Sensory Fibers

SENSORY ORGANS AND FIRST ORDER NEURONS

First order neurons for sensory functions of the glossopharyngeal nerve have their cell bodies located in either the superior or inferior glossopharyngeal ganglia, which are found at the jugular foramen. Taste from the posterior third of the tongue is detected by chemical stimulation of cells within taste buds and then is conveyed by the special afferent fibers. The first order neuron for taste has its cell

Figure 13-1 Cross section of the medulla. The ninth and tenth cranial nerve nuclei lie in the lateral portion of the medulla, in close juxtaposition with the eighth cranial nerve nucleus, the spinal nucleus of cranial nerve V, and the inferior cerebellar peduncles.

body located in the inferior glossopharyngeal ganglion. The central processes of these neurons project to the rostral nucleus solitarius. The specific means for detection of pain, temperature, and tactile sensation in areas served by the glossopharyngeal nerve are presumably similar to those elsewhere in the body. Pain, temperature, and tactile sensation from the posterior third of the tongue, upper pharynx, tonsils, skin behind the external ear, inner aspect of the tympanic membrane, and eustachian tube are conveyed by general somatic afferent fibers of the glossopharyngeal nerve. Cell bodies for these first order neurons lie in both the superior and inferior glossopharyngeal ganglia. The central processes of these neurons enter the medulla and descend in the spinal trigeminal tract and synapse within the caudal part of the spinal trigeminal nucleus. NUCLEAR AND SUPRANUCLEAR CONTROL

The axons of cells in the nucleus solitarius conveying taste from the posterior third of the tongue ascend and terminate at the ventral posterior nuclei of the thalamus. These neurons in turn project to the inferior third of the postcentral gyrus for taste perception. The secondary neuron cell bodies for pain, temperature, and tactile sensation are located in the spinal trigeminal nucleus. Their processes cross the midline in the medulla and ascend to terminate at the contralateral ventral posterior nucleus of the thalamus. From this point the third order neurons project to the postcentral, sensory cortex region.

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