Responses of neurons in the caudal medullary raphe nuclei of the cat to stimulation of the vestibular nerve.

In the decerebrate cat, recordings were made from neurons in the caudal medullary raphe nuclei to determine if they responded to electrical stimulation of the vestibular nerve and thus might participate in vestibulosympathetic reflexes. Many of these cells projected to the upper thoracic spinal cord. The majority (20/28) of raphespinal neurons with conduction velocities between 1 and 4 m/s received vestibular inputs; 13 of the 20 were inhibited, and 7 were excited. Since many raphespinal neurons with similar slow conduction velocities are involved in the control of sympathetic outflow, as well as in other functions, these cells could potentially relay vestibular signals to sympathetic preganglionic neurons. The onset latency of the vestibular effects was long (median of 15 ms), indicating the inputs were polysynaptic. In addition, 34 of 42 raphespinal neurons with more rapid conduction velocities (6-78 m/s) also received long-latency (median of 10 ms) labyrinthine inputs; 26 were excited and 8 were inhibited. Although little is known about these rapidly-conducting cells, they do not appear to be involved in autonomic control, suggesting that the function of vestibular inputs to raphe neurons is not limited to production of vestibulosympathetic reflexes. One hypothesis is that raphe neurons are also involved in modulating the gain of vestibulocollic and vestibulospinal reflexes; this possibility remains to be tested.