Identification of posterior cricoarytenoid motoneurons in the rat

The posterior cricoarytenoid (PCA) muscle is the sole abductor of the laryn x and is controlled by motoneurons located in the nucleus ambiguus. These m otoneurons receive inputs from a variety of interneurons, including those t hat impart respiratory modulation, and are responsible for the phasic inspi ratory activity of the PCA muscle. Identification of PCA motoneurons is the refore an essential initial step in understanding the mechanisms responsibl e for coordinated vocal cord abduction. We identified PCA motoneurons in th e rat model by retrograde labeling, and following antidromic activation. A total of 194 neurons were identified by retrograde labeling with cholera to xin B subunit (CTB). Labeling was exclusively ipsilateral where the contral ateral vagus and superior laryngeal nerves had been divided. The neurons we re multipolar, with dimensions of 33.2 +/- 6.4 mu m (mean +/- standard devi ation) in length and 22.3 +/- 3.4 mu m in width. The neurons were located w ithin a range of 0.6 to 2.4 mm caudal to the caudal pole of the facial nerv e, 1.2 to 1.7 mm lateral to the midline, and 1.5 to 2.3 mm deep to the dors al surface of the medulla. The PCA motoneurons were antidromically activate d by focal stimulation of the PCA muscle. The extracellular field was recor ded in 5 rats, and the PCA motoneurons were found within a range of 0.8 to 1.7 mm caudal to the caudal pole of the facial nerve, 1.5 to 2.0 mm lateral to the midline, and 1.9 to 2.4 mm deep to the dorsal surface of the medull a, The mean conduction velocity ranged from 37.0 +/- 5.8 to 68.6 +/- 5.0 m/ s. An extracellular antidromic field potential, which corresponds to the di stribution of the PCA motoneuron pool demonstrated by retrograde labeling w ith CTB, can be reliably obtained in a rat model following focal PCA muscle stimulation.