LOCATION AND AXONAL PROJECTION OF ONE TYPE OF SWALLOWING INTERNEURONSIN CAT MEDULLA

Extracellular recordings were made from a type of relay neurons of the superior laryngeal nerve (SLN) afferents in the vicinity of the retro facial nucleus (RFN) in either pentobarbitone-anesthetized or unanesth etized and decerebrate cats, which were paralyzed and artificially ven tilated. A total of 26 neurons that could be activated both orthodromi cally by electrical stimulation of the SLN and antidromically by stimu lation of the brainstem were analyzed. All 26 neurons were activated f rom the ipsilateral SLN and 13 were activated from the contralateral S LN with mean latencies of 7.7 ms and 11.4 ms, respectively. The majori ty of these neurons were located in the parvocellular reticular format ion dorsomedial to the RFN and to the rostral part of the nucleus ambi guus (AMB). Antidromic stimulation of the medulla showed that 22 of th e 26 neurons projected to the hypoglossal nucleus (HYP) and 19 neurons tested projected to the AMB. Of these, 15 neurons projected to both t he HYP and AMB and two projected to the lateral reticular nucleus as w ell. Seventeen neurons were tested for their behavior during fictive s wallowing which was elicited by continual electrical stimulation of th e SLN and monitored by the activity of the hypoglossal nerve. Twelve n eurons showed brief (100-200 ms) burst firing at the onset of swallowi ng; the firing of the other 5 neurons were suppressed during swallowin g. Both the swallowing-active and swallowing-inactive neurons projecte d to the HYP and AMB. Thus, the SLN relay neurons in the vicinity of t he RFN might participate in the early stage of SLN-induced swallowing by integrating inputs from SLN afferents.