Vasomotor Reflexes from Muscle

Casual observations of blood pressure changes during muscle stretch drew attention to the possible role of muscle proprioceptors in vasomotor control. An attempt has been made to throw some light on this problem by studying the effects of mechanical muscle stimulation, as also of electrical excitation of muscle afferents, on systemic arterial pressure and on regional hindlimb circulation, in anesthetized or decerebrated cats. Typical changes in systemic arterial pressure — viz. an initial depressor effect, followed or not by a more or less pronounced pressor effect — could be elicited by moderate stretch of different fore‐ and hindlimb muscles. The effects, which were dependent on intact nerve connections, appeared at tendon loadings of 100–300g and could also be produced by slight pressure on the muscle belly or tendon. The conclusion that these effects were reflex responses mediated from mechano‐receptors in muscle was further substantiated by comparative experiments which, i. a. , showed the divergent responses to various types of painful stimulation applied to the muscle. For eliciting systemic depressor effects by electrical stimulation of muscle afferents, stimulus strengths of more than 10 times the threshold of group I fibers were required; this implies that no depressor response was observed until, after maximal activation of group I and II fibers, group III fibers were being excited. By simultaneous recording of the local arterial pressure in the lower hindleg, perfused at constant rate, the occurrence of regional vasomotor effects — predominant vasodilatation or constriction — could be demonstrated during electrical or adequate activation of muscle afferents. Studies of blood flow in muscles by inserted thermocouples were also performed.