Skeletal Muscle

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Antagonistic Pairs

Skeletal muscles contract and relax to allow you to move your body. This is done using antagonistic pairs. Antagonistic pairs consist of an agonist and an antagonist.

Bones

Tendons attach skeletal muscles to bones.
The muscles work in a pair to move the bones.
A pair of muscles is called an antagonistic pair.
- In an antagonistic pair, one muscle contracts when the other muscle relaxes.

Antagonist

The muscle that is relaxing is called the antagonist.
Which muscle in a pair is the antagonist can vary depending on the movement.
E.g. When you bend your arm, your tricep muscle relaxes (it is the antagonist). When you straighten your arm, the tricep muscle contracts (it is the agonist).

Agonist

The muscle that is contracting is called the agonist.
Which muscle in a pair is the agonist can vary depending on the movement.
E.g. When you bend your arm, your bicep muscle contracts (it is the agonist). When you straighten your arm, the bicep muscle relaxes (it is the antagonist).

Muscle Fibres

The structure of skeletal muscles is specialised for contraction.

Muscle fibres

Skeletal muscle consists of many bundles of muscle fibres.
Muscle fibres are long, specialised cells.

Sarcolemma

The membrane of muscle fibres is called the sarcolemma.
The sarcolemma folds inwards to the sarcoplasm (muscle fibre cytoplasm) at certain points.
The inwards folds are called transverse (T) tubules. The tubules are very important in initiating muscle contraction.

Sarcoplasmic reticulum

The sarcoplasmic reticulum (SR) is an organelle in the sarcoplasm.
The SR is a store for calcium (Ca²⁺) ions. This is important in muscle contraction.

Mitochondria and nuclei

Muscle fibres also have many mitochondria and nuclei.
The mitochondria provide lots of ATP to power muscle contraction.

Myofibrils

Myofibrils are cylindrical organelles that run along the length of muscle fibres.
Myofibrils are the site of muscle contraction.

Myofibrils

Myofibrils are cylindrical organelles in muscle fibres. Myofibril structure is highly specialised for muscle contraction.

Sarcomere

Myofibrils are made of multiple units that run end-to-end along the myofibril. These units are called sarcomeres.
The end of a sarcomere is called the Z-line.

Myofilaments

Sarcomeres are made from two types of myofilaments.
The two myofilaments slide past each other. This movement is what makes muscles contract.
The two types of myofilaments are:
- Thick myofilaments - made of myosin protein.
- Thin myofilaments - made of actin protein.

Myosin filaments

Myosin and actin filaments are arranged in an alternating pattern in sarcomeres.
Thick myosin filaments overlap with the thin actin filaments at each end.
- The overlapping region is called the A-band.
- The region with only myosin filament is called the H-zone.

Actin filaments

Thin actin filaments only overlap with myosin filaments in the middle of the sarcomere. The middle is called the M-line.
The region with only actin filament is called the I-band.