Brainstem 3 Flashcards
(37 cards)
Describe the action and innervation of the following muscles:
- Superior rectus
- Inferior rectus
- Lateral rectus
- Medial rectus
- A: Superiorly; CN 3
- A: Inferiorly; CN 3
- A: Laterallyl; CN 6
- A: Medially; CN 3
What motion does the superior oblique muscle produce for the eyeball?
What nerve innervates this muscle?
- Intorsion, meaning movement of the upper pole of the eye inward
- Trochlear
What motion does the inferior oblique muscle produce for the eyeball?
What nerve innervates this muscle?
Extorsion, meaning movement of the upper pole of the eye outward.
Oculomotor
Describe the origin and insertion points of the rectus muscles.
These muscles originate in a common tendinous ring at the orbital apex and insert onto the sclera
Describe the origin and insertion of the superior and inferior obliques.
- Superior: originates on the sphenoid bone in the posterior medial orbit and passes anteriorly through the trochlea, a pulley-like fibrous loop on the medial superior orbital rim. It then inserts on the superior surface of the eye
- Inferior: originates along the anterior medial orbital wall and inserts on the inferior surface of the eye
Although the rectus muscles tend to produce horizontal and vertical movements and the obliques tend to produce torsional movements, there are times when the opposite can be true. Describe why this is the case.
The movement produced by an extraocular muscle depends on the direction in which the muscle pulls relative to the main axis of the eye. Therefore, as the eyes move by rotating in the orbit, the extraocular muscles can have different actions. Thus, depending on eye position, the rectus muscles can also produce torsional eye movements, and the oblique muscles can make important contributions to vertical eye movements
What is the action and innervation of the following muscles?
- Levator Palpebrae Superior
- Pupillary constrictor
- Pupillary Dilator
- Ciliary Muscle
- A: Elevates the eyelid; I: CN 3
- A: Causes pupil to beome smaller; I: CN3
- Causes pupil to become larger
- A: Adjusts the thickness of the lens in response to viewing distance; I: CN 3
What nerves enter through the superior orbital fissure?
CN 3, 4 and 6
What would right CN 3 palsy present?
How would left CN 4 palsy present?
Head tilt away from affected eye
Vertical diplopia
How would left CN 6 palsy present?
Horizonal diplopia
Describe the path taken by the oculomotor nerve as it runs through the brain.
Oculomotor nucleus –> oculomotor nerve –> subarachnoid space –> cavernous sinus –> superior orbital fissure –> orbit
The parasympathetic fibers in CN 3 are susceptible to compression from aneurysms from what artery?
Posterior communicating artery
Where is the trochlear nuclei located?
The trochlear nuclei are located in the lower midbrain at the level of the inferior colliculi.
Ventral to the periaqueductal gray matter and are bounded ventrally by the fibers of the medial longitudinal fasciculus
What types of injuries is the trochlear nerve particularly susceptible to?
- Compression from cerebellar tumors
- Very thin and are relatively easily damaged by shear injury from head trauma
Describe the path taken by the trochlear nerve as it runs through the brain.
Subarachnoid space –> cavernous sinus –> superior orbital fissure –> orbit –> superior oblique muscles
Where is the abducens nuclei?
The abducens nuclei lie on the floor of the fourth ventricle under the facial colliculi in the mid-to-lower pons
Describe the path taken by the abducens nerve as it runs through the brain.
Exit at the pontomedullary junction –> long course in the subarachnoid space –> exits the dura to enter Dorello’s canal, running between the dura and skull –> passes over the petrous tip of the temporal bone –> cavernous sinus –> superior orbital fissure –> orbit –> lateral rectus
What type of injury is the abducens nerve susceptible to and why?
Downward traction injury produced by elevated intracranial pressure due to long vertical course in skull
What is diplopia?
Double vision
What are some common causes of CN 3 palsy?
- Diabetic neuropathy
- Microvascular neuropathy associated with hypertension or hyperlipidemia
- Head trauma (shearing forces damage nerve)
- Compression of the nerve by intracranial aneurysms (Posterior communicating artery)
How can you differentiate between CN 3 palsy caused by aneurysm and CN 3 palsy caused by diabetic or microvascular neuropathy?
- There should be a high index of suspicion for aneurysms in patients presenting with CN 3 palsy. Aneurysms classically cause a painful oculomotor palsy that involves the pupil.
- A painless and complete oculomotor palsy that spares the pupil usually caused by diabetes or other microvascular neuropathy
