SENSORY ORGANS AND FIRST ORDER NEURONS
General somatic efferent fibers of the vagus nerve convey sensations of pain, temperature, and touch from the meninges of the posterior fossa, the back of the ear, the external auditory canal, the external tympanic membrane, the larynx, and the pharynx. Sensation from the vocal cords and the larynx below this level is carried by sensory fibers of the recurrent laryngeal nerve. Sensation above the vocal folds is conveyed by the internal laryngeal nerve, which is a branch of the superior laryngeal nerve. The cell bodies for sensation of the ear, external auditory meatus, and tympanic membrane are in the superior vagal ganglion (jugular ganglion); the remaining fibers have their cell
bodies in the inferior vagal ganglion (nodose ganglion). Both the superior and inferior vagal ganglia are located on the proximal vagus nerve intracranially at the jugular fossa near the jugular foramen. The central processes of both ganglia enter the medulla and descend in the spinal trigeminal tract to the trigeminal nucleus.
Visceral afferent fibers of the vagus nerve carry visceral sensation from the intestines, stomach, esophagus, tongue, part of the pharynx, lungs, bronchi, trachea, and larynx. These fibers also convey signals from chemoreceptors detecting blood oxygen tension located in the aortic body, and baroreceptors for blood pressure in the aortic arch. Cell bodies for all visceral afferent fibers are in the inferior vagal ganglion. Their central processes descend in the tractus solitarius in the medulla to reach the nucleus of the tractus solitarius.
After reaching the spinal trigeminal nucleus, impulses for pain, temperature, and tactile sensation are carried by second order neuron fibers, crossing in the medulla, to the contralateral ventral posterior nucleus of the thalamus. The third order neurons in the thalamus project to the sensory cortex via the internal capsule. The nucleus of the tractus solitarius, along with the reticular formation and hypothalamus, has connections to the dorsal vagal motor nucleus for reflexive control of cardiac, respiratory, and gastrointestinal functions.
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