The effect of sectioning cranial nerves V, VII, IXand X on the cardiac response of the dogfish Scyliorhinus canicula to environmental hypoxia.

1. Exposure of the dogfish to rapidly induced hypoxia caused an initial, large reduction in heart rate to 32 % of its initial normoxic value. When P o of inspired water (PIt Ot) reached its lowest level, heart rate increased to 65 % of its normoxic value and this rate was maintained until Px 0> was returned to normal. 2. Bilateral section of the branchial and pharyngeal branches of either cranial nerve IX or cranial nerve X had no significant effect on the cardiac response to rapidly induced hypoxia. Bilateral section of the pharyngeal and branchial branches of both of these cranial nerves abolished the initial large reduction in heart rate in response to rapidly induced hypoxia; heart rate fell steadily to 40 % of its initial normoxic value after 3 min. 3. Bilateral section of cranial nerves V and VII had a similar effect, with heart rate reaching 62 % of its normoxic value, 3 min after exposure to rapidly induced hypoxia. When the four cranial nerves were bilaterally sectioned together, there was no significant change in heart rate after 3 min exposure to rapidly induced hypoxia. 4. It is concluded that the oxygen receptors responsible for the bradycardia elicited in dogfish by environmental hypoxia are innervated by cranial nerves, V, VII, IX and X and that if receptors in the C.N.s. are involved in the response, their role is minimal.