CogKnoSys

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Physical

Muscle Contraction

Muscle contraction is the activation of tension-generating sites in muscle cells, as protein filaments slide past each other.

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The idea

Muscle contraction is the activation of tension-generating sites inside muscle cells. Contraction does not always mean that a muscle gets shorter. When you hold something heavy in one position, your muscle produces tension without changing length, which is called an isometric contraction. If the muscle shortens as it works, the contraction is concentric, and if it lengthens while resisting a load, it is eccentric. When a contraction ends, the muscle fibers relax back to a low-tension state.

Inside each muscle fiber are two kinds of protein filaments. Thin filaments are made mostly of actin, and thick filaments are made mostly of a motor protein called myosin. The sliding filament theory explains how they work together. Myosin heads attach to actin, forming links called cross-bridges, and the thick filaments slide past the thin ones. The filaments themselves stay about the same length, but they overlap more, so the muscle pulls.

Different kinds of muscle start contracting in different ways. Skeletal muscles, which move the skeleton, need signals from motor neurons, and a single motor neuron can make many fibers contract at the same time. Smooth muscle and heart muscle start their own contractions, although the autonomic nervous system can adjust them. How often nerve signals arrive decides whether a skeletal muscle gives a quick twitch or a strong, steady pull.

Check yourself

Question 1 of 3

What is a contraction called when a muscle produces tension but stays the same length?

Pick one. The first pick counts.

Composition

what it's made of

  • Actin — The protein that forms the thin filaments.
  • Myosin — The motor protein that forms the thick filaments and pulls on actin.
  • Cross-bridges — Links formed when myosin heads attach to actin filaments.
  • Motor neurons — Nerve cells whose signals make skeletal muscle fibers contract.
  • Relaxation — The return of muscle fibers to a low-tension state after a contraction ends.

Connection

how it relates

Under Muscular System

Related

  1. Nervous Systemstrength 80 of 100

    Signals from motor neurons are what make skeletal muscles contract.

  2. Autonomic Nervous Systemstrength 50 of 100

    The autonomic nervous system adjusts the contractions of heart and smooth muscle.

Cause and effect

  1. Long Bonesstrength 70 of 100

    Contracting skeletal muscles pull on long bones to move the arms and legs.

  2. Accelerationstrength 40 of 100

    The forces made by contracting muscles accelerate the body when it starts to move.

Change

how and why it evolves

Past
Before the 1950s, scientists debated several competing theories of muscle contraction, until two research teams independently introduced the sliding filament theory in 1954.
Now
Today the sliding filament theory is the widely accepted explanation of how muscles contract.
Next
Research on the cross-bridge cycle will likely help scientists understand muscle diseases and aging muscles better, though new treatments take time to develop.

Knowledge you can walk. Read the idea, then step up, across or down.

Muscle Contraction

Muscle contraction is the activation of tension-generating sites in muscle cells, as protein filaments slide past each other.

Learn, discuss, create, buy or invest

The idea

Muscle contraction is the activation of tension-generating sites inside muscle cells. Contraction does not always mean that a muscle gets shorter. When you hold something heavy in one position, your muscle produces tension without changing length, which is called an isometric contraction. If the muscle shortens as it works, the contraction is concentric, and if it lengthens while resisting a load, it is eccentric. When a contraction ends, the muscle fibers relax back to a low-tension state.

Inside each muscle fiber are two kinds of protein filaments. Thin filaments are made mostly of actin, and thick filaments are made mostly of a motor protein called myosin. The sliding filament theory explains how they work together. Myosin heads attach to actin, forming links called cross-bridges, and the thick filaments slide past the thin ones. The filaments themselves stay about the same length, but they overlap more, so the muscle pulls.

Different kinds of muscle start contracting in different ways. Skeletal muscles, which move the skeleton, need signals from motor neurons, and a single motor neuron can make many fibers contract at the same time. Smooth muscle and heart muscle start their own contractions, although the autonomic nervous system can adjust them. How often nerve signals arrive decides whether a skeletal muscle gives a quick twitch or a strong, steady pull.

Check yourself

3 quick questions, instant feedback.