On the location and size of laryngeal motoneurons in the cat and rabbit.

Motoneurons supplying the posterior crico-arytenoid (PCA), thyro-arytenoid (TA), lateral crico-arytenoid (LCA), and crico-thyroid (CT) laryngeal muscles were localized in the cat, the rabbit, and the 6-week-old kitten by using the technique of intramuscular injection of horseradish peroxidase. Each muscle was found to be innervated by a single, ipsilateral pool of motoneurons, a result which was reliably established only after controlling adventitious spread of the label to nontarget muscles by prior denervation of adjacent musculature. The laryngeal motoneuron column extended in the nucleus ambiguus for a distance of 5-6 mm caudally from the facial nucleus. CT motoneurons were located in the rostral third of this column while the PCA, TA, and LCA motoneurons were located more caudally. These results are in general agreement with earlier degeneration studies (Lawn, '66a; Szentágothai, '43). Although labelled cells were widely dispersed in the nucleus, particularly in the adult cat, a limited amount of topographical structure could still be discerned in the arrangement of recurrent laryngeal nerve motoneurons. In the cat, the PCA pool was located in the ventral part of the recurrent laryngeal nerve representation and did not extend as far caudally as the TA or LCA pools; the LCA pool was located in the caudal and dorsomedial part of the recurrent laryngeal nerve pool; TA motoneurons appeared to overlap the PCA and LCA pools on all three anatomical planes. TA motoneurons were more numerous than PCA or LCA motoneurons, the numbers of cells in the three pools being estimated at 170, 111, and 112, respectively. In the cat bilateral labelling of different pools pointed to certain differences in morphology between cells from these pools and also suggested a functional basis for such differences. The mean soma diameter for the PCA and CT motoneurons was each significantly smaller than that for the TA and LCA motoneurons. The rabbit data were similar. The findings on motoneuron morphology are considered in relation to anatomical and physiological characteristics known to have been established for individual laryngeal muscles and with which they appear to be consistent.