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Muscle Belly

Consists of thousands of muscle fibers arranged in bundles, each bundle is enclosed in connective tissue, while each muscle is attached to the bone via tendons ( tough, fibrous connective tissue ). Each fibre consists of many contractile units called myofibrils, while each myofybril contains 2 protein filaments called myofilaments. Myosin ( thick filament containing protein projections called cross bridges ) and Actin ( thin filaments ). Each myofybril is made up of units called sarcomeres Muscle Contraction Cross bridges attach to the actin filaments, pulling the actin towards the centre of the sarcomere. During stretching, the actin moves along the myosin increasing the length of the sacromere.

Role of Calcium

Sacroplasmic reticulum is the membraneous network surrounding the myofybrils which contains high concentration of calcium. In order for the muscles to relax, all calcium must be removed from around the filaments and pumped back into the Sacroplasmic reticulum. ie reacts with actin to expose active sites which act to attract myosin cross bridges.

Voluntary Muscular Contraction

Command signals ( action potentials ) generated in the brain, passing down the the spinal chord to the motor nerves and onto the muscle. Acetylcholine is released from nerve endings initiating the action potentials to pass along fibres to tranverse tubules stimulating the release of calcium from the Sacroplasmic reticulum.

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