Nature and Function of Muscular Contraction (All-or-None Law, Summation)
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A muscle shortens by shortening its fibers; each fiber fires completely or not at all, and force is graded by how many fibers fire and how their contractions sum.
Detail
When a muscle contracts, it simply becomes shorter. It does this by reducing the length of a number of the fibers that make up the body of the muscle. Muscle ends are made of dense, tough tissue known as tendons, which attach the muscles to the bones.
All-or-None Law of Muscle Contraction: when muscle fibers contract, they contract completely or not at all. The number of fibers that contract determines how much "strength" is exerted — for example, an individual would use fewer fibers to close a car hood than to push the car a block.
Muscles contract voluntarily because they are controlled by the Central Nervous System; they are under conscious control by a nerve impulse. When a muscle contracts, nerve impulses are sent to the fibers. When the fibers reach their level of contraction, they contract completely, then relax. Each successive time they contract, their level of tension is increased. Many successive contractions bringing about a prolonged contraction is known as the "summation of contractions."
Key facts
- A muscle becomes shorter by reducing the length of a number of its fibers.
- All-or-None Law: a fiber contracts completely or not at all.
- Force ("strength") is graded by the number of fibers recruited.
- Skeletal muscle is under voluntary CNS control via nerve impulses.
- Summation of contractions: many successive contractions producing a prolonged contraction.
Connections
- Structure of Skeletal Muscle — the fibers that shorten.
- Motor Unit / Muscle Fiber Recruitment — recruiting more fibers for more force.
- Three Basic Types of Muscle (Smooth, Cardiac, Skeletal) — skeletal muscle as the voluntary type.
- Muscle Fiber Types (Type I, IIa, IIx) — fiber types recruited.
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