Page 342 - Fundamentals of anatomy physiology

Spotlight EFixgucrieta10t–io10n-Contraction Coupling

Watch the > ™ on Excitation-Contraction Coupling. Action potential

1 Neural Control Axon terminal Excitation Sarcolemma
Excitation
A skeletal muscle fiber contracts when
stimulated by a motor neuron at a T tubule Cytosol
neuromuscular junction. The stimulus
arrives in the form of an action potential Sarcoplasmic reticulum
at the axon terminal.
Calcium
2 Excitation ion release

The action potential causes the release of Ca2+ Ca2+ ATP
ACh into the synaptic cleft, which leads
to excitation—the production of an
action potential in the sarcolemma.

3 Release of Calcium Ions Thick-thin
filament interaction
This action potential travels along the
sarcolemma and down T tubules to the Ca2+
triads. This triggers the release of calcium
ions (Ca2+) from the terminal cisternae of Myosin tail Cross-bridge
the sarcoplasmic reticulum. (thick filament) formation

4 Contraction Cycle Begins Tropomyosin Ca2+

The contraction cycle begins when Troponin Ca2+
the calcium ions (Ca2+) bind to troponin, G-actin
resulting in the exposure of the active (thin filament)
sites on the thin filaments. This allows Nebulin
cross-bridge formation and will
continue as long as ATP is available. Active site
(See Spotlight Figure 10-11 for the
details of the contraction cycle.) In a resting sarcomere, the When calcium ions enter the Cross-bridge
tropomyosin strands cover sarcomere, they bind to formation then
5 Sarcomere Shortening the active sites on the thin troponin, which rotates and occurs, and the
filaments, preventing swings the tropomyosin away contraction cycle
As the thick and thin filaments interact, the cross-bridge formation. from the active sites. begins.
sarcomeres shorten, pulling the ends of the
muscle fiber closer together. Muscle fiber
contraction

leads to

6 Generation of Muscle Tension Tension
production
During the contraction, the entire skeletal
muscle shortens and produces a pull, or
tension, on the tendons at either end.

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