Taste responses in the superior secondary gustatory nucleus of the carp, Cyprinus carpio L.

Single unit discharges in the superior secondary gustatory nucleus of the carp, Cyprinus carpio L., were studied electrophysiologically in response to chemical stimulation of the external chemoreceptors of the facial skin surface. Of 36 gustatory neurons recorded, 80.6% were facilitated by taste stimuli and 19.4% were inhibited. The gustatory neurons were classified according to their responsiveness to the 4 basic taste substances and, except the inhibitory type, did not differ remarkably from the primary and secondary gustatory neurons. More inhibitory type (19.4%) neurons occurred at higher levels of the gustatory system. As in the primary and secondary levels, sodium chloride and acetic acid solutions were more effective stimuli than quinine HCl and sucrose. The ascending secondary gustatory fibers project bilaterally to the superior gustatory nucleus of the carp. About 20% of the gustatory neurons respond to stimulation of only the contralateral facial skin while 27.8% respond to stimulation of either side of the face. The latter neuron type showed very complicated responses, and were classified into 'Uniform', 'Summation', 'Contra. greater than Ipsi.' and 'Quality field' types. The remaining 50% of the neurons respond only to stimulation on the ipsilateral side.