Extraocular muscles
EXTRAOCULAR MUSCLES
- The motor units are small, with only from 5 to 18 muscle fibers contact by each motor nerve.
- Anteroposterior stability of eyeball is provided by Superior oblique, Superior rectus & Orbital fat.
- The sheath of inferior rectus is thickened & blends with the sheath of inferior oblique as well as with check ligament, forming a hammock-like support for eye, suspensory ligament of lockwood. It provides inferior stability to eye.
- Whitnall’s (zygomatic) tubercle on the zygomatic bone just within orbital margin provides attachment to lateral check ligament.
- There are seven voluntary extraocular muscles. These are:
a. 4 recti (superior, inferior, medial, lateral)-
- arises from a common tendinous ring (Annulus of Zinn).
- Lateral rectus has an additional origin from greater wing of sphenoid (orbital surface).
- Superior rectus:Its’s Yoke muscle will be contralateral side of inferior rectus. Makes<230with orbital axis.
- Inferior rectus:Lies inferior to inferior oblique muscle. Anatgonist to superior rectus. Inferior rectus inserts 6.5 mm behind limbus
- Medial rectus: synergistic to ipsilateral side of superior rectus
b. 2 oblique (superior, inferior)
- Superior oblique arises from lesser wing of sphenoid.
- Inferior oblique arises from orbital surface of maxilla (only extraocular muscle that do not arise from apex of orbit)
- Superior oblique:Longest and thinnest intraorbital muscle, the muscle ends before the trochlea, tendon is 2.5 cm, smooth movement through trochlea. Makes<51oorbital axis.
- Inferior oblique: Shortest
c. Levator palpebrae superioris
- Arises from the lesser wing of sphenoid.
- Superior most muscle in the orbit.(Has voluntary and involuntary parts).
- Besides seven voluntary extraocular muscles, three small involuntary muscles are there:
- Superior tarsal muscle (of Muller):Assists in elevating the upper eyelid.
- Inferior tarsal muscle: Depresses the lower eyelid
- Orbitalis (Orbital muscle of muller): Fills the inferior orbital fissure
- These three muscles are supplied by postganglionic sympathetic fibers from superior cervical sympathetic ganglion.
|
MUSCLES |
NERVE SUPPLY |
ORIGIN |
INSERTION |
PRIMARY ACTION |
ACTION IN ADDUCTED EYE |
ACTION IN ABDUCTED EYE |
|
Superior rectus |
Oculomotor nerve |
Annulus of Zinn |
Eye |
Elevation |
Intorsion |
Elevation |
|
Inferior rectus |
Oculomotor nerve |
Annulus of Zinn |
Eye |
Depression |
Extorsion |
Depression |
|
Lateral rectus |
Abducent nerve |
Annulus of Zinn |
Eye |
Abduction |
|
|
|
Medial rectus |
Oculomotor nerve |
Annulus of Zinn |
Eye |
Adduction |
|
|
|
Superior oblique |
Trochlear nerve |
Lesser wing of sphenoid |
Eye |
Intorsion |
Depression |
Intorsion |
|
Inferior oblique |
Oculomotor nerve |
Orbital surface of maxilla |
Eye |
Extorsion |
Elevation |
Extorsion |
|
Levator palpabrae superioris |
Occulomotor nerve |
Lesser wing of sphenoid (orbital surface) |
Tarsal plate of upper eyelid |
Retracts and |
|
|
- Depending on the action of muscles in relation to other, muscles can be grouped into:
- It refers to the muscles having the same primary action in the same eye e.g. superior rectus & inferior oblique of the same eye act as synergistic elevators.
- These are the muscles having opposite actions in the same eye.
- For e.g. medial & lateral recti, superior & inferior recti & superior & inferior obliques are antagonists to each other in the same eye.
- It refers to the pair of muscles (one from each eye) which contract simultaneously during movements.
- For e.g, right lateral rectus & left medial rectus act as yoke muscles for dextroversion movements.
- Other pair of yoke muscles are:
- These are a pair of muscles (one from each eye) having opposite action: for e.g
- Superior oblique rotates the eye downward and away from midline.
- All the extra ocular muscles except inferior oblique arise from the apex of orbit.
- Anteroposterior stability of eyeball is provided by Superior oblique,Superior rectus & Orbital fat.
- The Superior oblique is the longest, thinnest extraocular muscle.
- Superior oblique muscle is involved in intorsion, depression and abduction of the eyeball.
- The superior oblique muscle is supplied by trochlear nerve.
- Superior rectus is supplied by superior division ofocculomotor nerve.
- Inferior rectus inserts 6.5 mm behind limbus
- Inferior rectus lies inferior to inferior oblique muscle.
- Inferior rectus is antagoinst to superior rectus.
- Inferior oblique is supplied by 3rd cranial nerve.
- Lateral rectus, Superior oblique are not supplied by occulomotor nerve.
- Downward and lateral gaze is action of superior oblique.
- Inferior oblique is the shortest extraocular muscle.
- The posterior end of inferior oblique Muscle lies near macula.
- The muscle which makes an angle of about 51° with the optical axis is superior oblique.
- The muscle which makes an angle of about 23° with the optical axis is Superior rectus.
- Lateral rectus is supplied by abducent nerve
- Action of superior rectus is elevtaion Adduction and intorsion.
- The extraocular muscles that does not arise from Annulus of Zinn is superior oblique.
- Muscle responsible for intorsion of the eye are superior oblique & superior rectus.
- Primary action of inferior oblique muscles are Extortion, Elevation, Abduction.
