THE EFFFCT OF FIBERS OF SPECIFIC TYPES IN THE VAGUS AND SYMPATHETIC NERVES ON THE SINUS AND ATRIUM OF THE TURTLE AND FROG HEART

Abstract

In the turtle vagus nerve (Pseudemys elegans, P. concinna, and P. scripta, no. not stated) fibers giving rise to separable potentials on conduction carry the impulses responsible for the negative inotropic and negative chronotropic effects in the sinus and atrium. These fiber groups have different thresholds and conduction rates. The fibers primarily responsible for inotropic changes are probably thinly myelinated, those responsible for chronotropic changes unmyelinated. In the bull frog sympathetic, unmyelinated fibers carry the impulses primarily responsible for both the positive inotropic and positive chronotropic changes in the sinus and atrium. They have a similar threshold. The experimental findings were consistent with the interpretation that in the sinus and atrium the mechanisms responsible for inotropic and chronotropic effects were influenced separately by fibers from extrinsic nerves. The chronotropic mechanism within the sinus and atrium indirectly might also influence the inotropic mechanism.