[Gustatory nervous pathway syndromes]

Although the lingual nerve and the chorda tympani are the components of the classic peripheral gustatory pathway, loss of taste in patients after surgery for trigeminal neuralgia supports for the existence of an accessory gustatory pathway through the trigeminal sensory root and the gasserian ganglion. Bell's palsy is the most common pathology of the peripheral gustatory pathway. The central gustatory pathway ascends from the solitary tract nucleus in the medulla up to the upper pons in the ipsilateral central tegmental tract, rather than in the medial lemniscus as proposed in the past. It is not possible to specify whether the central gustatory pathway decussates or not at the lower midbrain level. Interruption of the gustatory pathway in the brainstem usually occurs with stroke or demyelination. The thalamic gustatory relay is located in the most medial aspect of the ventroposteromedial nucleus, immediately adjacent to the somatosensory area for the oral cavity and fingers. Therefore, ageusia associated with the sensory cheiro-oral syndrome may occur with a thalamic lesion. The laterality of the gustatory representation in the thalamus remains unresolved. Studies on epileptic gustatory aura have demonstrated that the insula and the anteromedial temporal lobe are the primary and secondary gustatory cortex, respectively. Taste perception results in patients with corpus callosum section and strokes or tumors involving the insula support the hypothesis that there is a gustatory representation of both hemitongues in the left cerebral hemisphere, whereas only the right hemitongue is represented in the right hemisphere.