Axodendritic Synapses Research Articles

The ultrastructural characteristics and their functional significance of the arcuate nucleus and median eminence.

The ultrastructural characteristics of the arcuate nucleus and median eminence in rats have been studied by means of formaldehyde-osmium tetroxide fixation method. The observations showed that there are two kinds of neurons (dark and light) in the arcuate nucleus which might be responsible for producing both dopamine and releasing hormones. The tanycytes of the ependyma of the third cerebral ventricle pass longitudinally through the various zones of the median eminence and reach at pericapillary space of the portal vessels. The neurosecretory substance-containing nerve terminals may travel between ependymal cells and even enter the cavity of the third ventricle or end around the basal membrane of the capillaries of the median eminence. The axo-somatic and axodendritic synapses are formed at the perikaryon and dendrites of neurons in the arcuate nucleus. Both agranular type and granular type of axo-axonic synapses are encountered in the fibrous zone of the median eminence. There are also synaptic connections between the basic processes of tanycytes and the large granular vesicle-containing nerve terminals in the palisade zone of the median eminence. The ultrastructural characteristics mentioned above suggest that (1) the releasing (or inhibiting) hormones of the hypothalamus might be released through two routes: into the portal capillaries from nerve terminals directly or into the cerebrospinal fluid of the third ventricle first and uptaken by tanycytes, then transported to the portal capillaries by the basic processes of tanycyte; (2) each step of synthesis, storage, transport and release of the releasing (or inhibiting) hormones could be regulated by nervous mechanism.

Influence of ascending noradrenergic fibers on the neurotensin-like immunoreactive neurons in the rat paraventricular nucleus

Regulation of the neurotensin (NT)-producing neurons by ascending catecholamine fibers in the paraventricular nucleus of the rat hypothalamus was examined using fluorescence histochemistry and immunohistochemistry after the destruction of the ascending catecholamine pathway by 6-hydroxydopamine. On the ipsilaterally operated side, the fluorescence of the catecholamine terminals decreased remarkably in the paraventricular nucleus whereas an accumulation of catecholamine fluorescence was observed in the caudal mesencephalon rostral to the locus coeruleus. In addition, the number of neural perikarya with NT-like immunoreactivity was greatly decreased in the paraventricular nucleus on the operated side compared with the intact side as determined by peroxidase-antiperoxidase immunohistochemistry. This decrease in the NT-like immunoreactive neural perikarya may be due to the disappearence of noradrenergic input to the NT-like immunoreactive neurons through axo-somatic or axo-dendritic synapses.

Histochemistry of the magnocellular neurons in the posterior hypothalamus, with special reference to MAO activity and ability of 5HTP uptake and decarboxylation.

Morphological study at the light and electron microscopic levels was carried out on the posterior group of the magnocellular neurons in the hypothalamus of the rat as revealed by monoamine oxidase (MAO) histochemistry. A conspicuously high activity of type B MAO was found in the perikarya and dendritic processes of these neurons. The dendritic processes ramified into fine varicose fibers which ran in all directions throughout the hypothalamus. Axodendritic synapses were frequently observed on the MAO-positive dendritic trunks and varicosities, some of which were filled with the small round vesicles. The ventrally directed dendrites richly branched into a dense network at the basal surface of the posterior hypothalamus and penetrated the limiting astrocytes where they appeared to have a close association with cerebrospinal fluid. In addition, these neurons showed an ability to synthesize serotoin after administration of the immediate precursor, 5-hydroxytryptophan. The possible neuronal and humoral functions of the magnocellular neurons in the posterior hypothalamus is discussed.

The neuroanatomical and neurovascular organization of normal fetal hypothalamic explants in the third cerebral ventricle of Brattleboro rats with homozygous diabetes insipidus

This investigation has combined microangiography, immunocytochemistry, coupled with transmission and scanning electron microscopy to discuss the neuroanatomical interactions that occur in the brains of Brattleboro rats with diabetes insipidus, following stereotaxic placement of normal fetal hypothalamic fragments into the third cerebral ventricle. Following surgical placement of 17 day post-coitus hypothalamic fragments, host rats with chronic autosomal homozygous diabetes insipidus were killed and their brains were prepared for analysis. A significant degree of explants (68%) flourished and grew in the lumen of the third cerebral ventricle of recipient hosts. Explants were rapidly invaded by host vessels from two routes. (1) Vessels arose from the underlying mantle plexus of portal capillaries which remained fenestrated in the lower one-third of the explants and developed neurovascular (neurohemal) zones. (2) The second source of vessels arose from bed capillaries of the adjacent paraventricular nucleus and adjacent hypothalamus. In contrast to vessels arising from the contact zone, these latter vessels remained unfenestrated. Small clusters of immunocytochemically positive neurons (neurophysin positive) were seen throughout the explants. Numerous healthy magnocellular neurons harboring numerous dense core vesicles and exhibiting multiple axosomatic and axodendritic synapses were seen throughout the neuropil of explants. Axon profiles were noted to terminate upon the abluminal basal lamina of perivascular spaces surrounding fenestrated capillaries in the lower one third of explants. None of the host animals exhibited physiological return to normal parameters of urine output, drinking behavior, and/or urine osmolarity. However the growth and development of explants in the third cerebral ventricle of DI hosts coupled with the emergence of bonafide neurovascular zones supports a potential anatomical substrate for the central delivery of neuropeptide hormones in this experimental model.

An electron microscopic study of primary afferent terminals from slowly adapting type I receptors in the cat.

Primary afferent fibers transmitting impulses from slowly adapting (SA) Type I receptors in the glabrous skin of the hind paw of the cat were injected intraaxonally in the spinal cord with horseradish peroxidase (HRP). At the light microscopic level, terminal arborizations were observed in the medial dorsal horn extending up to 6 mm rostrocaudally in and near the seventh lumbar segment. Boutonlike swellings labelled with HRP were distributed in clusters in Rexed's laminae III-VI. There was a tendency for the most dorsal clusters from an individual fiber to be located rostrally and for the most ventral clusters to be located caudally. At the electron microscopic level, a combination of morphometric analysis and serial reconstruction revealed the following: (1) All the boutons labelled with HRP contained predominantly clear, round synaptic vesicles, 40-50 nm in diameter. (2) Labelled boutons (n = 75) had cross-sectional longest dimensions of 1.72 +/- 0.53 micron (Mean +/- S.D.), perimeters of 4.95 +/- 1.52 micron, and areas of 1.18 +/- 0.59 micron 2. Their shapes in section varied from rounded to elongated forms. (3) The sizes of labelled boutons decreased significantly and linearly with depth from lamina IV to VI. The shapes of the bouton cross sections also became rounder with depth in the dorsal horn. (4) About 72% of synaptic contacts associated with HRP-filled boutons were with dendritic spines and shafts; most of these synapses were of the asymmetric type. (5) The remainder (28%) of the appositions were synapselike contacts between labelled boutons and unlabelled structures containing flattened or pleomorphic vesicles, and occasional dense-cored vesicles. (6) We observed no unequivocal axosomatic contacts made by labelled boutons. (7) The lengths of synaptic appositions with dendritic spines (0.46 +/- 0.20 micron) or with dendritic shafts (0.51 +/- 0.18 micron) were significantly greater than the synapselike contacts with vesicle-containing unlabelled structures (0.29 +/- 0.09 micron). (8) Complex neuropilar organization was occasionally seen with labelled boutons as central elements, although simpler organizations were much more common. In summary, HRP-labelled fibers ended predominantly in boutons containing clear, round vesicles forming axospinous and axodendritic synapses.(ABSTRACT TRUNCATED AT 400 WORDS)

The fine structure of the caudal periaqueductal gray of the cat: Morphology and synaptic organization of normal and immunoreactive enkephalin-labeled profiles

Although the midbrain periaqueductal gray (PAG) is thought to have a major role in an endorphin-mediated analgesia system, little is known about its neuroanatomical organization. To determine the microcircuitry within the PAG through which exogenous and endogenous opiates may act, we analyzed the synaptic organization of normal and immunoreactive enkephalin (ENK)-labeled profiles in the caudal PAG, a region of particular interest because of its effectiveness in generating analgesia. Examination of the normal fine structure of this region demonstrated that there is no characteristic synaptic morphology that distinguishes individual regions of the caudal PAG (ventromedial, ventrolateral and dorsolateral) from one another. In all 3 regions of the caudal PAG, axodendritic synapses are the predominant form of synaptic interaction making up 93–97% of all synapses counted. Axosomatic synapses are much less common, as are presumed axoaxonic and dendrodendritic synapses. In the caudal ventral PAG, the largest population of ENK-labeled axonal boutons are found presynaptic to unlabeled, centrally placed dendrites. Much less frequently, immunoreactive ENK-containing boutons are found presynaptic to neuronal perikarya or vesicle-containing profiles. Thus, these results suggest that the dendrites of neurons intrinsic to the PAG are the most probable site of opiate action in the caudal ventral PAG.

Ultrastructural studies of vasopressin neurons of the paraventricular nucleus of the hypothalamus using a monoclonal antibody to vasopressin: Analysis of synaptic input

The ultrastructure of the vasopressin neurons of the paraventricular nucleus of the hypothalamus was studied by immunocytochemical techniques. Tissue antigen was detected in unembedded tissue sections using a monoclonal antibody that recognizes vasopressin but not oxytocin or vasotocin. 17 At the light-microscopic level, reaction product was seen to fill the cytoplasm of the neuron cell body as well as large portions of the dendrite and axon. Immunoreactive spines were seen on both somatic and dendritic surfaces and their presence was confirmed at the ultrastructural level. In the light-microscope, axonal processes do not have spines and are thinner and more varicose than dendritic processes. At the electron-microscopic level, both axons and dendrites of the vasopressin cells are filled with reactive neurosecretory granules. The presence of large numbers of these organelles made it difficult to distinguish proximal dendrites from Herring bodies (axonal swellings). At the ultrastructural level, reaction product was also observed in the cytoplasm of all segments of the vasopressin cells. The presence of reaction product outside of membranous compartments is undoubtably due to disruption of membranes by detergent treatment or exposure to basic pH. However, the staining procedure used did allow us to examine the synaptic input to the vasopressin cells. All portions of the vasopressin neuron receive a diverse innervation. The somata have synapses on their surfaces and on spines. These axo-somatic terminals are primarily, but not exclusively, symmetrical and the presynaptic elements contain spherical or elongate vesicles. On the dendrites, terminals again were observed on the surface or on spines. These axo-dendritic synapses were usually asymmetrical. The presynaptic elements contained clear spherical, elongate or pleomorphic vesicles. Occasional varicosities with dense-core granules were seen to make en passant contacts with dendrites; these contacts did not have obvious membrane specializations. Input to vasopressin axons was studied both along the paraventricular-neurohypophysial tract and in the median eminence. Vasopressin axons receive a synpatic input (axo-axonic), predominately of the asymmetric variety with clear, spherical vesicles in the presynaptic element. These findings demonstrate that the vasopressin neurons of the paraventricular nucleus receive a diverse innervation.

Cholecystokinin-immunoreactive neurons in rat and monkey cerebral cortex make symmetric synapses and have intimate associations with blood vessels.

Neurons displaying cholecystokinin-like immunoreactivity (CCK neurons) in rat and monkey cerebral cortex were examined by light and electron microscopic immunocytochemistry. CCK neurons were found to be mainly bipolar cells present in all layers and in all areas of the rat cerebral cortex. CCK neurons were also found in all regions examined in monkey cortex (pre- and post-central gyri and superior parietal lobule). The somata and the dendritic processes of CCK neurons receive relatively few synapses but both symmetric and asymmetric axosomatic and axodendritic synapses were found. The majority of axon terminals displaying CCK-like immunoreactivity formed symmetric synapses, most frequently with the somata and proximal dendrites of pyramidal and nonpyramidal neurons. The somata and processes of CCK neurons were also found to establish very close nonsynaptic associations with blood vessels and with other neurons, suggesting possible roles for the peptide in the maintenance of neuronal excitability and cerebral blood flow.

Some cellular characteristics of somatostatin neurons and terminals in the periventricular nucleus of the rat hypothalamus and median eminence. Electron microscopic immunohistochemistry

Somatostatin neuronal perikarya and their processes, presumably dendrites, in the periventricular nucleus of the rat hypothalamus and terminals in the median eminence were observed by electron microscopic immunohistochemistry. Neuronal perikarya and processes contained immunoreactive dense granules (100–120 nm in diameter) and other cellular components such as polysomes, rER membranes occasionally showed high electron density. Few axo-somatic terminals were found on the somatostatin neurons, but we could detect a number of preterminal axons on immunoreactive processes, presumbly dendrites. Therefore, we considered that somatostatin neurons receive mainly neuronal input through axo-dendritic synapses rather than through axo-somatic ones. In the somatostatin terminals in the external layer of the median eminence immunoreactivity was completely restricted on the granules.

Immunocytochemical localization of TRH and autoradiographic determination of 3H-TRH-binding sites in the arcuate nucleus-median eminence of the rat.

Immunocytochemical and autoradiographic localization of thyrotropin-releasing hormone (TRH)- and 3H-TRH-binding sites was studied in the arcuate nucleus-median eminence region of the rat. TRH-like immunoreactivity was found in dense granular vesicles (90-140 nm in diameter) in TRH-like immunoreactive nerve fibers and terminals. In the median eminence, the immunoreactive terminals were observed to be in direct contact with the perivascular basal lamina of the portal vessel and to form synaptoid contacts with tanycytes. In the arcuate nucleus, the immunoreactive terminals were often found to form axosomatic and axo-axonic, and/or axo-dendritic synapses. The uptake of tritiated TRH into the nerve fibers and terminals of the median eminence was also observed by autoradiography and the distribution and localization of silver grains in them were analyzed quantitatively by circle analysis. Thirty minutes after intraventricular infusion of 3H-TRH, radioactive labeling occurred in type-2 and 3-nerve fibers and terminals containing dense granular vesicles in the median eminence. It is therefore suggested that the neurons labeled after 3H-TRH infusion possess certain functions as physiological recognition sites or receptors for TRH.

The pineal organ of Raja clavata: opsin immunoreactivity and ultrastructure.

The pineal organ of Raja clavata was studied by light and electron microscopy, including the immunocytochemical antiopsin reaction. The pineal organ of the ray consists of three portions: (i) a large proximal pineal, (ii) a long tube-like connecting stalk, and (iii) a short distal terminal enlargement. This latter end-vesicle lies in the deep connective tissue layers of the braincase. All portions of the pineal are composed of pinealocytes, intrinsic neurons, ependymal/glial cells, and bundles of nerve fibers embedded in thin neuropil formations. The inner segments of the pinealocytes protrude into the lumen in all parts of the organ and usually contain basal bodies and numerous mitochondria. Often, two outer segments were found to arise from the basal bodies of a single inner segment. By means of light-microscopic immunocytochemistry the outer segments showed a strong antiopsin reaction. The axons of the pinealocytes form ribbon-containing synapses on dendrite-like profiles, which appear to belong to the intrinsic pineal neurons. There are other axo-dendritic synapses established by presynaptic terminals lacking ribbons and containing granular and synaptic vesicles. Pineal neurons may contain granular vesicles approximately 60-100 nm in diameter; their processes contribute to the bundles of unmyelinated axons. The fine structural organization of the pineal organ and the opsin immunoreactivity of the outer segments of the pinealocytes indicate a photoreceptive capacity of the organ. The double outer segments represent a peculiar multiplication of the photoreceptor structures.

Ultrastructural organization of the neuropil in layer I of the cat cerebral cortex

The ultrastructure of layer I in the middle ectosylvian gyrus (area 22) of the cat's cerebral cortex was investigated. Beneath the subpial astrocytic layer most of the neuropil in layer I was shown to be occupied by nerve fibers and their terminals, terminal branches, dendritic spines, and astrocytic processes surrounding them. More than 90% of the presynaptic terminals contained spherical synaptic vesicles. The predominant types of interneuronal junctions are axo-spinous and axo-dendritic synapses of asymmetrical type. Presynaptic terminals, which contain flattened and pleomorphic synaptic vesicles, take part in the formation of all symmetrical junctions, accounting for 6% of the total number of synapses. Large polymorphic outgrowths filled with vacuoles — so-called multivacuolar sacs — are described. These structures were invaginated into varicose expansion of the terminal branches of apical dendrites of pyramidal neurons. They are shown to be outgrowths of presynaptic terminals. Dependence of synaptic function on the shape of the synaptic vesicles is examined.

A quantitative ultrastructural study of the effects of phenylacetate on synaptic organization in the developing rat cerebral cortex.

Male rats were injected with either phenylacetate (PA; a phenylalanine metabolite) or saline (control). Osmicated tissue from the parietal cortex was analyzed qualitatively and quantitatively, attention being focused on possible synaptic changes in the molecular layer. PA-treated rats differed from controls in the following ways: a 20% decrease in body weight at 21 days, and a reduction in brain weight at 10 and 21 days; the presence of 'atypical' profiles (possibly derived from axonal growth cones) in the neuropil; a lag in the accretion of synapses per 100 micron2 of tissue, and of vesicles per micron2 of presynaptic terminal; the presence of a 'crossover' in the developmental trend for several synaptic organizational parameters between days 15 and 21; an elevation in synaptic density at day 21; an increased proportion of negatively curved synapses at 21 days; a significant difference in the preponderance of axodendritic synapses over days 7-21. This suggests that the metabolism of some neurons in the PA brain is initially different from normal, with a lag in synaptic development being evident. The increase in synaptic density and the relative preponderance of other synaptic parameters at 21 days signifies that an initial lag in development may alter the developmental scheme. This may be coupled with changes in synaptic efficacy and connectivity.

Reciprocal connections between the amygdala and parabrachial nuclei: ultrastructural demonstration by degeneration and axonal transport of horseradish peroxidase in the cat

An ultrastructural analysis of the connections of the amygdala with the parabrachial nuclei (PBN) was carried out after electrolytic lesions or injections of horseradish peroxidase in the amygdala. Degenerating and labeled terminals originating from the amygdala formed axosomatic and axodendritic synapses in the PBN. In some instances, anterograde labeled terminals synapsed with retrogradely labeled neurons. These results demonstrate that the amygdala and PBN have close reciprocal synaptic connections.

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. In Eigenmannia, 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 of Eigenmannia, in comparison with Sternarchus 5 (see also ref. 16), suggests increased modulation possibilities in the former fish's EOD behaviour.

Morphological studies on neuroglia. V. Microglial cells in the cerebral cortex of the rat, with special reference to their possible involvement in synaptic function.

Electron-microscopic survey of selectively stained microglial cells in the cerebral cortex of the rat reveals that the processes of this cell type often encircle axo-dendritic synapses. Enzyme-histochemical methods for thiamine pyrophosphatase (TPPase) or nucleoside diphosphatase (NDPase) were used for the selective marking of the microglial cells; TPPase and NDPase activities were observed in the plasma membrane of microglial cells. The synapses encircled by microglial processes displayed presynaptic structures containing round clear vesicles (50 nm in diameter) and a prominent thickening of the postsynaptic membrane. In vitro, the above-mentioned enzymatic activities were completely suppressed by neuroactive agents such as catecholamines and phenothiazine derivatives. Examination using enzyme-histochemical techniques suggests that a single enzyme may be responsible for both microglial cells in the normal central nervous tissue is discussed.

Morphology of radial glia, ependymal cells, and periventricular neurons in the optic tectum of goldfish (Carassius auratus).

There are three populations of cells in the deep layers of the optic tectum in a normal adult goldfish: the periventricular neurons, the ependymal cells, and the radial glia. The characteristic morphological features which distinguish the three cell populations are examined at light and electron microscopic levels in the present work. A radial glial cell has a deeply invaginated nucleus located in a subependymal layer. Its cytoplasm contains mitochondria with 35-nm dark granules and 20-nm microtubules but no intermediate filaments. Its prominent radial process extends through the superficial tectal layers. In contrast, the processes of ependymal cell ramify and interweave within the ependymal region. The cytoplasm of an ependymal cell contains prominent bundles of intermediate filaments but not microtubules. Its soma lies at or near the ventricular surface. A periventricular neuron has a round nucleus and a smooth dendrite which extends toward the superficial tectal layers. Its cytoplasm contains microtubules and agranular mitochondria. Axosomatic and axodendritic synapses are found on periventricular neurons. The morphological characteristics of these cell types are considered in relation to previous descriptions of teleost tectal cytology and with regard to the atypical natures of the cytoskeletal elements of the ependymal and radial glial cells.

Quantitative study of synapse formation in the duck olfactory bulb.

Synaptic emergence and development in the duck olfactory bulb was quantitatively studied by electron microscopy from the 14th day of incubation (E 14) to the adult stage. Overall synaptic density in this bulb grew considerably during the last weeks of embryonic life and the first postnatal week. The pattern of synaptic density development was similar in the four main architectonic layers of the bulb. However, lower density values were observed in the mitral and inner granule cell layers. In the glomerular layer (GL), axodendritic synapse density was always higher than dendrodentritic synapse density. In the external plexiform layer, most synapses were dendrodendritic and were established between the gemmules of the granule cells (GC) and the dendrites of the mitral cells (MC) or tufted cells (TC). Synapses established by MC and TC on GC gemmules, or by GC on MC and TC dendrites had densities very similar to each other at all the stages studied. Reciprocal synapses already appeared at E 14; their density grew until a week after birth (P7) and thereafter remained stable. In the internal granular layer, the density of asymmetrical synapses was always higher than that of symmetrical synapses. Excitatory synapses formed earlier on MC and TC than inhibitory synapses. The ratio of inhibitory-to-excitatory synapses rose rapidly after birth, reaching 2.5 in the adult duck. The density of excitatory synapses received by granule cells was as high in the external plexiform layer as in the inner granule layer, at all stages of GC development. However, the ratio of received-to-formed synapses fell in these cells from 8.42 at E 14 to 2 after birth. These results are discussed as a function of the evolution of the different synaptic balances during olfactory bulb development. Synaptic development in the duck olfactory bulb at birth is relatively close to the adult state. It appears sufficiently advanced to enable the olfactory system to function in a way compatible with the relatively independent behavior displayed by the duckling.

An experimental electron microscopical study of a direct retino-pulvinar pathway in the tree shrew

After unilateral enucleation in the tree shrew (Tupaia glis), axo-dendritic synapses degenerate within two secondary visual centers (intergeniculate and lateral pulvinar nuclei). Degenerated optic terminals were found in both pulvinar nuclei on the contralateral side much more often than on the ipsilateral one.

Thin-section and freeze-fracture studies of crayfish stretch receptor synapses including the reciprocal inhibitory synapse.

The crayfish slow-adapting abdominal stretch receptor organ is innervated by three inhibitory and several excitatory axons. A previous study by Tisdale and Nakajima ('76) showed that under certain fixation conditions inhibitory and excitatory synapses can be distinguished on the basis of synaptic vesicle structure. Using this morphological criterion we describe six types of synapses in the receptor: (1) the inhibitory axo-dendritic synapse, (2) the excitatory neuromuscular synapse, (3) the inhibitory neuromuscular synapse, (4) the axo-axonic synapse which suggests presynaptic inhibition of the excitatory synapse, (5) the axo-axonic synapse which suggests presynaptic inhibition on the inhibitory synapse, (6) the reciprocal inhibitory axo-axonic synapse, which is a new type of synapse. The presence of these six types of synapse suggest that inhibitory and excitatory axons interact synaptically in a complicated manner, resulting in a delicate control of receptor function. In freeze fracture we have observed the presynaptic membrane structures of inhibitory and excitatory synapses. The active zone of the inhibitory synapse has ridges with loosely aggregated particles on the tops of the ridges and indentations (vesicle attachment sites) along their sides. The active zone of the excitatory neuromuscular synapse consists of bands of particle aggregates which are situated on slightly elevated membrane regions and surrounded by wide, relatively particle-free, flat membrane areas.