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The Nervous System - Basic Version / The Cranial Nerves
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Cranial nerves can be thought of as modified spinal nerves, since the “general” functional fiber types found in spinal nerves also are found in cranial nerves but are supplemented by “special” afferent or efferent fibers. fibers conveying olfaction (in cranial nerve I) and taste (in cranial nerves VII, IX, and X) are classified as special visceral afferent, while the designation of special somatic afferent is applied to fibers conveying vision (cranial nerve II) and equilibrium and hearing (cranial nerve VIII). Skeletal muscles that arise from the branchial (pharyngeal) arches are innervated by fibers of cranial nerves V, VII, IX, and X; these are classified as special visceral efferent fibers.
The 12 pairs of cranial nerves are commonly identified either by name or by Roman or Arabic numeral.
The ninth cranial nerve, which exits the skull through the jugular foramen, has both motor and sensory components. Cell bodies of motor neurons, located in the nucleus ambiguus in the medulla, project as special visceral efferent fibers to the stylopharyngeal muscle. The action of the stylopharyngeus is to elevate the pharynx, as in gagging or swallowing. In addition, the inferior salivatory nucleus of the medulla sends general visceral efferent fibers to the otic ganglion via the lesser petrosal branch of the ninth nerve; postganglionic otic fibers distribute to the parotid salivary gland.
Among the sensory components, special visceral afferent fibers convey taste sensation from the back third of the tongue via lingual branches of the nerve. General visceral afferent fibers from the pharynx, the back of the tongue, parts of the soft palate and eustachian tube, and the carotid body and carotid sinus have their cell bodies in the superior and inferior ganglia, which are situated, respectively, within the jugular foramen and just outside the cranium. Sensory fibers in the carotid branch detect increased blood pressure in the carotid sinus and send impulses into the medulla that ultimately produce a reduction in heart rate and arterial pressure; this is known as the carotid sinus reflex.
The vagus nerve has the most extensive distribution in the body of all the cranial nerves, innervating structures as diverse as the external surface of the eardrum and internal organs of the abdomen. The root of the nerve exits the cranial cavity via the jugular foramen. Within the foramen is the superior ganglion, containing cell bodies of general somatic afferent fibers, and just external to the foramen is the inferior ganglion, containing visceral afferent cells.
Pain and temperature sensations from the eardrum and external auditory canal, and pain fibers from the dura of the posterior cranial fossa, are conveyed on general somatic afferent fibers in the auricular and meningeal branches of the nerve. Taste buds on the root of the tongue and on the epiglottis contribute special visceral afferent fibers to the superior laryngeal branch. General visceral afferent fibers conveying sensation from the lower pharynx, larynx, trachea, esophagus, and organs of the thorax and abdomen to the left (splenic) flexure of the colon converge to form the posterior (or right) and anterior (or left) vagal nerves. Right and left vagal nerves are joined in the thorax by cardiac, pulmonary, and esophageal branches. In addition, general visceral afferent fibers from the larynx below the vocal folds join the vagus via the recurrent laryngeal nerves, while comparable input from the upper larynx and pharynx is relayed by the superior laryngeal nerves and by pharyngeal branches of the vagus. A vagal branch to the carotid body usually arises from the inferior ganglion.
Motor fibers of the vagus nerve include special visceral efferent fibers arising from the nucleus ambiguus of the medulla and innervating pharyngeal constrictor muscles and palatine muscles via pharyngeal branches of the vagus as well as the superior laryngeal nerve. All laryngeal musculature (excluding the cricothyroid but including the muscles of the vocal folds) are innervated by fibers arising in the nucleus ambiguus. Cells of the dorsal motor nucleus in the medulla distribute general visceral efferent fibers to plexuses or ganglia serving the pharynx, larynx, esophagus, and lungs. In addition, cardiac branches arise from plexuses in the lower neck and upper thorax, and, once in the abdomen, the vagus gives rise to gastric, celiac, hepatic, renal, intestinal, and splenic branches or plexuses.
Damage to one vagus nerve results in hoarseness and difficulty in swallowing and speaking. Injury to both nerves results in increased heart rate, paralysis of pharyngeal and laryngeal musculature, atonia of the esophagus and intestinal musculature, vomiting, and loss of visceral reflexes. Such a lesion is usually life-threatening, as paralysis of laryngeal muscles can result in asphyxiation.
The accessory nerve is formed by fibers from the medulla (known as the cranial root) and by fibers from cervical levels C1–C4 (known as the spinal root). The cranial root originates from the nucleus ambiguus and exits the medulla below the vagus. Its fibers join the vagus and distribute to some muscles of the pharynx and larynx via pharyngeal and recurrent laryngeal branches of that nerve. For this reason, the cranial part of the accessory nerve is, for all practical purposes, part of the vagus nerve.
fibers that arise from spinal levels exit the cord, coalesce and ascend as the spinal root of the accessory nerve, enter the cranial cavity through the foramen magnum, and then immediately leave through the jugular foramen. The accessory nerve then branches into the sternocleidomastoid muscle, which tilts the head toward one shoulder with an upward rotation of the face to the opposite side, and the trapezius muscle, which stabilizes and elevates (or shrugs) the shoulder.
The hypoglossal nerve innervates certain muscles that control movement of the tongue. From the hypoglossal nucleus in the medulla, general somatic efferent fibers exit the cranial cavity through the hypoglossal canal and enter the neck in close proximity to the accessory and vagus nerves and the internal carotid artery. The nerve then loops down and forward into the floor of the mouth and branches into the tongue musculature from underneath. Hypoglossal fibers end in intrinsic tongue muscles, which modify the shape of the tongue (as in rolling the edges), as well as in extrinsic muscles that are responsible for changing its position in the mouth.
A lesion of the hypoglossal nerve on one side of the head would result in paralysis of intrinsic and extrinsic musculature on the same side. The tongue would atrophy and, on attempted protrusion, would deviate toward the side of the lesion. Take this preliminary to see if your condition could respond to treatment.
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