Integrative role of medullary neurons of the cat during exercise

The co-ordinated cardiovascular and respiratory responses observed during exercise are mediated by both afferent activation arising in contracting skeletal muscles and descending central drive originating in brain regions rostral to the medulla. Even though integration of these two mechanisms must occur in order to produce alterations in cardiorespiratory activity proportional to the intensity of the exercise, little is known about how this integration occurs. The purpose of the present study was to examine the role of medullary sites in the integration of exercise drives to the cardiovascular and respiratory systems. The responses of neurons in the ventrolateral medulla and the lateral tegmental fields to contraction of hindlimb muscles (elicited by stimulation of the L7 S1 ventral roots) and to activation of simulated central command (caudal hypothalamic stimulation) were examined in anaesthetized cats. Muscular contraction evoked an increase in discharge frequency in 59% (24/41) of the medullary neurons; 58% (14/24) of these neurons excited by muscular contraction displayed an immediate increase and 42% (10/24) had a more delayed increase in discharge frequency. Eighty-three per cent (10/12) of neurons rapidly excited by hindlimb muscle contraction were inhibited by hypothalamic stimulation. In contrast, most neurons which showed a delayed increase in discharge frequency during muscle contraction were either excited by hypothalamic stimulation (5/10) or unaffected (4/10). The majority of medullary neurons studied possessed a basal discharge related to the cardiovascular, respiratory and/or sympathetic cycles. These findings demonstrate that both feedback from contracting muscles and descending central command impinge upon individual medullary neurons. This is consistent with the idea that medullary neurons are important in integrating both the central drive and peripheral reflexes controlling the cardiorespiratory responses to exercise.