Muscle Tissue

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Please make liberal use of your text in using these notes. You will need to view the diagrams available there.

Muscle functions include:

motion

locomotion

maintenance of posture or position

production of heat

circulation of the blood

There are 3 types of muscle:

Skeletal Muscle:

These muscles function in pairs or groups, each having the opposite function of the other. In other words, muscles that flex the forearm, for example, must be complemented by muscles that extend the same structure. One end of each muscle is generally termed the origin, the other the insertion, and the muscle mass itself is sometimes referred to as the belly.

Origin: the end of the muscle that is relatively stationary during contraction.

Insertion: the end of the muscle that moves toward the origin during contraction.

Rarely, a muscle origin can act as the insertion and vice versa. An example is the rectus abdominus where the xiphoid process and the pubic symphysis can interchangeably be the origin or insertion depending on the action being performed.

While most muscles have a layer of shiny connective tissue called superficial fascia surrounding them, the outermost layer of the muscle is the epimysium. Deep to the epimysium and surrounding the fasciculi is the perimysium. Within each fasciculus and surrounding the muscle fibers that lie inside is the muscle layer known as the endomysium. The fasciculus can be viewed as a cylindrical area within the muscle containing parallel-running muscle fibers. Please refer to your textbook diagram at this point if you are not already! Remember, a muscle fiber is a muscle cell. The ultrastructure (fine structure viewable with an electron microscope) of the muscle fiber has been very much studied. Your text gives a good explanation of the important findings. Several terms are summarized below. Please be certain that you do not confuse a diagram of a fasciculus in cross-section with that of a muscle fiber similarly cut. Check the caption for each diagram!

Terms to Know

Sarcomere: the portion of a muscle fiber between two successive Z-lines.

Z-Line: the partitions within a muscle fiber that section it into sarcomeres. Thin myofilaments attach to these.

A-Band: the comparatively dark area entirely within a sarcomere. This area is composed of thick filaments and thin filaments. This area does not shrink during muscle contraction.

I-Band: the lighter areas alternating with A-Bands; these areas straddle the Z-line and are composed of thin myofilaments (the terms myofilament and filament are used interchangeably). During contraction the I-Band shrinks in size.

H-Zone: a subdivision of the A-Band in the center of the sarcomere where only thick filaments are present. This area shrinks during muscle contraction.

Thick Filaments: these filaments, which span A-Band, are composed of the protein myosin.

Thin Filaments: these filaments, which attach to the Z-line and interdigitate with the thick filaments, are composed of three proteins:

f-actin

tropomyosin

troponin (attaches to the end of each tropomyosin strand

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The release of Calcium ions to initiate muscle contraction allows Ca to bind to troponin.  When this happens, troponin moves slightly and allows myosin to bind with tropomyosin.  This permits the movement (sliding) of the thick and thin filaments over each other.

During muscle contraction the thick and thin filaments overlap eachother more and more. Movement of the filaments is described in the Sliding Filament Theory. During the contraction, the thick filaments appear to maintain their position in the center of the sarcomere, but the sarcomere itself becomes smaller as the Z-lines "move in" on the thick filaments. Since all of the sarcomeres in a muscle contract but the origin remains stationary, the result is that the insertion is moved toward the origin.

Note: You will need to review your in-class notes and the assigned reading to understand the sliding filament theory.

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