The influence of hypothalamic thermoceptive structures on the electroencephalogram and gamma motor activity

The influence of hypothalamic and body temperature on the EEG and the gamma motor system was studied in cats and rabbits. The index of gamma motor activity was obtained by recording muscle spindle discharge from a muscle at constant tension. Moderate hypothalamic heating inhibited the gamma motor activity in both cats and rabbits. Excessive hypothalamic heating induced increased discharge frequency in muscle spindles in rabbit. Moderate heating inhibited shivering in both species and induced cutaneous vasodilatation as well as other autonomic activities. Excessive heating induced restlessness in rabbits. Moderate hypothalamic heating synchronized the EEG in both cats and rabbits. Excessive heating desynchronized, “aroused”, the EEG in the rabbits. Simultaneous records of muscle spindle activity and EEG have demonstrated a close correlation between these two events. Anesthetics abolished more readily the influence of hypothalamic heating on the EEG than on the gamma motor activity. Other exceptions from the correlation are also discussed. It is concluded that the effects of hypothalamic heating are mediated by common relays influencing both gamma motor and cortical activity. Hypothalamic thermoceptive structures may thus, like most peripheral receptors, project upon the activating relay system of the brain stem. The muscle spindle test has been found to be a useful index of that system. The significance of the results for homeostasis of body temperature are discussed.