The Snellen chart ( ) demonstrates visual acuity by presenting standard Roman letters in a variety of sizes. Testing vision relies on the tests that are common in an optometry office. However, the taste of food can be improved by adding ingredients (e.g., salt) that stimulate the gustatory sense. Anosmia means that food will not seem to have the same taste, though the gustatory sense is intact, and food will often be described as being bland. The enjoyment of food is largely based on our sense of smell. Olfaction is not the pre-eminent sense, but its loss can be quite detrimental. If the nerve is not completely severed, the anosmia may be temporary as new neurons can eventually reconnect. If all of the fibers are sheared when the brain moves within the cranium, such as in a motor vehicle accident, then no axons can find their way back to the olfactory bulb to re-establish connections. The axons from these neurons grow back into the CNS by following the existing axons-representing one of the few examples of such growth in the mature nervous system. The neurons in the olfactory epithelium have a limited life span, and new cells grow to replace the ones that die off. The short axons of the first cranial nerve regenerate on a regular basis. Loss of the sense of smell is called anosmia and can be lost following blunt trauma to the head or through aging. The patient should be able to recognize the smell of coffee or mint, indicating the proper functioning of the olfactory system. Testing smell is straightforward, as common smells are presented to one nostril at a time. The trigeminal nerve is a mixed nerve that carries the general somatic senses from the head, similar to those coming through spinal nerves from the rest of the body. Taste sensation is relayed to the brain stem through fibers of the facial and glossopharyngeal nerves. The olfactory, optic, and vestibulocochlear nerves (cranial nerves I, II, and VIII) are dedicated to four of the special senses: smell, vision, equilibrium, and hearing, respectively. Tests of these functions can provide insight into damage to specific regions of the brain stem and may uncover deficits in adjacent regions.

Preganglionic parasympathetic nerve fibers that control pupillary size, salivary glands, and the thoracic and upper abdominal viscera are found in four of the nerves. The movement of the eyes, face, tongue, throat, and neck are all under the control of cranial nerves. The special senses are served through the cranial nerves, as well as the general senses of the head and neck. The vagus nerve (cranial nerve X) has autonomic functions in the thoracic and superior abdominal cavities. The twelve cranial nerves serve the head and neck. The cranial nerve exam allows directed tests of forebrain and brain stem structures. Finally, one nerve controls the muscles of the neck, assisting with spinal control of the movement of the head and neck. Four nerves make up the cranial component of the parasympathetic nervous system responsible for pupillary constriction, salivation, and the regulation of the organs of the thoracic and upper abdominal cavities. Four nerves connect to muscles of the face, oral cavity, and pharynx, controlling facial expressions, mastication, swallowing, and speech. Three nerves are connected to the extraocular muscles resulting in the control of gaze. Three of the nerves are strictly responsible for special senses whereas four others contain fibers for special and general senses. The nerves can be categorized by functions, and subtests of the cranial nerve exam can clarify these functional groupings. But knowing the names of the nerves in order often leaves much to be desired in understanding what the nerves do. The twelve cranial nerves are typically covered in introductory anatomy courses, and memorizing their names is facilitated by numerous mnemonics developed by students over the years of this practice. Relate cranial nerve deficits to damage of adjacent, unrelated structures.Suggest diagnoses that would explain certain losses of function in the cranial nerves.Match the regions of the forebrain and brain stem that are connected to each cranial nerve.Describe the functional grouping of cranial nerves.By the end of this section, you will be able to:

Gag reflex cranial nerve test