Cranial Nerves (Part 2)-أ.د.عمرو حسن الحسني أستاذ المخ و الأعصاب

Cranial nerve V (trigeminal nerve)

Cranial Nerve V is the trigeminal nerve responsible for the general somatic sensory innervation (GSA) of the face through its three main branches, V1, V2, and V3 (ophthalmic, maxillary, and mandibular, respectively). This cranial nerve (via V3) is also responsible for motor innervation (SVE) of the muscles of mastication, the anterior belly of the digastric, mylohyoid, and two tiny tensor muscles: the tensor veli palatini and tensor tympani. While no autonomic fibers travel with the fifth cranial nerve as it exits the pons, parasympathetic fibers from the other mixed cranial nerves will join with peripheral branches of cranial nerve V to innervate their respective target structures, such as the lacrimal, parotid, submandibular and sublingual glands. In this sense, central nuclear or supranuclear lesions may result in ipsilateral sensory or motor deficits, but parasympathetic functions will only be impaired by peripheral nerve damage to the respective branches.[9]

Cranial nerve VII (facial nerve)

Cranial nerve VII (facial nerve) has both motor and autonomic fibers with minor somatosensory components. 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 (facial palsy). General visceral efferents (GVE) and special visceral afferents (SVA) fibers initially exit the brainstem as nervus intermedius, a separate nerve bundle that joins with the other components of the facial nerve within the facial canal. The GVE components from the superior salivary nucleus are responsible for parasympathetic innervation of the glands and mucosae of the face, with the exception of the parotid gland and the smaller buccal and labial glands. 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. Depending on the location of the lesion, these visceral components may also be impacted in lesions of the facial nerve. Somatic afferents (GSA) provide sensory innervation from the auricle and a small external portion of the auditory canal.[10]

Cranial nerve VIII (vestibulocochlear nerve)

Cranial nerve VIII, the vestibulocochlear nerve, is responsible for the auditory sense and the vestibular sense of orientation of the head.

Cranial nerve IX (glossopharyngeal nerve)

Cranial nerve IX (glossopharyngeal nerve) is responsible for motor (SVE) innervation of the stylopharyngeus and the pharyngeal constrictor muscles receive sensory information from the carotid body and carotid sinus, and taste from the posterior third of the tongue to synapse on the solitary nucleus. The sensory afferents (GSA) receive information from the skin over the tongue, oropharynx, middle ear cavity, and auditory canal.[12]

Cranial nerve X (vagus nerve)

Cranial nerve X is the vagus nerve. The parasympathetic efferents (GVE) fibers from the dorsal vagal nucleus to the thoracic and abdominal viscera to the splenic flexure of the colon represent its major neural component. These fibers form a comprehensive plexus that travels along the esophageal serosa to the viscera. It also has a considerable motor output (SVE) from the nucleus ambiguous to the pharyngeal and soft palate muscles and the intrinsic laryngeal muscles via the superior and recurrent laryngeal nerves. Somatic afferents (GSA) supply the posterior cranial dura and a portion of the ear and external auditory canal epithelium. Visceral afferents (GVA) from the pharynx, larynx, aorta, thoracic and abdominal viscera

Cranial nerve XI (Accessory nerve)

Cranial nerve XI, the spinal accessory nerve, is responsible for the general somatic efferent (GSE) motor innervation of the trapezius and sternocleidomastoid muscles by way of the spinal nucleus of the accessory nerve. The spinal nucleus of the accessory nerve is located within the cervical spinal cord from the levels of C1 through approximately C5/6. The fibers emerge as independent roots, separate from the anterior or dorsal spinal roots of the central spinal grey matter, and ascend through the foramen magnum to enter the cranial cavity. These fibers then exit via the jugular foramen along with cranial nerves IX and X.

Cranial nerve XII (Hypoglossal nerve)

Cranial nerve XII, the hypoglossal nerve, is responsible for the general somatic efferent (GSE) innervation of the intrinsic and extrinsic muscles of the tongue, except the palatoglossus muscle, from the nerve’s synonymous nucleus. This includes the genioglossus, geniohyoid, hyoglossus, and styloglossus muscles. Fibers from the hypoglossal nucleus exit the medulla from the sulcus between the pyramids and the olives as a collection of fibers that coalesce before entering the hypoglossal canal to exit the cranium.[13]