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Summary neurology 2- usmlerx bricks
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Cranial Nerve II andPupillary Control
By ScholarRx
Updated September 11, 2020
access_time14 min
Learning Objectives (4)
After completing this brick, you will be able to:
● Describe and state the cause of the abnormal pupillary findings of
Argyll Robertson pupil and Horner syndrome.
● 1
● Describe the accommodation (near triad) reflex.
● 2
● Describe the pupillary reflex and the neuroanatomical circuitry
underlying it.
● 3
● Discuss the roles of the sympathetic and parasympathetic divisions
of the autonomic nervous system in regulating pupillary diameter.
● 4
cableCASE CONNECTION
Sixteen-year-old DF is undergoing chemotherapy for advanced Ewing
sarcoma with bony metastases. She has had a great deal of difficulty
tolerating the treatment, including cyclical nausea and vomiting. Today
she developed blurry vision in her right eye, with an inability to focus
close up. She has no eye pain or double vision. Her medications are
topotecan, cyclophosphamide, and a scopolamine patch. On exam,
her right pupil is widely dilated and sluggishly reactive to light. The left
,eye is normal, and she has no other neurologic findings. Her distance
vision is normal in both eyes. DF’s mother asks, “Is this something
caused by the tumor?”
Is the dilated pupil caused by the tumor? Consider your answer as
you read, and we’ll revisit at the end of the brick.
Sympathetic Dilation of the Pupil
We will start with dilation, also known as mydriasis. The sympathetic
nervous system is responsible for dilation of the pupil. The
sympathetic pathway to the pupil begins in the posterolateral
hypothalamus (Figure 1).
,Figure 1
Credit: ©ScholarRx
Let’s follow the pathway of these nerve fibers, starting at the
posterolateral hypothalamus:
● From the posterolateral hypothalamus, first-order neurons
descend through the lateral brainstem to the C8-T2 region of the
intermediolateral cell column (IML) of the spinal cord, also
known as the ciliospinal center of Budge.
, ● Here these first-order neurons synapse onto second-order
neurons—also known as preganglionic sympathetic neurons
because they travel to a ganglion.
● The preganglionic axons leave the spinal cord, ascend in the
cervical sympathetic trunk, and eventually synapse on third-
order neurons in the superior cervical ganglion.
● The superior cervical ganglion lies at about the level of the
second or third cervical vertebrae, deep to the internal carotid
artery and internal jugular vein. Superior cervical ganglion
neurons are also known as the postganglionic neurons.
● The postganglionic fibers leave the superior cervical ganglion,
continuing to travel along the internal carotid artery to enter the
skull.
● The fibers then join the nasociliary nerves (branch of the
ophthalmic nerve) and travel with the latter nerve into the orbit
to the eye.
● Once in the eye, the sympathetic neurons release norepinephrine
at the iris dilator muscle.
● Norepinephrine causes contraction of the radially arranged
fibers of the iris dilator muscle, resulting in dilation of the pupil,
or mydriasis.
If this pathway were interrupted, what would you expect to see? The
answer is no dilation or in fact pupillary constriction (miosis).
Damage anywhere along this pathway will result in tonic pupillary
constriction due to the unopposed action of the parasympathetically
innervated pupillary constrictor muscles. Because damage is typically
unilateral, the pupils will also be of unequal sizes (anisocoria).
peopleCLINICAL CORRELATION
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