Figure 7-17. Facial nerve, special sensory component.
Consists of afferent fibers which convey taste information from the anterior 2/3 of the tongue and the hard and soft palates.
Figure 7-18a. Chorda tympani and the lingual nerve.
Chemoreceptors of the taste buds located on the anterior 2/3 of the tongue and hard and soft palates initiate receptor (generator) potentials in response to chemical stimuli.
The taste buds synapse with the peripheral processes of special sensory neurons from CN VII. These neurons generate action potentials in response to the taste bud's receptor potentials. The peripheral processes of these neurons follow the lingual nerve and then chorda tympani to the petrous portion of the temporal bone (similar to the path followed by the efferent visceral motor fibers).
The cell bodies of these primary afferent neurons reside in the geniculate ganglion:
Figure 7-19. CN7 special sensory component, central course.
The central processes of the special sensory neurons pass from the geniculate ganglion through the facial canal and enter the brainstem as part of the nervus intermedius portion of CN VII.
The fibers then join the caudal portion of tractus solitarius and ascend to synapse in the rostral portion of the nucleus solitarius - also referred to as the gustatory nucleus:
Ascending secondary neurons originating from nucleus solitarius project both ipsilaterally and contralaterally to the ventral posteromedial (VPM) nucleus of the thalamus.
Tertiary neurons from the thalamus project via the posterior limb of the internal capsule to the area of the cortex responsible for taste.