The hypothesis that the nexus is a specialized structure allowing current flow between cell interiors is corroborated by concomitant structural changes of the nexus and changes of electrical coupling between cells due to soaking in solutions of abnormal tonicity Fusiform frog atrial fibers are interconnected by nexuses The nexuses, desmosomes, and regions of myofibrillar attachment of this muscle are not associated in a manner similar to intercalated discs of guinea pig cardiac muscle Indeed, nexuses occur wherever cell membranes are closely apposed Action potentials of frog atrial bundles detected extracellularly across a sucrose gap change from monophasic to diphasic when the gap is shunted by a resistor This indicates that action potentials are transmitted across the gap when sufficient excitatory current is allowed to flow across the gap When the sucrose solution in the gap is made hypertonic, propagation past the gap is blocked and the resistance between the cells in the gap increases Electron micrographs demonstrate that the nexuses of frog atrium and guinea pig ventricle are ruptured by hypertonic solutions

/pdf/propagation-of-action-potentials-and-the-structure-of-the-5fbspehd37.pdf

Propagation of Action Potentials and the Structure of the Nexus in Cardiac Muscle

Nexuses, that is, fusions of plasma membranes of adjacent cells, are described in mammalian smooth and cardiac muscle, median giant axon of earthworm, frog skin, and rat submandibular gland. In smooth muscle they usually occur where a process from one cell either meets a process of, or projects into a neighboring cell. On the other hand, in mammalian heart muscle and in earthworm giant axon the nexuses occur along the intercalated disc and intercellular segmental septa, respectively. Their occurrence between these excitable cells is correlated with propagation of action potentials by an electrical rather than chemical mechanism. Since the nexuses may offer pathways for electric current between cell interiors, it seems possible that they constitute a link in the structural basis for electrical transmission in these systems. In epithelia, nexuses usually appear as part of a terminal bar complex. This is true in the rat salivary gland studied here. In the epidermis of frog skin, nexuses are less numerous between the basilar columnar cells than between the subjacent squamous cells. The nexuses which occur in epithelia in frog skin and rat salivary gland are distributed as though to provide seals against electrochemical backleaks and sites of chemical exchange between cell interiors.

/pdf/a-study-of-the-structure-and-distribution-of-the-nexus-3wem1gd8x1.pdf

A study of the structure and distribution of the nexus.

The ciliary ganglion of the bird is of particular interest because of the unusual morphology of its synaptic connexions. At many of these a single presynaptic fibre ends on the ganglion cell in the form of a cup-like termination, or calix. The calix embraces the ganglion cell near its axonal pole, sending out large finger-like processes extending for a considerable length over the cell body. On other cells, presynaptic fibres end in diffusely branching terminal networks. The calices were first reported by Carpenter (1911) as being the most abundant type of ending present in ciliary ganglia obtained from hens, ducks and pigeons. Similar endings were described by Terzuolo (1951) as being present only in young birds. Later they were the subject of investigation with the electron microscope by de Lorenzo (1960). De Lorenzo's electron micrographs of preparations from young chicks suggested that the calices might be large enough to permit penetration by a micropipette without undue injury, thus permitting direct intracellular recording from presynaptic nerve terminals. The physiology of the ganglion has received scant attention. Consiglio (1900) reported that transmission through it was not blocked by nicotine, in contrast to the mammalian ciliary ganglion (Langley & Anderson, 1892) and Langendorff (1900) noted that transmission through the avian ganglion was unusually persistent following death of the animal by bleeding. Carpenter (1911) suggested that persistence of ganglionic transmission in these adverse circumstances might be related to the presence of caliciform nerve terminals. In the experiments to be reported here records were obtained from both ganglion cells and presynaptic nerve terminals. The results indicated that at many of the synapses there was, in addition to chemical synaptic transmission, direct electrical coupling between presynaptic and post-synaptic elements and that this coupling was bi-directional. Brief reports of these findings have appeared elsewhere (Martin & Pilar, 1962a, b).

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1963.sp007202

Dual mode of synaptic transmission in the avian ciliary ganglion

Identified neurons and behavior of arthropods

A type of excitatory synaptic transmission which is novel for the vertebrate brain has been found in the ner neuron (M-cell) by means of the passive spread of their action currents across the synaptic membrane. After stimulating the ipsilateral eighth cranial nerve, an excitatory postsynaptic potential (EPSP) appears in the M-cell with a latency which is very brief ( about 0.1 msec) and which proba; bly represents a negligible synaptic delay. This response is attributed to the club endings: there were steep gradients of potential along the lateral dendrite of the M-cell during activity and the early EPSP was maximal in the distal part of the dendrite where the club endings predominate. Potential changes in the M-cell spread (passively) backwards into certain eighth-nerve fibers (probably club endings) indicating the presence of special low-resistance connections between them and the M-cell.

http://science.sciencemag.org/content/sci/144/3620/878.full.pdf

H. Grundfest

Papers

Postsynaptic electrogenesis in septate giant axons. I. Earthworm median giant axon.