Discharge patterns of ventrolateral medullary neurons during muscular contraction

Static muscular contraction is known to elicit reflex increases in arterial pressure. However, the areas of the central nervous system that mediate this reflex remain elusive. A recent study from this laboratory suggested that neurons in the ventrolateral medulla (VLM) participate in the pressor reflex to muscular contraction. In the present study, we sought to ascertain whether extracellular single-unit activity of VLM neurons is altered by static contraction of hindlimb muscles. In anesthetized cats, muscular contraction elicited by stimulation of L₇ and S₁ ventral roots evoked increases in arterial pressure, heart rate, and minute ventilation. The firing frequency in 33 of 50 VLM units increased > 70% during muscular contraction. VLM units displayed two types of discharge patterns in response to contraction: 1) a rapid onset response (0.5-3 s) and 2) a delayed onset response (10-20 s). Computer averaging analysis showed that 14 of 28 VLM units tested had a cardiac-related rhythm and 10 of those 14 also responded to muscular contraction. Muscular contraction had no effect on the discharge patterns of most neurons located outside the VLM. These findings suggest that neurons in the ventrolateral medulla respond to muscular contraction and may have a role in the pressor reflex to muscular contraction.