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

Abstract

SUMMARY1. In cats anaesthetized with Althesin, the efferent descending pathway from the brain-stem defence areas has been traced through the medulla by identifying sites at which electrical stimulation evoked the characteristic pattern of the visceral alerting (defence) 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.2. The efferent pathway runs as a narrow strip, about 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 was found to lie 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.3. On bilaterial application of glycine to the sensitive area of the ventral medulla, the visceral alerting response evoked by stimulation in the defence 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.4. 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.5. It is suggested (1) that 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 defence areas to the preganglionic sympathetic neurones, and (2) that the neurones which receive an input from the alerting (defence) 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.

Attenuated defense response and low basal blood pressure in orexin knockout mice

“…However, previous studies enabled us to suppose key sites where orexin acted as a mediator of defense response. The most apparent site seems to be the RVLM, since inhibition of RVLM or subjacent ventral surface of the medulla (glycine-sensitive area) attenuated sympathetic and defense responses evoked by stimulation to the lateral hypothalamic area (12,24,52). A doublevirus transneuronal labeling technique revealed that not only perifornical neurons but also RVLM neurons provided a dual input to the sympathetic outflow systems that regulate cardiac and adrenal medullary functions, indicating these neurons were the "central command neurons" of defense response (28).…”

Central respiratory modulation of subretrofacial bulbospinal neurones in the cat.

“…nucleus has been shown by chemical inhibition or lesions to contain neurones essential for the maintenance of blood pressure as well as centrally or reflexly-evoked pressor responses in anaesthetized animals (Guertzenstein & Silver, 1974;McAllen, Neil & Loewy, 1982;Hilton, Marshall & Timms, 1983). In this respect it behaves like a 'final common pathway ' (McAllen, 1985).…”

Chemoreceptors and cardiovascular control in acute and chronic systemic hypoxia

“…In cats and dogs it was largely mediated by sympathetic cholinergic fibres, but was attributable also to inhibition of sympathetic noradrenergic activity to muscle and to the dilator influence of circulating adrenaline. This cardiovascular pattern of response was accompanied by other autonomically mediated changes including pupillary dilatation, retraction of the nictitating membrane, piloerection and even urination and defaecation (33)(34)(35)(36)(37).…”

“…They concluded from experiments in which brain sites were stimulated electrically that the regions that integrate the defence response run from the amygdala via an efferent pathway to the ventral hypothalamus, that a contiguous or further region runs through the central grey matter of midbrain and medulla and that all of these regions feed through a single pathway that runs through the ventral medulla. More recently, this pathway was found to synapse onto the neurones of the rostral ventrolateral medulla (RVLM) that is now recognised as being of major importance in setting the resting level of vasomotor tone and in integrating the major cardiovascular reflexes (35)(36)(37). The pattern of response that was evoked in anaesthetised animals by electrical stimulation within the integrating regions for the defence response and which was, in fact, crucially important in allowing these regions to be identified, characteristically included a rise in arterial pressure, tachycardia, vasoconstriction in cutaneous renal and splanchnic regions, but vasodilatation in skeletal muscle.…”