The activity in sensory and motor nerves of the gills was recorded from selected branches of the vagus nerve in decerebrate dogfish, Scyliorhinus canicula. Vagal motoneuronal activity was observed at the start of the rapid pharyngeal contraction and was followed by sensory nerve activity which preceded the slow expansion phase. Rhythmical vagal motoneuronal activity was still present after all movements had been prevented by curare paralysis although the frequency of the rhythm was higher than in the ventilating fish. Electrical stimulation of vagal sensory fibres had 3 effects on the ventilatory movements. (1) It evoked a reflex contraction of several gill muscles after a latency of about 11 ms. (2) It could reset the respiratory cycle because a stimulus given during expansion delayed the onset of the subsequent contraction. (3) The stimulus could entrain the rhythm if it was given continuously at a frequency close to that of ventilation. The vagal motor rhythm was disrupted by trigeminal nerve stimulation in the paralyzed fish but not if the motor rhythm was being entrained by vagal nerve stimulation. Vagal sensory activity may be important, therefore, in maintaining the stability of the generating circuits.

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Data on the interaction between respiration, coughing, feeding, vision and oculomotor control in fish have been analyzed. A self-contained respiratory rhythm generator in the brainstem reticular formation is, at times, interrupted by a coughing generator, which is partly autorhythmic and partly under vagal afferent control. A number of cranial muscles are shared by the respiratory and masticatory systems. Others are predominately allocated to one of these systems but are recruited by the other under heavy loading conditions. The effect of retinal image displacements caused by respiration-induced eye movements appears to be centrally corrected during normal respiration. In addition, excessive eye movements are reduced by extraocular muscle contraction during intense respiration and cough. Both the central visual and the oculomotor corrections are based on open-loop control through input from the respiratory center.

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