APPENDIX Cranial Nerves: Anatomy and Physiology We have 12 cranial nerves; some are sensory nerves, some are motor nerves, and some are part of the autonomic nervous system. I. Olfactory Sensory: Smell II. Optic Sensory: Vision III. Oculomotor Motor: Eye Movements: Innervates all extraocular muscles, except the superior oblique and lateral rectus mus- cles. Innervates the striated muscle of the eyelid. Autonomic: Mediates pupillary constriction and accommodation for near vision. IV. Trochlear Motor: Eye Movements: Innervates superior oblique muscle. V. Trigeminal Sensory: Mediates cutaneous and proprioceptive sensations from skin, muscles, and joints in the face and mouth, includ- ing the teeth and the anterior two-thirds of the tongue. Motor: Innervates muscles of mastication. VI. Abducens Motor: Eye Movements: Innervates lateral rectus muscle. VII. Facial Motor: Innervates muscles of facial expression. Autonomic: Lacrimal and salivary glands. Sensory: Mediates taste and possible sensation from part of the face (behind the ear). Nervous intermedius: Pain around the ear; possibly taste. VIII. Vestibulocochlear Sensory: Hearing Equilibrium, postural reflexes, orientation of the head in space. IX. Glossopharyngeal Sensory: Taste Innervates taste buds in the posterior third of tongue. Sensory: Mediates visceral sensation from palate and posterior third of the tongue. Innervates the carotid body. Motor: Muscles in posterior throat (stylopharyngeal muscle). Autonomic: Parotid gland. X. Vagus Sensory: Mediates visceral sensation from the pharynx, larynx, thorax, and abdomen. Innervates the skin in the ear canal and taste buds in the epiglottis. Autonomic: Contains autonomic fibers that innervate smooth mus- cle in heart, blood vessels, trachea, bronchi, esopha- gus, stomach, and intestine. Motor: Innervates striated muscles in the soft palate, pharynx, and the larynx. XI. Spinal accessory Motor: Innervates the trapezius and sternocleidomastoid muscles. XII. Hypoglossal Motor: Innervates intrinsic muscles of the tongue. 343 344 Intraoperative Neurophysiological Monitoring CN VI. Abducens nerve: Controls eye FUNCTIONS OF THE CRANIAL movements from the midline toward the side. NERVES Lesion to CN VI prevents movements of the eye from the midline and outward. CN I. Olfactory nerve: Communicates chemical airborne messages to the brain. CN VII. Facial nerve: Controls the face. CN II. Optic nerve: Communicates optic CN VII is often monitored intraoperatively information. Variations in contrast are the most because it is at risk in all operations to powerful stimulations of the visual system. remove acoustic tumors and it is involved in diseases such as hemifacial spasm. The auto- CN III. Oculomotor nerve: Controls all of nomic fibers of CN VII control both tear the extraocular eye muscles, except the glands and salivary glands. A loss of facial trochlearis and the lateral rectus muscles; thus, function is cosmetically important and makes it innervates the superior, the inferior, the it difficult to eat, and the lack of tears and the medial rectus, and the inferior oblique muscles. inability to close the eye might result in This muscle moves the eye in all directions; injures to the cornea. therefore lesions to CN III affect essentially all Nervus intermedius: Perhaps taste. Deep eye movements and cause the eye to be devi- ear pain (geniculate neuralgia). ated downward and outward. It also innervates the eyelid and makes it possible to close the eye CN VIII. Vestibulocochlear nerve: The when lying down. Lesions to CN III cause pto- two parts of this nerve communicate auditory sis (partial closure of the eyelid). CN III con- information and information about head move- tains autonomic fibers that control the size of ments. Whereas the covering of the nerve fibers the pupil and stretches the lens to achieve of most of the brainstem cranial nerves changes accommodation. Lesions to the CN III essen- from peripheral myelin to central myelin a few tially make the eye useless. millimeters from the brainstem, the transitional zone for CN VIII is in the internal auditory CN IV. Trochlearis nerve: Controls the meatus, which means that CN VIII throughout trochlear muscle, and contraction of this mus- its entire intracranial course is covered with cle causes the eye to move downward when it central myelin and it has no epineurium. This is in a position medial to the midline. Lesions means that CN VIII has mechanical properties of CN IV affect downward and inward move- similar to those of the brain, making it more ments of the eye. fragile than other cranial nerves. The vestibular portion of CN VIII communi- CN V. Trigeminal nerve: This nerve’s sen- cates to the brain information gathered by the sory portion — the portio major — innervates the inner ear about the position of the head. In fact, skin of the face and the cornea. This portion of we can do quite well without the vestibular por- CN V thereby communicates sensory informa- tion of CN VIII, but if it is injured on one side tion about touch and pain from the face and the only, severe balance disturbances can result; mouth. CN V is the nerve that causes toothache however, one can adapt to such dysequilibrium and the severe pain of trigeminal neuralgia. depending on one’s age (better when younger Lesions to the sensory portion of CN V cause a than when older). loss of sensation of the face. Loss of corneal sen- sation could result in corneal bruises. CN IX. Glossopharyngeal nerve: Com- The motor potion of CN V –– the portio municates sensory information from the throat minor –– controls the muscles of mastication. to the brain and information about blood pres- Lesions to the motor portion of CN V cause sure to the cardiovascular centers. The motor atrophy of the mastication muscles. portion of CN IX controls the stylopharyngeal Appendix Cranial Nerves: Anatomy and Physiology 345 muscle. Lesions of CN IX will cause a loss of effect on the cardiovascular system, but the effect gag reflex on the affected side and a risk of of bilateral severance of the vagal nerve is severe. choking on food. Lesions on one side will likely The vagus nerve might carry more complex sen- have little effect on cardiovascular function, but sory information from the lower body. a loss of CN IX on both sides is fatal. CN XI. Spinal accessory nerve: Controls CN X. Vagus nerve: This nerve’s name muscles in the neck and shoulder (sternocleido- means the “vagabondering” nerve, descriptive in mastoid and trapezoid muscles). Lesions of CN that it travels around in a large portion of the XI cause atrophy of the muscles that are inner- body. This nerve conveys parasympathetic input vated by that nerve. to the entire chest and abdomen. The vagus nerve also controls the vocal cords, the heart, and the CN XII. Hypoglossal nerve: Controls diaphragm. The most noticeable effect of unilat- movements of the tongue. Unilateral lesions to eral lesions to CN X is hoarseness, because the CN XII cause deviation of the tongue and atro- vocal cord on the affected side cannot close. phy of the tongue on the affected side. Bilateral Although CN X carries information to and from lesions make it almost impossible to speak and the heart, unilateral lesions to CN X have little swallowing is impaired. Abbreviations μS: Microseconds ICC: Central nucleus of the inferior AAF: Anterior auditory field colliculus ABI: Auditory brainstem implants IPL: Interpeak latency ABR: Auditory brainstem response ISI: Inter stimulus interval AI: Primary auditory cortex kHz: Kilohertz AICA: Anterior inferior cerebellar artery kOhm: Kiloohm AII: Secondary cortex LED: Light-emitting diodes AP: Action potentials LGN: Lateral geniculate nucleus AVCN: Anterior ventral cochlear nucleus LL: Lateral lemniscus CAP: Compound action potentials mA: Milliampere CCT: Central conduction time ma: Milliampere cm: Centimeter MAC: Minimal end-alveolar concentration CM: Cochlear microphonics MCA: Middle cerebral artery CMAP: Compound muscle action potential MEP: Motor evoked potentials CMN: Centromedian nucleus MGB: Medial geniculate body CN I-XII: Cranial nerves I-XII MGP: Medial segment of globus pallidus CN: Cochlear nucleus MI: Primary motor cortex CNAP: Compound nerve action potentials mm: Millimeter CNS: Central nervous system MOhm: Megaohm CPA: Cerebellopontine angle ms: Millisecond CPG: Central pattern generator MSO: Medial superior olivary nucleus CSF: Cerebrospinal fluid mv: Millivolts CT: Corticospinal tract MVD: Microvascular decompression DAS: Dorsal acoustic stria (operations) dB: Decibel NF2: Neurofibromatosis type 2 DBS: Deep brain stimulation NIHL: Noise induced hearing loss DC: Direct electric current NMEP: Neurogenic motor evoked potentials DCN: Dorsal cochlear nucleus NTB: Nucleus of the trapezoidal body DPV: Disabling positional vertigo PAF: Posterior auditory field DRG: Dorsal root ganglia PD: Parkinson’s disease ECoG: Electrocochleographic PeSPL: Peak equivalent sound pressure level EEG: Electroencephalographic PICA: Posterior inferior cerebellar artery (potentials) PMC: Premotor cortical (areas) EKG: Electrocardiogram (or pps: Pulses per second electrocardiographic) PVCN: Posterior ventral cochlear nucleus EMG: Electromyographic (potentials) REZ: Root exit zone (or root entry zone) EPSP: Excitatory postsynaptic potential RMS: Root mean square GPe: Globus pallidus external part SI: Primary somatosensory cortex CPG: Central pattern generator SMA: Supplementary motor area GPi: Globus pallidus internal part SNc: Substantia nigra pars compacta GPN: Glosso-pharyngeal