Skeletal muscles are effectors - AL only (3.6.3)

Muscles are able to flex, causing joints to bend, this is done by flexor muscles.
Muscles are also able to extend, causing the joints to straighten, this is caused by extensor muscles.

Muscles can only pull the skeleton, so they work in antagonistic pairs.

There are several different muscle types:

• Skeletal muscle: these muscles control movement.
• Cardiac muscle: specialised muscle only found in the heart.
• Smooth muscle: involved in involuntary processes, found in the bladder, blood vessels, and digestive tract.

Skeletal muscle is made up of a bundle of muscle fibres. This bundle is made up of millions of single muscle fibres which are long, multinucleated cells. Each fibre contains thousands of myofibrils (contractile units).

Muscle fibres share sarcoplasm (cytoplasm) and nuclei found around the periphery of the cell surface membrane.

Muscle fibres contain myofibrils, contractile threads with repeating units (sarcomeres). These are protein-based organelles specialised for contraction. Myofibrils are laid down in parallel and combined, are very strong.

Myofibrils contain two types of protein filaments:

• Actin is a thinner filament, made up of two twisted strands. Actin has binding sites for myosin, which are blocked by tropomyosin and held in place by troponin.
• Myosin filaments are thicker, long, rod-shaped fibres with hinged projections to the sides. Each head has one binding site for actin and one for ATP.

The sliding filament model explains how muscle fibres contract:

• Myosin filaments pull actin filaments towards the centre of the sarcomere, sliding past one another.
• The I-band and H-zone become narrower.
• The A-band remains the same.
• The Z-lines move closer together so the sarcomere shortens.

As many sarcomeres shorten simultaneously, the muscle fibre contracts, causing the whole muscle to contract and produce movement.

Skeletal muscle contraction occurs when actin and myosin filaments slide past one another, shortening the sarcomere. This is known as the sliding filament theory:

• A nerve impulse triggers release from the sarcoplasmic reticulum into the sarcoplasm.
• binds to troponin, causing tropomyosin to move off actin’s myosin binding sites.
• Myosin heads attach to exposed actin, forming actinomyosin bridges.
• ADP and P_i on myosin is released.
• Myosin heads bend forward and actin slides over the myosin.
• ATP binds to myosin heads and actin is released.
• ATPase on myosin head hydrolyses ATP to ADP + P_i.
• The myosin head returns to its original position.

is actively transported back into the endoplasmic reticulum; this process requires ATP.

This allows tropomyosin to block the actin filament, so the muscle contraction eases

Myosin heads are no longer able to bind to actin.

The hydrolysis of ATP releases energy.

Muscle contraction requires a lot of oxygen for respiration to occur and to supply ATP. ATP is required to move myosin heads during cross-bridge cycling and for the active transport of back into the endoplasmic reticulum.

During intense exercise, ATP demand can exceed the supply. When this occurs, small amounts of ATP can be generated by glycolysis.

It is also possible for ATP to be regenerated using phosphocreatine (PCr):

The PCr system is anaerobic and very fast but lasts only a few seconds, making it useful for short bursts of high-intensity activity.

Phosphocreatine is then replenished during a period of rest.

Slow twitch muscle fibres are adapted for endurance exercise.

• They contain a lot of myoglobin, which stores oxygen.
• Slow twitch muscle fibres have a rich supply of blood vessels for oxygen and glucose delivery, which can be used to produce ATP during respiration.
• They also contain many mitochondria.

Fast twitch muscle fibres are adapted for short periods of intense exercise.

• They are thicker and contain more myosin filaments for faster contraction.
• They have a store of glycogen, so more glucose can be released.
• Fast twitch muscle fibres have a higher concentration of enzymes involved in anaerobic respiration.
• They also have a store of phosphocreatine to produce ATP during periods of intense exercise.