Extraocular muscles and Semi-circular canals Flashcards
How many extraocular muscles are there and what are they called?
7
Lateral Rectus
Medial Rectus
Superior Rrectus
Inferior Rectus
Superior Oblique
Inferior Oblique
Levator Palpabrae Superioris
What does conjugate movement mean?
Co-ordinated movement of both eye to form one image
What causes diplopia (double vision)?
Paralysis of muscles
Which extraocular muscles do the oculomotor nerve (CN III) innervate?
Superior rectus, inferior rectus, medial rectus, inferior oblique and levator palpabrae
What nerve innervates the Superior oblique?
Trochlear nerve (CN IV)
The superior oblique is held in place by a trochlea, a pulley, so think trochlear.
Which nerve innervates the Lateral Rectus?
Abducens (CN VI)
The lateral rectus abducts, so think abducens
Where does the Levator palpabrae superioris insert onto?
Upper eyelid
Where do the Rectus and Oblique muscles insert to?
The sclera
Why do the muscle fibres not coincide with the optical axis?
The orbit has an apex medially so it’s straight line forward (orbital axis) isn’t the same as what the eyeball’s straight line forward is (optic axis)
What does injury to CN III (oculomotor) result in
Ptosis (drooping eyelid)
Medial Rectus becomes weak or paralysed so the eye deviates laterally. This results in double vision (diplopia)
Is the Levator Palpabrae Superioris innervated by sympathetic or parasympathetic fibres?
Sympathetic fibres
What action does the medial rectus make?
Moves eye medially (adducts) via CN III (oculomotor)
What does the lateral rectus do?
Moves eye laterally (abducts) via CN VI (abducens)
What does injury to CN VI (abducens) cause?
Lateral rectus becomes weak / paralysed. Eye deviates medially. Patient experiences double vision (diplopia)
What does the superior rectus do?
Primary action: elevates eye
Secondary action: adducts and medially rotates (intorsion)
What dos the inferior rectus do?
Primary action: depresses eye
Secondary action: adducts and laterally rotates
What does the superior oblique do?
Primary action: medially rotates the eye
Secondary action: depresses and abducts
What does the inferior oblique do?
Primary action: laterally rotates the eye
Secondary action: elevates and abducts
Which muscle works with the inferior rectus to look down?
The superior oblique (cancels out the adduction and extorsion)
Which muscle works with the superior rectus to look up?
The inferior oblique (cancels out the inversion and adduction)
How do you examine the extraocular muscles?
Patient follows your finger. Examine the right eye. Then repeat movements and examine the left eye.
Test medial and lateral rectus, then superior and inferior recti, then oblique muscles.
To test superior and inferior recti, move finger laterally and then up to test recti not oblique. (Superior/inferior rectus causes elevation/depression and abduction whereas the obliques causes elevation/depression and adduction).
What formula can help remember which muscle is innervated by which nerve
LR6 SO4 3
Lateral recuts - CN VI
Superior oblique - CN IV
All the others - CN III
What can help to remember movements of the superior/inferior recti and obliques?
RAD SIN
Rectus = ADduction
Superior = Intorsion
Therefore, oblique = abduction and inferior = extorsion.
Semicircular ducts
At right angles to each other.
Contain fluid (emdolymph). This fluid empties into sac called the utricle
Which parts contain sense organs for balance?
Semicircular ducts and utricle
What does the Crista ampullaris contain?
Cupula, endolymph, hair cells, supporting cells and sensory nerve fibres
If your head moves in one direction…
Endolymph, cupula and hair cells in ampulla bend in opposite direction.
Info is sent centrally from right and left SC ducts
Via CN VIII (vestibulocochlea)
To nuclei in medulla
Vestibular nuclei
When reflex not present, head moves, eyes move with it.
Can indicate brainstem lesion.
