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Variably afferent and efferent, somatic and visceral, the 12 cranial nerves are functionally more complex than their ordered number would suggest (Table 3–1). Cranial nerves 4, 6, and 12 are solely somatic efferent. Cranial nerves 1 and 8 are solely afferent, but 8 conveys two very different kinds of sensory information. Cranial nerve 2, while solely afferent, is actually a central nervous system tract (accounting, among other things, for its frequent involvement in multiple sclerosis). Cranial nerve 11, while solely efferent, is anatomically an aberrant spinal nerve; its motor neurons reside in the upper cervical spinal cord, accounting for its involvement by lesions at or just below the foramen magnum. The other cranial nerves—3, 5, 7, 9, and 10—are multifunctional. Nonetheless, for clinical purposes, the examination of the cranial nerves is usually straightforward. How many components of each nerve are assessed will depend on the clinician’s diagnostic index of suspicion.

Table 3–1.The cranial nerves.


The first cranial nerve is concerned with the sense of smell. Chemoreceptors of the olfactory epithelium are located high in the nasopharynx (Figure 3–1), and the first step in assessing olfaction is to look into the nose for possible obstruction of airflow. Each nostril is then tested separately, using nonnoxious odorants such as coffee, peppermint, or soap. (Pungent substances such as ammonia will stimulate trigeminal nociceptors.) Failure to smell anything is termed anosmia. Unpleasant distortion of the stimulus is termed parosmia. Inability to identify the stimulus more likely reflects inexperience than true olfactory agnosia.

Figure 3–1.

The olfactory system. Odorants are detected by olfactory sensory neurons in the olfactory epithelium, which lines part of the nasal cavity. The axons of these neurons project to the olfactory bulb, where they terminate on the dendrites of mitral and tufted cell relay neurons within glomeruli. In turn, the axons of the relay neurons project to the olfactory cortex, where they terminate on the dendrites of pyramidal neurons whose axons project to other brain areas. (Reproduced with permission from Kandel ER, Koester JD, Mack SH, Siegelbaum SA. 2021. Principles of Neural Science, 6th ed. New York: McGraw Hill.)

The cerebral representations for olfaction are multiple, and ...

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