Central connections of aortic depressor and carotid sinus nerves

Abstract

Medullary vasomotor regions of the cat were explored for units responding with changes in discharge frequency to stimulation of the carotid sinus nerve or aortic depressor nerve. Neurons excited after a short (< 10 msec) latency by stimulation of the aortic depressor nerve or carotid sinus nerve were located in the regions of nucleus tractus solitarius and medial or paramedian reticular formation. Less than half of these neurons were excited by stimulation of the vagus nerves or the dorsolateral columns of the spinal cord. Inhibition, seen either as cessation of spontaneous discharge or as IPSPs, often followed the short latency excitation from the aortic depressor nerve or carotid sinus nerve and was of short (< 40 msec) duration. Long latency (> 10 msec) excitatory responses and complex excitatory-inhibitory patterns were evoked in neurons of the medial, paramedian and lateral ventral reticular areas by stimulation of the carotid sinus nerve and aortic depressor nerve. Most of these neurons also were influenced orthodromically by vagal nerve and dorsolateral column stimulation. A frequently occurring pattern in response to stimulation of all sites was excitation followed by a long period (> 100 msec) of inhibition of spontaneous discharge. Of the neurons in the lateral ventral reticular region, over half were fired antidromically from the ipsilateral dorsolateral columns. The responses of these neurons to stimulation at all sites resembled the responses to the same inputs previously recorded from sympathetic preganglionic and postganglionic nerves. Thus, it was tentatively concluded that this group of lateral ventral reticular neurons is the final common pathway to preganglionic sympathetic neurons in the intermediolateral columns of the spinal cord. Of the few cardio-synchronous neurons encountered, some had discharge patterns similar (in timing) to those seen in the carotid sinus nerve or aortic depressor nerve but others displayed the inverse of this pattern. This observation, together with the protracted postexcitatory inhibition commonly seen in many of the neurons in the vasomotor regions, could account for considerable modification of the primary afferent signal and may help to explain why cardio-synchronous neurons are so rarely found in the medulla.