A study of the immobility reflex by experimental and mathematical techniques

Abstract

The Immobility Reflex (IR ), also known as animal hypnosis and commonly exhibited by many species of animals, is a state of involuntary immobility characterized by decreased muscle tone, inhibited spinal reflexes and relative unresponsiveness to external stimuli. It is completely and spontaneously reversible and can be induced in rabbits in a laboratory by rapid inversion in to a V-shaped trough and holding in this position for several seconds. This study was undertaken to elucidate and to be able quantitatively to describe the interrelation between the mechanisms involved in the reflex. An experimental set up was used in which the proprioceptive input was kept constant while the tactile input was varied. Spontaneous durations of IR were remarkably consistent within and among rabbits (0.36 to 0.70 min /trial), where rabbits were "habituated" by 25 pre -trials and when IR termination was scored on the basis of any attempt, even abortive, at righting. Arousing (IR -disrupting) thresholds of electric current applied to the ear declined progressively as the duration of a given episode progressed. Throughout a given trial, hippocampal EEG theta activity persisted, but its frequency increased just prior to IR induction and decreased immediately afterwards. During IR, theta frequency increased progressively in short-duration trials but in long-duration trials it oscillated, with a period about the same as the "normal" (short) IR duration. These observations support a theory that when IR is triggered, reverberating circuits (in the reticular formation of the pons and medulla) are hyperactivated, and their output inhibits spinal motoneurons. The relatively uniform durations observed and the progressive decrease in IR depth could reflect the damping constant for the reverberatory (motor inhibitory) activity...