Centrally evoked sympathetic discharge: a functional study of medullary vasomotor areas

Abstract

SUMMARY1. The medulla oblongata was stimulated electrically in decerebrate cats following bilateral vagotomy. Changes in splanchnic nerve discharge evoked by stimulation of pressor and depressor areas in the medulla are correlated with changes in arterial pressure. An interaction of baroreceptor afferents with the efferent sympathetic system at the level of the medulla and/or spinal cord is demonstrated. Evidence is presented which suggests that it is possible to alter activity selectively in a given division (splanchnic) of the sympathetic outflow.2. Evoked responses were categorized as follows: (a) Sustained splanchnic activation; an immediate increase in total splanchnic activity occurred with only a small, gradual decline in activity during the stimulation period. Mild stimulation ,tA) raised systolic pressure by 60-80 mm Hg and stronger stimulation (300-400 ,uA) increased pressure by 100-150 mm Hg. When comparable pressor responses were elicited from points on either side of the mid line, splanchnic excitation was greater with homolateral stimulation (left side).(b) Unsustained splanchnic activation; an immediate increase in activity occurred which declined sharply as systolic pressure rose by 60-100 mm Hg during stimulation. Such a decline was attributed to the influence of baroreceptor afferents. Thus, if previous carotid occlusion was maintained during stimulation, the decline in activity was minimized although the pressor response increased. It is suggested that the structures yielding unsustained responses differ from those yielding sustained responses in their functional relation with the baroreceptor afferents.(c) Reduced splanchnic discharge during direct medullary stimulation;22-2 NORMAN KAHN AND ELLIOTT MILLS a partial or complete inhibition of splanchnic activity was obtained, which was sometimes accompanied by a 20-40 mm Hg reduction in systolic pressure. However, the direction and magnitude of the systemic pressure change were not as consistently related to the splanchnic response as they were during activation. Even if splanchnic efferent activity were elevated by carotid occlusion, stimulation still reduced discharge.

The effects of stimulation of the left atrial receptors on sympathetic efferent nerve activity

“…Thus this reflex activation of the sympathetic system is highly specific and differential. Previous reports of differential activation of the sympathetic system (Folkow, Johansson & Lofving, 1961;Folkow, 1962;Johansson, 1962;Green & Heffron, 1966;Kahn & Mills, 1967;Brender & Webb-Peploe, 1969) have described changes in the same direction but of differing magnitude. So far as we are aware, the present study is the first in which directionally different activation of fractions of the efferent sympathetic nerves has been produced by a specific physiological stimulus.…”

Pulse and Respiratory Grouping of Sympathetic Impulses in Human Muscle Nerves

“…Second, the pulse-and respiratory driving of the bursts creates a most characteristic pattern of "spontaneous" activity, previously seen and described by many workers recording from various sympathetic nerves in the cat (for ref. see Heymans and Neil 1958, Kahn and Mills 1967, Biscoe and Purves 1967 but never encountered in somatic efferent nerve fibres. To our knowledge, there are no reports concerning spontaneous activity of this type recorded from sympathetic fibres in peripheral muscle nerves of the cat but one might expect that an adequate search for such activity would be rewarding.…”

Central neural mechanisms of the cerebral ischemic response. Characterization, effect of brainstem and cranial nerve transections, and simulation by electrical stimulation of restricted regions of medulla oblongata in rabbit.

“…In many studies by other workers the medulla has been stimulated in a similar fashion and arterial pressure changes measured (Wang and Ranson, 1939;Alexander, 1946;Amoroso et al, 1954;Chai and Wang, 1962;Kahn and Mills, 1967;Gootman and Cohen, 1971;Coote et al, 1973;Trouth et al, 1973). However, our approach differs from previous work in two major respects.…”

“…First, whereas in some studies a single parameter of regional vasomotor activity was measured, such as, for example, splanchnic nerve activity (Kahn and Mills, 1967;Gootman and Cohen, 1971), cardiac sympathetic activity (Alexander, 1946), or skeletal muscle blood flow (Coote et al, 1973), in none of these studies were regional vasomotor changes measured within more than one vascular bed in response to stimulation of the medulla. Second, in nearly all previous studies either heart rate was not recorded (Wang and Ranson, 1939;Alexander, 1946;Amoroso et al, 1954) or, if recorded, the vagi were cut (Chai and Wang, 1962;Kahn and Mills, 1967;Gootman and Cohen, 1971). Since the regional vasomotor and heart rate changes evoked by cerebral ischemia were so characteristic and stereotyped, and differed so clearly from those associated with other pressor responses, such as the defense reaction (Coote et al, 1973) or the response to carotid occlusion (McGiff and Aviado, 1961), it was necessary to measure these variables as well as arterial pressure.…”