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We identified a 'semicircular' compartment of the rat thyropharyngeus muscle at the pharyngoesophageal junction and used the glycogen depletion method to determine how the fibers of this muscle (as well as all others of the pharynx and larynx) are innervated by different cranial nerve branches. The semicircular compartment appears anatomically homologous to the human cricopharyngeus muscle, an important component of the upper esophageal sphincter. While we found very little overlap in the muscle targets of the pharyngeal, superior laryngeal and recurrent laryngeal nerves within the pharynx and larynx, the semicircular muscle receives a dual, interdigitating innervation from two vagal branches: the pharyngeal nerve and a branch of the superior laryngeal nerve we call the dorsal accessory branch. After applying horseradish peroxidase to either of these two nerves, we compared the distribution and number of cells labeled in the brainstem. The dorsal accessory branch conveys a more heterogeneous set of efferent fibers than does the pharyngeal nerve, including the axons of pharyngeal and esophageal motor neurons and parasympathetic preganglionic neurons. The observed distribution of labeled motor neurons in nucleus ambiguus also leads us to suggest that the semicircular compartment is innervated by two subsets of motor neurons, one of which is displaced ventrolateral to the main pharyngeal motor column. This arrangement raises the possibility of functional differences among semicircular compartment motor neurons correlated with the observed differences in brainstem location of cell bodies.

To determine the sources and targets of auditory pathways that bypass the inferior colliculus in the mustache bat, we injected WGA-HRP in the medial geniculate body and related auditory nuclei of the thalamus as well as in the lower brainstem. We used electrophysiological methods to verify that the injection electrode was in an area responsive to sound. The only thalamic injections that produced retrograde transport to cells in auditory nuclei caudal to the inferior colliculus were those that included the suprageniculate nucleus. These injections labeled a group of large multipolar cells lying between the ventral nucleus of the lateral lemniscus and the superior olivary complex. Neurons in this cell group have also been shown to project to the deep layers of the superior colliculus in the mustache bat. The pathway revealed by these studies is almost identical to the "central acoustic tract" in which fibers course medial to the lateral lemniscus and bypass the inferior colliculus to reach the deep superior colliculus and the suprageniculate nucleus.

The mustache bat, Pteronotus parnellii, depends on echolocation to navigate and capture prey. This adaptation is reflected in the large size and elaboration of brainstem auditory structures and in the minimal development of visual structures. The superior colliculus, usually associated with orienting the eyes, is nevertheless large and well developed in Pteronotus. This observation raises the question of whether the superior colliculus in the echolocating bat has evolved to play a major role in auditory rather than visual orientation. The connections of the superior colliculus in Pteronotus were studied with the aid of anterograde and retrograde transport of wheat germ agglutinin conjugated to HRP. These results indicate that the superior colliculus of Pteronotus is composed almost entirely of the layers beneath stratum opticum. The retinal projection is restricted to a very thin zone just beneath the pial surface. Prominent afferent pathways originate in motor structures, particularly the substantia nigra and the deep nuclei of the cerebellum. Sensory input from the auditory system originates in three brainstem nuclei: the inferior colliculus, the anterolateral periolivary nucleus, and the dorsal nuclei of the lateral lemniscus. The projections from these auditory structures terminate mainly in the central tier of the deep layer. The most prominent efferent pathways are those to medial motor structures of the contralateral brainstem via the predorsal bundle and to the ipsilateral midbrain and pontine tegmentum via the lateral efferent bundle. Ascending projections to the diencephalon are mainly to the medial dorsal nucleus and zona incerta. Thus, the superior colliculus in Pteronotus possesses well-developed anatomical connections that could mediate reflexes for orienting its ears, head, or body toward objects detected by echolocation.