AJP - Regulatory, Integrative and Comparative Physiology, Vol 268, Issue 5 1249-R1256, Copyright © 1995 by American Physiological Society
Physiological correlates of vagal nerve innervation in lower vertebrates
E. Preston and G. P. Courtice
School of Physiology and Pharmacology, University of New South Wales, Sydney, Australia.
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.
