Muscle Physiology

Muscle Physiology

The human body contains over 600 muscles, contributing to about 40% of its mass. The main purpose of muscles is to provide movement, to protect the body, to produce body heat and to support the posture of the skeleton. Of those 600 muscles, there are 215 skeletal muscles specifically responsible to work cooperatively to produce force and movement in a joint. Skeletal muscles are voluntary muscles, they are attached to bone by tendons and they are striated. In order for a muscle to produce force or movement, it must contract, which requires the tissue to have several properties; excitability, contractility, extensibility and elasticity. The muscle must be able to respond to stimulus to develop tension to contract; this stimulus is sent to the muscle via the central nervous system. The sliding filament theory of muscle contraction will guide us through the events that cause a muscle to contract. In order to fully understand how the sliding filament theory works, we must look closely at the anatomy of a muscle and investigate each integral part.

All 215 of the body’s skeletal muscles contain muscle tissue, connective tissue, blood vessels and nerves. The outer covering of the muscle and tendon is fibrous and is called the epimysium. Tendons are responsible for connecting the belly of the muscle to the bone, where they attach to the periosteum, the outer covering of the bone. The outer membrane that surrounds muscle fiber is called the sarcolemma, and it has membrane potential allowing it to send neuron signals down the muscle fiber, signaling the muscle to contract. The belly, or gaster of a muscle is made up of muscle fibers, and these fibers are grouped into bundles, or fasciculi, of up to 150 fibers. The fasciculi are surrounded by connective tissue called the perimysium, and the fibers within each bundle are surrounded by more

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connective tissue called endomysium.

Each individual fiber within the fasciculi is called a myofibril. Myofibrils are cylindrical shaped structures that extend along the entire length of the muscle fiber. There are hundreds of myofibrils in each muscle fiber, and they make up about 60% of myofiber protein. Myofibrils contain 2 types of protein filaments; thick filaments and thin filaments. These myofibrils are arranged in a very particular pattern and are striated or striped in appearance, which is a typical appearance of skeletal and cardiac muscles. The myofibrils are broken down into small units and line themselves up end to end to create the sarcomere.

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Within the sarcomere, thin filaments fill in the ends, and thick filaments are found in the middle. This is where the appearance of striation comes from; the alternation of the thin filaments which are light colored in appearance, and the thick filaments are dark colored in appearance. The light or thin filaments make up the I band, and each I band, located on the ends of the sarcomere, extends across 2 adjacent sarcomeres. The Z line separates each sarcomere, anchoring one end of the thin filaments. The proteins of the Z line “act to keep the myofibrils within a muscle fiber in register.” (MW King, PHD, Indiana State) The thick filaments, or A bands, are found in the center of the sarcomere. The I Bands are located between A bands, which is the darker part of the striated muscle. The H zone is the center of the sarcomere.

There are proteins that make up these thin and thick filaments. The thin filaments or I bands, are composed of Actin molecules, Troponin and Tropomyosin, and together they form a helix structure.

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Actin, a contractile protein molecule, is a major component in the production of force. With its combination of strength and sensitivity actin has the ability to continuously build up and breakdown as needed for the muscle tissue to function correctly. Adenosine Triphosphate (ATP) helps to control the character of Actin by binding to it, keeping it stable. Troponin is the major regulator of force production. It has 3 subunits that lie in the groove of the filament and block the myosin binding site. Tropomyosin is a protein that stabilizes the actin. Together these 3 proteins play a key role in the contraction of muscle.

A Bands, or thick filaments, are made of the protein Myosin, a contractile protein. There are approximately 400 myosin molecules in the A Band, divided equally by the M line (where adjacent myosin filaments