Muscle Microstructure And Contraction Flashcards
What are the types of muscle?
Smooth - involuntary control from autonomic nervous system
Cardiac - can contract autonomously but is under the influence of the autonomic nervous system and circulating chemicals
Skeletal - under voluntary control. Usually attached to bones and contract to bring about movement
What are some types of arrangement of muscle fibres?
Parallel
Fusiform
Triangular
Pennate: unipennate, bipennate, multipennate
What is the structure of skeletal muscles?
Muscle (surrounded by epimysium)->
Fascicles (bundle of muscle fibres) (surrounded by perimysium) ->
Myofibre (surrounded by endomysium) (multi uncleared ass they are made from fused muscle cell) ->
Myofibril ->
Myofillament
What is the structure of myofibres?
Covered by a plasma membrane - sarcolemma
T tubules tunnel into centre
Cytoplasm called sarcoplasm - myoglobin and mitochondria present
Network of fluid filled tubules called sarcoplasmic reticulum
Composed of many myofibrils
What is the structure of myofibrils?
1-2 mu m in diameter
Extend along the entire length of myofibres
Composed of two main types of protein - actin (thin) and myosin (thick)
The light and dark bands gove muscle striated appearance - dark band is myosin, light is actin
These don’t extend along the length of myofibres
Overlap and are areanged in compartments called sarcomeres
What is the structure of the myofillaments (with relation to bands)?
Sarcomeres are separated by dense protein bands - Z discs
Dark bands (myosin) - A band
Light bands (actin) - I band
Actin and myosin fillaments overlap
The M line is the centre line down the dark band of myosin
The H zone is the portion of the A band that does not overlap with the I band
What is the structure of myosin?
Golf club
Two globular heads
Single tail formed by two alpha helices
Tails of several hundred molecules form one filament (all in same direction)
What is the structure of actin?
Actin molecules twisted into a helix
Each molecule has a myosin binding site
Fillaments also contain troponin and myosin (these move and uncover binding sites)
What happens (basically) in sliding filament theory?
Z discs get closer together
I Bands get shorter
A bands stay the same length
H zone disappeared or narrowed
How does initiation of muscle contraction occur?
Occurs at neuromuscular junctions
- Action potential opens voltage Ca2+ channels
- Calcium enters presynaptic terminal
- Calcium triggers exocytosis of vesicles
- Acetylcholine diffuses across cleft
- Binds to acetylcholine receptors and induces action potentials in muscles
- Local currents flow from depolarised region and adjacent region. So action potential spreads along surface of muscle fibre membrane
- Acetylcholine is broken down by acetylcholine esterase. Muscle fibres response to that molecule of ACH ceases
How does activation of muscle contraction occur?
- Action potential propagates along surface membrane and into T tubules
- Dihydropyridine (DHP) receptor in T tubule membrane senses the change in voltage and changes shape of the protein linked to Ryan prime receptor. This opens the ryanodine receptor calcium channel in the sarcoplasmic reticulum.
This causes Ca2+ to be released from sarcoplasmic reticulum into the space around the fillaments
- Ca2+ binds to troponin, so tropomyosin moves revealing the binding site on actin allowing…
- Cross bridges to attach to actin
- Calcium is actively transported to the sarcoplasmic reticulum continuously while action potentials contine. ATP driven pump
What happens during excitation contraction coupling (interaction between actin and myosin)?
- In the presence of calcium - 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 and 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
What is the neural control of muscle contraction?
Voluntary neural control from upper and lower motor neurones
Upper motor neurones in brain - synapses with Lower
Lower motor neurone in brainstem or spinal cord - this synapses with a muscle (NMJ)
What is a motor unit?
The name given to a single motor neurone and all the muscle fibres that it innervates
On average each motor unit supplies about 600 muscle fibres
What are the types of motor unit?
Slow (type I):
Smallest diameter cell bodies
Small dendritic trees
Thinnest axons
Slowest conduction velocity
Fast, fatigue resistant (FR, IIa):
Large diameter cell bodies
Large dendritic trees
Thicker axons
Faster conduction velocities
Fast, fatiguable (FF, IIb):
Same as above
How are the muscle fibres types distributed?
Randomly throughout the muscle
Muscles have different proportions of slow and fast twitch muscles
They vary in myoglobin content, colour, aerobic capacity and anaerobic capacity
(Muscles involved in posture control have a larger proportion of slow twitch)
What are the properties of motor units?
Classified on amount of tension generated, speed of contraction, and fatigueability of the motor unit
Type I: slow twitch. Moderate force. Fatigue resistant
Type IIA: fast twitch. Moderate force. Fatigue resistant
Type IIB: fast twitch. High force. High fatigue
How is muscle force regulated?
Recruitment:
Motor units are not randomly recruited, there is an order
Governed by the “size principal “. Smaller units are recruited first (generally slow twitch)
As more force is required, more units are recruited. This allows fine control, under which low force levels are required
Rate coding:
The 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
Summation occurs when units fires at a frequency too fast to allow the muscle to relax between arriving action potentials
What is the order of muscle fibres recruitment?
Slow
Fast, fatigue resistant
Fast fatiguable
The fibre recruited last is also the first fibre to be de recruited
Muscle force can be regulated by The number of motor units recruited
What are neurotrophic factors?
A type of growth factor that prevents neuronal death and promotes growth of neurones after injury
Motor unit and fibre characteristics are dependant on the nerve which innervates the
If any fast and slow twitch muscle or cross innervated, The slow one becomes fast and vice versa
The motor neuron has some effect on the properties of the muscle fibres it innervates
What are the types of muscle contraction?
Concentric - shortens muscle to produce movement (lifting the dumbbell)
Eccentric - muscle produces force but the muscle gets longer (the dumbell is too heavy to lift but you’re still trying)
Isometric - provides force but doesn’t change length
What is the plasticity of motor units/muscle fibres?
Fibre types can change properties under many different conditions
Type IIB (fatiguable) to IIA (resistant) is the most common change during training
Type I to II is possible in cases of severe deconditioning or spinal cord injury. Microgravity during space flight results in shift from slow to fast muscle fibres
Ageing associated with loss of type I and II muscle fibres. But preferential loss of type II. So elederly people have slower contraction times
