Autonomic afferents at T1 in elicitation of volume-induced tachycardia in the dog.

Intravenous infusion of blood elicited tachycardia in anesthetized dogs with beta-receptor blockade (dogs with vagal efferents intact but cardiac sympathetic efferents blocked) and bradycardia in dogs with combined beta-receptor blockade and rhizotomy at T1 (dogs with vagal efferents intact but cardiac sympathetic efferents plus autonomic afferents at T1 blocked). This suggests that tachycardia elicited by volume in infusion may be partly due to a reflex with its afferent pathway at the T1 segment of the spinal cord and its efferent pathway in the vagi. Moreover, infusion evoked bradycardia in dogs with right-sided rhizotomy at T1, which was not significantly different from the response elicited in dogs with bilateral rhizotomy at T1. Additionally, the tachycardia response induced in dogs with ventral or dorsal rhizotomy at T1 was significantly different from the bradycardia response elicited in dogs with both dorsal and ventral rhizotomy at T1. These results suggest that the afferent pathway predominates on the right side, and its entry at T1 may be via both dorsal and ventral roots.