11.2 muscle contraction

Muscle

There are 3 types of muscle, that vary slightly in structure and properties:

skeletal (voluntary), smooth (involuntary) and cardiac.

End view of a muscle fibre

muscle fibres are ‘full’ of long parallel protein

structures

- the myofibrils

The cell membrane (sarcolemma) of the muscle fibre links with the sarcoplasmic reticulum, which extends

throughout the muscle fibre

Muscle tissue is not made up of individual ‘cells’, but giant muscle fibres

As embryonic muscle tissue differentiates, individual cells fuse together, creating multinucleate structures, the muscle fibres

Within a muscle fibre are many long, banded structures, the myofibrils.

These myofibrils extend the whole length of a muscle fibre.

Myofibrils have a regular repeating pattern. Each repeating ‘unit’ is called a sarcomere

A sarcomere is composed of 2 overlapping types of fibrous proteins, actin and myosin

Muscle fibre contractions are controlled from the CNS by neurons that synapse at neuromuscular junctions

A neuromuscular junction is a synapse Acetylcholine (Ach) is the

neurotransmitter

The arrival of an impulse releases Calcium ions allowing myosin/ actin cross

links to form

actin

myosin

An action potential is transmitted

to the muscle fibre’s

sarcolemma and spreads throughout the muscle fibre along

its sarcoplasmic reticulum

A muscle contraction is caused by the interlocking actin and myosin fibres sliding over one another, shortening the muscle.

The arrival of a nerve impulse, and its spread throughout the muscle fibre causes this ‘sliding’contraction

Myosin molecules have a head and ‘tail’, and occur in ‘bundles’ or filaments

Actin molecules are globular and occur in chains

In a resting muscle, any reaction between actin & myosin is prevented by tropomyosin, which blocks actin’s binding site

when a nerve impulse stimulates a muscle to contract....

The action potential spreads throughout the muscle fibre, along its sarcoplasmic reticulum Releasing Calcium ions into the cytoplasm Calcium ions allow myosin ‘heads’ to form cross links with actin The myosin molecule pulls the actin molecule ‘back’, shortening the overall length of the fibreATP provides the energy to release the myosin head and change its angle, ready to bind again

So long as the actin binding sites are ‘open’, myosin will continue to bind, contract and move the actin fibres along. This process requires energy as ATP

1. Stages in muscle contraction

The muscle fibre is at rest;Myosin is prevented from forming cross links with actin

2. Stages in muscle contraction

When Calcium ions are present, actin sites are ‘unblocked’

3. Stages in muscle contraction

Cross- bridges can form

4. Stages in muscle contractionThe myosin head pulls the actin

‘back’

Summary• Energy provided by ATP is needed for any contraction to occur • a muscle is always “ready” to contract, but this is prevented (or ’inhibited’) by a lack of Ca2+ ions• Ca2+must be present to unblock actin’s binding sites.• AFTER a contraction, Ca2+ is pumped back into the sarcoplasmic reticulum• So, in the absence of Ca2+, the muscle relaxes

Animations•Sliding filaments•Actin and myosin binding model