Efferent discharges of sympathetic and parasympathetic nerve fibers during increased intracranial pressure in anesthetized cats in the absence and presence of pressor response

Efferent discharges of the cervical sympathetic cardiovascular and vagal type 1 fibers in response to increased intracranial pressure (ICP) were simultaneously recorded in cats anesthetized with pentobarbitone and ventilated artificially. Sympathetic outflow of renal nerve fibers was also recorded in some animals. The type 1 fibers were assumed to be cardiac vagal fibers, from the response behavior such a pulse-synchronicity to respiratory and heart rhythm, reflex activation from arterial baroreceptors and reciprocal relationship of the activity to sympathetic ones during slower fluctuations of hemodynamic changes, and which occur spontaneously during Mayer waves. The vagal type 1 discharges increased to various amplitudes with increase in ICP and in the absence and the presence of pressor response. Efferent outflow of the renal and cervical sympathetic fibers frequently decreased with a moderate increase in ICP. There was a slight decrease or no apparent change in the blood pressure, and a higher elevation of ICP ensued. Heart rates decreased with increase in ICP, while the rate frequently increased with levels of ICP over about 120 mm Hg. Changes in the vagal and sympathetic discharges always began at a time before the initiation of cardiovascular response to the elevated ICP. However, when ICP was repeatedly increased, the increase in vagal discharges progressively decayed and was accompanied by vigorous sympathetic firings and a marked pressor response. The sympathetic outflow also decayed following the decrease in vagal activities. The present findings of changes in the vagal type 1 discharges demonstrate clear participation of parasympathetic as well as sympathetic nerve activity in the occurrence of cardiovascular responses to increased ICP. Changes in both these autonomic nerve responses may explain the initial fall in arterial blood pressure and pressor responses associated with bradycardia or tachycardia, at different levels of elevated ICP.