Studies on nerves of the upper respiratory tract in the ferret and the mink

Abstract

The central localization of the superior laryngeal nerve (SLN), recurrent laryngeal nerve (RLN) and pharyngeal nerve (PHAR) in the ferret ( Mustela putorius furo) and mink ( Mustela vison) was determined by neuronal tract tracing techniques. The anterograde transport of horseradish peroxidase (HRP)/wheat germ aggluttinin conjugated HRP mixtures (WGA-HRP) revealed afferent fibres of the SLN projecting insilaterally in the tractus solitarius (TS) before terminating in the ipsilateral nucleus of the tractus solitarius (nTS). In both mustelids large concentrations of terminal reaction product were observed in the dorsal/dorsolateral and medial subnuclei of the nTS; however, at levels near obex significant projections of the SLN to the interstitial subnucleus were also observed. Caudal to obex, sparse terminal labelling was identified bilaterally in the nucleus commissuralis (n com). There were no labelled afferent projections of the SLN to the spinal trigeminal complex in either mustelid; neither was afferent terminal reaction product detected in the medulla oblongata following labelling of the RLN (in either the ferret or the mink) or PHAR (in the ferret). Projection of the SLN to areas of the nTS associated with upper airway functions, like swallowing and respiration, suggest a substantial role for this nerve in the initiation and control of airway reflexes. Extensive labelling of the nucleus ambiguus (nA) was revealed following HRP/WGA-HRP application to the PHAR, RLN and SLN and there was some evidence of a topographical arrangement of neuronal cell bodies in the nA, relative to the different nerves, in the ferret. In the case of the SLN, retrogradely labelled cells were also identified in the dorsal motor nucleus of the vagus (DmnX), nTS and reticular formation (rf). Electrical stimulation of the SLN produced periods of apnoea/ataxic breathing in both mustelids. As well as respiratory effects, stimulation of the SLN often caused bradycardia in the mink, but not in the ferret, in which heart rate generally remained unchanged. This difference did not reflect anatomical differences in the central localization of the SLN.