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PubMed1976

Olfactory epithelium of Necturus maculosus and Ambystoma tigrinum.

The morphological study presented here provides a general description of the elements of the olfactory epithelium in the mud puppy and tiger salamander,, and gives evidence about their dynamic activity and interrelationships. There are morphological indications of local bursts of reduplication and a continual line of differentiation of receptor cells from basal cell progenitors through stages of mature development to senescence (indicated by the accumulation of pigment granules) and cell death and disposal (by expulsion of pycnotic cell nuclei and by phagocytosis by macrophages). The supporting cells probably play several roles: a secretory role which supplements the activity of Bowman's glands, a minor insulating role in which some dendrites are shielded from the surrounding milieu, and a skeletal role in which they facilitate the efficient displacement of dendrites. The dendrites are regularly arranged in organized relationships with one another and are for the most part in direct apposition, separated only by a 200 A intercellular gap, thus suggesting the possibility of functional interrelationships. This study emphasizes the fact that efficient planning of experimental investigations must include knowledge and consideration of the thickness of the particular olfactory epithelium under study. It also suggests that because of the large receptor-cell size, the mud puppy and/or tiger salamander would make good model systems for single cell recording. Further, the olfactory epithelia of these species are suggested as favorable targets for studies of the aging process in nerve cells.

Ambystoma↗
PubMed1976

Visual identification of two kinds of nerve cells and their synaptic contacts in a living autonomic ganglion of the mudpuppy (Necturus maculosus).

1. Many of the nerve cells comprising the cardiac parasympathetic ganglion of the mudpuppy are spread out in a thin, transparent sheet of tissue, enabling one to see cellular details in living preparations with differential interference contrast optics. The aim of this study was twofold: to establish the morphology of the nerve cells and their synaptic connections by light and electron microscopy, and to determine which aspects of the ganglion's structure could be reliably identified in the living tissue. 2. There are two types of neurones in the ganglion: (a) principal cells that send post-ganglionic axons to cardiac muscle fibres, and (b) interneurones whose processes are confined to the ganglion. 3. Interneurones are distinguished from principal cells by the presence of numerous granular vesicles seen with the electron microscope, and by intense formaldehyde-induced fluorescence. The interneurones are thus similar to catecholamine-containing interneurones in autonomic ganglia of other vertebrates. 4. Principal cells are innervated by processes that terminate mainly on the cell body, forming up to forty-five synaptic boutons and covering, on the average, 5% of the perikaryal surface. The synaptic terminals are derived from three sources: (a) axons from the vagus nerves, (b) interneurones and (c) other principal cells. Vagal terminals contacting principal cells contain agranular vesicles typical of preganglionic cholinergic endings. At regions of contact between processes of interneurones and principal cells, the interneurones have granular vesicles focused at membrane specializations; in addition there are small areas of close plasma membrane apposition, probably gap junctions. Some of the contacts between principal cells are characterized by gap junctions; others are structurally similar to vagal endings but persist after vagal degeneration. 6. Interneurones are innervated by axons that make contact mainly with their processes. The axon terminals on processes of interneurones contain agranular vesicles similar to vagal terminals on principal cells. 7. In live preparations principal cells are distinguished from interneurones by their size and the appearance of their organelles. Synaptic contacts on principal cells could often be identified and, in some cases, large contacts from interneurones or those from other nearby principal cells could be traced back to their cell bodies of origin. The validity of these identifications was confirmed by subsequent electron microscopic examination of the same cells.

Animals↗
PubMed1976

An electrophysiological study of chemical and electrical synapses on neurones in the parasympathetic cardiac ganglion of the mudpuppy, Necturus maculosus: evidence for intrinsic ganglionic innervation.

1. The cardiac ganglion of the mudpuppy is situated on a thin sheet of tissue. Two nerve cell types can be distinguished readily in the living preparation - principal cells and smaller interneurones which synapse with the principal cells. The purpose of this study was to investigate synaptic transmission and the functional organization of neuronal connections of ganglion cells with intracellular micro-electrodes. 2. Stimulation of the preganglionic, vagus, nerves evoked a large excitatory response in principal cells. About three quarters of these neurones were innervated by a single vagal axon. The remaining cells received two or more preganglionic nerve fibres. 3. The quantum content of vagal excitatory post-synaptic potentials (e.p.s.p.s) was measured. Normally, the e.p.s.p. was suprathreshold and consisted of about twenty-two quanta, whereas only about nine quanta were required to reach threshold and initiate an action potential. 4. Intracellular stimulation of principal cells evoked e.p.s.p.s in neighbouring principal cells. The responses were blocked by cholinergic antagonists. These potentials were caused by excitation of principal cell axon collateral synapses. 5. Principal cells also formed electrical junctions with each other. These electrical junctions were very weak. Although they transmitted slow potential changes, only a small response was recorded in one cell when an electrically coupled neighbouring cell fired an impulse. The resistance of the electrical junction between principal cells was calculated to be about 5-8 X 10(8) omega. 6. Stable penetrations of interneurones were only rarely achieved, making it difficult to study their functional relationship to principal cells. Action potentials were recorded from interneurones in a few instances. 7. These data demonstrate that parasympathetic ganglion cells in the heart of the mudpuppy receive innervation from more than one source involving both chemical and electrical synapses, and that some of the synapses are intrinsic to the ganglion.

Action Potentials↗