Spontaneous efferent activity in branches of the vagus nerve controlling heart rate and ventilation in the dogfish

Abstract

Efferent activity was recorded from cranial nerves in the decerebrate dogfish (Scyliorhinus canicula) before and after injection of paralysing drugs. The recordings were made from the mandibular (Vth) and glossopharyngeal (IXth) nerves and the branchial (respiratory) and cardiac branches of the vagus (Xth) nerve. All the respiratory branches (Vth, IXth and Xth) and both cardiac branches fired rhythmic bursts of activity, synchronous with ventilation, which continued (at a higher rate) following paralysis, indicating that they originated in the CNS rather than arising reflexly from stimulation of pharyngeal mechanoreceptors. A burst of activity in the Vth nerve was followed by a burst in the IXth then, after a 30-ms delay, simultaneous bursts in the three respiratory branches of the Xth. The bursts in the branchial cardiac branches had a fixed phase relationship with activity in the respiratory branches, the onset of each burst preceding that in the immediately adjacent branch (branchial III), whereas the bursts in the visceral cardiac branches had a variable phase relationship with all other branches. The branchial cardiac branches alone contained units which fired sporadically between the bursts and increased their rate of firing during hypoxia. Both the bursting and non-bursting units responded to mechanical stimulation of the gill area. Separate oscillatory inputs driving the Vth, IXth and Xth respiratory motoneurones and an excitatory input to the bursting cardiac vagal motoneurones from expiratory motoneurones or the respiratory rhythm generator are implied by these relationships. The sporadically firing units in the branchial cardiac nerves clearly receive non-oscillatory inputs.