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, 28 (5), 284-97

The Mammalian Diving Response: An Enigmatic Reflex to Preserve Life?


The Mammalian Diving Response: An Enigmatic Reflex to Preserve Life?

W Michael Panneton. Physiology (Bethesda).


The mammalian diving response is a remarkable behavior that overrides basic homeostatic reflexes. It is most studied in large aquatic mammals but is seen in all vertebrates. Pelagic mammals have developed several physiological adaptations to conserve intrinsic oxygen stores, but the apnea, bradycardia, and vasoconstriction is shared with those terrestrial and is neurally mediated. The adaptations of aquatic mammals are reviewed here as well as the neural control of cardiorespiratory physiology during diving in rodents.

Conflict of interest statement

No conflicts of interest, financial or otherwise, are declared by the author(s).


Homeostatic control of cardiovascular hemodynamics and respiration are disrupted during underwater submersion A: note the ∼80% drop in HR when a laboratory rat (top left photograph) volunteers to dive underwater (down arrow), which persists until it surfaces (up arrow). The transitory increase in arterial blood pressure due to sympathetic activation can also be seen. B: respirations also cease during diving, inducing radical changes in blood chemistry. Po2 falls while underwater, whereas Pco2 rises dramatically. Despite this, respiratory chemoreceptors that normally would increase ventilation are muted. The hypoxia in tissues deprived of blood after the selective peripheral vasoconstriction induces anerobic metabolism, with an increase in lactic acid as by-product. Note, however, that its release into the bloodstream does not occur until after the animal surfaces, when the stringent vasoconstriction of muscular, splanchnic, and cutaneous circulations is released. B is adapted from Ref. (and is used with permission) showing such changes in a seal (bottom left photomicrograph) during a dive.
Aquatic mammals develop several changes to augment their intrinsic oxygen stores The changes aquatic mammals develop to increase oxygen stores and decrease its utilization include increased blood volume, enhanced hematocrit, hemoglobin, and myoglobin, as well as hypothermia. Although such adaptations seldom are seen in land-bound mammals, both aquatic and terrestrial mammals share the bradycardia, vasoconstriction, and apnea characterizing the diving response.
Work on diving rodents suggest paranasal areas (shaded blue) innervated by the anterior ethmoidal and infraorbital nerves are important for initiating the diving response These nerves project [A1 and A2 show transport of an HRP cocktail (colored gold) transported transganglionically from the anterior ethmoidal and infraorbital nerves, respectively] into the rostral medullary dorsal horn (MDH) overlapping the caudal subnucleus interpolaris (Sp5I). Note the band of neuropil just dorsal to the Sp5I (arrows) is labeled from either nerve. Neurons activated with cFos (A3; small black nuclei) induced by diving are found in similar neuropil. Moreover, small, bilateral injections of lidocaine (blue squares) or kynurenate (red circles) made into similar locations (A4) blocked the cardiorespiratory responses of nasal stimulation. The hallmark of the diving response is the dramatic bradycardia (see FIGURE 1A); many neurons surrounding the rostral nucleus ambiguus are labeled with cFos after diving (B1), and some of these are preganglionic cardiac motoneurons (B2; arrows point to double-labeled neurons containing cFos and a retrograde tracer injected into the pericardial sac). There also is a massive but selective peripheral vasoconstriction during diving in rats mediated by neurons in the rostral ventrolateral medulla (C1 shows cFos-labeled neurons in the RVLM induced by diving); many such neurons are monoaminergic (C2 showing double labeled neurons with antibodies against cFos and tyrosine hydroxylase). The third neuronal reflex induced by underwater submergence is a profound apnea, which is maintained despite gross disruption of blood chemistry, suggesting inhibition of the respiratory chemoreceptor reflex. Few neurons were activated in the medullary ventral respiratory column (see C1, C2), but projections from the MDH to the ventral surface of the caudal medulla at the spinomedullary junction (D2; approximately −14.6 mm from bregma) overlap where neurons/glia activated by diving are found (D1, arrows; small black profiles show cFos activation). Arrows in D2 point to presumptive neurons with juxtaposed BDA fibers. Injection of a retrograde tracer, which included the retrotrapezoid nucleus labeled small neurons in neuropil similar to that labeled by paranasal primary afferent fibers (D3, arrow). Anterograde transport of tracers injected into these areas of the MDH resulted in extremely small labeled fibers with swellings (D4, arrows) in the retrotrapzoid nucleus ventral to the facial motor nucleus. Similar neurons/glia have long been suspected to be chemoreceptors sensitive to high Pco2, but details of how they interact with central respiratory neurons is lacking. Other studies (188) have shown the neuronal circuitry driving the diving response is contained within the medulla and spinal cord.

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