Paraventricular nucleus of the hypothalamus: an electrophysiological investigation of neurons projecting directly to intermediolateral nucleus in the cat.

Experiments were done in chloralosed, paralyzed and artificially ventilated cats to identify electrophysiologically single units in the paraventricular nucleus of the hypothalamus (PVH) projecting directly to 'cardiovascular' responsive sites in the region of the intermediolateral nucleus (IML) in the upper thoracic cord. Action potentials evoked antidromically by electrical stimulation of the IML were recorded from 41 histologically verified single units in the PVH. Single units responded with a mean latency of 43.7 +/- 5.6 ms (range, 16-230 ms) corresponding to a mean conduction velocity of 4.1 +/- 0.2 m/s (range, 0.6-7.4 m/s). Single units were located primarily in the medial aspect of the anterior component, and in the anteromedial aspect of the dorsal component of the PVH. These data provide electrophysiological evidence of a direct pathway consisting of unmyelinated and small myelinated fibers, from neurons in the PVH to the IML, and suggest that this pathway is involved in the control of the circulation.