Neural control of the heart: significance of double innervation re-examined.

Our recent studies on the activity of cardiac sympathetic and vagal nerves in anesthetized dogs have revealed that control of these two autonomic nerves having 'antagonistic' action on the heart is not always reciprocal; both reciprocal and non-reciprocal controls exist in reflex as well as in adaptive responses. We have evidence which also suggests the functional significance of non-reciprocal control. The co-activation of both autonomic efferents to the heart is particularly useful in increasing cardiac output as, for example, in the chemoreceptor reflex. On the other hand, under 'normal' conditions the respiration related rhythmic activity of both sympathetic and vagal nerves is almost completely reciprocal. The loss of the reciprocity occurs under subnormal conditions in which sympathetic discharges shift so that both nerves fire simultaneously. This tends to reduce respiration-induced fluctuations in heart rate. During cardiac rhythms seen in the efferent nerve activity, the peak of vagal firing does not strictly coincide with the inhibitory period of the sympathetic activity; vagal impulses reach the heart slightly earlier. However, when the heart rate is irregular and slow, the vagal activity shifts in time so that it coincides more definitely with the period of sympathetic nerve inhibition. This tends to further augment rhythmic fluctuations in heart rate. These studies show that the centers involved employ both reciprocal and non-reciprocal control of cardial nerve activity in reactions regulating heart function. The relationship between discharge patterns of both autonomic nerves is involved in production of fluctuation of heart rate in normal as well as in subnormal conditions.