EOMs: cell biology & physiology Flashcards
how many EOMs does each eye possess
7
what are 6 of the 7 EOMs and what are they responsible fro
oculo-rotatory muscles
responsible for movements of the globe in different directions
if 6 of the EOMs are oculo-rotatory muscles, what is the 7th muscle
levator palpebrae superiors which elevates the upper eyelid
list what 4 of the 6 oculo-rotatory muscles are
4 recti: superior rectus medial rectus inferior rectus lateral rectus
what are 2 of the 4 oculo-rotatory muscles
2 obliques:
superior oblique
inferior oblique
what is the globe of the eyeball suspended/separated from the orbital bones by
network of specialised connective tissue capsules & fibrous septa (orbital fascia) which holds the eyeball in the orbit
what type of joint is the network of specialised connective tissue capsules & fibrous septa (orbital fascia) which holds the eyeball in the orbit similar to
ball & socket joint
what does the network of specialised connective tissue capsules & fibrous septa (orbital fascia) which suspends the eyeball from the orbital bones allow
allows the eye to rotate in different axes i.e. horizontally, vertically & torsionally, in a relatively low friction environment, with only minimal translational (backward-forward) movements
what is the action of the medial rectus
ADduction - towards the nose
what is the action of the lateral rectus
ABduction - away from the nose
what is the action of the superior rectus
elevation - up
what is the action of the inferior rectus
depresses - down
what is the action of the superior oblique
intorsion - top of eyeball moves towards the nose
what is the action of the inferior oblique
extorsion - top of eyeball moves away from the nose
what is required between the muscles in order to maintain a straight ahead primary gaze
well balanced contraction of paired antagonists (3 pairs)
ie MR-LR, SR-IR & SO-IO
what is the inner surface of the orbital bones covered by which is the same case for all bones in the body
a layer of tough connective tissue called periosteum or periorbital fascia
in addition to the orbital bones being covered by a layer of tough connective tissue called periosteum or periorbital fascia, what three additional unique substances cover the surface of the orbital bones (not found anywhere else in the body)
- tenon’s capsule
- medial and lateral check ligaments
- suspensory ligament of lockwood
what is tenon’s capsule
fascial sheath surrounding the globe from corneal limbus which wraps itself back and goes around the optic nerve
what is the tenon’s capsule separated from the sclera by
epi-scleral space
what does the epi-scleral space contain inside it
epi-scleral fluid
what does epi-scleral fluid allow with tenon’s capsule
smooth (low-friction) rotation of the globe in tenon’s capsule i.e. little resistance
what can friction from movement of the eyeball within globe cause
heating up of eye
what are medial & lateral check ligaments
extensions of the fascial sheaths of the MR & LR muscles anchored/inserted into the periosteum of the lacrimal and zygomatic bones
as one side of the medial and lateral check ligaments insert into the periosteum of the lacrimal and zygomatic bones , what does the other side of the medial and lateral check ligaments insert
into the sleeve around the medial and lateral recuts muscles called epimyosin
what do the medial and lateral check ligaments prevent
retraction of the globe into the orbital cavity when the eye is moving in different directions
what is the suspensory ligament of lockwood
blending on tenon’s capsule with extensions of the fascial sheaths (i.e. thickening of connective tissue/suspensory ligaments) enclosing epimyosin of the inferior IR & IO muscles
what so the suspensory ligaments of lockwood do
suspends the globe ‘hammock’ style
so prevents the eye from dropping down into the globe from the force of gravity
what can an orbital blow which damages the suspensory ligament of lockwood cause
eye to fall down into orbit which causes vertical diplopia
what is duane’s syndrome
a convergence-retraction syndrome
when try to converge the eyes, they fall back into the orbit
what is the cause of duane’s syndrome
errors in muscle contraction which over powers the check ligaments causing the eyes to translate back
what is the difference of EOM cytology compared to skeletal muscles of the rest of the body
- contain smaller muscle fibres
- have 2 separate layers containing different fibre types with distinct contractile properties
- possess pulleys which the outer layer is directly connected (apart from the superior oblique)
what size are the smaller muscle fibres diameters of the EOMs, compared to skeletal muscles of the body
10-20um compared to 100um
5-10x smaller
what is the inner layer of muscle fibre closer to
the globe
what type of fibres does the inner layer of the muscle contain
fast contracting muscle cells/fibres called fibrillenstrucktur
what is the outer layer of muscle fibre closer to
the orbital bones
what type of fibres does the outer layer of the muscle contain
mix of fast and slow felderstrucktur fibres
what is the name of the fast contracting muscle fibre in the EOMs
fibrillenstrucktur
what is the name of the slow contracting muscle fibre in the EOMs
felderstrucktur
how many % do fibrillenstrucktur fibres account for
80%
what size fibres are fibrillenstrucktur
large fibres 20um
what size and type of myofilaments do fibrillenstrucktur fibres have
small & well defined
how much sarcoplasmic reticulum & mitochondria do fibrillenstrucktur fibres have
abundant
why so fibrillenstrucktur fibres have abundant mitochondria
they’re energy dependent
what do fibrillenstrucktur fibres mediate
fast twitch (rapid/phasic) contractions of the muscle
how many % do felderstrucktur fibres account for
20%
what size are felderstrucktur fibres
smaller
what size and type of myofilaments do felderstrucktur fibres have
larger & less distinct
how much sarcoplasmic reticulum & mitochondria do felderstrucktur fibres have
less amount
why do felderstrucktur fibres have less mitochondria
they’re not as active
what do felderstrucktur fibres mediate
tonic sustained contractions of the muscle in order to maintain fixation
what other muscles are felderstrucktur fibres correspond to
to specialised skeletal muscle cells, abundant in axial muscles of the trunk involved in postural control
what does a longitudinal section of the EOMS show in relation to the muscle fibres
small diameter felderstrucktur muscle fibres
&
larger diameter fibrillenstructur muscle fibres
what does a transverse section of the EOMs show in relation to muscle fibres
felderstrucktur fibres are not as common, less organised and smaller diameter
&
fibrillenstrucktur fibres are more common, myofilaments inside them are more organised and larger diameter
in a transverse section of the EOMs, what is found between the muscle cells/fibres
fine connective tissue called endomysium
what does the inner global region of the EOM contain
densely packed mass of large diameter fast twitch fibrillenstrcktur fibres
what does the outer orbital region of the EOM contain
less dense mixture of fast and slow twitch, fibrillensctrucktur and felderstrucktur fibres for tonic contractions
which EOMs have pulleys
4 rectus & inferior oblique
where are the pulleys located
close to where the muscles insert in the sclera
what is the composition of the pulleys of the 4 rectus and IO muscles
rings of fibro-elastic connective tissue made of collagen and elastin & smooth muscle fibres
what are the pulleys of the 4 rectus and IO muscles made of
collagen, elastin and smooth muscle fibres
what do the outer orbital layer of felderstrucktur muscle fibres insert into
the pulley itself
what does the fact that the felderstrucktur muscle fibres insert into the pulley itself allow for
being adjustable: they move backwards during muscle contraction, modifying the forces applied to the eye
what do the inner global layer of fibrillenstrucktur muscle fibres insert into
pass through the hole in the pulley and insert into the sclera
what is it important not to do during a squint surgery
do not cut the pulleys
what is the name of the superior oblique pulley
trochlea
what is the pulley (trochlea) of the superior oblique composed of
ring made out of cartilage
how is the EOMs similar to skeletal muscle cells with contraction
their contraction is both caused by acetylcholine release from the motor-end plates of somatic (voluntary) motor neurons inside the brain stem
what do EOMs generate much less of than skeletal muscles
force
why do EOMs generate much less force than skeletal muscles
they only contract against a fairly constant & low-resistance load (eye only weighs 100g) and eye is suspended in orbit in a friction free environment
what are the difference between EOM compared to skeletal muscle motor units
EOMs have much smaller motor units 1:100
what significance does an EOM have in terms of smaller motor units <1:10
they have much finer neural control & a wider dynamic range increasing their precision of contraction
what does the motor unit ratio refer to/mean
the number of muscle fibres innervated by a single motor neuron i.e. for EOMs this ratio is uniquely small
what is meant by EOMs having a small motor neuron ratio
each ocular motor neuron (in the brain stem) innervates/exerts control over very few (1-10) fibres in am EOM
what is the number of active motor neurons supplying an EOM increases known as
recruitment
when more and more motor neurons = need to recruit more muscle cells
what happens to the contraction when more and more motor neurons are recruited
the force of the contraction increases gradually
what happens as a result of motor neuron recruitment
very fine adjustments can be made to the state of EOM contraction, which are needed for accurate eye movements & stable fixation
what does a motor neuron ratio of 1:1 refer to
one motor neuron in the brain stem controls one muscle cell in the EOM
what does a motor neuron ratio of 1:100 in skeletal muscles refer to
one motor neuron controls a 100 skeletal muscle cells
what did an experiment measuring frequency force relations prove about EOMs compared to skeletal muscles
for the same increments in nerve stimulation frequency, the force of contraction of the EOM is 10x less (a 10th of that of a diaphragm cell) & more gradual (over 20-250Hz range), compared to the typical skeletal muscle
between EOMs and skeletal muscle, which has a wider dynamic range
EOM from 0-300Hz (EOM muscle exerts force more gradually)
what is a fast twitch fibrillenstrucktur fibre innervated by
thick, heavily myelinated motor axons
what type of end plate do fibrillenstrucktur fibres which are innervated by thick, heavily myelinated motor axons form
a single en plaque = typical motor end plate (found in all skeletal muscles)
what is slow tonic felderstrucktur fibres innervated by
thinner motor axons
what type of end plate do felderstrucktur fibres innervated by thinner motor axons form
grape like en grappe = unique motor end plates/endings which do not generate action potentials in the muscle, but are thought to mediate sustained contractions
what do en grappe/grape like motor end plates of felderstrucktur fibres not generate and do mediate
do not generate action potentials
do mediate sustained contractions
what is the tendon region of felderstrucktur fibres innervated by
unique proprioceptors (on the tendon) endings
what is the name of the unique proprioceptor endings on the tendon of felderstrucktur fibres called
palisades
what is the function of palisades, which are proprioceptors found on the tendon of felderstrucktur fibres
monitor muscle tension signalling eye position in the orbit to the brain
what type of nerve is palisades proprioceptive endings of the felderstrucktur fibres
sensory nerve
what do intracellular recordings to maximal nerve stimulation show about fast twitch fibrillenstrucktur fibres
short duration from response to end of action potential of <10ms
amplitude was large 10mv
what do intracellular recordings to maximal nerve stimulation show about slow twitch felderstrucktur fibres
long slow response to end of action potential of ~200ms
amplitude was low but multiple (summating responses) with the highest to be 7.5mv
over which bone is the medial check ligament attached
lacrimal
over which bone is the lateral check ligament attached
zygomatic
briefly list a summary of the slow tonic muscle fibres
- thin motor nerve fibers
- multiply innervated en grappe
- large poorly delineated muscle fibrils (felderstrucktur)
- no conduction of action potentials
- slow sustained contraction (tonic)
- predominantly in orbital layer
briefly list a summary of fast twitch muscle fibres
- thick motor nerve fibres
- singly innervated en plaque
- small, well-delineated muscle cell fibrils (metabolic activated cells - fibrillenstrucktur)
- conduction of action potential
- fast contraction (phasic)
- predominantly in central/inner bulbar layer/global
