Caudal ventrolateral medullary cells responsive to muscular contraction

The pressor reflex evoked by muscular contraction (exercise pressor reflex) is held to be an important mechanism in producing the cardiovascular adjustments to static exercise. Recent experiments using lesioning and metabolic labeling methods have indicated that the caudal ventrolateral medulla may be a key integrative site for the reflex evoked by muscular contraction induced by ventral root stimulation. Therefore, we sought to determine whether cells in this region could be associated with the cardiovascular reflex accompanying muscular contraction through analysis of their discharge characteristics. Eighty cells were characterized as to their response to ventral root stimulus-induced static muscular contraction, intra-arterial capsaicin (selective groups III and IV stimulus), and mechanical probing. The cells' receptive fields were also determined by mechanical probing. The receptive fields were usually large, often including all four limbs and the trunk. Four response patterns were observed to static contractions: 1) a brisk initial discharge followed by a gradual return toward control levels (slowly adapting), 2) a brief onset and cessation response, 3) a brief inhibition followed by a slowly adapting discharge, and 4) inhibition alone. Virtually all cells tested were responsive to capsaicin. Histological analysis verified the position of the recorded cells. It is suggested that the cells most likely to participate in the pressor response to muscular contraction were 1) those cells in the general region of the lateral reticular nucleus which responded with an initial and sustained discharge and 2) the cells that were inhibited in the region of the nucleus ambiguus (possible inhibition of vagal outflow).