The Extra-Ocular Muscles: Cell Biology & Physiology Flashcards
Define recti
Its latin for straight and refers to the insertion of muscles
Define oblique
To be at or of an angle … refers to muscles being inserted at an angle
What are the three orbital connective tissues and what are their purposes?
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]
How many extraocular muscles surround the eye?
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
What is a periosteum?
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]
What is episcleral fluid and what is it’s purpose?
In the episcleral space. It is to allow the eyeball to rotate within Tenon’s capsule in relatively low friction conditions.
Where is the episclera?
Between Tenon’s capsule and the sclera is a space - called the episclera.
What is an example of a Convergence retraction syndrome and why may it occur?
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
True or False- EOMs are specialized Skeletal Muscles
True - EOMs are specialized Skeletal Muscles, which their cytology closely resembles.
What are the main differences between EOM cytology and skeletal muscle cytology?
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.
Describe the two layer structure of EOMS
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
Describe the similarities and differences between the two muscle fibre types in EOMS
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
In this transverse histological section identify which muscle fibre type is which in this EOM
Symbols are given in the picture
What is the pulley for the superior oblique muscle?
The SO pulley is the cartilagenous ‘trochlea’,
which we’ve known about for ages!
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?
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]