Neurophysiological mechanisms involved in tonic immobility (TI).

This chapter summarizes the main neurophysiological characteristics of tonic immobility (TI), in many susceptible species of mammals and birds. During TI, cortical EEG shows high voltage slow waves whose amount is affected by events preceding TI induction and is positively correlated with TI duration. The pattern of hippocampal activity helps to predict TI onset and TI termination. Both polysynaptic flexor and monosynaptic heteronymous reflexes are depressed independently from the EEG activity. Brain metabolism, signaled by glycogen mobilization and glucose utilization, indicates a reduced neuronal activity during TI. Learned avoidance responses to shock can be extinguished during TI and recover after TI. Moreover, during TI animals may learn how to avoid the shock by a motor response that may be followed by TI interruption. Decortication, decerebellation and telencephalic sections do not affect TI characteristics, whereas ponto-mesencephalic sections abolish both righting reflexes and TI.