Evoked responses in brain motor areas during the immobility reflex (“Animal hypnosis”)

Abstract

The immobility reflex (IR) is a behavior in which spinal reflexes and movements are profoundly inhibited. Subcortical motor systems control this behavior, and two important motor areas, the medullary reticular formation (MRF) and caudate nucleus, were studied for changes in excitability during the IR. Single and paired stimulation of several sites (caudate, septum, hippocampus, and MRF) were used to evoke responses which were averaged by computer. In all stimulus conditions, MRF and caudate responses contained distinct and relatively short-latency components, suggesting relatively direct interconnections, although neuroanatomical evidence for some of these paths is scanty. Under paired stimulus conditions, the early components of 2nd responses were conspicuously attenuated in MRF responses to septal and hippocampal stimulation and caudate responses to septal and MRF stimulation. Delayed, or secondary, components of responses were notably indistinct and inconspicuous in all MRF responses, whereas they were conspicuously distinct and sustained during paired stimulation in caudate responses to septal and hippocampal stimulation. Unexpectedly, in no case did the IR clearly affect amplitude or waveform of the initial components of the responses. However, the delayed components were always altered by the IR. This and other evidence suggests that the IR is a behavior that is characterized by profound inhibition of movement without corresponding decrease in primary excitability and discriminative capacity, although secondary neural processing reactions are affected in a wide variety of neural systems.