Muscle Contraction Health Article

The nature of the biochemistry of the contraction

Muscle contraction involves the sliding of thick filaments of myosin past thin filaments of actin. The interaction of myosin and actin begins when a high-energy molecule of ATP located in the head of the myosin filament is hydrolyzed into an inorganic phosphate (P_i) molecule and ADP. The myosin head subsequently attaches to an actin filament forming a crossbridge. The ADP and P_i are then released, and the myosin head undergoes a conformational change that causes the actin filament to move relative to the myosin filament. Then, ATP once again binds to the myosin head and causes myosin to dissociate from the actin filament. These steps are repeated very rapidly, causing the myosin head to "walk" along the actin filament, resulting in a muscle contraction. Only when ATP is present can the myosin head detach from the actin filament to continue the process. If ATP is not present, then the muscle will become stiff and unable to relax as is seen in rigor mortis.

KEY TERMS

ADP (adenosine diphosphate)—A molecule that accepts phosphate groups in biochemical reactions.

ATP (adenosine triphosphate)—A high energy molecule that releases a phosphate group, providing energy to power reactions.

Voluntary movement—Movement as a result of conscious effort.

Work—Describes the amount of force used to move an object a certain distance.

The nature of control of the contraction

Muscle cells contain a highly excitable membrane called the sarcoplasmic reticulum, which can be excited to release calcium ions and produce an action potential. Most stimulation occurs through motor neurons that originate in the somatic portion of the central nervous system and innervate the muscles at the myoneural junction. When the motor neuron nears the muscle it branches to innervate several different muscle fibers. Many different nerves innervate muscles responsible for fine and precise motor movements, each nerve innervating only a couple muscle fibers. Conversely, only a few nerves innervate muscles responsible for large, imprecise movements, each nerve branching many times to innervate many muscle fibers.

The nerve side of the myoneural junction makes up the presynaptic portion. Muscle is located on the other side of the junction, forming the postsynaptic portion. As an action potential travels down the nerve and reaches the axon terminal, extracellular calcium ions enter the terminal. Neurotransmitter vesicles in the axon terminal migrate to the axon membrane, fusing with it to release acetylcholine into the synaptic cleft. Molecules of acetylcholine diffuse across the cleft and bind to receptors on the postsynaptic membrane of the muscle. Then, ion channels in the postsynaptic membrane open, allowing potassium and sodium ions to enter. This creates an electrical potential (end-plate potential) and depolarization of the postsynaptic membrane, which then travels down the entire muscle membrane, resulting in a muscle action potential. As an action potential travels down the muscle fiber, a membrane system located within the muscle called the sarcoplasmic reticulum releases calcium ions, which are stored within the membrane system. The calcium ions diffuse into an area of actin and myosin filaments where they bind to troponin molecules associated with the actin filaments. Then the actin filaments are enabled to interact with the myosin filaments and the result is a muscle contraction.

In order to halt a contraction after the initial action potential is fired, acetylcholine diffuses away from the receptor in the postsynaptic cleft, and an enzyme called cholinesterase hydrolyzes acetylcholine into choline and acetate. Choline is taken back into the presynaptic cleft and recycled into more acetylcholine in the neurotransmitter vesicles.