Muscles

Properties of muscles

• Contractability

• Extensibility

• Flexibility

• Excitability

Tendons

Fibrous inelastic connective tissue that attaches muscle to bone.

Antagonists

Pairs of muscles where one muscle provides movement in one direction and the other muscle in the other direction.

Origin

The end of the muscle fixed to the stationary bone. Doesn’t move in the action.

(E.g. Scapula - when the  bicep contracts, the forearm moves but shoulder is stationary)

Insertion

The end of the muscle fixed to the moveable bone. Moves in the action.

(E.g. Forearm (radius) moves when bicep contracts)

Belly

The fleshy portion of the muscle between the tendons of origin and insertion.

(E.g. The main portion of the bicep)

Agonist (prime mover)

The muscle that causes the desired action. (E.g. The bicep, as it is the contracting muscle)

Antagonist

Has the opposite effect to the agonist.

(E.g. The triceps, as it relaxes when the bicep contracts)

Synergists

Muscles that help indirectly to steady the joint during the movement by preventing unwanted movement.

(E.g. If the wrist did not include these it would flex every time you clenched your fist)

Fixator

When a synergist immobilises the joint it is called this.

(E.g. Muscles of the scapula hold it steady when the bicep and triceps contract)

Muscle fibres

Muscle cells which are cylinder in shape.

• Each muscle cell lies parallel to the next and are about 10 to 100 micrometres in diameter and a few millimetres to several centimetres long.

• Contain many nuclei.

Sarcolemma

The thin plasma membrane surrounding the muscle cells.

Sarcoplasm

Cytoplasm contained within the sarcolemma.

Myofibrils

Thread like structures inside the sarcoplasm which lie parallel to each other. There can be hundreds to many thousands in each fibre.

Myofilaments

• Myofibrils are composed of these smaller structures.

• Made of protein.

• Allow contraction of the muscle, making them very important.

• Two types of these.

Myosin

Thick myofilaments

Actin

Thin myofilaments

Myofibril shortening

Thick and thin filaments (myosin and actin) slide past each other when:

• Stimulated by a nerve impulse

• And if there is enough energy.

Sarcomeres

Units that myofibrils can be divided into. The arrangement of thick and thin filaments gives a balanced effect to the muscle.

• Contain actin and myosin in certain patterns, which gives skeletal and cardiac muscle its striated appearance.

Sliding filament model

Explains how muscle contraction occurs.

• It is a simplified representation of the idea/process.

• The basis of the model surrounds actin and myosin filaments sliding over each other.

1. SFM

The thin actin filaments slide over the thick myosin filaments.

2. SFM

The “Z Lines” (anchor points for actin) are drawn closer together and the sarcomere is shortened, thus shortening the whole muscle.

3. SFM

The myofilaments stay the same length, but they overlap each other to shorten the muscle.

4. SFM

When the muscle is relaxed, the actin and myosin are pulled back in the opposite direction, returning the muscle to its original state.

Thin filaments

• Contain tropomyosin and troponin

• Double-stranded coil

• Composed mostly of actin

Thick filaments

• Rod-shaped (tail) with head

• Composed mostly of myosin

Energy

Comes from the breakdown of ATP in the muscle cells.

Required for shortening of the muscle fibres.

ATP breakdown

Results in release of energy. Turns into adenosine diphosphate (ADP) and a phosphate group.

ATP reformation

Occurs when energy is again available from cellular respiration. It can then transfer energy from cellular respiration to processes such as muscle contraction.

Muscle tone

Maintaining partial contraction of skeletal muscles. Holds many body parts in position, e.g., the head is held up while standing or sitting.

Partial contraction

Tightens the muscle, but not enough fibres are contracting at once to produce movement.

Shift work

Fibres take turns contracting and relaxing.

Posture

Depends on muscle tone.