsynaptic cistern comparable to previous descriptions of C-type boutons. Sixty-eight percent of spindle afferent boutons synapsed with large or medium-sized, retrogradely labeled motoneuron dendrites, and 32% synapsed with retrogradely labeled somata. In numerous instances, spindle afferent boutons synapsed with trigeminal motoneuron dendritic or somatic spines. Most of the synapses between spindle afferent boutons and trigeminal motoneuron dendrites were asymmetric, and the greatest percentage of axosomatic synapses between spindle afferents and trigeminal motoneurons were symmetric. Approximately 24% of spindle afferent boutons constituted the intermediate element of a axoaxodendritic or axoaxosomatic assemblage, implying that some jaw-muscle spindle afferent synapses with trigeminal motoneurons are presynaptically modulated.

Synaptic terminal coverage of primate triceps surae motoneurons

This study examined the synaptic terminal coverage of primate triceps surae (TS) motoneurons at the electron microscopic level. In three male pigtail macaques, motoneurons were labeled by retrograde transport of cholera toxin-horseradish peroxidase that was injected into TS muscles bilaterally and visualized with tetramethylbenzidine stabilized with diaminobenzidine. Somatic, proximal dendritic, and distal dendritic synaptic terminals were classified by standard criteria and measured. Overall and type-specific synaptic terminal coverages and frequencies were determined. Labeled cells were located in caudal L5 to rostral S1 ventral horn and ranged from 40 to 74 microns in diameter (average, 54 microns). The range and unimodal distribution of diameters, the label used, and the presence of C terminals on almost all cells indicated that the 15 cell bodies and associated proximal dendrites analyzed here probably belonged to alpha-motoneurons. Synaptic terminals covered 39% of the cell body membrane, 60% of the proximal dendritic membrane, and 40% of the distal dendritic membrane. At each of these three sites, F terminals (flattened or pleomorphic vesicles, usually symmetric active zones, average contact length 1.6 microns) were most common, averaging 52%, 56%, and 58% of total coverage and 56%, 57%, and 58% of total number of cell bodies, proximal dendrites, and distal dendrites respectively. S terminals (round vesicles, usually asymmetric active zones, average contact length 1.3 microns) averaged 24%, 29%, and 33% of coverage and 33%, 35%, and 36% of number at these three sites, respectively. Thus, S terminals were slightly more prominent relative to F terminals on distal dendrites than on cell bodies. C terminals (spherical vesicles, subsynaptic cisterns associated with rough endoplasmic reticulum, average contact length 3.5 microns) constituted 24% and 11% of total terminal coverage on cell bodies and proximal dendrites, respectively, and averaged 11% and 6% of terminal number at these two locations. M terminals (spherical vesicles, postsynaptic Taxi bodies, some with presynaptic terminals, average contact length 2.7 microns) were absent on cell bodies and averaged 3% and 7% of total coverage and 2% and 5% of terminals on proximal and distal dendrites, respectively. Except for M terminals, which tended to be smaller distally, terminal contact length was not correlated with location. Total and type-specific coverages and frequencies were not correlated with cell body diameter. Primate TS motoneurons are similar to cat TS motoneurons in synaptic terminal morphology, frequency, and distribution. However, primate terminals appear to be smaller, so that the fraction of membrane covered by them is lower.

Alpha and gamma motoneurons in the peroneal nuclei of the cat spinal cord: an ultrastructural study.

The aim of the present study was to investigate whether ultrastructural features can be used as a guide to identify alpha- and gamma-motoneurons among the intermediate-size neurons of the peroneal motor nuclei. The peroneus brevis and peroneus tertius muscles of adult cats were injected with horseradish peroxidase, and motoneurons labeled by retrograde axonal transport were examined by electron microscopy. In both nuclei, the distributions of cell-body diameters, measured in the light microscope, were bimodal covering the range of 28-84 microns, with a trough around 50 microns. The sample of 25 motoneurons selected for the ultrastructural study included not only large (presumed alpha) and small (presumed gamma) neurons but also intermediate-size cell bodies with diameters in the 40-60 microns range. For each motoneuron, 2-5 profiles were reconstructed from ultrathin sections taken at 6-8 microns intervals. Synaptic boutons were counted and their lengths of apposition were measured. On the basis of three criteria, namely: (1) bouton types present on the membrane, (2) percentage of membrane length covered by synapses, and (3) the aspect of the nucleolus, all the examined motoneurons, including those with intermediate sizes, fell into one of two categories. Fourteen motoneurons, with cell-body diameters in a range of 55-84 microns, were contacted by all types of boutons (mainly S-type with spherical vesicles, F-type with flattened vesicles, and C-type with subsynaptic cistern); the synaptic covering of the somatic membrane was over 40% and the nucleus contained a vacuolated nucleolus. These were considered alpha-motoneurons. Eleven motoneurons, with only S and F boutons, a synaptic covering under 30%, a compact nucleolus and a cell-body diameter ranging between 28 and 50 microns, were considered gamma-motoneurons. No other combination of the three criteria was observed. These results show that unequivocal distinction of alpha- and gamma-motoneurons is possible in the peroneal nuclei, on the basis of morphological differences independent of cell-body size.

Freeze‐fracture organization of hair cell synapses in the sensory epithelium of guinea pig organ of Corti

The fine structure of both the afferent and efferent hair cell synapses in the sensory epithelium of guinea pig organ of Corti was examined by freeze-fracture electron microscopy. In the afferent synapse, barlike aggregates of intramembrane particles (IMPs) of about 10 nm in diameter were seen on the P-face of the afferent presynaptic membrane directly beneath the presynaptic dense projection which is located in the active zone of the presynaptic membrane. Small and large depressions have been seen on the presynaptic membrane. The former were observed in the proximity of the barlike aggregates, while the latter were observed some distance from the aggregate. In outer hair cells, IMPs of about 10 nm in diameter were seen on the P-face of the afferent postsynaptic membrane at a density of 3,000/microns 2. In the efferent synapse, many aggregates composed of from several to tens of large IMPs of 13 nm in diameter were observed on the presynaptic membrane. These aggregates were localized to small membrane depressions, which tended to be deeper as particle number per aggregate increased. Dense populations of IMPs of about 9 nm in diameter were observed on the P-face of the efferent postsynaptic membrane at a density of 4,000/microns 2. A fenestrated subsynaptic cistern completely covers the efferent postsynaptic membrane. Moreover, the subsynaptic cistern spans several efferent postsynaptic membranes when efferent synapses are gathered in a group. In the afferent and efferent synapses of hair cells, specializations of the synaptic membranes were represented by marked aggregates characteristic of IMPs. In the efferent synapse, IMP movement inside the synaptic membrane was proposed in relationship to retrival of synaptic vesicle membrane. Structural relationship between the subsynaptic cistern and efferent postsynaptic membrane was revealed.

Structurally based new functional interpretations of the subsurface cisternal network in human outer hair cells

The system of subsurface cisterns (SSC) in human outer hair cells (OHC) was studied by high-resolution transmission electron microscopy (TEM). The SSC consist of only a single layer of cisterns, either flattened or tubular. The subsynaptic cisterns of efferent synapses are an integral part of the SSC. The outer leaflet of the SSC facing the cell membrane is connected to the OHC wall by pillars arranged in a highly regular and geometric pattern. The inner leaflet facing the cytoplasm displays an identical pattern of particles arranged in a star-like configuration so that pores are formed centrally. Pores are also identified at the outer leaflet and at the insertion of the pillars into the cell membrane. It is hypothesized that this highly specific organization of SSC, so far described only in the human OHC wall, is controlled by the efferent acetylcholine-mediated innervation.

Feline ‘C’-type terminals possess synaptic sites associated with a hypolemmal cistern and Nissl body

Previous findings suggest that axosomatic ‘C’-type terminals on rat spinal motoneurones possess ‘active’ synaptic sites associated with the characteristic subsynaptic cistern and postsynaptic Nissl body, but ‘C’-terminals in cat do not. A re-examination of feline ‘C’-terminals undertook to compare the synaptic ultrastructure in aldehyde-fixed material stained by either osmium or ethanolic phosphotungstic acid (E-PTA). In agreement with previous findings osmicated cat ‘C’-terminals failed to reveal synaptic complexes in regions possessing the subsynaptic cistern (‘cisternal regions’). In contrast, cisternal regions of E-PTA stained ‘C’-terminals exhibited a linear array of presynaptic dense projection all opposed by a single extended postsynaptic density which abutted directly onto both postsynaptic membrane and the cistern. This close topographical relationship is suggested to be functionally significant in the trans-synaptic trophism previously demonstrated for feline ‘C’-terminals.

Acetylcholinesterase activity and type C synapses in the hypoglossal, facial and spinal-cord motor nuclei of rats. An electron-microscope study.

Using the electron-microscope technique of Lewis and Shute, we studied the localization of the acetylcholinesterase (AChE) activity in the hypoglossal, facial and spinal-cord motor nuclei of rats. The technique used selectively detects synapses with subsynaptic cisterns (type C synapses) as well as heavy deposits of reaction products in the rough endoplasmic reticulum, in fragments of the nuclear envelope, in some Golgi zones and on parts of the pericaryal plasma membrane, the axolemma and the dendritic membrane. In C synapses, AChE activity was located in the synaptic cleft and on the membrane of presynaptic boutons. Some C synapses exhibited distinct synaptic specialization in the form of multiple 'active zones'. These zones were characterized by dense presynaptic projections, short dilations of the synaptic cleft, and postsynaptic densities localized between the postsynaptic membrane and the outer membrane of the subsynaptic cistern. Within the postsynaptic densities, rows of rod- or channel-like structures were observed. The subsynaptic cisterns were continuous with the positive rough endoplasmic reticulum. The results are discussed in terms of the possible role of C synapses in the regulation of AChE synthesis in postsynaptic cholinergic neurons and/or in the regulation of AChE release into the extracellular space as well as in the establishment of new synaptic contacts.

Inputs to motoneurones in the hypoglossal nucleus of the rat: Light and electron microscopic immunocytochemistry for choline acetyltransferase, substance P and enkephalins using monoclonal antibodies

Light and electron microscopic peroxidase-antiperoxidase immunocytochemistry has been used to localize choline acetyltransferase, substance P and enkephalin in the hypoglossal nucleus of the rat. Choline acetyltransferase immunoreactivity was observed in motoneurone cell bodies and proximal dendrites, in large varicosities in the surrounding neuropil and in nerve terminals in synaptic contact with immunostained motoneurones. Most choline acetyltransferase immunostained terminals which made synaptic contact with motoneurone cell bodies and proximal dendrites possessed prominent subsynaptic cisterns and belong to the terminal type referred to in the literature as C or L. Substance P and enkephalin immunoreactivity did not occur in motoneurones but was seen in fibres and synaptic terminals. Substance P immunoreactive fibres made multiple axosomatic contacts while enkephalin immunoreactive terminals made synaptic contact mainly with large and small dendrites. C terminals were not stained for either substance P or enkephalin. This study provides immunocytochemical support for the classic identification of hypoglossal motoneurones as cholinergic and in addition shows that these neurones are innervated by a number of morphologically and chemically distinct terminal types. C terminals have previously been shown to contain cholinesterase and our demonstration that these terminals contain choline acetyltranferase thus provides additional evidence for their cholinergic nature and for a cholinergic innervation of hypoglossal motoneurones. The origin of the immunoreactive terminals was not identified in this study but possible candidates include the raphe nuclei for substance P and propriobulbar interneurones for choline acetyltransferase.

A freeze-fracture study of afferent and efferent synapses of hair cells in the sensory epithelium of the organ of Corti in the guinea pig.

Afferent and efferent synapses of hair cells in the organ of Corti of the guinea pig have been examined in freeze-fracture replicas. Afferent synapse. In the inner hair cells, intramembranous particles 10 nm in diameter are aggregated on the ridge on the P-face of the presynaptic membrane directly beneath the synaptic rod. In the outer hair cells, in which the synaptic rod is located in the presynaptic cytoplasm underneath the presynaptic membrane, small aggregations of intramembranous particles 10 nm in diameter can be found on the P-face of the presynaptic membrane corresponding to the site of the presynaptic dense projection. Intramembranous particles 10 nm in diameter are also densely aggregated on the P-face of the postsynaptic membrane of the outer hair cells. Efferent synapse of the outer hair cells. Large intramembranous particles 13 nm in diameter are distributed in clusters composed of four to ten particles on the P-face of the presynaptic membrane. In the P-face of the postsynaptic membrane, disc-like aggregations of intramembranous particles 9 nm in diameter are found. The subsynaptic cistern covers the cytoplasmic surface of the postsynaptic membrane of the efferent synapse; it may cover more than one postsynaptic membrane when several efferent synapses are in close proximity to one another.

An ultrastructural study of the development of afferent and efferent synapses on outer hair cells of the guinea pig organ of Corti.

The guinea pig organ of Corti was studied using transmission electron microscopy, the second turn of the cochlea being examined at various ages between 20 days before birth and 30 days postnatal. Outer hair cells were examined at each of these ages. At all ages studied, the efferent (presynaptic) terminals are large and are packed with synaptic vesicles, whereas the afferent (postsynaptic) terminals are generally smaller, with a relatively small number of vesicles. During development, the subsynaptic cistern changes from a fragmented, diffuse profile extending over 50-70% of the length of the efferent contact zones, to a continuous, compact structure spanning neighbouring synapses. Synaptic vesicles in the efferent terminals are predominantly rounded in early development, flattened vesicles appearing postnatally. The synaptic bodies at afferent synapses do not change noticeably during development. Quantitative analysis revealed that the area of efferent terminals and the length of their active zone increase with increasing age, the same parameters decreasing in afferent terminals. Synaptic vesicles in the efferent terminals decrease in diameter, but remain constant in afferent terminals, with increasing age. The number of hair cell membrane invaginations decreases as development proceeds.

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.

The synaptic organization of the motor nucleus of the trigeminal nerve in the opossum

The motor nucleus of the opossum trigeminal nerve consists of a main body and a small dorsomedial cell cluster. The cell bodies form a unimodal population with areas that range from 150-2700 mum2. Golgi impregnations reveal that each neuron has three to six primary dendrites which radiate in all planes from the cell body. Within 300 mum from the soma, the primary dendrites divide into secondary branches and these, in turn, bifurcate into thinner distal dendrites. The overall diameter of the dendritic tree often extends as much as 1 mm, with a rare branch leaving the confines of the nucleus to enter the neighboring reticular formation. Somatic and dendritic spines are often present and are either sessile or complex appendage forms. The perikarya and initial dendritic trunks of trigeminal neurons are contacted by four types of presynaptic terminals which cover more than 40% of the membrane. Most endings are 1-3 mum long and contain either spherical (S) or pleomorphic (P) synaptic vesicles. Another, less common, type of bouton is marked by large dense-core (DC) vesicles. Approximately 8% of the terminals on trigeminal cell bodies are large (2-5 mum) with spherical synaptic vesicles and are always associated with a subsynaptic cistern (C-boutons). These terminals very often interdigitate with adjacent synaptic endings. S-, P-, and C-boutons synapse on the dendritic tree of trigeminal neurons in the following characteristic pattern: proximal dendrites (greater than 5 mum in diameter) are contacted by all three types of terminals; intermediate-sized dendrites (between 2.5 and 5.0 mum in diameter) are most often contacted by S-boutons although P-boutons are also present; and small, distal dendrites (less than 2.5 mum in diameter) are almost always contacted by S- boutons. Both S- and P-boutons contact spines. In order to determine the ultrastructural identity of some of the major afferent systems to the trigemina motor nucleus, adult opossums were subjected to two different types of lesions. Three and 5 days subsequent to lesions which destroyed most of the trigeminal mesencephalic nucleus, degenerating terminals containing spherical vesicles were found. These endings were S-boutons on more distal parts of the dendritic tree while on the cell body and proximal dendrites they were C-boutons. Seven days after a mesencephalic lesion, expanded glial processes approximated the trigeminal cell membrane. Two days subsequent to lesions which transected commissural fibers from the contralateral trigeminal complex, degenerating S- and P-boutons were found in contact with intermediate and distal parts of the trigeminal dendritic tree.

Fine structure of the sensory epithelium of the guinea pig organ of corti: afferent and efferent synapses of hair cells

The fine structure of afferent and efferent synapses on the hair cells of guinea pig organ of Corti was observed. The afferent synapses of the inner and outer hair cells had basically similar structures. The afferent synapse was composed of an active and a periactive zone. The active zone of the presynaptic membrane was always accompanied by a synaptic rod, and striations were found within the synaptic rod. This structure was surrounded by synaptic vesicles, and a presynaptic dense projection was seen on the presynaptic membrane beneath it. In the efferent synapse, pre- and postsynaptic membranes showed a symmetrical structure. Synaptic vesicles were accumulated at the presynaptic dense projection of the presynaptic membrane, and a flattened subsynaptic cistern was seen below the postsynaptic membrane. Efferent synapses were also found on the inner hair cell.