Mechanical sensitivity of the facial nerve fibers innervating the anterior palate of the puffer,Fugu pardalis, and their central projection to the primary taste center

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Abstract
Mechanical and chemical sensitivity of the palatine nerve, ramus palatinus facialis, innervating the anterior palate of the puffer,Fugu pardalis, and their central projection to the primary taste center were investigated. Application of horseradish peroxidase (HRP) to the central cut end of the palatine nerve resulted in retrogradely labeled neurons in the geniculate ganglion but no such neurons in the trigeminal ganglion, suggesting that the palatine nerve is represented only by the facial component. Tracing of the facial sensory root in serial histological sections of the brain stem suggested that the facial sensory nerve fibers project only to the visceral sensory column of the medulla. Peripheral recordings from the palatine nerve bundle showed that both mechanical and chemical stimuli caused marked responses. Mechanosensitive fibers were rather uniformly distributed in the nerve bundle. Intra-cranial recordings from the trigeminal and facial nerves at their respective roots revealed that tactile information produced in the anterior palate was carried by the facial nerve fibers. Elimination of the sea water current over the receptive field also caused a marked response in the palatine nerve bundle or facial nerve root while this did not cause any detectable responses in the trigeminal nerve root. Single fiber analyses of the mechanical responsiveness of the palatine nerve were performed by recording unit responses of 106 single fibers to mechanical stimuli (water flow), HCl (0.005M), uridine-5′-monophosphate (UMP, 0.001M), proline (0.01M), CaCl2 (0.5M), and NaSCN (0.5M). All these fibers responded well to one of the above stimuli; however, most taste fibers did not respond well to the inorganic salts. The palatine fibers (n=36), identified as mechanosensitive, never responded to any of the chemical stimuli, whereas chemosensitive fibers (n=70) did not respond to mechanical stimuli at all. The chemosensitive units showed a high specificity to the above stimuli: they tended to respond selectively to hydrochloric acid, UMP, or proline. The responses of the mechanosensitive units consisted of phasic and tonic impulse trains and the sensitivity of the units varied considerably. The results reveal that the facial nerve fibers innervating the anterior palate of the puffer contain two kinds of afferent fibers, chemosensory and mechanosensory respectively, and suggest that the convergence of the tactile and gustatory information first occurs in the neurons of the primary gustatory center in the medulla.