The pontine taste area in the rat

The pontine taste area relays gustatory information from the rostral pole of the solitary nucleus to both the thalamus and ventral forebrain. An electrophysiological investigation of this area was carried out in 3 stages. First, multiunit responses from the dorsal pons were mapped using sapid, thermal, and tactile stimuli applied to the anterior tongue. The gustatory zone lies within and just dorsal and ventral to the brachium conjunctivum as it enters the pons from the cerebellum. Second, gustatory stimuli were applied independently to the anterior and posterior tongue to determine whether receptors in both fields are represented in the pons. Responses with characteristics similar to those obtained from the glossopharyngeal nerve were located on the dorsal edge of the pontine gustatory zone. More ventrally the responses from the posterior tongue mimicked anterior tongue responses, but were of lesser amplitude than the largest anterior responses occurring at the ventral edge of the gustatory zone. Third, 71 single units were isolated in the dorsal pons, and tested for sensitivity to gustatory stimulation of the anterior and posterior tongue separately. More than half the units responded to gustatory stimuli-some from the anterior tongue alone, some from the posterior alone but most responded to stimuli applied to either field. In the latter instance 7 of 10 units tested continued to respond after anesthetizing the chorda tympani with Xylocaine instilled into the middle ear, thus demonstrating a true glossopharyngeal input. This proves that gustatory information from two distinct receptive fields may converge on the same central neuron.