Physiological correlates of vagal nerve innervation in lower vertebrates

In lower vertebrates, cardiac vagal innervation shows less anatomic complexity and specialization than in mammals. To assess the physiological development of vagal specialization in the vertebrates, we investigated cardiac chronotropic effects of electrical stimulation of left and right vagus nerves separately and the interactions between both nerves in anesthetized animals from three vertebrate groups, toad (Bufo marinus), 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 stimulation were the same as the sum of left and right responses. In sharks and lizards, no slowing after atropine was detected (10 Hz stimulation). In toads, after atropine, cardiac slowing was elicited equally by left or right vagal stimulation > 2 Hz. Simultaneous stimulation of both vagi after atropine caused significantly greater slowing than the sum of left and right responses. The results suggest even distribution of left and right vagal nerve endings to pacemaker cells, and limited competition for cardiac receptor sites in lower vertebrates.