Reciprocal synapses between cholinergic postganglionic axon and adrenergic interneuron in the cardiac ganglion of the turtle

Morphology of Synapses at the Base of Hair Cells in the Organ of Corti of the Chimpanzee

The synaptic morphology of inner and outer hair cells of the organ of Corti of the chimpanzee was evaluated by serial section electron microscopy. The morphology of nerve terminals and synapses at both sites was very similar to that of human and other mammalian species. Two types of nerve terminals, nonvesiculated and vesiculated, with distinct synaptic morphology were found. In addition, between some nonvesiculated endings and outer hair cells, a reciprocal synaptic relationship was seen. In such terminals there was morphologic evidence for transmission from hair cell to neuron and from neuron to hair cell between a single neuron and an outer hair cell.

Patterns of innervation of outer hair cells in a chimpanzee: I. Afferent and reciprocal synapses

Three varieties of synaptic specialization, afferent, efferent, and reciprocal, have been demonstrated at the base of outer hair cells of one chimpanzee. The purpose of the present study was to investigate the innervation density of afferent and reciprocal synapses in the three rows and three turns of the organ of Corti. The data presented is based on light and electron microscopy from one aged chimpanzee using serial section electron microscopy. Afferent fibers make contact with outer hair cells as either terminal swellings or en passant contacts. In addition to membrane specialization, presynaptic bodles were present at the majority of afferent synapses. The mean innervation density of afferent endings was highest in the middle turn. Reciprocal endings were found on 74% of all outer hair cells. The average number of reciprocal endings increased from the base to apex and, except in the apical turn, from the first to third row of outer hair cells. On the basis of morphological criteria, endings with a reciprocal synapses were more similar to afferent than to efferent endings.

Synaptic morphology of inner and outer hair cells of the human organ of Corti

Innervations of inner and outer hair cells of the organ of Corti of the human cochlea were studied by serial section electron microscopy. At the base of inner hair cells, presumed afferent fibers were of varying size and demonstrated synaptic specialization consisting of a presynaptic body, vesicles, and asymmetrical synaptic membrane specialization. Two types of neurons, vesiculated presumably efferent and nonvesiculated presumably afferent, synapsed at the base of outer hair cells. The synaptic specialization of afferent fibers included presynaptic body, vesicles, and asymmetrical membrane thickening, whereas efferent synapses demonstrated presynaptic vesicles and a subsynaptic cisterna. Some presumably afferent nerve terminals formed a reciprocal synapse with outer hair cells in both the human and the chimpanzee. Such a synaptic relationship demonstrated morphologic specialization consistent with both hair cell-to-neuron and neuron-to-hair cell transmission between the same outer hair cell and nerve terminal. The innervation density of inner and outer hair cells and the comparative anatomy of the afferent and efferent innervation are discussed.

Small, intensely fluorescent cells and the paraneuron concept

Sympathetic ganglia contain large principal nerve cells and, in addition, many smaller cells that resemble the endocrine cells of the adrenal medulla in morphology and chromaffinity. The advent of the formaldehyde-induced fluorescence technique proved to be an invaluable tool for studying this unique cell type, and it was this method that accounted for their descriptive name of small, intensely fluorescent cells, now universally abbreviated to SIF cells. Electron microscopy also proved of great importance in detailing the structure of SIF cells and their relationship with neighbouring neurones. Fine structural observations revealed that the cells contained numerous dense-cored granules, and this led to their electron microscopic name of small, granule-containing cells. SIF cells are most abundant, and very well studied, in the rat superior cervical ganglion, where they both receive and give synapses. Early researchers suggested that SIF cells were interneurones appropriately situated between pre- and postganglionic elements and thus capable of influencing ganglion signals. SIF cells also are known to exist in the form of richly vascularized, compact clusters of varying size. Clustered chromaffin cells do not necessarily give rise to processes that would contact the principal neurones. The existence of singly occurring as well as clustered SIF cells has given rise to a proposed designation of type I and type II cells, with I representing the interneuronal-like form and II possibly performing as an endocrine-like component. Despite a wealth of knowledge concerning SIF cells, their exact role(s) in the overall functioning of the autonomic nervous system is still not completely understood.

A light and fluorescence cytochemical and electron microscopic study of granule-containing cells in the intrapulmonary ganglia of Pseudemys scripta elegans

In the lung of the red-eared turtle, large numbers of intramural ganglia located in the intraparenchymal connective tissue are demonstrated. Numerous cells in close proximity to the principal ganglionic neurons displayed a bright blue-white formaldehyde-induced fluorescence. Microspectrofluorometric analysis revealed the presence of dopamine (DA) in all cells measured. Subsequent light histochemical staining of the fluorescent sections showed the DA-containing cells to display argentaffinity. Electron microscopy of serial sections revealed cells characterized by dense-cored vesicles corresponding to the intensely formaldehyde-induced fluorescent cells. The argentaffin technique performed directly on ultrathin sections selectively stained the dense-cored vesicles. After fixation with glutaraldehyde followed by dichromate, x-ray microanalysis showed the chromium to be incorporated into the dense granules. Cholinergic-type nerve endings formed axosomatic synaptic contacts with the DA-containing cells, which can therefore be considered as intrinsic postganglionic elements. No efferent synapses from the granule-containing cells to the principal ganglionic neurons could be observed. The granule-containing cells occurred solitarily and in clusters, partially invested with satellite cells, and usually located near fenestrated capillaries; they displayed cytoplasmic processes and indicated emiocytotic granule release. Adjacent granule-containing cells were separated by spaces about 20 nm wide, gradually widening to form intercellular channels with apically projecting microvilli and primary cilia. It is concluded that the intrapulmonary granule-containing cells of the red-eared turtle belong to the APUD system. Furthermore, morphologically these cells appeared to possess a special sensory apparatus which designates them as paraneurons. The possible physiological significance of these intrapulmonary granule-containing cells is discussed.

Incidence of reciprocal synapses on outer hair cells of the human organ of Corti

The incidence of reciprocal synapses at the base of outer hair cells of the human organ of Corti was studied by electron-microscopic serial section reconstruction. Reciprocal synapses were found in each of six ears from five individuals. A total of 1,386 serial sections and the neural poles of 25 outer hair cells in two individuals were examined. Fourteen of the 25 outer hair cells (56%) possessed reciprocal synapses. The incidence of reciprocal synapses increased from the first to the third rows of outer hair cells, as determined by the percentage of outer hair cells and afferent nerve terminals of each row demonstrating reciprocal synapses and the average number of reciprocal synapses per hair cell in each row. Hair cells with reciprocal synapses had more afferent and fewer efferent terminals per hair cell than hair cells without reciprocal synapses.

Nerve endings in the pulmonary trunk, ductus arteriosus and aorta of intact and decapitated pig fetuses

Nerve fibres reactive to acetyl-thiocholine, and tissues showing catecholamine fluorescence were examined in the pulmonary trunk, ductus arteriosus and aorta of 28 pig fetuses between 31 and 113 days of gestation (term = 114 +/- 1 days). Eight additional fetuses, which had been decapitated in utero at 40-43 days, were also studied at ages between 51 and 114 days of gestation. Spherical micro-networks of nervous tissue reactive to acetyl-thiocholine are present in the adventitia on the cranial aspect of the pulmonary trunk and ductus arteriosus, between the aorta and pulmonary trunk, and on the caudal aspects of the pulmonary trunk and the pulmonary arteries. These fibres invest spherical clusters of catecholamine containing cells which are well supplied with blood vessels. Nerve fibres which fluoresce are also found in association with these cells. Decapitation in utero does not appear to affect the distribution of morphology of these structures. The observations show that structures are present in the major arteries of the fetal pig which may act as sensory receptors, and that these structures are unaffected by chronic vagotomy of the fetus produced by decapitation early in gestation.

Synaptic organisation of the pelvic ganglion in the guinea-pig

A semi-quantitative electron-microscopic study of neuronal cell bodies, nerve profiles and synapses in the anterior pelvic ganglia of the guinea-pig has been carried out following in vivo labelling of adrenergic nerve endings with 5-hydroxydopamine. Ganglion cells of three main types have been distinguished: 1) the majority (about 70%) not containing granular vesicles, probably cholinergic elements; 2) those containing large granular vesicles of uniform size (80-110 nm), with granules of medium density and prominent halos; and 3) those containing vesicles of variable size (60-150 nm), with very dense eccentrically placed granular cores. Some non-neuronal 'granule-containing' cells were present, mainly near small blood vessels. Some 95% of the total axon profiles within the ganglia were cholinergic, the remaining 5% were adrenergic. However, 99% of synapses (i.e. axons within 50 nm of nerve cell membrane with pre- and post-synaptic specialisations) were cholinergic, and 1% were adrenergic. Only three examples of nerve cell bodies exhibiting both cholinergic and adrenergic synapses were observed. Unlike the para- and prevertebral ganglia, the pelvic ganglia contained large numbers of axo-somatic synapses. As many as 20% of the nucleated neuronal cell profiles displayed two distinct nuclei.

Reciprocal synapses at the base of outer hair cells in the organ of corti of man

Reciprocal synapses have been found between nerve terminals and the outer hair cells in the human organ of Corti. A single nerve ending of the nonvesiculated type may possess two types of synaptic specialization of opposite polarity. The first is typical of the "afferent" synapse with a presynaptic body in the hair cell and pre- and postsynaptic membrane thickening.. The second consists of a small collection of presynaptic vesicles in the neural cytoplasm near the plasma membrane facing the hair cell and a subsynaptic cisterna within the hair cell cytoplasm. The second type of specialization is similar to the synapses seen in "efferent" endings. This suggests that both an afferent (hair cell to neuron) and efferent (neuron to hair cell) synaptic relationship may exist between an outer hair cell and a single nerve terminal.

Reciprocal synapses between hair cells and first order afferent dendrites in the crista ampullaris of the bullfrog

Reciprocal synapses were found between afferent dendrites and hair cells of the crista ampullaris in the vestibular system of the bullfrog. The reciprocal synapse consisted of two components forming a dual synaptic complex between a single hair cell and a single dendritic process such that the receptor cell was both presynaptic and postsynaptic to the afferent dendrite. The first component, a typical afferent synaptic complex, consisted of a synaptic sphere surrounded by a single layer of clear vesicles, and was located within the hair cell. In addition, 3-4 arciform structures were located between the sphere and the plasma membrane of the hair cell. The second component of the reciprocal synapse, located approximately 1.5 micron from the first, was recognized by a localized accumulation of clear membrane vesicles within the neuronal process and a short membrane profile within the cytoplasm of the adjacent hair cell. Additional characteristic features included continuity of the arciform structures with the plasma membrane of the hair cell, and within the neuronal process, a greatly reduced number of vesicles when compared with the efferent axonal terminal. This latter was important in distinguishing a first-order afferent dendritic process from an efferent neuronal process. The possible functional significance of these reciprocal synapses has been discussed in terms of the two components having an opposite effect on the membrane polarization of the hair cell.

A special type of small granule-containing cell in the abdominal para aortic region of the frog

Light and electron microscopic studies have been made of a special type of small granule-containing cell (termed Type IV cell) in the frog abdominal para aortic region. These cells contain numerous dense granular vesicles (100--150 nm in diameter) and are considerably smaller (10--20 mu) than neighbouring nerve cells, although they have many features in common with them. They do not resemble chromaffin cells as do Types I, II and III cells. The cell bodies are completely ensheathed by satellite cells and are isolated from neighbouring cells of the same type. Type IV cells have long processes which usually become incorporated in bundles containing 2--20 processes, including some cholinergic nerve fibres, and are loosely enveloped by perineurium. The termination of the processes of Type IV cells do not appear to form efferent synapses on nerve cells at least within the para aortic region or in paravertebral sympathetic ganglia. A close topographical relationship is not found between these processes and blood vessels. It is suggested that the small Type IV granule-containing cells in the frog abdominal para aortic region are not interneurons or neurosecretory cells, but are a special type of sympathetic nerve cell.