Anatomy and Physiology I

Lab: 01

May 4, 2008

Skeletal Muscle Physiology

Introduction:

Skeletal muscles are discrete organ that consists of hundreds to thousands of muscle cells, connective tissues, blood vessels, and nerve fibers. A skeletal muscle contracts with varying force and for different periods of time in response to stimuli of varying frequencies and intensities (Marieb & Hoehn, 2007). When skeletal muscle contracts, it will generate a certain amount of force that enable the body to do work.

A twitch contraction is a quick shortening when a firing of an action potential reaches the muscle. There are three phases of a twitch contraction: the latent period, the contraction period, and the relaxation period. The latent period is the period where electrical stimulus was applied. During this period, calcium is released from sarcoplasmic reticulum to flow in and bind to troponin complex, causing a change in tropomyosin. The change in shape of the tropomyosin enables the actin-binding site to be exposed so that the myosin head can attach to the actin molecule, which introduces the beginning of contraction period. This is the period where cross-bridge formation and cycling occurs with the presence of calcium. First, ATP is split into ADP and inorganic phosphate, which energized the myosin head. Then the energized myosin head binds to the unblocked myosin-binding site on the actin molecule in cross-bridge formation. An inorganic phosphate is released from the myosin head, initiating the power stroke that allows the myosin head to rotates and bends as it pulls the thin filament toward the center of the sarcomere. As a result of the power stroke, ADP is released from the mysosin head. Then another ATP attaches to the myosin head, breaks down to ADP and inorganic phosphate, which provides enough energy for the myosin head to lift-off and end the cross bridge so that it’ll be free to reattach to the next actin molecule. It is when the myosin head detach and reattach to a different actin molecule is called cross-bridge cycling. The relaxation period occurs when intracellular calcium levels drop, the stimulus is no longer on effect, and calcium is re-sequestered back into the sarcoplasmic reticulum. (Marieb & Hoehn, 2007)

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The contraction of muscle fibers and muscles are similar. There are two types of muscle contractions, isometric contraction and isotonic contraction. Isometric contractions are contractions where tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens.  Isometric contractions occur if the load is greater than the tension, then the muscle is able to develop. In isotonic contractions, the muscle changes in length and moves the load. There are two types of isotonic contractions, concentric and eccentric isotonic contractions. Concentric contractions is when the muscle shortens and does work, whereas eccentric contractions is when the muscle contracts as ...

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