Figure 8.

Hypothesized interpretation for the effects of cross-regeneration of the chorda tympani (CT) and glossopharyngeal (GL) nerves on discrimination performance. A. In the intact case NaCl selectively stimulates the amiloride-sensitive channels on taste receptor cells found in the anterior tongue (Ant.) as well stimulating less cation-selective amiloride-insensitive channel(s) on taste receptor cells in both the anterior and posterior tongue (Post.). Receptors on the palate,, some of which are amiloride-sensitive, supplied by the greater superficial petrosal (GSP) nerve also contribute to normal salt discrimination. The cation specificity of the amiloride-sensitive GSP fibers remains unknown, however, and it is assumed here to be similar to the anterior tongue Growing evidence suggests that branches of the facial nerve (i.e., CT and GSP) including the chorda tympani provide necessary and sufficient input to neural circuits subserving qualitative taste discrimination. Thus, the GL provides irrelevant information for this particular task. B. When the CT cross-reinnervates the posterior tongue in the absence of the GL, there is no longer a discriminable signal arising from the taste receptor cells of the anterior tongue that unequivocally distinguishes between NaCl and KCl. Input is still provided from the palate by the intact GSP, but the brain is now receiving changed total peripheral signals leading to some change in salt perception. C. When the GL cross-reinnervates the anterior tongue in the absence of the CT, although a discriminable signal is available from the taste receptor cells it is channeled through neural circuits that do not contribute to discriminative function. Although the GSP still provides some discriminable signal, the circuits involved in qualitative taste discrimination are now receiving a changed peripheral signal leading to changed salt perception. The framework (B & C) shown here assumes that phenotype of the regenerated taste receptor cells is maintained in the face of reinnervation from a different gustatory nerve. Moreover, it is assumed that there is not massive reorganization of central circuits. The superior laryngeal branch of vagus which innervates taste buds in the laryngeal epithelium thought to be primarily involved with protection of the airways is not shown. The presence of an intact GSP is likely responsible for keeping performance above chance when the CT is transected and serves as the basis for the recovery of function with extended postsurgical testing as animals learn a new taste discrimination based on the changes in salt perception which occurred as a function of the changes in peripheral input. The perceptual changes need not be qualitative per se but could also involve decreases in intensity, sensation rise and decay times, or simply changes in the oral locus of the signal.