EOM- cell biology and pathology

Question 1

What do each eye contain ?

Answer 1

7 EOM

Question 2

What are the 6OM of the eye?

Answer 2

• 6 are Oculo-Rotatory muscles

Question 3

What are oculo rotary muscles responsible for ?

Answer 3

• for movements of the globe in different directions
  (Plus the levator palpebrae superioris muscle,
  which elevates the upper eyelid)

Question 4

What do the 6 Oculo-Rotatory muscles comprise of ?

Answer 4

4 recti: superior, medial, inferior & lateral

& 2 obliques: superior & inferior

Question 5

Why ‘Rotatory’ Movements?

Answer 5

-The globe of the eyeball is suspended from the
orbital bones by an elaborate network of
specialized connective tissue capsules &
fibrous septa (orbital fascia)

-This suspending apparatus is analogous to a
‘ball-&-socket’ joint. It allows the eye to rotate in
different axes in a relatively low-friction
environment, with only minimal translational
(backward-forward) movements

Question 6

How is normal straight ahead gaze maintained ?

Answer 6

with balanced contraction of paired antagonists (i.e - MR-LR, SR-IR, & SO-IO )

Question 7

What are the actions for the right eye of the muscles ?

Answer 7

1. MR: Adduction
2. LR: Abduction
3. SR: Elevation
4. IR: Depression
5. SO: Intorsion
6. IO: Extorsion

Question 8

What are the 3 major types of orbital connective tissue ?

Answer 8

1. Tenon’s Capsule
2. Medial & Lateral Check ligaments
3. Suspensory Ligament of Lockwood

Question 9

What is the tenons capsule ?

Answer 9

• (Fascial/continous sheath surrounding globe from corneal limbus to optic nerve & separated from sclera by epi-scleral space
• Allows smooth (low-friction) rotation of the globe))- episclera

Question 10

What is the episclera?

Answer 10

the tenons capsule is separated from the sclera underneath (narrow space underneath) filled with fluid

Question 11

What does the fluid in the episclera do ?

Answer 11

lets eye rotate smoothly when muscles are contracting

Question 12

What are the Medial & Lateral Check ligaments

?

Answer 12

• Extensions of fascial sheaths of the MR & LR muscles anchored to the lacrimal & zygomatic bones
• Prevent retraction of the globe into the orbital cavity (the Medial check ligaments make sure dont move backwards into globe)

Question 13

What is the pathology if your eyeball does move backwards into globe ?

Answer 13

; convergence-retraction syndromes (Duane’s); orbital blow-out fractures

Question 14

What are EOM’s?

Answer 14

are specialized Skeletal Muscles, which their cytology closely resembles.

Question 15

What are the main differences of EOMs?

Answer 15

• contain smaller muscle fibres (10-20µm diameter) than most skeletal muscles (often up to 100µm diameter)
• Uniquely: have 2 separate layers containing different fibre types with distinct contractile properties
• inner layer (next to globe) has typical fast contracting fibres (Fibrillenstrücktur), but the outer layer (next to orbit) has a mix of fast & slow (Felderstrücktur) fibres
• Also Uniquely possess ‘pulleys’ to which the outer layer is directly connected (apart from the superior oblique

Question 16

What do EOMs contain ?

Answer 16

2 Striated Cell/Fibre Types

Question 17

What are the 2 Striated Cell/Fibre Types of EOMs?

Answer 17

1. Fibrillenstrücktur fibres

2. Felderstrücktur fibres

Question 18

What are Fibrillenstrücktur fibres ?

Answer 18

(~80% of total)
Large fibres, with small well-defined myofilaments & abundant sarcoplasmic reticulum + mitochondria
Mediate fast twitch (rapid/phasic) contractions of the muscle

Question 19

What are Felderstrücktur fibres ?

Answer 19

(the other 20%)
Smaller fibres, with larger less distinct myofilaments & less sarcoplasmic reticulum + mitochondria
Mediate tonic (sustained) contractions of the muscle
[Correspond to specialized skeletal muscle cellss, abundant in axial muscles of the trunk involved in postural control

Question 20

What do the 4 Rectus & IO Muscles have ?

Answer 20

Pulleys

Question 21

WHAT is the location of the pulleys?

Answer 21

: close to where the muscles insert in the sclera

Question 22

What is the composition of the pulleys?

Answer 22

rings of Fibro-Elastic connective tissue (collagen, elastin) & smooth muscle fibres

Question 23

How do the pulleys work?

Answer 23

Outer (Orbital) layer of felderstrücktur muscle fibres insert into the pulley itself!
Inner (Global) layer of the fibrillenstrcktur muscles fibres pass through it to insert into the sclera
Adjustable: they move backwards during muscle contraction, modifying the forces applied to the eye
But they were only discovered about 20 years ago, so their precise function is still to be elucidated
Important, though, for squint surgery; don’t cut them

Question 24

What is the SO pulley ?

Answer 24

cartilagenous ‘trochlea’,

Question 25

What is the EOM Physiology?

Answer 25

is also similar to Skeletal Muscle cells, with contraction caused by acetylcholine release from the motor-end plates of somatic (voluntary) motor neurons.

Question 26

What are the main differences of EOM in physiology ?

Answer 26

- they generate much less force than skeletal muscles as they only contract against a fairly constant & low-resistance load (the eye, only weighs ~100g!) - they have much smaller motor units (<1:10) than most skeletal muscles (>1:100), so they are under much finer neural control & have a wider dynamic range increasing their precision of contraction

Question 27

What does the ‘Motor Unit’ ratio refer to?

Answer 27

the number of muscle fibres innervated by a single motor neuron.

Question 28

For EOMs this ratio is uniquely small. What does this mean?

Answer 28

- each ocular motor neuron innervates/exerts control over very few (1-10) fibres in an EOM - as the number of active motor neurons supplying an EOM increases (aka ‘recruitment’), the force of contraction increases gradually - as a result, very fine adjustments can be made to the state of EOM contraction, which are needed for accurate eye movements & stable fixation

Question 29

What are the differences in Innervation between the 2 EOM Cell/Fibre Types?

Answer 29

Fast twitch, Fibrillenstrücktur fibres Slow tonic, Felderstrücktur fibres

Question 30

What is the Fast twitch, Fibrillenstrücktur fibres ?

Answer 30

Innervated by thick, heavily myelinated motor axons, forming a single (en plaque= typical) motor end-plate

Question 31

What is the Slow tonic, Felderstrücktur fibres?

Answer 31

Innervated by thinner motor axons forming multiple grape-like (en grappe = unique) endings which do not generate action potentials in the muscle, but are thought to mediate sustained contractions Tendon region also innervated by unique proprioceptor (palisade) endings, thought to monitor muscle tension signaling eye position in the orbit

Question 32

x

Answer 32

Eye Movements*: - Activation of Fast Twitch fibres in both layers of the EOMs Sustained Fixations+: - Activation of only Tonic, sustained fibres in orbital part of EOMs, with sensory feedback from palisade endings (fast fibres inactive)