Electrophysiology of caudal neurosecretory cells in the skate and fluke

The caudal neurosecretory cells of two species of skate (Raja erinacea and R. ocellata) and of the fluke (Paralichthys dentatus) were studied by intracellular microelectrode techniques. The cells were capable of producing spikes and of being synaptically activated. The neurosecretory cells of the skate were intermingled with other neurons within the spinal cord and could not be penetrated under visual control. however, a class of responses was recorded that were readily distinguishable from those of motoneurons and myelinated fibers and which very likely came from the much larger neurosecretory cells. The spikes were longer in duration and had pronounced undershoots. Nearby stimulation on the surface of the cord activated the cell bodies directly rather than evoking antidromic spikes. Brief stimuli, intra- or extracellular, could produce direct spikes of very long latency, thus indicating a low degree of accommodation. The presynaptic pathway was slowly conducting and of high threshold. The neurosecretory cells of the fluke were anatomically isolated from other cell bodies, but recordings from axons and somata had to be distinguished electrophysiologically. Recordings from cell bodies were characterized by the occurrence of postsynaptic potentials, which could be adequate to initiate spikes. The spikes were rather long in duration and usually followed by an undershoot. Those in response to antidromic stimulation had an inflection on the rising phase caused by delay of propagation at the axon hillock. Invasion of the soma could be blocked by hyperpolarization or refractoriness, leaving the axonal component of the spike. The threshold in the initial part of the axon was not significantly lower than in the soma. There was little accommodation, as indicated by the long latencies of responses to brief threshold stimuli. The antidromic responses in a cell often had several distinct latencies when stimulation was over the urohypophysis. The properties of the longer latency responses suggested that they were initiated in the terminals of the axons and that the delay resulted from very slow propagation in these structures. Antidromic conduction could be blocked in the axon at points some distance from the soma as well as at the axon hillock. Spikes recorded from the neurosecretory axons were similar in duration to those from the cell bodies. They were characterized by the occurrence either of synaptically evoked spikes without recording of p.s.p.'s adequate to iniate them or of direct responses to stimulation on each side of the recording site. Hyperpolarization could cause block of propagation along the axon which was associated with block of a later component of the recorded spike. The presynaptic pathway ran anteriorly to the neurosecretory cells. Its conduction velocity suggested that it consisted of small myelinated fibers. The individual presynaptic fibers appeared to innervate both rostral and caudal cells. Intravenous injections of hypotonic solutions could cause sufficient activity in the presynaptic fibers to initiate impulses in the neurosecretory cells.