11.2 muscle contraction
TRANSCRIPT
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