Ventral medullary relay neurones in the pathway from the defence areas of the cat and their effect on blood pressure.

Abstract

In cats anesthetized with Althesin, the efferent descending pathway from the brain-stem defense areas was traced through the medulla by identifying sites at which electrical stimulation evoked the characteristic pattern of the visceral alerting (defense) response. This response includes an increase in arterial blood pressure resulting from increased heart rate and cardiac output and vasoconstriction in renal and splanchnic beds, accompanied by active vasodilatation in skeletal muscle. The efferent pathway runs as a narrow strip, .apprx. 3 mm from the mid line, ventral to the superior olive and the nucleus of the trapezoid body, extending caudally to the rostral portion of the inferior olive where it lies ventral to the facial nucleus. It lies very close to the ventral medullary surface just rostral to and within the area at which bilateral topical application of glycine results in a profound fall in arterial blood pressure and cessation of respiration. On bilaterial application of glycine to the sensitive area of the ventral medulla, the visceral alerting response evoked by stimulation in the defense areas of the amygdalo-hypothalamic complex, or the mid-brain central grey or tegmentum, was attenuated in parallel with the fall in arterial pressure, the vasoconstrictor responses being most strongly reduced. As soon as arterial blood pressure had fallen to its lowest level the visceral alerting response was virtually abolished. A small radio-frequency lesion made in the ventral medullary efferent pathway, in the rostral part of the glycine-sensitive area, had the same effect as that produced by unilateral application of glycine: it resulted in little respiratory or cardiovascular effect itself, but application of glycine to the contralateral area then produced the full effect otherwise seen only on bilateral application of glycine. The effects of glycine result from blockade of a synaptic relay, close to the ventral surface of the medulla, in the efferent pathway from the defense areas to the preganglionic sympathetic neurons. The neurons which receive an input from the alerting (defense) areas normally provide an essential, tonic excitatory drive to the sympathetic output and probably to respiration also. After sudden withdrawal of this drive, vasomotor tone and the normal level of arterial blood pressure are not maintained.