Effect of anesthesia of the chorda tympani nerve on taste perception in humans

Individuals who have sustained considerable damage to parts of the taste system often fail to experience changes in everyday taste experience. The two halves of the tongue are independently innervated: the chorda tympani (branch of the facial or VIIth cranial nerve) innervating the anterior two-thirds and the glossopharyngeal (IXth cranial nerve) innervating the posterior one-third. Anesthesia of the chorda tympani nerve on one side produced increased taste intensities for some stimuli on the area innervated by the glossopharyngeal nerve on the other side. Because this effect occurs across the midline and taste projects ipsilaterally, the effect must occur in the central nervous system (CNS). This supports Halpern and Nelson's release-of-inhibition hypothesis that the area to which the chorda tympani projects in the CNS must normally inhibit that of the glossopharyngeal nerve. Anesthesia of the chorda tympani abolishes that inhibition and leads to perception of increased taste intensities from areas innervated by the glossopharyngeal nerve.