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Anatomy of IOM
Filaments of
olfactory nerve (I)
Frontal lobe
Olfactory bulb
Olfactory tract
Temporal lobe
Optic nerve (II)
Optic chiasma
Optic tract
nerve (III)
nerve (IV)
Facial nerve (VII)
nerve (V)
nerve (VIII)
nerve (IX)
nerve (VI)
Vagus nerve (X)
Accessory nerve (XI)
Hypoglossal nerve (XII)
Medulla oblongata
Fig. 2.19 Ventral view of the brain showing the 12 cranial nerves
has a unique name but may also be designated by
its number. The numerical designation takes the
form of “CN” followed by the Roman numeral
assigned to the nerve. For example, the facial
nerve is also designated CNVII. You should be
familiar with both the names and numerical designations of all 12 cranial nerves.
Table 2.1 lists all of the cranial nerves as well
as information about each nerve that you should
know. There is only one cranial nerve that cannot
be monitored and that is CNI, the olfactory nerve,
which mediates the sense of smell.
Two of the sensory cranial nerves can be
monitored by special evoked potentials. The
optic nerve (CNII) and the visual pathway are
monitored with visual evoked potentials (VEPs).
VEPs are most often carried out in the clinic and
are primarily used to diagnose optic neuritis and
multiple sclerosis. Intraoperatively, VEPs are
used to protect the optic nerve when there is a
risk of damaging the nerve during a craniotomy,
such as for removal of a tumor near the optic
nerve or optic chiasm. Because VEPs are highly
sensitive to anesthesia, reliable recordings are
often difficult to obtain during surgery.
The vestibulocochlear nerve (CNVIII) is the
other cranial nerve that is monitored with the use
of special evoked potentials. CNVIII is actually
two nerves in one (Fig. 2.20). The vestibular
branch innervates the semicircular canals and is
important for balance, while the auditory branch
innervates the cochlea and mediates hearing. The
auditory branch is monitored with the use of
brainstem auditory evoked potentials (BAEPs).
BAEPs are discussed in another chapter of
this book.
Three cranial nerves innervate the extraocular
muscles (Fig. 2.21). The oculomotor nerve
(CNIII) controls most of the movements of the
eye and also governs the pupillary reflex and
accommodation. CNIII innervates all of the
extraocular muscles except two. The ones innervated by CNIII are the superior rectus, medial
rectus, and inferior rectus. A lesion of CNIII produces oculomotor palsy characterized by a lateral
and downward deviation in the gaze known as