The Extra-Ocular Muscles: Cell Biology & Physiology

Question 1

Define recti

Answer 1

Its latin for straight and refers to the insertion of muscles

Question 2

Define oblique

Answer 2

To be at or of an angle … refers to muscles being inserted at an angle

Question 3

What are the three orbital connective tissues and what are their purposes?

Answer 3

1. Tenon’s Capsule

–Fascial sheath surrounding globe from corneal limbus to optic nerve & separated from sclera by epi-scleral space

–Allows smooth (low-friction) rotation of the globe

2. Medial & Lateral Check ligaments

–Extensions of fascial sheaths of the MR & LR muscles anchored to the lacrimal & zygomatic bones

–Prevent retraction of the globe into the orbital cavity

3. Suspensory Ligament of Lockwood

–Blending of Tenon’s Capsule with extensions of the fascial sheaths enclosing the inferior (IR & IO) muscles

–Suspends the globe ‘hammock’style

[Essentially the connective tissues suspend the globe within the orbit - see picture]

Question 4

How many extraocular muscles surround the eye?

Answer 4

Each eye possesses 7 Extra-Ocular Muscles:

•6 are Oculo-Rotatory muscles, responsible for movements of the globe in different directions

Plus the levator palpebrae superioris muscle,

which elevates the upper eyelid

Question 5

What is a periosteum?

Answer 5

a dense layer of vascular connective tissue enveloping the bones except at the surfaces of the joints.

[This exists in the bones of the orbit too]

Question 6

What is episcleral fluid and what is it’s purpose?

Answer 6

In the episcleral space. It is to allow the eyeball to rotate within Tenon’s capsule in relatively low friction conditions.

Question 7

Where is the episclera?

Answer 7

Between Tenon’s capsule and the sclera is a space - called the episclera.

Question 8

What is an example of a Convergence retraction syndrome and why may it occur?

Answer 8

Duanes’s.

A convergence retraction syndrome may occur if you have something wrong with a check ligament and so when you try to extort your eyes they retract back into the orbit

Question 9

True or False- EOMs are specialized Skeletal Muscles

Answer 9

True - EOMs are specialized Skeletal Muscles, which their cytology closely resembles.

Question 10

What are the main differences between EOM cytology and skeletal muscle cytology?

Answer 10

EOM muscle fibres are much smaller than skeletal muscle fibres (10-20µm diameter VS often up to 100µm diameter)

EOMs have two seperate layers containing different fibre types with distinct contractile properties whereas skeletal muscles only have the one layer. (SEE picture)

EOMS also possess ‘pulleys’ to which the outer layer is directly connected (apart from the superior oblique) whereas skeletal muscles do not.

Question 11

Describe the two layer structure of EOMS

Answer 11

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

Question 12

Describe the similarities and differences between the two muscle fibre types in EOMS

Answer 12

EOMs contain 2 Striated Cell/Fibre Types:

1. Fibrillenstrücktur fibres (~80% of total muscle)

–Large fibres, with small well-defined myofilaments & abundant sarcoplasmic reticulum + mitochondria

–Mediate fast twitch (rapid/phasic) contractions of the muscle

2. Felderstrücktur fibres (the other 20%)

–Smaller fibres, with larger less distinct myofilaments & less sarcoplasmic reticulum + mitochondria

–Mediate tonic ( they cause sustained) contractions of the muscle

Question 13

In this transverse histological section identify which muscle fibre type is which in this EOM

Answer 13

Symbols are given in the picture

Question 14

What is the pulley for the superior oblique muscle?

Answer 14

The SO pulley is the cartilagenous ‘trochlea’,

which we’ve known about for ages!

Question 15

What is the difference between the pulleys of the 4 recti muscles and IO as a pose to the pulley of the superior oblique muscle?

Answer 15

Pulleys of the E recti muscles + IO are rings of Fibro-Elastic connective tissue (collagen, elastin) & smooth muscle fibres.

Whereas the pulley of the SO is cartilagenous (made of cartiledge)

[Fun fact- trochlea translates as pulley in latin and so does the trochlea nerve which supplies the SO]

Question 16

Are pulleys adjustable or fixed?

Answer 16

•Adjustable: they move backwards during muscle contraction, modifying the forces applied to the eye

[Thus it is important in squint surgery not to cut them!]

Question 17

How are pulleys attached to the eye via EOMs?

Answer 17

• 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

Question 18

Describe the similarities and differences between EOM physiology and Skeletal muscle physiology

Answer 18

They are both similar as:

as contraction of both is caused by acetylcholine release from the motor-end plates of somatic (voluntary) motor neurons.

The main differences are that EOMs:

• generate much less force than skeletal muscles as they only contract against a fairly constant & low-resistance load (the eye, only weighs ~100g!)
• 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 19

What is a motor unit ratio?

Answer 19

The ‘Motor Unit’ ratio refers to the number of muscle fibres innervated by a single motor neuron.

Question 20

Comment on the motor unit ratio of EOMs and its affect.

Answer 20

EOMs have a uniquely small motor unit ratio . This means:

• each ocular motor neuron innervates/exerts control over very few (1-10) muscle 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 21

What is the difference in innervation between the two types of muscle fibres in EOMs?

Answer 21

Fast twitch aka Fibrillenstrücktur fibres

•Are Innervated by thick, heavily myelinated motor axons, forming a single motor end-plate

Slow tonic aka Felderstrücktur fibres

•Are Innervated by thinner motor axons forming multiple grape-like endings which do not generate action potentials in the muscle and so can mediate sustained contractions

Question 22

Describe the similarities and differences between the two muscle fibre types found in EOMS

Answer 22

Question 23

What is the functional significance of having two types of muscle fibres in EOMs?

Answer 23

Activation of Fast Twitch fibres in both layers of the EOMs allow for rapid eyemovements.

Activation of only Tonic, sustained fibres in orbital part of EOMs, with sensory feedback from palisade endings allows for sustained fixation.

Question 24

Over which bone is the medial check ligament attached?

a) Ethmoid
b) Lacrimal
c) Lesser Wing of the Sphenoid
d) Zygmatic

Answer 24

B) Lacrimal