Location and axonal projection of one type of swallowing interneurons in cat medulla

Extracellular recordings were made from a type of relay neurons of the superior laryngeal nerve (SLN) afferents in the vicinity of the retrofacial nucleus (RFN) in either pentobarbitone-anesthetized or unanesthetized and decerebrate cats, which were paralyzed and artificially ventilated. A total of 26 neurons that could be activated both orthodromically by electrical stimulation of the SLN and antidromically by stimulation of the brainstem were analyzed. All 26 neurons were activated from the ipsilateral SLN and 13 were activated from the contralateral SLN with mean latencies of 7.7 ms and 11.4 ms, respectively. The majority of these neurons were located in the parvocellular reticular formation dorsomedial to the RFN and to the rostral part of the nucleus ambiguus (AMB). Antidromic stimulation of the medulla showed that 22 of the 26 neurons projected to the hypoglossal nucleus (HYP) and 19 neurons tested projected to the AMB. Of these, 15 neurons projected to both the HYP and AMB and two projected to the lateral reticular nucleus as well. Seventeen neurons were tested for their behavior during fictive swallowing which was elicited by continual electrical stimulation of the SLN and monitored by the activity of the hypoglossal nerve. Twelve neurons showed brief (100-200 ms) burst firing at the onset of swallowing; the firing of the other 5 neurons were suppressed during swallowing. Both the swallowing-active and swallowing-inactive neurons projected to the HYP and AMB. Thus, the SLN relay neurons in the vicinity of the RFN might participate in the early stage of SLN-induced swallowing by integrating inputs from SLN afferents.