Control of reciprocal and non-reciprocal action of vagal and sympathetic efferents: study of centrally induced reactions.

The mechanism of control of reciprocal and non-reciprocal action between parasympathetic and sympathetic nerves was investigated. Simultaneous recordings were made from both vagal and sympathetic nerves innervating the heart of responses evoked by hypothalamic stimulation in chloralose-anesthetized dogs. Four patterns of responses could be elicited by repetitive stimulations of various sites within the hypothalamus: (1) A reciprocal pattern of cardiac vagal and sympathetic discharges accompanied by a rise in blood pressure, heart rate and a 2 to 8-fold increase in muscle blood flow. The vagal activity completely ceased, while sympathetic discharges were greatly augmented. These changes occurred quickly and often lasted throughout the stimulation, preventing baroreceptor reflex from breaking through. This pattern is similar to the cardiovascular component of the "defense reaction". (2) A reciprocal pattern of discharges accompanied by a depressor response and bradycardia; the vagal discharges increased while those of the sympathetic efferents decreased. (3) A non-reciprocal pattern of response in which activity of the two efferents increased. The blood pressure was elevated and heart rate decreased. (4) A non-reciprocal pattern of action of the two efferents in which discharges of both nerves were depressed; the blood pressure decreased and heart rate increased. Factors affecting these responses patterns were found to be: (a) secondarily occurring reflex responses due to baroreceptor activation or deactivation. These baroreceptor-related reactions always showed reciprocal changes between vagal and sympathetic nerve activity; (b) the level of tonic activity of the autonomic nerves, e.g. during a high level of tonic activity inhibitory action became less effective while the excitatory effect was greater. Thus central states maintaining influenced the pattern of reaction and the relationship between activity in these two efferent nerves. Single pulse stimulations of the hypothalamic regions from which different patterns were evoked, when stimulated repetitively, yielded patterns which were basically reciprocal and biphasic; in vagus efferents an inhibitory phase was followed by an excitation of tonic activity, while in sympathetic an excitation was followed by an inhibition. The degree and duration of these excitatory and inhibitory phases differed, depending on the site of stimulation, but only to a minor degree, and the basic pattern remained unchanged. The implication of these findings is that the hypothalamus can organized both reciprocal and non-reciprocal patterns of activity in the two autonomic limbs in response to varied afferent stimuli. Probably only subtle changes in the central states or influences on neurons of the autonomic system suffice to result in different response pattern. No pattern of response is fixed within the central control complex.