Distribution of the facial nerve to taste receptors in the rat

The distribution of facial sensory axons to the tongue and palate of the rat was determined in relation to the spatial distribution of taste buds. Gross dissections and serial reconstruction of silverstained material revealed five divisions of the chorda-lingual nerve in the tongue and four divisions of the greater petrosal nerve in the palate. Axon counts were made in normal and de-efferented chorda tympani and greater petrosal nerves from montages of electron micrographs. De-efferented facial nerves were prepared by intracranial section of the nervus intermedius and motor root prior to sacrifice. Cell bodies were counted in the geniculate ganglion and the total number of axon profiles in the chorda tympani and greater petrosal nerves exceeded the number of ganglion cells by 30%. The greater petrosal nerves contained about three times as many profiles as the chorda tympani nerves with the principal disparity among unmyelinated axons. Taste bud numbers on the palate and anterior tongue were nearly equivalent; however, their densities per unit of surface area varied 150 fold between regions. On the tongue, taste bud density seemed not to be correlated with the number of axons directed to a region as there was a nearly linear distribution of axons from the base to the apex.