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Gag reflex cranial nerve1/22/2024 This nerve conveys special sensory afferents (SSA) from the inner ear to the cochlear nuclei and the vestibular nuclei in the caudal medulla oblongata. The vestibulocochlear nerve (CN VIII) is responsible for auditory sensation and the vestibular sense of orientation of the head. General somatic afferents (GSA) provide sensory innervation from the auricle and a small external portion of the auditory canal. Depending on the location of the lesion, these visceral components may also be impacted in lesions of the facial nerve. Taste fibers from the anterior two-thirds of the tongue travel centrally as the chorda tympani nerve to their cell body of origin in the geniculate ganglion before synapsing centrally in the solitary nucleus. The GVE components from the superior salivary nucleus are responsible for parasympathetic innervation of the glands and mucosae of the face, except for the parotid gland and the smaller buccal and labial glands. This separate nerve bundle joins with the other components of the facial nerve within the facial canal. General visceral efferents (GVE) and special visceral afferents (SVA) fibers initially exit the brainstem as the nervus intermedius. Special visceral efferent (SVE) motor innervation is to the muscles of facial expression and exits the skull through the stylomastoid foramen deep to the parotid gland. Damage to these fibers results in ipsilateral facial paralysis. The facial nerve (CN VII) has motor and autonomic fibers with minor somatosensory components. The trigeminal nerve also provides somatic efferents to the muscles of mastication, anterior belly of the digastric, mylohyoid, tensor veli palatini, and tensor tympani muscles. The trigeminal nerve (CN V) provides the general sensory function to the face through 3 divisions: the ophthalmic (V1), maxillary (V2), and mandibular (V3). The trochlear nerve innervates the superior oblique muscle, and the abducens nerve innervates the lateral rectus muscle. The oculomotor nerve innervates the superior, inferior, and medial recti muscles and the inferior oblique muscle. The oculomotor and abducens nerves then pass through the common tendinous ring, while the trochlear nerve remains outside the ring. Each nerve originates from its like-named nucleus in the brainstem and travels through the superior orbital fissure. The oculomotor (CN III), trochlear (CN IV), and abducens (CN VI) nerves are grouped and tested together as they all function in the motion of the eye. Collaterals are also given off centrally to innervate the superior colliculus, responsible for the pupillary light reflex. Fibers from the medial visual fields travel posteriorly without crossing at the optic chiasm, while fibers of the lateral visual fields cross within the chiasm. The regions of the visual field are retinotopically organized within the optic nerve and at the synapses in the lateral geniculate nucleus. Next, the optic nerve fibers traverse the optic canal and enter the cranium. Optic nerve (CN II) fibers are formed from the projections of ganglion cells, whose cell bodies are located deep in the retina. The secondary olfactory cortex comprises the insula and entorhinal cortex (hippocampus input area connected to the parahippocampal cortex). These fibers travel to the secondary olfactory cortex. At the same time, other fibers travel laterally to the amygdala and piriform cortex, also known as the primary olfactory cortex, where conscious odorant sense is processed. The central processes of these olfactory bulb neurons project through the olfactory trigone medially to the septal area and the contralateral bulb via the anterior commissure. Chemosensory receptors in the olfactory mucosal lining bind to odorant molecules and conduct a signal through the nerves traveling through the cribriform plate of the ethmoid bone to synapse on the neurons of the olfactory bulb. The sense of smell is 95% of chemo-sensation, with taste comprising only 5%. The olfactory nerve (CN I) is composed of special visceral afferents, which provide the sense of smell and contribute to the flavor-discriminatory aspect of taste. The olfactory bulb lies within the cranial vault on the inferior portion of the frontal lobes. ![]() A lesion in the brainstem may produce multiple cranial nerve deficits and motor and sensory abnormalities of the extremities. Generally, a singular cranial nerve deficit indicates a peripheral nerve lesion. Ĭranial nerves have motor, sensory, and autonomic functions. ![]() Abnormalities in cranial nerve function help localize the lesion to a specific level of the brain or brainstem. The olfactory and optic nerves exit from the cerebrum, while the remaining cranial nerves exit from the brainstem. Cranial nerves innervate different structures in the head, neck, and trunk.
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