| Abstract: | The immobility reflex, more commonly called animal hypnosis, is a state of involuntary immobility characterized by decreased muscle tone, inhibited spinal reflexes, and relative unresponsiveness to external stimuli. The immobility reflex (IR) is induced in rabbits by rapid inversion into a supine position and holding in this position for several seconds. This study was undertaken to clarify the mechanisms involved in induction and maintenance of the IR. Bilateral labyrinthectomies never prevented induction and had no consistent influence on IR duration. Integrated multiple-unit activity (action potentials from a small pool of neurons) from the medial portion of the medullary reticular formation, normally increased during the IR relative to wakefulness, increased during IR after labyrinthectomy. Vestibular input, therefore, has no significant effect on the IR. After hair removal from the backs and sides of rabbits, the IR durations decreased markedly, and in one animal the state could no longer be induced. Therefore, tactile receptors are a principal cause of induction of the IR. Rabbits were also treated with a tranquilizer, promazine, before and after hair removal since promazine normally enhances IR durations. Promazine still enhanced IR durations after hair removal, indicating that its effects are probably due to non-specific motor depression rather than to direct enhancement of IR mechanisms. ... |
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