Extraocular Muscles Extraocular Muscles (EOM) & Eye movement(EOM) & Eye movement

Muscle Fibers - cellular & molecular nuts & Muscle Fibers - cellular & molecular nuts & boltsbolts

Motor Units - how is tension developed?Motor Units - how is tension developed?

Eye movements - definitions of stimulus & Eye movements - definitions of stimulus & actionaction

Dr. Busatini has an entire course on this subject, so we Dr. Busatini has an entire course on this subject, so we can skip pages 143 -152, and 176-185.can skip pages 143 -152, and 176-185.VS 112 Ocular Anatomy

I. Just what sort of movements I. Just what sort of movements do your eyes make? do your eyes make?

Those which are you aware of, those that are Those which are you aware of, those that are voluntaryvoluntary (or at least the initiation of them is (or at least the initiation of them is voluntary).voluntary).

Reflex eye movements - automatic adjustment Reflex eye movements - automatic adjustment of the eye position to stabilize an image on the of the eye position to stabilize an image on the eye.eye.

Eye movementsEye movements

Smooth trackingSmooth tracking (pursuit) (pursuit) Combined head and eye trackingCombined head and eye tracking

SaccadesSaccades – quick jumps, – quick jumps, saccadic eye saccadic eye movements movements (< 50 msec) (up to 20°) voluntary (< 50 msec) (up to 20°) voluntary initiationinitiation Mini saccades (see fixation).Mini saccades (see fixation).

FixationFixation – holding a steady gaze (which we – holding a steady gaze (which we never actually do – see figure 4.1).never actually do – see figure 4.1).

Reflex eye movements. Reflex eye movements. The stimulus can be head/body movementThe stimulus can be head/body movement

ii)) VORVOR vestibular ocular reflexes – if you vestibular ocular reflexes – if you move your head the eyes can maintain fixation move your head the eyes can maintain fixation upon the target.upon the target.

iiii)) Optokinetic reflexOptokinetic reflex : stimulus = rapid : stimulus = rapid motion of the world, such as motion of a stream, or motion of the world, such as motion of a stream, or motion of world outside of the side car window.motion of world outside of the side car window.

Nystagmus (alternating slow & quick phases of Nystagmus (alternating slow & quick phases of movement) movement)

Reflex eye movements.Reflex eye movements. The stimulus can be movement, motion of an The stimulus can be movement, motion of an

object of interest.object of interest.

iiiiii)) Smooth pursuitSmooth pursuit - one object is targeted and - one object is targeted and remains clear on the retina the rest of the visual remains clear on the retina the rest of the visual world is in motion – the standard stimulus for the world is in motion – the standard stimulus for the optokinetic reflex,(only seen in foveate animals)optokinetic reflex,(only seen in foveate animals)

iviv)) FixationFixation - holding a steady gaze on a target - holding a steady gaze on a target of interest. NOT just a special form of pursuit.of interest. NOT just a special form of pursuit.

FixationFixation – holding a steady gaze – holding a steady gaze

The eyes are in constant motionThe eyes are in constant motion Slow driftSlow drift MinisaccadesMinisaccades Tremor (high frequency jitter in position) Tremor (high frequency jitter in position)

aka micronystagmus.aka micronystagmus.

In fact, if you could perfectly stabilize an image on In fact, if you could perfectly stabilize an image on the retina the retina it would fade and disappear.it would fade and disappear. Some retinal Some retinal motion is necessary, that is some movement of the motion is necessary, that is some movement of the image across the retina must be present or the image image across the retina must be present or the image fades.fades.

Eye Movements:Eye Movements:

See figure 4.1 over headSee figure 4.1 over head

Motion of an EyeMotion of an Eye

To describe eye motions we need a set of To describe eye motions we need a set of defined axes (Fick’s Axes - defined axes (Fick’s Axes - draw on boarddraw on board)) X axis : nasal -> temporal X axis : nasal -> temporal Y axis: anterior -> posteriorY axis: anterior -> posterior Z axis: superior -> inferiorZ axis: superior -> inferior

These axes intersect at the center of rotation - These axes intersect at the center of rotation - a fixed point, defined as 13.5 mm behind a fixed point, defined as 13.5 mm behind cornea.cornea.

Ductions Ductions (single eye movements)(single eye movements)

Rotation about the Z axis (Z axis runs vertically Rotation about the Z axis (Z axis runs vertically superior to inferior)superior to inferior)

Medial Rotation - adduction Medial Rotation - adduction toward midlinetoward midline Lateral Rotation - abduction Lateral Rotation - abduction away from midlineaway from midline

Rotation about the X axis (X axis runs Rotation about the X axis (X axis runs horizontally, from nasal to temporal)horizontally, from nasal to temporal)

Upward, elevation (supraduction)Upward, elevation (supraduction) Downward, depression (infraduction)Downward, depression (infraduction)

Torsion - cyclorotationsTorsion - cyclorotations Rotation about the Y axis (Y axis runs Rotation about the Y axis (Y axis runs

horizontally, from anterior to posterior)horizontally, from anterior to posterior) These are described with respect to a point at These are described with respect to a point at

12 o‘clock on the superior limbus12 o‘clock on the superior limbus Intorsion (incyclorotation) rotation nasallyIntorsion (incyclorotation) rotation nasally

Extorsion (excyclorotation) rotation of the 12 Extorsion (excyclorotation) rotation of the 12 o’clock position temporally.o’clock position temporally.

Counteracting head tilt (up to 7-9°)Counteracting head tilt (up to 7-9°)

Version & VergencesVersion & Vergences

Some eye movements are paired, that is both Some eye movements are paired, that is both eyes do the same thing. . . . Versionseyes do the same thing. . . . Versions

Sometimes eyes move in the opposite Sometimes eyes move in the opposite directions simultaneously. . . Vergencesdirections simultaneously. . . Vergences

VergencesVergences

Disjunctive eye movements (opposite left- right Disjunctive eye movements (opposite left- right movments). Non-yolked motionmovments). Non-yolked motion

Convergence (simultaneous movement nasally)Convergence (simultaneous movement nasally) Divergence (simultaneous temporal movement)Divergence (simultaneous temporal movement) Encyclovergence (intorsion)Encyclovergence (intorsion) Excyclovergence (extorsion)Excyclovergence (extorsion)

Versions Versions (conjugate eye movement)(conjugate eye movement)

Dextroversion - rightward gaze (demo)Dextroversion - rightward gaze (demo) Levoversion - leftward gazeLevoversion - leftward gaze Supraversion - elevationSupraversion - elevation Infraversion - depressionInfraversion - depression

Also up and right, up and leftAlso up and right, up and left Down and right, down and leftDown and right, down and left

ALL BEHAVIOR IS THAT OF YOLKED EYESALL BEHAVIOR IS THAT OF YOLKED EYES

Extraocular MusclesExtraocular Muscles

4 rectus muscles - origin is in the common 4 rectus muscles - origin is in the common tendous ring (annulus of Zinn)tendous ring (annulus of Zinn) Oval ring of connective tissueOval ring of connective tissue Continuous with periorbitaContinuous with periorbita Anterior to optic foramenAnterior to optic foramen

Medial and lateral rectus attached to both the Medial and lateral rectus attached to both the upper and lower tendon limbsupper and lower tendon limbs

The muscles traveling from the this tendon The muscles traveling from the this tendon ring to the insertions create ring to the insertions create muscle cone.muscle cone.

The Muscle ConeThe Muscle Cone

Spiral of TillauxSpiral of Tillaux

The rectus muscle pass through tenon’s The rectus muscle pass through tenon’s capsule and insert into the sclera.capsule and insert into the sclera.

The muscles insert at different distances from The muscles insert at different distances from the cornea.the cornea.

The insertion pattern is a spiral with the medial The insertion pattern is a spiral with the medial rectus closest to the cornea (5.5 mm) and the rectus closest to the cornea (5.5 mm) and the superior rectus the furthest away from the superior rectus the furthest away from the cornea (7.4 mm).cornea (7.4 mm).

Common Tendon of ZinnCommon Tendon of Zinn

Medial RectusMedial Rectus

Originates on both the upper and lower limb of Originates on both the upper and lower limb of the common tendous ring and the optic nerve the common tendous ring and the optic nerve sheath.sheath.

Inserts along a vertical line 5.5 mm from the Inserts along a vertical line 5.5 mm from the cornea. The horizontal plane of eye bisects the cornea. The horizontal plane of eye bisects the insertion.insertion.

Fascial expansion from muscle sheath forms Fascial expansion from muscle sheath forms the medial check ligament and attach to medial the medial check ligament and attach to medial wall of orbit.wall of orbit.

Medial Rectus Medial Rectus cont.cont.

Innervation is via cranial nerve III, the Innervation is via cranial nerve III, the oculomotor nerve, and the specific branch runs oculomotor nerve, and the specific branch runs along the inside of the muscle cone, on the along the inside of the muscle cone, on the lateral surface.lateral surface.

The superior oblique, ophthalmic artery and The superior oblique, ophthalmic artery and nasociliary nerve all lie above the medial nasociliary nerve all lie above the medial rectus.rectus.

Spiral of TillauxSpiral of Tillaux

5.5 mm

6.7 mm6.9 mm

7.4 mm

Lateral rectusLateral rectus

Originates on both the upper and lower limb of Originates on both the upper and lower limb of the common tendous ring. . .AND a process of the common tendous ring. . .AND a process of the greater wing of the sphenoid bone.the greater wing of the sphenoid bone.

Inserts parallel to medial rectus 6.9 mm from Inserts parallel to medial rectus 6.9 mm from the cornea. (Tendon 9.2 mm wide, 8.8 long).the cornea. (Tendon 9.2 mm wide, 8.8 long).

Fascial expansion from muscle sheath forms Fascial expansion from muscle sheath forms the lateral check ligament and attach to lateral the lateral check ligament and attach to lateral wall of orbit at Whitnalls tubercle.wall of orbit at Whitnalls tubercle.

Lateral Rectus Lateral Rectus cont.cont.

Innervated by the abducens nerve, Cranial n Innervated by the abducens nerve, Cranial n VI which enters the muscle on the medial VI which enters the muscle on the medial surface.surface.

The lacrimal artery and nerve run along the The lacrimal artery and nerve run along the superior border.superior border.

The abducens n., ophthalmic artery and ciliary The abducens n., ophthalmic artery and ciliary ganglion lie medial to the lateral rectus and ganglion lie medial to the lateral rectus and between it and the optic nerve.between it and the optic nerve.

Superior RectusSuperior Rectus

Originate on superior limb of the tendonous Originate on superior limb of the tendonous ring, and optic nerve sheath.ring, and optic nerve sheath.

Muscle passes forward underneath the levator, Muscle passes forward underneath the levator, but the two sheaths are connected resulting in but the two sheaths are connected resulting in coordinated movements.coordinated movements.

Insertion 7.4 mm from limbus, and obliquely.Insertion 7.4 mm from limbus, and obliquely. The angle from the origin to the insertion is The angle from the origin to the insertion is

23° beyond the sagital axis. (see overhead)23° beyond the sagital axis. (see overhead)

Superior Rectus Superior Rectus cont.cont.

Frontal nerve runs above the s. rectus & levat.Frontal nerve runs above the s. rectus & levat. The nasociliary nerve and ophthalmic artery The nasociliary nerve and ophthalmic artery

run below.run below. The tendon for insertion of the superior The tendon for insertion of the superior

oblique muscle runs below the anterior part of oblique muscle runs below the anterior part of the superior rectus.the superior rectus.

Innervationis via superior division of CN III, Innervationis via superior division of CN III, from the inferior surface; additional branches from the inferior surface; additional branches make their way to the levator.make their way to the levator.

Action of Superior RectusAction of Superior Rectus

Primary action is elevation . . But since the Primary action is elevation . . But since the insertion on the globe is insertion on the globe is laterallateral as well as as well as superior, contraction will produce rotation superior, contraction will produce rotation about the vertical axis about the vertical axis toward midlinetoward midline

Thus secondary action is adductionThus secondary action is adduction Finally, because the insertion is oblique, Finally, because the insertion is oblique,

contraction produces torsion nasally Intorsion.contraction produces torsion nasally Intorsion. (overhead figure 10-13A)(overhead figure 10-13A)

Superior view of Sup. RectusSuperior view of Sup. Rectus

23°

Because the muscle runs at an angle to the Fick’s axes, contraction is not confined to one axis

Inferior rectusInferior rectus

Originates on lower limb of common Originates on lower limb of common tendonous ring.tendonous ring.

Inserts 6.7 mm from limbus, insertion is an arcInserts 6.7 mm from limbus, insertion is an arc It is parallel to superior rectus, making a 23° It is parallel to superior rectus, making a 23°

angle beyond the sagittal axis.angle beyond the sagittal axis. Innervated by inferior division of CN III Innervated by inferior division of CN III

which runs above it (within the muscle cone).which runs above it (within the muscle cone). Below is the floor of the orbit and inf. obliqueBelow is the floor of the orbit and inf. oblique

Inferior Rectus Inferior Rectus cont.cont.

Fascial attachments below attached to inferior Fascial attachments below attached to inferior lid coordinate depression and lid opening.lid coordinate depression and lid opening.

Fascia below Inf. Rectus and Inf. Oblique Fascia below Inf. Rectus and Inf. Oblique contribute to the suspensory ligament of contribute to the suspensory ligament of lockwood.lockwood.

Primary Action downward gaze depressionPrimary Action downward gaze depression 2° Adduction, as is the case for sup. Rectus2° Adduction, as is the case for sup. Rectus Also extorsion due to oblique arc of insertion.Also extorsion due to oblique arc of insertion.

Vectors of Sup & Inf RectiVectors of Sup & Inf Recti

Superior ObliqueSuperior Oblique

Anatomical origin is on the lesser wing of the Anatomical origin is on the lesser wing of the sphenoid bone. The physiological origin is the sphenoid bone. The physiological origin is the trochlea, a cartilagenous “U” on the superior trochlea, a cartilagenous “U” on the superior medial wall of the orbit.medial wall of the orbit.

Longest thinnest EOM, the muscle ends before Longest thinnest EOM, the muscle ends before the trochlea, tendon is 2.5 cm, smooth the trochlea, tendon is 2.5 cm, smooth movement through trochlea.movement through trochlea.

Innervation by CN IV, the trochlear nerve Innervation by CN IV, the trochlear nerve posterior in the orbit.posterior in the orbit.

Action of Sup. ObliqueAction of Sup. Oblique

Primary action is intorsion _ rotation of 12 Primary action is intorsion _ rotation of 12 o’clock position toward midline.o’clock position toward midline.

Because the insertion of the oblique muscle is Because the insertion of the oblique muscle is in the lateral, posterior quadrant the secondary in the lateral, posterior quadrant the secondary actions are actions are Rotating the back half of the globe from lateral to Rotating the back half of the globe from lateral to

medial (the anterior pole will move away) medial (the anterior pole will move away) ABDUCTIONABDUCTION

Also depression (posterior superior quadrant of the Also depression (posterior superior quadrant of the globe being pulled upward).globe being pulled upward).

Inferior ObliqueInferior Oblique

Originates on the maxillary bone inferior to Originates on the maxillary bone inferior to the nasolacrimal fossa. The ONLY EOM the nasolacrimal fossa. The ONLY EOM originating in the anterior orbit.originating in the anterior orbit.

Inserts on the posterior lateral aspect of globe Inserts on the posterior lateral aspect of globe mostly inferior, below the ant.-post. horizontal mostly inferior, below the ant.-post. horizontal plane.plane.

Innervation from inferior division of CN III Innervation from inferior division of CN III inserts on the upper surface (within muscle inserts on the upper surface (within muscle cone.)cone.)

Origins/Insertions of Oblique musclesOrigins/Insertions of Oblique muscles

Action of Inf. ObliqueAction of Inf. Oblique

Primary is extorsionPrimary is extorsion 2° is due to posterior, lateral, inferior insertion 2° is due to posterior, lateral, inferior insertion

being pulled around, underneath globe and being pulled around, underneath globe and toward the anterior inferior insertion medially. toward the anterior inferior insertion medially.

Rotation about the Z axis will be nasal to Rotation about the Z axis will be nasal to temporal (abduction).temporal (abduction).

Rotation about the X axis will be elevation Rotation about the X axis will be elevation (see overhead figure 10.14)(see overhead figure 10.14)

And from here it’s complicatedAnd from here it’s complicated

There is a balance of tension in the pairs of muscle to There is a balance of tension in the pairs of muscle to start with. . . .start with. . . .

Actions of the muscle can and do depend on the Actions of the muscle can and do depend on the starting position.starting position.

For example elevation of the eye in the straight-For example elevation of the eye in the straight-ahead position and lateral position is accomplished by ahead position and lateral position is accomplished by the sup. rectus.the sup. rectus. But when the eye is medial rotated elevation is But when the eye is medial rotated elevation is

accomplished by the inferior oblique muscle accomplished by the inferior oblique muscle action. (see fig 10-17 over head).action. (see fig 10-17 over head).

Defining Muscle groupsDefining Muscle groups

What is a muscle, how does it What is a muscle, how does it work?work?

Thick & thin filamentsThick & thin filaments

Elements within, between the Z-lines make up a contractile unit

Ratchet ModelRatchet Model

Myosin head binds to actin Myosin head binds to actin filament.filament.

The ratchet motion moves the The ratchet motion moves the two filament about 12 nm two filament about 12 nm with respect to each other.with respect to each other.

It takes only 5 msIt takes only 5 ms Because of the large number Because of the large number

of z-line segments or of z-line segments or contractile units along a fiber, contractile units along a fiber, a fast motion is attained.a fast motion is attained.

Thick and thin filament Thick and thin filament specializations with EOMspecializations with EOM

Each motor fiber is innervated by only one nerve

Extraocular muscles are specialExtraocular muscles are special

The motor units are small, with only from 5 to 18 muscle fibers contact by each motor nerve

A motor nerve can and does contact more than one fiber usually 100’s

EOM’s are specialEOM’s are special

THICK FIBERSTHICK FIBERS Striated muscleStriated muscle Singly innervatedSingly innervated

1 nerve, 1 branch1 nerve, 1 branch Motor end plateMotor end plate

Terminaison en plaqueTerminaison en plaque Includes both fast and Includes both fast and

slow twitch fibersslow twitch fibers All or none contractionAll or none contraction

THIN FIBERSTHIN FIBERS Striated musleStriated musle Multiply innervatedMultiply innervated

Many branches 1 nerveMany branches 1 nerve En grappe end plateEn grappe end plate

Terminaisons en grappeTerminaisons en grappe Thought to be slow Thought to be slow

sustained (tonic)sustained (tonic) Graded contractionsGraded contractions

Thick and thin filament Thick and thin filament specializations with EOMspecializations with EOM

Each motor fiber is innervated by only one nerve

Fiber types segregate in the muscleFiber types segregate in the muscle

A single nerve impulseA single nerve impulse Generates a muscle AP and Generates a muscle AP and

contraction - tension.contraction - tension. Multiple nerve AP’sMultiple nerve AP’s Tensions sum over timeTensions sum over time

Repetitive firing results in a Repetitive firing results in a sustained contraction.sustained contraction.

Tetanus (unresolved Tetanus (unresolved individual twitches).individual twitches).

3 Basic types of motor units3 Basic types of motor units

Slow Fast-fatigue resistant Fast- fatigable

Cat gastroc muscleCat gastroc muscle ATPase activity at neurtral pHATPase activity at neurtral pH

ATPase activity at acidic pHATPase activity at acidic pH

NADH dehydrogenase stain NADH dehydrogenase stain

Recruitment within the motor Recruitment within the motor nervenerve

Groups of cell bodies innervating the same Groups of cell bodies innervating the same muscle make up a muscle make up a MOTOR NUCLEUSMOTOR NUCLEUS

. . . .and give rise to a . . . .and give rise to a motor nervemotor nerve.. Nerves within a motor nerve are activated in a Nerves within a motor nerve are activated in a

characteristic sequence - characteristic sequence - the size principlethe size principle.. Smaller fibers, fire first, and larger laterSmaller fibers, fire first, and larger later

There is a characteristic pattern of recruitment, tension There is a characteristic pattern of recruitment, tension is added and removed in a repeated pattern.is added and removed in a repeated pattern.

Muscle Spindle & Golgi Tendon Muscle Spindle & Golgi Tendon OrgansOrgans Specialized sensory organs Specialized sensory organs

within the muscle provide feed within the muscle provide feed back to the brain.back to the brain.

How much tension is in the How much tension is in the muscle?muscle?

Is there any stretch imposed? Is there any stretch imposed? What is muscle length?What is muscle length? EOM’s do not have the typical EOM’s do not have the typical

stretch reflexstretch reflex