How Muscles Contract
Nerves connect the spinal column to the muscle. The nerve and muscles meet at the
neuromuscular junction. Acetylcoline is released by the axon into the synapse, where it then
stimulates the target sarcolemma. Inside the muscle fibers, the signal stimulates the flow of
calcium, which causes the thick and thin myofilaments to slide across one another. When this
occurs, it causes the sarcomere to shorten, which generates force. When billions of sarcomeres
in the muscle shorten all at once, it results in a contraction of the entire muscle fiber. Its main
purpose is to move you. It connects to the bones, and makes them move or not. Tendons attach
muscle to bone.
Muscle Contraction
- Motor nerve cell (neuron) carries stimulus to motor end plate.
- Acetylcholine crosses the neuromuscular junction and stimulates sarcolemma.
- After acetylcholine reaches muscle fiber, it stimulates a release of calcium in the cell,
which triggers action of the troponin and tropomysin. - Cross bridges pull thick and thin myofilaments over each other, shortening the length of
entire fiber. - One neuron may innervate up to one hundred muscle fibers.
- Motor unit: unit of muscle fibers innervated by one single neuron.
- The less muscle fibers innervated by one neuron, the more fine control over the muscle
(fine movements). - Force is generated in two ways: 1) increasing the amount of motor units firing; 2) increasing
the speed of neuro-motor stimuli.
When a muscle is stimulated to work, it can only shorten. Muscles pull on the objects they
are connected to.
Concentric Contraction
- Muscle contraction as fibers shorten.
- Usually active, voluntary.
Eccentric Contraction
- Muscles contract as fibers lengthen.
- Usually involuntary, in order to protect the joint.
- Usually antagonistic - purpose is to decelerate the agonist (this usually occurs at the end
range of the joint). - Strength training is more effective when includes eccentric actions.
- More muscle injuries occur during eccentric than concentric phase.
Isotonic Contraction
- Muscle contraction with movement around the joint.
Isometric Contraction
- Muscle contraction with no movement around the joint.