AQA Biol 4 Muscle Contraction

The Components of a Muscle

(http://www.macroevolution.net/images/sarcomere.gif)

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An action potential arrives at the neuromuscular junction. This causes calcium ion channels to open and Ca2+ ions to diffuse in. Vescicles bind to the muscle end plate and release acetylcholine.
Acetylcholine binds to receptors on the muscle fibre membrane, causing Na+ to diffuse in, causing depolarisation.
Depolarisation wave travels down T Tubules (T system)
This depolarisation leads to Ca2+ release from the sarcoplasmic reticulum in the muscle
Ca2+ binds to the proteins in the muscle - troponin - which causes tropomyosin to move and reveal myosin-binding sites, this leads to muscle contraction
Acetylcholinesterase in the cleft rapidly breaks down acetylcholine so that contraction only occurs when impulses arrive continuously
Acetyl and choline diffuse back across the cleft.

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Skeletal muscles cause the skeleton to move at joints
They are attached to the skeleton by tendons, which transmit the muscle force to the bone and can also change the direction of the force
Tendons are made of inelastic fibres and are very strong and stiff
The skeleton provides leverage, magnifying either the movement or the force
Muscles are either relaxed or contracted
In the relaxed state muscle is compliant (can be stretched)
In the contracted state muscles exert a pulling force, causing it to shorten, or generate force
Since muscles can only pull (not push), they work in pairs called antagonistic muscles. The muscle that bends (flexes) the joint is called the flexor muscle, and the muscle that straightens (extends) the muscle is called the extensor muscle.

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The action potential moves through the fibres by travelling through T-tubules that branch through the sarcolemma.
The action potential moves through the tubules until it reaches the sarcoplasmic reticulum.
This opens the calcium ion channels in the sarcoplasmic reticulum.
Calcium ions diffuse out into the muscle.
Calcium ions cause troponin to change shape so that the binding sites on the actin filament are exposed.
The myosin head forms a cross bridge with actin by binding with the receptor site.
An ATP molecule attaches to the myosin head and thus causes it to detach.
Calcium ions activate the enzyme ATPase which hydrolyses the ATP and releases energy to allow the myosin to resume its natural shape.
The myosin head now has a new ADP molecule that will allow it to bind with a new receptor site somewhere along the actin filament.

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Fast contraction speed
Powerful contraction power
Short length of contraction
Adapted for short bursts of energy, ie sprinting, weight lifting
Found mainly in upper thigh and upper arms to move faster
Has anaerobic respiration because aerobic takes too long
Adaptions
- Thicker and more numerous myosin filaments
- High concentration of enzymes involved in anaerobic respiration
- A store of phosphocreatin, a molecule that can rapidly generate ATP from ADP in anaerobic respiration
- Produces lactate so fatigues quickly

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Slow contraction speed
Short contraction power
Long length of contraction
Adapted for endurance activities, for long periods of time
Found mainly in calf and lower arms
Has aerobic respiration
Adaptions
- Large store of myoglobin
- A supply of glucose to produce a source of metabolic energy
- Rich supply of blood vessels to deliver oxygen and glucose
- Numerous mitochondria to produce ATP

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