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.
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