EMG activities of masticatory muscles during licking in rats

EMG activities of the jaw closers and the jaw opener were recorded during licking of water and varying taste solutions in rats. The rats showed a continuous and rhythmic licking pattern (about 7 Hz) when they took water, 0.5 M sucrose and 0.1 M NaCl. A clear reciprocal relation of the jaw closers and opener was recognized during licking. The digastric muscle (jaw opener) started to work about 60 msec before the contact of the tongue with the liquid and ceased its activity at the moment of the contact, while the temporalis and masseter muscles (jaw closers) started to work about 50 msec after the contact. On the other hand, the rats rejected 1 M NaCl and 5 × 10^-4 M quinine-HCl, and stopped licking within a few seconds. However, EMG activities of the masticatory muscles continued more than 20 sec after cessation of licking. A characteristic jaw opening (gaping) was observed after licking quinine or after an oral infusion of 0.3 M CaCl₂. To presentations of 0.03 M HCl and 0.3 M NaCl, the licking pattern was the intermediate between the above-mentioned licking patterns shown by acceptable and rejective taste stimuli, i.e., the rats tended to show intermittent lickings to these HCl and NaCl solutions and the interlick interval varied greatly. The present results suggested that licking patterns and EMG activities of the masticatory muscles were characterized by the hedonic aspect or palatability of taste (acceptance-rejection dichotomy) rather than the taste quality (whether a taste is sugar, salt, acid, etc.).