REFLEXES FROM THE LIMBS AS A FACTOR IN THE HYPERPNEA OF MUSCULAR EXERCISE

Abstract

Active movements of the hind limbs, produced by electrical stimulation of cut ventral roots of spinal cord, in cats and dogs anesthetized with morphine-chloralose or barbital, uniformly led to respiratory stimulation. The hyperpnea occurring in the dog was due chiefly to an increase in rate of respiration, was reflex in origin (for it disappeared upon section of nerves or of the spinal cord) and did not depend upon accumulation of tissue metabolites locally or upon their liberation into the blood stream (for occlusion or release of the femoral blood vessels did not materially alter the response to "exercise"). The reflex appeared to originate mainly in proprioceptors about the knee joint for passive movements of the limbs led to similar hyperpnea, which was abolished by injecting procaine into and around the knee joints. The corresponding hyperpnea occurring in the cat involved an increase in both rate and depth and appeared to be due to liberation of metabolites into the blood stream for occlusion of the blood vessels greatly reduced the response to "exercise" and marked hyperpnea followed restoration of the circulation; furthermore the response was reduced only slightly by nerve section, and though passive movements produced hyperpnea, this response was slight and inconsistent compared with that of the dog. In neither sp. could physiologically important chemosensitive receptors, other than pain receptors, be demonstrated in the limbs; the effects of infusion or transfusion of anoxic, hypercapnic or acidic blood, and of intrafemoral arterial and intramuscular injections of numerous chemicals and tissue metabolites supported this conclusion. Expts. in man showed that passive movements of one leg at the knee usually led to significant hyperpnea; this was markedly reduced- or abolished by spinal anesthesia. Active contraction of the muscles of one forearm in man also caused a hyperpnea which was in-tensified by ischemia, indicating that it was not due to liberation of chemical products into the circulation; the concomitant changes in blood pressure were insignificant unless severe pain was experienced.