Afferent neurons in the hypoglossal nerve of the zebra finch (Poephila guttata): localization with horseradish peroxidase.

Hypoglossal efferent fibers are known to innervate the vocal organ (syrinx) in songbirds. In order to determine the existence of afferent fibers from the syrinx in the zebra finch, horseradish peroxidase (HRP) was applied to the cut peripheral branch of the hypoglossal nerve that innervates the left side of the syrinx. Cell bodies in the ganglion of the left vagus nerve were labeled, but no transganglionic (anterograde) transport into the CNS was observed at transport intervals of 1 to 5 days. (In comparison applying HRP to the cut descending branch of the vagus produced labeled cell bodies in the vagal ganglion as well as heavy anterograde label extending into the solitary nucleus.) Injection of HRP conjugated to wheat germ agglutinin into the intrinsic syringeal muscles also labeled cell bodies in the vagal ganglion, and again no transganglionic label was observed. Application of HRP more proximally to the cut hypoglossal truck resulted in the appearance of labeled cell bodies in the vagal ganglion as well as the anterograde label extending from the descending trigeminal tract to the principal sensory nucleus of V. These results indicate that hypoglossal afferents have cell bodies in the vagal ganglion, enter the medulla dorsally with vagal fibers, and terminate in the trigeminal complex. The identification of these afferent fibers raises the important question of their role in vocal learning.