The Nervous Control of Cardiac Activity in the Tench (Tinca tinca) and the Goldfish (Carassius auratus)

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Abstract
The branch of the vagus innervating the heart of fish contains efferent cholinergic fibers. Activity in these fibers inhibits heart rate. There are intraspecific variations in the level of tonic activity in the cardiac ramus of the vagus, being almost zero in the goldfish. In the tench activity normally present in the nerve plays a role in establishing synchrony between heart beat and the breathing movements. Action potentials were recorded during the mouth opening phase of the breathing movements, inhibiting the heart beat and thus establishing a relationship between the heart beat and the mouth closing phase of the breathing cycle. It is suggested that this relates maximum flows of blood and water at the respiratory surface. The frequency of firing and the duration of the activity increased when the water passing over the gills was deoxygenated, resulting in a marked bradycardia. It is suggested that the synchrony between heart beat and the breathing movements and the bradycardia associated with anoxia are mediated via the same nervous pathways in fish and are related to the level of deoxygenation of the water passing over the respiratory surface. An increase in nervous activity in the cardiac branch of the vagus results in inhibition of the heart during a respiratory cough.

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