Studies on the termination of sinus nerve afferents.

Compound action potentials evoked by electrical stimulation in the brainstem of anaesthetized, paralyzed cats and rabbits were recorded distally in the carotid sinus nerve. The latencies of the components of the responses were indicatives of fibres with conduction velocities between 0.5 and 32 m/s. These include both myelinated and unmyelinated fibres. Using histological reconstruction of the 'responsive sites', the termination of the primary afferent fibres of the sinus nerve within the medulla was mapped. In both cat and rabbit these responsive sites were restricted to the doromedial medulla in the vicinity of the NTS 0--3 mm rostral to the obex. Although in the cat the fast conducting myelinated fibres (conduction velocities greater than 12.5 m/s) were discretely localized to the ventrolateral area of the NTS, the slower conducting myelinated and unmyelinated fibres were found more diffusely, spreading into areas dorsal and medial to the NTS. In the the rabbit the terminals of all types of fibre were mainly confined to the NTS although some C fibres appeared to be grouped in the juxta-alar region. Responses were never evoked from the medial reticular formation or nucleus ambigus in either cats or rabbits. The organization of the sinus nerve input to the medulla is discussed in the light of these results.