Structure of Skeltal Muscle

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Attached to bone and acts under concious control

Made up of millions of myofibrils

Muscle is composed of smaller units bundled into progressively larger ones

Seperate cells fuse together in muscle fibres to increase overal strength of muscle. they share a nuclei and cytoplasm (sacroplasm) which is found around the circumference of the fibre

Sarcoplasm contains large concentration of mitochondria and endoplasmic reticulum

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Microscopic Structure of Skeletal Muscle

Myofibrils are made up of actin (thinner and consists of 2 strands twisted around each other) and myosin (thicker and consists of long rod shaped fibres with bulbous heads that project to the side)

Myofibrils appear striped due to theor alternating light and dark coloured bands

Light bands= isotrophic bonds. They appear lighter because actin and myosin don't overlap

Dark bands= anisotrophic bands. They appear darker because actin and myosin overlap

At the centre of each anisotropic band is a lighter coloured region called the h-zone

At the centre of each isotrophic band is a line called the z-line. Distance between adjacent z-lines is called the sacromere

When the muscle contracts, sacromeres shorten and the pattern of light and dark bands changes

Other 2 portiens in muscles are the tropomyosin (forms a fibrous strand around  the actin filament) and troponin (globular protien involved in muscle contraction)

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Type of muscle fibre- Slow Twitch

Contract more slowly and provide less powerful contractions, but over long periods of time

Suitable for endurance work

More common in calf muscle (has to contract constantly to maintain body in upright position)

Adapted for aerobic respiration

Have a large store of myoglobin (stores O2)

Have a supply of glycogen to provide a sorce of metabolic energy

Have a rich supply of blood vessels to deliver O2 and glucose

Have numerous mitochondia to produce ATP

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Type of Muscle Fibre- Fast Twitch

Contract more rapidly and produce powerful contractions, but over short periods of time

Suitable for intense exercise

More common in biceps (which may do short bursts of energy)

Adapted for anaerobic respiration

Have a thicker and more numerous myosin filaments

Have a concentration of enzymes involved in anaerobic respiration

Have a store of phosphocreatine (a molecule that can rapisly generate ATP from ADP in anaeroobic conditions and provide energy for muscle contraction)

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Neuromuscular Junctions

Where a neurone meets a skeletal muscle fibre

There are many junctions along the muscle in order for muscles to contract simutanously and fast movement

All muscle fibres supplied by a single motor neurone act together as a single functional unit (motor unit). This arrangment gives control over the force that the muscle exerts. If only slight force needed, only a few units are stimulated

When nerve impulse is recieved at neurotransmitter junction, the synaptic vesicles fuse with the pre-synaptic membrane and release their acetylcholine

Acetylcholine diffuses to post-synaptic membrane, altering its permablilty to sodium ions, which enter rapidly, depolarising membrane

Acetylcholine is broken down by acetylcholinesterase to ensure muscle isn't over stimulated

Resulting choline and ethanoic acid diffuse back into neurone, where its recombined to form acetylcholine using energy provided by mitochondria

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