ORBITAL MUSCLES Extrinsic muscles of eyeball. Involved in movement of eyeball. Intrinsic muscles Controls shape of lens and size of pupil.
Extraocular muscles are special The motor units are small, with only from 5 to 18 muscle fibers contact by each motor nerve
Motion of an Eye To describe eye motions we need a set of defined axes (Fick’s Axes - draw on board) X axis : nasal -> temporal Y axis: anterior -> posterior Z axis: superior -> inferior These axes intersect at the center of rotation - a fixed point, defined as 13.5 mm behind cornea.
Ductions (single eye movements) Rotation about the Z axis (Z axis runs vertically superior to inferior) Medial Rotation - adduction toward midline Lateral Rotation - abduction away from midline Rotation about the X axis (X axis runs horizontally, from nasal to temporal) Upward, elevation (supraduction) Downward, depression (infraduction)
Torsion - cyclorotations Rotation about the Y axis (Y axis runs horizontally, from anterior to posterior) These are described with respect to a point at 12 o‘clock on the superior limbus Intorsion (incyclorotation) rotation nasally Extorsion (excyclorotation) rotation of the 12 o’clock position temporally. Counteracting head tilt (up to 7-9°)
Version & Vergences Some eye movements are paired, that is both eyes do the same thing. . . . Versions Sometimes eyes move in the opposite directions simultaneously. . . Vergences
Versions (conjugate eyemovement) Dextroversion - rightward gaze (demo) Levoversion - leftward gaze Supraversion - elevation Infraversion - depression Also up and right, up and left Down and right, down and leftALL BEHAVIOR IS THAT OF YOLKED EYES
VOLUNTARY MUSCLES7 in number Levator palpebrae superioris Superior rectus Inferior rectus Medial rectus Lateral rectus Superior oblique Inferior oblique
LEVATOR PALPEBRAESUPERIORIS Superior most muscle in the orbit. Has voluntary and involuntary parts.
Origin-Inferior surface of lesser wing of sphenoid. Insertion- 1. Upper lamina-Anterior surface of superior tarsus and skin of upper eyelid.2. Middle lamina-superior margin of superior tarsus.3. Lower lamina-Superior conjunctival fornix NERVE SUPPLY- Upper division of occulomotor nerve.
PtosisDrooping of upper eyelid. Complete ptosis-injury to occulomotor nerve. Partial ptosis-disruption of postganglionic sympathetic fibres from superior cervical sympathetic ganglion.
SUPERIOR RECTUS MUSCLE Origin-Superior part of common tendon of zinn. Insertion-inserted into sclera by flat tendinous insertion(10mm broad)about 7.7 mm behind sclero- corneal junction. Nerve supply-superior division of occulomotor nerve.
Frontal nerve runs above the superior rectus & levator. The nasociliary nerve and ophthalmic artery run below. The tendon for insertion of the superior oblique muscle runs below the anterior part of the superior rectus.
Action of Superior Rectus Primary action is elevation . . But since the insertion on the globe is lateral as well as superior, contraction will produce rotation about the vertical axis toward midline Thus secondary action is adduction Finally, because the insertion is oblique, contraction produces torsion nasally Intorsion.
INFERIOR RECTUS Origin-inferior part of common tendon of zinn Insertion-in the sclera 6.5 mm behind sclero corneal junction. Nerve supply-inferior division occulomotor nerve.
Fascial attachments below attached to inferior lid coordinate depression and lid opening. Fascia below Inf. Rectus and Inf. Oblique contribute to the suspensory ligament of lockwood. ACTIONS- Primary depressor. Subsidiary actions are adduction and extorsion.
MEDIAL RECTUS Origin-annulus of zinn and from optic nerve sheath. Insertion-in sclera 5.5mm behind sclero-corneal junction. Nerve supply-lower division of occulomotor nerve. Fascial expansion from muscle sheath forms the medial check ligament and attach to medial wall of orbit.
Innervation is via cranial nerve III, the oculomotor nerve, and the specific branch runs along the inside of the muscle cone, on the lateral surface. The superior oblique, ophthalmic artery and nasociliary nerve all lie above the medial rectus. ACTION- Primary adductor of the eye.
LATERAL RECTUS Origin-annulus of zinn. Insertion-in the sclera 6.9mm behind sclerocorneal junction. Nerve supply-abducens nerve which enters the muscle on the medial surface.
The lacrimal artery and nerve run along the superior border. The abducens nerve, ophthalmic artery and ciliary ganglion lie medial to the lateral rectus and between it and the optic nerve. ACTION- Primary abductor of eye.
SUPERIOR OBLIQUE Longest and thinnest intraorbital muscle, the muscle ends before the trochlea, tendon is 2.5 cm, smooth movement through trochlea. Origin-body of sphenoid above and medial to optic canal.Passes along superomedial part of orbit and ends in a tendon. Insertion-Posterosuperior quadrant of sclera behind equator of eyeball. Nerve supply-trochlear nerve entering it approximately one third of the distance from the origin to the trochlea.
ACTIONS Primary action-intorsion. Subsidiary actions-abduction and depression. Adducted position-depression.
INFERIOR OBLIQUE Origin-Anteromedial part of orbital floor lateral to nasolacrimal groove. Insertion-posteroinferior surface of globe near the macula. Nerve supply-inferior division of occulomotor nerve enters the muscle laterally at the junction of the inferior oblique and inferior rectus muscles.
ACTIONS Primary action-extorsion. Subsidiary actions-elevations and abduction. Causes elevation only in adducted position of eyeball.