Toshifumi Morimoto

Role of submandibular and sublingual saliva in maintenance of taste sensitivity recorded in the chorda tympani of rats

1. To evaluate the role of saliva in the maintenance of taste sensitivity, the activities in the rat chorda tympani innervating taste buds in the anterior part of the tongue were analysed. The effects of chronic extirpation of the submandibular and sublingual salivary glands were tested and compared with results after chronic oral administration of artificial saliva. 2. Removal of the salivary glands sharply decreased chorda tympani responses to four different taste stimuli by 7 days post‐desalivation, while a stable response to cold water was observed by at least 28 days. 3. This selective decrease in taste responses was considerably recovered by 7‐day‐oral injection of artificial saliva (containing NaHCO3, KCl and/or mucin) or distilled water. However, the injection of the salt‐containing artificial saliva induced significantly larger sucrose and smaller NaCl, HCl and quinine responses than did the injection of distilled water. 4. In our salivary manipulations, an alteration in the number of the functional sweet receptors was suggested by the cross‐adaptation technique using NaHCO3, whereas sensitivity to the epithelial sodium transport blocker, amiloride, was stable in the NaCl response. 5. Salivary water and electrolytes which may participate in forming the external environment of the taste receptor cells modulated taste sensitivity in the chorda tympani.

Neural activity of chorda tympani mechanosensitive fibers during licking behavior in rats.

The chorda tympani nerve, supplying the anterior two-thirds of the tongue, contains gustatory and mechanosensitive afferent fibers. We have analyzed discharge patterns in rats of various fibers recorded from dissected nerve filaments during licking behavior of which 4 were taste-sensitive and 12 mechanosensitive. The incidence of these two types were estimated electrophysiologically under anesthesia and their conduction velocity measured. Recordings in freely moving animals showed that the mechanosensitive fibers innervating the dorsal part of the tongue gave two burst discharges per lick, suggesting that contact of the tongue with the upper incisors and/or lip occurred during tongue protrusion and retraction. The fibers from the tip of the tongue showed one burst discharge per lick, which was the response to contact with a drinking spout. No rhythmical discharges synchronized with lick signals were observed in the fibers from the lateral part of the tongue or the taste-sensitive fibers. Such mechanoreceptor discharges were difficult to detect in recordings from the whole chorda tympani nerve. This masking of responses was due mainly to activation of a small number of mechanosensitive fibers by licking-induced mechanical stimulation. The lubricating action of saliva also decreased mechanoreceptor sensitivity. Despite their small number, the mechanosensitive fibers had axons with faster conduction velocities (larger diameter) than the taste-sensitive fibers. This was probably the reason why dissected nerve bundles more frequently showed mechanical than taste responses in conscious rats.

Application of polynomial regression modeling to automatic measurement of periods of EMG activity

We have developed a new algorithm for automatic detection and measurement of on/off periods of EMG burst and examined validity and reliability of the measuring technique. Mean EMG amplitude (M) during a semi-stationary state of an EMG data array [EMG] is calculated. Because M was determined to be significantly correlated with g(T(on)) (or g(Tend)) which represent amplitude on a polynomial regression curve g(t) which best-fitted to the [EMG], the estimate ]g(T(on) (or ĝ(T(end)) is calculated by substituting M into a regressive equation f(M) which explains the association between the M and g(T(on)) (or g(T(end)). T(on) and T(end) are human-determined on/off burst times for the [EMG]. The on/off periods of the EMG burst are finally computed as roots of the g(t) when ĝ(T(on)) and ĝ(T(end)) are subtracted from the constant of the polynomial. Application of the current method to the human masticatory muscle activity during chewing revealed that the absolute differences between human- and computer-determined measurements were smaller than 10 ms, and these measurements did not differ significantly. We conclude that the proposed algorithm is useful and effective for automatic detection and measurement of on/off periods of EMG burst.