The olfactory nerve
The filaments of the olfactory nerve run through the cribriform plate of the ethmoid bone to reach
the olfactory bulb. These fine nerve fibres are particularly liable to injury resulting in changes to
the sense of smell
The optic nerve
The optic nerve <1> runs from the back of the eyeball through the optic
canal <2> in the sphenoid bone.
Just anterior to the pituitary stalk the optic nerves come together to form the optic chiasma.
Injury of the optic nerve usually occurs in the optic canal where the nerve is held by the bone. The
nerve can also be compressed by edema in the optic canal. The optic nerve can be tested by
examining the visual field. The ophthalmic artery (4) lies below the
optic nerve after it arises from the internal carotid artery (3). The
pituitary stalk (5) lies behind the optic chiasma.
The oculomotor nerve
The oculomotor nerve (1 in the diagram) arises from the brainstem and
enters the dura to lie on the side wall of the
cavernous sinus. The nerve enters the orbit through the superior orbital fissure. In the orbit the
nerve supplies the levator palpebrae superioris, superior rectus, inferior rectus, medial rectus and
inferior oblique muscles. The nerve also carries parasympathetic fibres to the ciliaris and sphincter
pupilae muscles. The nerve is at risk where it enters the dura. At this point it becomes fixed, and
so can be stretched. The nerve can be easily tested. Complete loss of the nerve results in inability
to constrict the pupil to light, drooping of the upper eyelid (ptosis), inability to look up, and a
depression of the globe at rest.
The trochlear nerve
The trochlear nerve (2 on the diagram) pierces the dura to enter the
cavernous sinus. The nerve lies in the lateral
wall of the cavernous sinus and enters the orbit through the superior orbital fissure. In the orbit
the nerve supplies the superior oblique muscle. The trochlear nerve is particularly liable to
traumatic injury in that it has the longest course of any cranial nerve within the skull. Patients
with trochlear nerve injury typically present with outward rotation of the affected eye and vertical
diplopia.
The trigeminal nerve
The trigeminal nerve (1) lies in the floor of the middle cranial fossa,
on the petrous temporal bone. It
forms the trigeminal ganglion (2) from which its three branches diverge.
The mandibular nerve (3) passes
out of the skull through the foramen ovale. It is a sensory nerve to the skin over the mandible, the
mandibular teeth, tongue and floor of the mouth, and motor to the muscles of mastication, the
mylohyoid, tensor tympani and palati and the anterior belly of digastric. The lingual branch of the
mandibular carries taste fibres for the anterior two-thirds of the tonge. These taste fibres originate
in the facial nerve as the chorda tympani. The maxillary nerve (4) passes
along the lateral wall of the
cavernous sinus to leave the skull through the foramen rotundum in the sphenoid bone. The nerve
is entirely sensory and innervates the skin over the maxilla, the maxillary teeth, the mucous
membrane of the nose and maxillary sinus, and the palate. The ophthalmic
nerve (5) passes along the
side of the cavernous sinus to pass into the orbit through the superior orbital fissure. The nerve
supplies sensory fibres to the cornea, eyelids, mucous membrane of the air sinuses and nasal
cavity, and the skin on the nose. Direct injuries due to fractures of the base of the skull and
penetrating objects are the two main causes of injury to the trigeminal ganglion. The ophthalmic
and infraorbital nerves may be injured in trauma to the face. The mandibular division is rarely
injured except for its inferior alveolar nerve in cases of mandibular fracture.
The distribution of the three
branches of the trigeminal nerve is shown at right. 
The abducent nerve
The abducent nerve enters the dura medial to the trigeminal nerve. It runs within the cavernous
sinus to enter the orbit through the superior orbital fissure. The nerve supplies the lateral rectus
muscle of the eye. Injury to the sixth nerve results in paralysis of the ipsilateral rectus muscle and
diplopia on attempted horizontal gaze toward the side of the paralyzed muscle. The affected eye
will tend to be deviated inward because of the unopposed action of the medial rectus muscle. The
nerve is susceptible to injury in that it has a long course before entering the dura where it is fixed,
and because it passes through the cavernous sinus. Sudden caudal displacement of the brainstem
causes avulsion of the nerve. Cavernous sinus injury can be caused by
sphenoidal fracture or carotid-cavernous fistula.
The facial nerve
The facial nerve has motor and sensory roots. These enter the temporal bone through the internal
auditory meatus (1) together with the vestibulocochlear nerve. In the
temporal bone the nerve forms
the genicular ganglion (2). Within the bone the facial nerve sends a motor
branch to the stapedius
muscles and forms the greater petrosal nerve and the chorda tympani. The greater petrosal nerve
leaves the temporal bone to run in the pterygoid canal (3) of the sphenoid
bone. It is secretomotor to
the lacrimal gland and the glands of the nose and palate. The chorda
tympani (4) runs in the middle
ear over the handle of the malleus on the tympanic membrane. The nerve leaves the skull to join
the lingual nerve (6)from the mandibular root of the trigeminal nerve. The
fibres of the chorda
tympani are secretomotor to the submandibular and sublingual glands, and carry taste sensation
from the anterior two thirds of the tongue. The main part of the facial nerve leaves the temporal
bone in the facial canal through the stylomastoid foramen (5). As the nerve
exits the skull it divides
into five branches which fan out through the parotid gland to innervate the muscles of facial
expression, the stylohyoid and the posterior belly of digastric. The facial nerve is the most
commonly injured motor cranial nerve. The nerve is prone to injury in fracture of the temporal
bone and through swelling in the bony canals through which its branches run. Proximal
interruption of the facial nerve results in paralysis of one side of the face, loss of taste sensation on
one half of the tongue and loss of lacrimation.
THe motor branches of the facial passing through the parotid
gland are:
 | 1. Temporal |
| 2. Zygomatic |
| 3. Buccal |
| 4. Marginal mandibular |
| 5. Cervical |
The vestibulocochlear nerve
The vestibular and cochlear nerves arise from ganglia within the middle ear, and travel to the
brainstem through the internal auditory meatus. Cochlear and vestibular dysfunction is common
after head injury. Some of the problems may be related to disruption of innervation, but most are
likely due to fracture of the temporal bone disrupting the ossicles, the cochlea or the semicircular
canals.
The glossopharyngeal, the vagus and the spinal accessory
These three cranial nerves leave the skull together through the jugular foramen with the internal
jugular vein. The glossopharyngeal nerve gives off a branch, the lesser petrosal, which supplies
the parotid gland, then leaves the skull to send sensory fibres to the posterior one third of the
tongue, pharyngeal mucosa, the carotid sinus and carotid body. The motor fibres innervate the
stylopharyngeus muscle. The vagus nerve acquires the fibres of the cranial accessory before
leaving the skull. These fibres are destined for striated muscle of the pharynx and larynx supplied
through the pharyngeal plexus, superior laryngeal and recurrent laryngeal nerves. The vagus
continues down the neck in the carotid sheath. The spinal accessory nerve is the motor nerve to
the sternocleidomastoid and trapezius muscles. It runs below the sternocleidomastoid and crosses
the posterior triangle superficially to supply the trapezius. The majority of injuries to these three
cranial nerves occur at the level of the skull base or after a penetrating neck injury. Injury to the
glossopharyngeal nerve results in loss of sensation to half of the posterior tongue. Unilateral
lesions of the vagus produce minor clinical symptoms related to the palate, pharynx and larynx.
Lesions of the spinal accessory cause paralysis of sternocleidomastoid and trapezius.
The hypoglossal nerve
The twelfth cranial nerve leaves the skull through the hypoglossal canal to supply the muscles of the tongue. The nerve can be injured in fractures of the occipital bone. The protruded tongue will deviate towards the injured side.
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