RESPONSES OF DISTINCT TYPES OF SYMPATHETIC NEURON TO STIMULATION OF THE SUPERIOR LARYNGEAL NERVE IN THE CAT

Stimulation of afferents in the superior laryngeal nerve (SLN) leads t o apnea and evokes reflexes in sympathetic neurons. It is not clear wh ether these reflexes are secondary to changes in the brainstem respira tory network or due directly to the afferent input on neurons belongin g to central sympathetic pathways. To clarify this question, single th oracic preganglionic sympathetic neurons projecting into the cervical sympathetic trunk (CST) were classified as described previously and th en tested for their responses to electrical stimulation of the superio r laryngeal nerve (SLN) in chloralose-anesthetized, paralysed and arti ficially ventilated cats. SLN stimulation was performed with intensiti es sufficient to suppress central inspiratory activity detected by phr enic and recurrent laryngeal nerve recordings. Sympathetic neurons wer e tested under different levels of respiratory drive. Thirteen group I (putative muscle vasoconstrictor) neurons were mostly activated by SL N stimulation when respiratory drive was low, but depressed when it wa s high; this was due to the change in inspiration-related activity. Te n of eleven group II (putative cutaneous vasoconstrictor) neurons were depressed during SLN stimulation. This inhibition was independent of central respiratory drive. Inhibition also occurred in those neurons w hich predominantly discharged during postinspiration. Eight group III neurons which showed a discharge confined to inspiration were inhibite d but mostly not silenced by SLN stimulation. Group IV (functionally u nclassified) neurons either showed no response (n = 5) or were slightl y inhibited (n = 2). The responses of group I neurons, but trot the re sponses of group II and group III neurons, showed a significant positi ve correlation with those of systemic blood pressure. The observed res ponses corroborate the classification made previously. The results als o demonstrate that the responses of sympathetic neurons to SLN stimula tion are not merely due to the respiratory modulation of their activit y, but rather consist of two components, one occurring independently o f and the other secondary to, the changes in respiration. (C) 1997 Els evier Science B.V.