The cranial nerves emerge from or enter the skull (the cranium), as opposed to the spinal nerves which emerge or enter the vertebral column. Cranial nerves come directly from the brain through the skull. The cranial nerves are nerves of the brain. There are 12 cranial nerves each of which is accorded a Roman numeral and a name:
- Cranial nerve I: The olfactory nerve,
- Cranial nerve II: the optic nerve,
- Cranial nerve III: the oculomotor nerve,
- Cranial nerve IV: the trochlear nerve,
- Cranial nerve V: the trigeminal nerve,
- Cranial nerve VI: the abducent nerve,
- Cranial nerve VII: the facial nerve,
- Cranial nerve VIII: the vestibulocochlear nerve,
- Cranial nerve IX: the glossopharyngeal nerve,
- Cranial nerve X:the vagus nerve,
- Cranial nerve XI: the accessory nerve, and
- Cranial nerve XII: the hypoglossal nerve.
Let’s take them one by one.
No titleCranial nerve I: The first nerve to emerge from or enter the skull
(the cranium). The first cranial nerve is the olfactory nerve
which permits the sense of smell.
Cranial nerve II: The second cranial is the optic nerve, the
nerve that connects the eye to the brain and carries the impulses formed by
the retina – the nerve layer that lines the back of the eye, senses light and
creates the impulses – to the brain which interprets them as images.
Cranial nerve III: The third cranial nerve is the oculomotor nerve
which branches to four of the six tiny muscles which control the movement
of the orbit (eyeball).
Cranial nerve IV: The fourth cranial nerve, the trochlear nerve,
is the nerve supply to the superior oblique muscle of the eye, one of
the muscles that moves the eye. Paralysis of the trochlear nerve results in
rotation of the eyeball upward and outward (and, therefore, double vision).
Cranial nerve V: The fifth cranial nerve is the trigeminal nerve.
The trigeminal nerve is quite complex. It functions both as the chief nerve
of sensation for the face and the motor nerve controlling the muscles of mastication (chewing). Problems with the sensory part of the trigeminal nerve result
in pain or loss of sensation in the face. Problems with the motor root of the
trigeminal nerve result in deviation of the jaw toward the affected side and
trouble chewing. The term "trigeminal" comes from the Latin trigeminus
meaning "threefold," referring to the three divisions (ophthalmic,
maxillary and mandibular) of this nerve.
Cranial nerve VI: The sixth cranial nerve is the abducent nerve.
It is a small motor nerve that has one task: to supply a muscle called the
lateral rectus muscle of the eye that moves the eye outward. Paralysis of
the abducent nerve causes inward turning of the eye (internal strabismus)
leading to double vision. The word "abducent" comes from the Latin
"ab-", away from + "ducere", to draw = to draw away. The
abducent (or abducens) operates the lateral rectus muscle that draws
the eye toward the side of the head. The abducent nerve is also called the abducens
Cranial nerve VII: The facial nerve is the seventh cranial
nerve. The facial nerve supplies the muscles of facial expression. Paralysis
of the facial nerve causes a characteristic picture with drooping of one side
of the face, inability to wrinkle the forehead, inability to whistle, inability
to close the eye and deviation of the mouth toward the other side of the face.
Paralysis of the facial nerve is called Bell’s palsy.
Cranial nerve VIII: The eighth cranial nerve is the vestibulocochlear
nerve. The vestibulocochlear nerve is responsible for the sense of hearing
and it is also pertinent to balance, to the body position sense. Problems with
the vestibulocochlear nerve may result in deafness, tinnitus (ringing
or noise in the ears), dizziness, vertigo and vomiting.
Cranial nerve IX: The ninth cranial nerve is the glossopharyngeal
nerve. The glossopharyngeal nerve supplies the tongue, throat, and one of
the salivary glands (the parotid gland). Problems with the glossopharyngeal
nerve result in trouble with taste and swallowing.
Cranial nerve X: The tenth cranial nerve, and one of the most important,
is the vagus nerve. The vagus nerve originates in the medulla oblongata,
a part of the brain stem. The vagus nerve is a remarkable nerve that relates
to the function of numerous structures in the body. The vagus nerve supplies
nerve fibers to the pharynx (throat), larynx (voice box), trachea
(windpipe), lungs, heart, esophagus and most of the intestinal tract (as far
as the transverse portion of the colon). And the vagus nerve brings sensory
information back from the ear, tongue, pharynx and larynx. The term "vagus"
(Latin for "wandering") is apt because the vagus nerve wanders all
the way down from the brainstem to the colon, a long wandering trek. Complete
interruption of the vagus nerve causes a characteristic syndrome. The back part
of the palate (the soft palate) droops on that side. The capacity to
gag (the gag reflex) is also lost on that side. The voice is hoarse and
nasal. The vocal cord on the affected side is immobile. The result is dysphagia
and dysphonia (trouble swallowing and trouble speaking). One of the best
known branches of the vagus nerve is the recurrent laryngeal nerve. After
leaving the vagus nerve, the recurrent laryngeal nerve goes down into the chest
and then loops back up to supply the larynx (the voice box). Damage to
the recurrent laryngeal nerve can result from diseases inside the chest (intrathoracic
diseases) such as a tumor or an aneurysm (ballooning) of the arch of
the aorta or of the left atrium of the heart. The consequence is laryngeal
palsy, paralysis of the larynx (the voice box), on the affected side. Laryngeal
palsy can also be caused by damage to the vagus nerve before it gives off the
recurrent laryngeal nerve.
Cranial nerve XI: The eleventh cranial nerve is the accessory nerve.
The accessory is so-called because, although it arises in the brain, it receives
an additional (accessory) root from the upper part of the spinal cord. The
accessory nerve supplies the sternocleidomastoid and trapezius muscles. The
sternocleidomastoid muscle is in the front of the neck and turns the head.
The trapezius muscle moves the scapula (the wingbone), turns the face
to the opposite side, and helps pull the head back. Damage to the accessory
nerve can be isolated (confined to the accessory nerve) or it may also involve
the ninth and tenth cranial nerves which exit through the same opening (foramen)
from the skull . Accessory neuropathy (nerve disease) can sometimes occur
and recur for unknown reasons. Most patients recover. Paralysis of the accessory
nerve prevents rotation of the head away from that side and causes drooping
of the shoulder.
Cranial nerve XII: The twelfth cranial nerve is the hypoglossal
nerve. The hypoglossal nerve supplies the muscles of the tongue.