Possible relationships between orienting and diving reflexes.

Abstract

THE diving reflex has been studied extensively in aquatic organisms that breathe air and submerge to obtain food or to escape from predators such as ducks, seals and alligators1. This reflex seems to have two phases: an initial phase which may be mediated by the vagus and trigeminal nerves2, and a later one which follows prolonged holding of the breath during voluntary or forcible submersion3. The initial phase includes bradycardia1,4, apnoea (specifically, an interruption of respiration)1,4, peripheral vasoconstriction4,5 and cephalic vasodilation5. Gaunt and Gans6 have reported that bradycardia in Caiman crocolilius is minimal during spontaneous dives but great during the approach of an investigator or during handling. They suggest that early diving bradycardia is mainly “psychogenic” in origin and results from “threatening” stimuli. We have been studying (ref. 7 and unpublished results) the responses of the wholly aquatic salamander Necturus maculosus (mudpuppy) and have noted similarities to the early components of diving, specifically bradycardia and an interruption of respiration. (Necturus breathes through large external gills beating back and forth at 0.5–1.0 beats/s.) Insofar as it does not dive, we suggest here that these responses of Necturus are components of the orienting reflex8,9 rather than the initial diving reflex, and that both reflexes may reflect some common neural substrate involving adaptive responses to external stimulation.