Muscle microstructure and contraction Flashcards
what are Muscle types
Smooth
Cardiac
Skeletal
what is smooth muscle controlled by
is under involuntary control from the autonomic nervous system
control of cardiac
muscle can contract autonomously, but is under the influence of the autonomic nervous system and circulating chemicals
control of skeletal
Skeletal muscles are under voluntary control, usually attached to bones and contract to bring about movement
what is a fusiform
larger in the middle and tapered at the end
what are pennate muscles
where muscle fibres are placed at angles to the tendon
unipennate
same direction from the tendon
bipennate
2 direction from the tendons
multipennate
multiple tendons and multiple fibres
what is a Fascicles
– bundles of muscle fibres
what is a fascicle made up of
myofibre –> myofibril –> myofilament
how many connective tissues are muscles made up of
3
what does the epimysium made up of
muscle fibre
what are muscle fascicle bounded by
perimysium
what do endomysium sorround
muscle fibre
what is a sacrolemma
the plasma membrane covering myofibres
what is the cytoplasm in the muscle fibre
sacroplasm
what does the sacroplasm contain
myoglobin and mitochondria
sarcoplasmic reticulum
Network of fluid filled tubules
what makes up a myofibre
myofibrils
what is a sacomere
a repeating unit
a z disk to another
what gives striated (striped) appearance
Light and dark bands
what is the thin filament/light band
actin
thick filament
myosin
what separates sacromeres
Z-discs separate sarcomeres
how are dark bands represented
A band (thick - myosin)
and light
Light bands – I band (thin - actin)
what causes muscle contraction
movement of actin over myosin
strcuture of myosin
Two globular heads
Single tail formed by two α-helices
Tails of several hundred molecules form one filament
Structure of skeletal muscles - actin
Actin molecules twisted into helix
iniation of muscle contraction
- Action potential opens voltage gated Ca2+ channels
- Ca2+ enters pre-synaptic terminal
- Ca2+ triggers exocytosis of vesicles
- Acetylcholine diffuses across cleft
- Binds to acetylcholine receptors and induces action potentials (AP) in muscle
- Local currents flow from depolarized region and adjacent region
AP spreads along surface of muscle fibre membrane - Acetylcholine broken down by acetylcholine esterase. Muscle fibre response to that molecule of acetylcholine ceases.
what happens in the muscle
- Action potential propagates along surface membrane and into T-tubules
- Dihydropyridine (DHP) receptor in T-tubule membrane: senses V & changes shape of the protein linked to ryanodine receptor,
opens the ryanodine receptor Ca2+ channel in the sarcoplasmic reticulum (SR)
Ca2+ released from SR into space around the filaments - Ca2+ binds to troponin & tropomyosin moves allowing..
- Crossbridges to attach to actin
- Ca2+ is actively transported into the SR continuously while action potentials continue. ATP- driven pump (uptake rate < or = release rate).
Excitation contraction coupling
In the presence of Ca2+ movement of troponin from tropomyosin chain
Movement exposes myosin binding site on surface of actin chain
‘Charged’ myosin heads bind to exposed site on actin filament
This binding & discharge of ADP causes myosin head to pivot (the ‘power stroke’) pulling actin filament towards centre of sarcomere
ATP binding releases myosin head from actin chain
ATP hydrolysis provides energy to ‘recharge’ the myosin head
where do upper motor nuerons have their cell bodies
motor cortex
The motor unit
Name given to a single motor neuron together with all the muscle fibres that it innervates.
non of those muscle fibres is also innervated by another any other branches of another motor unit
muscles which have fine control have many or few muscles fibres innervated by a motor neurone
few
large muscles which are needed for large force have how many many or few muscle fibres innerveated by a single nuerone
many
what are the types of motor units
Slow (S, type I) Fast, fatigue resistant (mantain contractions for long periods of time) (FR, type IIA) Fast, fatiguable (FF, type IIB)
what are the charatistcis of slow/type 1
smallest diameter cell bodies
small dendritic trees
thinnest axons
slowest conduction velocity
fast, fatigue resistant
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
fast gatiguable
larger diameter cell bodies
larger dendritic trees
thicker axons
faster conduction velocity
how are muscle fibres distributed throughout the muscle.
Randomly
Muscles have different proportions of slow and fast twitch muscles
Myoglobin content Colour Aerobic capacity Anaerobic capacity of slow
Myoglobin content High Colour Red Aerobic capacity High Anaerobic capacity Low
Fast, fatigue resistant (FR, type IIA) Myoglobin content Colour Aerobic capacity Anaerobic capacity
Myoglobin content High
Colour Pink
Aerobic capacity Moderate
Anaerobic capacity High
Fast, fatiguable (FF, type IIB) Myoglobin content Colour Aerobic capacity Anaerobic capacity
Myoglobin content Low Colour White Aerobic capacity Low Anaerobic capacity High
muscles involved in postural control have a greater proportion of slow or fast twitch fibres
greater proportion of slow twitch
how much force and how fast would typw 1 produce
low force
long time
type iia
moderate force
in moderate time
type iib
high force
quickly
what muscle fibres are high fatigue
type iib
fatigue resistant
iia i
what are the two mechanisms by which the brain regulates the force that a single muscle can produce.
Recruitment
Rate coding
recruitement
how many motor nuerones are being bought in or involved to fire in order to produce a contraction
more morot unit recruited = more force produced
Motor units are not randomly recruited, there is an order.
Motor units are not randomly recruited, there is an order.
what is the order
Governed by the “size principle”. Smaller units are recruited first (these are generally the slow twitch units).
more force is required, more units are recruited.
This allows fine control (e.g. when writing), under which low force levels are required.
Rate coding
what frequency are action potentials are sent down the nuerone
A motor unit can fire at a range of frequencies. Slow units fire at a lower frequency.
As the firing rate increases
the force produced by the unit increases.
when does summation take place
when units fire at frequency too fast to allow the muscle to relax between arriving action potentials.
