Muscle Contraction – Skeletal Muscles

Sliding Filament Theory

1. Motor Neuron sends nerve impulse to the muscle cell.  They can carry messages to contract or inhibit contraction.
   Nerve impulse signals the release of acetylcholine (neurotransmitter) from the synaptic vesicles into the synaptic vessel
2. Acetylcholine moves across the synaptic cleft and starts the action potential that moves from the end plate to the sarcolemma
3. The Acton Potential enters T tubules and sarcoplasmic reticulum releasing calcium into the sarcoplasm
4. Calcium uncovers binding sites on the actin
5. Receptor sites on actin attach to the myosin cross bridges
6. Energy from ATP breakdown cause the myosin cross brides to swivel causing the actin to slide past the myosin
7. Zdiscs are pulled together and the muscle shortens.
8. Acetycholinesterase breaks down acetylcholine
9. Calcium is pumped back into the sarcoplasmic reticulum.  Low levels of calcium stop the myosin from moving.
10. The sarcomere returns to its resting length.

All or None Principle

Muscle fibers of motor units will contract to their fullest extent when they are stimulated to the threshold.

Motor neuron sends stimulus and all muscle fibers respond to contract at the same time.  Applies to motor units only, not the whole muscle.