Muscles

Skeletal Muscle

Overview:

• Muscles act in antagonistic pairs against an incompressible skeleton
• The skeleton provides the basic structure of levers and joints to which the muscles are attached
• Muscle fibres and cells have a cell surface membrane called a sarcolemma, an endoplasmic reticulum called a sarcoplasmic reticulum and a cytoplasm called a sarcoplasm
• Muscles contain many myofibrils, one myofibril consists of many protein filaments, thick myosin filaments and thin actin filaments

Neuromuscular Junction:

The place where a motor neurone synapses with a muscle fibre.

When an action potential reaches the junction, synaptic vesicles containg the neurotransmitter acetylcholine, fuse with the presynaptic membrane and releases the neurontransmitter into the synaptic gap. Acetylcholine diffuses across the gap and attaches to the plasma membrane on the sarcolemma (the muscle cell surface membrane).

The sarcolemma is depolarised and an action potential is transmitted along the muscle fibre.

The action potential causes the release of calcium ions into the sarcoplasm (the cytoplasm of the muscle cell). The prescence of the calcium ions cause a protein called tropomyosin to uncover the myosin head binding sites on the actin filaments. Myosin heads begin to form cross bridges with the actin filamets, called actino-myosin cross bridges.

This allows the actin filaments to be pulled closer together by the myosin filaments, thus making the sarcomeres of the muscle shorten or contract. This is known as the sliding filament theory, ATP is necessary for the contraction of the muscle fibres.

Muscle Contraction:

• Sliding filament mechanism -
  • Actin filaments have binding sites for the myosin heads
  • Myosin filaments are bound together…