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THE BRAINSTEM AND CRANIAL NERVES

As a result of an infection, a 56-year-old woman experiences a loss of taste affecting the front of her tongue and the ability to smile. If the sensory loss involves damage of cell bodies, which of the following specific group of neurons would be so affected?

a. Otic ganglion

b. Nodose (inferior) ganglion

c. Pterygopalatine ganglion

d. Geniculate ganglion

e. Trigeminal ganglion

The answer is D. (Afifi, pp 307-309.) Taste associated with the anterior two-third of the tongue is mediated by the facial nerve (cranial nerve VII). The geniculate ganglion contains the cell bodies associated with the sensory (gustatory) component of the seventh nerve. The somatic motor component of the seventh nerve mediates the muscles of facial expression. Thus, the sensory and motor components of the seventh nerve affected in this individual can be characterized as special visceral afferent (because this afferent contains chemoreceptors) and special visceral efferent (because the motor component innervates skeletal muscle and is derived from a branchial arch), respectively.

A 55-year-old man, who has been suffering from hypertension for the past 8 years, experiences attacks of pain in the regions of the pharynx and ear which are usually preceded by swallowing and coughing spells. These attacks, each of which lasts for an average of 1 minute, occur a number of times. Ultimately, this condition showed remission. Although the neurological examination is basically normal, a subsequent magnetic resonance imaging (MRI) reveals an abnormality at the base of the skull. Which of the following cranial nerves is most likely involved in this disorder?

a. Cranial nerve V

b. Cranial nerve VII

c. Cranial nerve IX

d. Cranial nerve XI

e. Cranial nerve XII

The answer is C. (Afifi, pp 91-92, 99. Siegel and Sapru, pp 235-237.) Cranial nerve IX, the glossopharyngeal nerve, innervates the skeletal muscles of the pharynx. The motor component involved arises from the nucleus ambiguus of the medulla. This cranial nerve also contains afferents, a component of which arises from the superior ganglion. These sensory neurons convey somatosensory sensation, including pain afferents that ultimately synapse in the spinal trigeminal nucleus. The motor component of the glossopharyngeal nerve mediating swallowing and coughing constitutes a special visceral efferent (because it is derived from a visceral arch), and the sensory component conveying pain is referred to as a general somatic afferent fiber.

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