Changes in R‐R interval at the start of muscle contraction in the decerebrate cat.

Abstract

SUMMARY1. The effect on R-R interval of a brief hindlimb contraction, elicited by electrical stimulation of L7 ventral roots, was investigated in decerebrate cats. The first series of experiments was performed at both low and high carotid sinus pressure to vary the level of vagal tone. When carotid sinus pressure was elevated to increase vagal tone, contraction commenced 1 s later.2. The change in R-R interval at low carotid sinus pressure was expressed as the difference between the mean of the five R-R intervals immediately preceding contraction and the mean of the last five R-R intervals at the end of a 5 s contraction. At high carotid sinus pressure, the change was expressed as the difference between the mean of the last five R-R intervals at the end of a 5 s contraction and the mean of five R-R intervals at an equivalent time after raising pressure alone.3. Hindlimb contraction at low carotid sinus pressure produced a significant reduction in R-R interval from 359+25 (mean+s.E.M. n = 8) to 336+24 ms (P < 0005). At high carotid sinus pressure the response was enhanced with contraction producing a reduction in R-R interval from 474 + 45 to 419 + 47 ms (P < 0-001).4. The shortening of R-R interval produced by hindlimb contraction at high carotid sinus pressure, 55 + 8 ms, was significantly greater than that observed at low sinus pressure, 23 + 5 ms (P < 0-001, n = 8, paired t test). This pattern of response was also seen at stimulation frequencies as low as 10 Hz.5. In a second series of experiments, designed to determine the latency of the cardiac acceleration, the minimum latency between the onset of L7 ventral root stimulation and the end of the first shortened R-R interval was 687 + 29 ms (n = 5).6. Atropine (0-4 mg kg-', i.v.) prevented a 5 s contraction from producing any change in R-R interval.7. These results indicate that afferent information originating from receptors in contracting muscles is responsible for producing an immediate shortening of R-R interval, which is mediated by vagal withdrawal. The possibility that the shortening of R-R interval at the start of contraction is linked to a reduction in arterial baroreceptor reflex sensitivity, possibly via inhibitory effects on neurones forming the central pathway of the baroreceptor reflex, is discussed.

Exercise and diving, two conflicting stimuli influencing cardiac vagal tone in man.

“…We interpret this as indicating that the inputs associated with muscle contraction are not gating the trigeminal input but rather are algebraically summating with it. This is an arrangement unlike that proposed for the baroreceptor reflex pathway where arousal and possibly exercise block transmission of the primary afferent activity early in the reflex pathway (Jordan et al 1988;McMahon & McWilliam, 1992). Bergman et al (1972) tested the heart rate response to immersion of the face in water with dynamic exercise on a bicycle ergometer or with isometric exercise (hand grip).…”

Spontaneously hypertensive rats exhibit altered cardiovascular and neuronal responses to muscle contraction

“…However, the exact impact arterial baroreflexes may have had on the cardiovascular and neuronal responses to muscle contraction is clouded by findings showing that muscular contraction alters baroreflex-mediated regulation of cardiovascular function (McWilliam & Yang, 1991;McMahon & McWilliam, 1992;Raven et al 1997).…”

Training-related changes in the R-R interval at the onset of passive movements in humans

“…At the onset of exercise, there is an immediate and rapid decrease in R-R interval mediated purely by the abrupt withdrawal of cardiac vagal activity (1)(2)(3). This instantaneous decrease in cardiac vagal activity is induced by central signals from the higher brain (central command) and by neural feedback arising from mechanically sensitive skeletal muscle receptors (the muscle mechanoreflex) (4)(5)(6).…”