The mechanisms of differential control in the sympathetic system studied by hypothalamic stimulation

In an attempt to study the mechanisms of differential action observed in the autonomic nervous system, response patterns evoked in two sympathetic nerves by stimulation of the hypothalamus were investigated. Recordings were made from inferior cardiac and vertebral nerves (mainly vasoconstrictors of the forearm muscles), both originating from stellate ganglia, in chloralose anesthetized cats. Repetitive stimulation of various hypothalamic regions produced 4 types of responses often in single preparations: (1) a response in which differential action between cardiac and vertebral nerves was characterized by an increase in vertebral nerve activity and a decrease in cardiac nerve discharges; (2) a response in which the activities of these two nerves were opposite to that described above; (3) a generalized response of the two nerves; both vertebral and cardiac nerve activity being greatly augmented. This response was accompanied by a great increase in forearm blood flow and could be classified as a part of "defense reaction"; and (4) another type of generalized response in which augmented activity of both nerves was accompanied by a decrease in muscle blood flow. Stimulation of the same hypothalamic area with a single or a short train of pulses evoked in sympathetic neurons an excitation of a short duration, followed by a long-lasting "silent period". The magnitude and duration of these two phases of responses of cardiac and vertebral nerves differed, depending on the site of stimulation. Stimulation of certain hypothalamic points could affect discharges of the left and the right cardiac nerves differently in some animals. The mechanism of differential control in the sympathetic system and some conditions which alter this pattern was discussed.