Cranial Nerves

Cranial Nerves

Dr. Kennedy
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.

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    Cranial 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
    . 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
    . 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
    , 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
    . The hypoglossal nerve supplies the muscles of the tongue.


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