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Preganglionic vagal motoneurones supplying the heart of the dogfish have been located in the medulla by antidromic stimulation of the central cut end of the branchial cardiac branch of the vagus. They supplied axons with conduction velocities between 4.75 and 16.3 m s-1, which is similar to mammalian B fibres. Motoneurones were found in two locations: the rostromedial (N = 5) and lateral (N = 12) divisions of the vagal motor column. Their measured depths and rostrocaudal distributions with respect to obex corresponded with the location of branchial cardiac motoneurones determined by horseradish peroxidase (HRP) histochemistry. All the neurones located in the rostromedial division of the vagal motor column were spontaneously rhythmically active. Their activity contributed to the rhythmic, respiratory-related bursts in peripheral recordings of efferent activity from the branchial cardiac vagus. They could be induced to fire in a prolonged burst by mechanical stimulation of the gill arches. The neurones located lateral to the rostromedial division of the vagal motor column could be divided into three categories: (1) spontaneously, continuously active cells which could be induced to fire more frequently by mechanoreceptor stimulation, (2) silent cells which could be induced to fire by mechanoreceptor stimulation, (3) silent cells which did not respond to mechanoreceptor stimulation. It is concluded, from the response of the medial and two categories of lateral cells to mechanoreceptor stimulation (which results in a transient bradycardia), that branchial cardiac motoneurones from both these central locations exert a chronotropic influence on the heart.

The Characteristics of Cardiac Vagal Preganglionic Motoneurones in the Dogfish