E&E-Structural properties and activation of muscle PDF

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Exercise and environment

Structural properties and activation of muscle Function of muscle:  Mobility  Maintain posture (sit, stand)  Breath, talk, eat  Hold structures in body together  Slow movement (act as brake)  Source of heat  Metabolic store Basic composition of skeletal muscle Musculoskeletal system (muscle, bone, connective tissue) comprises -75% of the body mass of a healthy person. It is made up of:  75% water  20% protein Muscle  Myosin (40%)  Actin (20%)  Tropomyosin  5% inorganic salts and other substances Structure of muscle and muscle fibre  

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Inside muscle fibres, there are tiny units called myofibrils Within myofibrils there are dark and white bands called sarcomeres From one dark band to another is one sarcomere Within sarcomeres there are proteins which are important for muscle contraction

In muscle tissue there are muscle fibres There is connective tissue (endomysium) around muscle fibres for protection

Electron microscope picture of myofibrils with sarcomeres

Exercise and environment Sarcomere

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Different proteins are found in different sarcomeres The two most important proteins found are actin and myosin Myosin is thick filament and actin is thin filament Mutations in sarcomere can lead to severe muscle weakness and may be put on ventilator because diaphragm is not strong enough

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Myosin, actin interaction

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Sliding theory (myosin and actin slide past) Sliding provokes shortening and stretching of sarcomere Sarcomere is not rigid (constantly moving)

Myosin head

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Myosin heads are found on myosin filaments Myosin heads bind to actin (this causes a pulling force) which causes contraction of the sarcomere Release allows for stretching

Exercise and environment Single muscle fibre twitch

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Muscle fibre of a frog When electrical current is passed through muscle, there is a mechanical force Small electrical stimulus = small force response and vice versa The force is called an isometric twitch

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Isometric twitch- A mechanical response to a single electrical impulse      

No change in muscle fibre length during contraction = isometric contraction There is always a delay before the force response =EMD (electromechanical delay) Peak point= Pt (peak twitch) Activation =TPT (time to peak tension) Decrease= ½ RT (relaxation time) total relaxation is never recorded because it does not reach 0 All these parameters are used to compare different muscle fibres, different species etc

The neuromuscular junction   

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Action potential occurs = electrical stimulus AP goes along membrane of muscle fibre and into the T-tubule This causes an activation of the DHP receptor= activates the sarcoplasmic reticulum This activation opens the gates and causes influx of calcium ions to the muscle fibres Calcium ions then bind to the actin filaments Binding will promote actin attachment to the myosin head Sliding of actin and myosin filaments Muscle contraction

Exercise and environment Tetanus- The mechanical response to multiple stimuli (lots of twitches)  

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Many stimulus Unfused response- consecutive responses after short delay (one response, wait, another one etc) the force is not added because during the delay, the muscle fibres have time to relax Unfused response- Where there is a shorter delay, the muscle fibres cannot completely relax (when there is shorter delay, the force is added and becomes bigger Fused response- where there is very short delay between stimulus, so the forces get added (no time to relax)

What happens to calcium? Unfused tetanus  

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Unfused tetanus- during a large delay, calcium has time to bring calcium and release When frequency of stimulus is increased there is no time to release the calcium= no relaxation (calcium builds up) It takes 80 milliseconds to remove half of calcium

Fused Tetanus What happens to the force?

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A high rate of impulses results in consistently high levels of calcium in the cytoplasm This allows cross-bridge (myosin-actin interactions) cycling to continue “uninterrupted”

Exercise and environment Which muscle fibres can generate the most force?

Diagram shows two muscle fibres  Red- many sarcomeres in series  Blue- short sarcomeres going in parallel  Blue fibre can generate the most force (thicker fibre)  Red muscle fibre is good for high velocity (moving at a faster rate) Isometric contraction vs concentric contraction

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During isometric contraction, the muscle fibre does not change length Myosin tries to contract but they can’t E.g. if trying to lift a very heavy item, muscle cannot contract

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During a concentric contraction, the muscle fibre shortens E.g. if item is light, the muscle can contract and lift

During eccentric contraction the muscle fibre stretches E.g. if item is lifted but heavy you put it back down (active muscle is lengthening under load)

Force-velocity relationship   

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There is no movement in isometric contraction so there is no velocity For high forces, movement is very slow and vice versa E.g. when lifting heavy items, you cannot walk fast whereas with a pencil you can Relationship between force and velocity is a negative correlation

Exercise and environment

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A smaller proportion of myosin heads are attached and contribute to force generation When muscle is trying to shorten at a higher velocity, some of the myosin heads cannot bind to the actin Red muscle fibre is good for velocity (being fast) long muscle is needed

Weight lifters more likely to have thicker (blue) muscle fibres and sprint runners are more likely to have long (red) muscle fibres Single muscle fibre power Power= force x velocity Power can be measured during concentric contractions but not isometric conditions (because in isometric contraction, there is no velocity) How power looks      

Red shows the force-velocity curve Yellow dotted line represents power Person A is likely to be a body builder (can produce a lot of force but not much velocity) Person B is likely to be a sprint runner (lower force but higher velocity) A and B are producing the exact same amount of power Power is a combination of force and velocity not one or the other

High levels of power require high levels of thick muscle fibre (blue) AND long muscle fibre (red) Sarcomeres in parallel:  Forces add up (more force is being produced)  each element feels only part of the whole force  movements do not add up  each element experiences the whole movement Sarcomeres in series:  forces do not add up (more velocity)  each element feels the whole force  movements do add up  movement is shared between the elements Influence of fibre type

Exercise and environment

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In muscle, there are different fibre types  Type I fibres (slow fibres) time to reach peak force is 110ms  Type II fibres (fast fibres) time to reach peak force is 50ms Fibre type is defined by the myosin heavy chain isoform composition

Muscle fibre recruitment (aka motor unit recruitment)    

A motor unit is composed of a single motor neuron and all the muscle fibres it innervates Number of muscle fibres in a motor unit varies e.g.: Gastrocnemius (forms half of calf muscle) has 2000 muscle fibres per motor neuron (important for posture, fine adjustment is not required) Extraocular muscle (controls movement of eye) has...