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Introduction
Examination of the cranial nerves allows one to "view" the brainstem all
the way from its rostral to caudal extent. The brainstem can be divided
into three levels, the midbrain, the pons and the medulla. The cranial
nerves for each of these are: 2 for the midbrain (CN 3 & 4), 4 for the
pons (CN 5-8), and 4 for the medulla (CN 9-12).
It is important to remember that cranial nerves never cross (except for
one exception, the 4th CN) and clinical findings are always on the same
side as the cranial nerve involved.
Cranial nerve findings when combined with long tract findings (corticospinal
and somatosensory) are powerful for localizing lesions in the brainstem.
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Cranial
Nerve 1
Olfaction is the only sensory modality with direct access to cerebral cortex
without going through the thalamus. The olfactory tracts project mainly
to the uncus of the temporal lobes. |
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Cranial
Nerve 2
This cranial nerve has important localizing value because of its "x" axis
course from the eye to the occipital cortex. The pattern of a visual field
deficit indicates whether an anatomical lesion is pre- or postchiasmal,
optic tract, optic radiation or calcarine cortex. |
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Cranial
Nerve 3 and 4
These cranial nerves give us a view of the midbrain. The 3rd nerve in particular
can give important anatomical localization because it exits the midbrain
just medial to the cerebral peduncle. The 3rd nerve controls eye adduction
(medial rectus), elevation (superior rectus), depression (inferior rectus),
elevation of the eyelid (levator palpebrae superioris), and parasympathetics
for the pupil.
The 4th CN supplies the superior oblique muscle, which is important to looking
down and in (towards the midline). |
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Pontine
Level
Cranial nerves 5, 6, 7, and 8 are located in the pons and give us a view
of this level of the brainstem. |
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Cranial
Nerve 6
This cranial nerve innervates the lateral rectus for eye abduction.
Remember that cranial nerves 3, 4 and 6 must work in concert for conjugate
eye movements; if they don't then diplopia (double vision) results.
The medial longitudinal fasciculus (MLF) connects the 6th nerve nucleus
to the 3rd nerve nucleus for conjugate movement. |
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Major Oculomotor
Gaze Systems
Eye movements are controlled by 4 major oculomotor gaze systems,
which are tested for on the neurological exam. They are briefly outlined
here:
- Saccadic
(frontal gaze center to PPRF (paramedian pontine reticular formation)
for rapid eye movements to bring new objects being viewed on to the
fovea.
- Smooth Pursuit
(parietal-occipital gaze center via cerebellar and vestibular pathways)
for eye movements to keep a moving image centered on the fovea.
- Vestibulo-ocular
(vestibular input) keeps image steady on fovea during head movements.
- Vergence
(optic pathways to oculomotor nuclei) to keep image on fovea predominantly
when the viewed object is moved near (near triad- convergence,
accommodation and pupillary constriction)
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Cranial
Nerve 5
The entry zone for this cranial nerve is at the mid pons with the motor
and main sensory (discriminatory touch) nucleus located at the same level.
The axons for the descending tract of the 5th nerve (pain and temperature)
descend to the level of the upper cervical spinal cord before they synapse
with neurons of the nucleus of the descending tract of the 5th nerve. Second
order neurons then cross over and ascend to the VPM of the thalamus. |
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Cranial
Nerve 7
This cranial nerve has a motor component for muscles of facial expression
(and, don't forget, the strapedius muscle which is important for the acoustic
reflex), parasympathetics for tear and salivary glands, and sensory for
taste (anterior two-thirds of the tongue).
Central (upper motor neuron-UMN) versus Peripheral (lower motor neuron-LMN)
7th nerve weakness- with a peripheral 7th nerve lesion all of the muscles
ipsilateral to the affected nerve will be weak whereas with a "central 7th
", only the muscles of the lower half of the face contralateral to the lesion
will be weak because the portion of the 7th nerve nucleus that supplies
the upper face receives bilateral corticobulbar (UMN) input. |
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Cranial
Nerve 8
This nerve is a sensory nerve with two divisions- acoustic and vestibular.
The acoustic division is tested by checking auditory acuity and with the
Rinne and Weber tests.
The vestibular division of this nerve is important for balance. Clinically
it be tested with the oculocephalic reflex (Doll's eye maneuver) and oculovestibular
reflex (ice water calorics). |
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Medullary
Level
Cranial nerves 9,10,11, and 12 are located in the medulla and have
localizing value for lesions in this most caudal part of the brainstem. |
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Cranial
nerves 9 and 10
These two nerves are clinically lumped together. Motor wise, they innervate
pharyngeal and laryngeal muscles. Their sensory component is sensation for
the pharynx and taste for the posterior one-third of the tongue. |
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Cranial
Nerve 11
This nerve is a motor nerve for the sternocleidomastoid and trapezius muscles.
The UMN control for the sternocleidomastoid (SCM) is an exception to the
rule of the ipsilateral cerebral hemisphere controls the movement of the
contralateral side of the body. Because of the crossing then recrossing
of the corticobulbar tracts at the high cervical level, the ipsilateral
cerebral hemisphere controls the ipsilateral SCM muscle. This makes sense
as far as coordinating head movement with body movement if you think about
it (remember that the SCM turns the head to the opposite side). So if I
want to work with the left side of my body I would want to turn my head
to the left so the right SCM would be activated. |
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Cranial
Nerve 12
The last of the cranial nerves, CN 12 supplies motor innervation for
the tongue. |
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Traps
A 6th nerve palsy may be a "false localizing sign". The reason for this
is that it has the longest intracranial route of the cranial nerves, therefore
it is the most susceptible to pressure that can occur with any cause of
increased intracranial pressure. |
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Pearls
Rules of Diplopia
- Diplopia is maximum
in the direction of action of the paretic muscle
- The most peripherally
seen image is the false image and comes from the eye with the paretic
muscle.
- The diplopia is
horizontal if the medial or lateral recti are involved and vertical
if the elevator or depressor muscles are involved.
Intranuclear ophthalmoplegia
(INO)
A lesion of the MLF causes nystagmus of the abducting eye with absent
adduction of the other eye. The lesion is on the side of the eye that
should be adducting. There can be a bilateral INO in which case neither
eye adducts with horizontal gaze.
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