PHYSIOLOGICAL CORRELATES OF VAGAL NERVE INNERVATION IN LOWER-VERTEBRATES

In lower vertebrates, cardiac vagal innervation shows less anatomic co mplexity and specialization than in mammals. To assess the physiologic al development of vagal specialization in the vertebrates, we investig ated cardiac chronotropic effects of electrical stimulation of left an d right vagus nerves separately and the interactions between both nerv es in anesthetized animals from three vertebrate groups, toad (Bufo ma rinus), shark (Heterodontus portusjacksoni), and lizard (Physignathus lesueurii). Atropine-sensitive slowing was effected equally by left or right vagi in all species, and chronotropic effects of simultaneous s timulation were the same as the sum of left and right responses. In sh arks and lizards, no slowing after atropine was detected (10 Hz stimul ation). In toads, after atropine, cardiac slowing was elicited equally by left or right vagal stimulation > 2 Hz. Simultaneous stimulation o f both vagi after atropine caused significantly greater slowing than t he sum of left and right responses. The results suggest even distribut ion of left and right vagal nerve endings to pacemaker cells, and limi ted competition for cardiac receptor sites in lower vertebrates.