Muscular system

The muscular system is the biological system of humans that allows them to move. The muscular system in vertebrates is controlled through the nervous system, although some muscles (such as the cardiac muscle) can be completely autonomous.

Human Muscles
Muscle tissue is composed of a series of fibers, similar to neurons in shape, that operate in a coordinated manner under the supervision of the nervous system to: (1) support movement in the body, and (2) assist in maintaining body temperature through shivering to create heat. There are three forms of muscles in the human body: (1) Skeletal muscle, voluntary, multinucleated, striated (2) Smooth muscle, involuntary, uninucleated, non-striated (3) Cardiac muscle, involuntary, uninucleated, striated (*with intercalated disc)

Skeletal Muscle Structure
Skeletal muscle fibers are multinucleated, with the cell's nuclei located just beneath the plasma membrane. The cell is comprised of a series of striped or striated, thread-like myofibrils. Each myofibril there are protein filament that are anchored by dark Z lines. The fiber is one long continous thread-like structure. The smallest cross section of skeletal muscle is called a sarcomere which is the functional unit within the cell. It extends from one Z line to the next attached Z line. The individual sarcomere has alternating thick (myosin and thin actin protein filaments. Myosin forms the center or middle of each sarcomere the exact center is designated the M line. Thinner actin filaments form a zig zag pattern along the anchor points or Z line.

Upon stimulation by an action potential, skeletal muscles perform a coordinated contraction by shortening each sarcomere. The best proposed model for understanding contraction is the sliding filament model of muscle contraction. Actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped heads that project toward the actin filaments.

Larger structures along the myosin filament called myosin heads are used to provide attachment points on binding sites for the actin filaments. The myosin heads move in a coordinated style, they swivel toward the center of the sarcomere, detach and then reattach to the nearest active site of the actin filament. This is called a rachet type drive system. This process consumes large amounts of adenosine triphosphate (ATP).

Energy for this comes from ATP, the energy source of the cell. ATP binds to the cross bridges between myosin heads and actin filaments. The release of energy powers the swiveling of the myosin head. Muscles store little ATP and so must continiously recycle the discharged adenosine diphosphate molecule (ADP} into ATP rapidly. Muscle tissue also contains a stored supply of a fast acting recharge chemical, creatine phosphate which can assist initially producing the rapid regeneration of ADP into ATP.

Calcium ions are required for each cycle of the sarcomere. Calcium is released from the sarcoplasmic reticulum into the sarcomere when a muscle is stimulated to contract. This calcium uncovers the actin binding sites. When the muscle no longer needs to contract, the calcium ions are pumped from the sarcomere and back into storage in the sarcoplasmic reticulum.

Control of Muscle Contraction
Neuromuscular junctions are the focal point where a motor neuron attaches to a muscle. Acetylcholine, (a neurotransmitter used in skeletal muscle contraction) is released from the axon terminal of the nerve cell when an action potential reaches the miscoscopic junction, called a synapse. A group of chemical messengers cross the synapse and stimulate the formation of electrical changes, which are produced in the muscle cell when the acetylcholine binds to receptors on its surface. Calcium is released from its storage area in the cell's sarcoplasmic reticulum. An impulse from a nerve cell causes calcium release and brings about a single, short muscle contraction called a muscle twitch. If there is a problem at the neuromuscular junction, a very prolonged contraction may occur, tetanus. Also, a loss of function at the junction can produce paralysis.

Skeletal muscles are organized into hundreds of motor units, each of which involves a motor neuron, attached by a series of thin finger-like structures called axon terminals. These attach to and control discrete bundles of muscle fibers. A coordinated and fine tuned response to a specific circumstance will involve controlling the precise number of motor units used. While individual muscle units contract as a unit, the entire muscle can contract on a predetermined basis due to the structure of the motor unit. Motor unit coordination and control frequently come under the direction of the cerebellum of the brain. This allows for complex muscular coordination with little conscious effort, such as when one drives a car without thinking about the process.

Muscle activity in an anaerobic vs aerobic environment
At rest, the body produces small amounts of ATP in an anaerobic production model through glycolysis in the cytoplasm of muscle cells. As activity increases to a sustained higher activity level such as in running, the body can shift to aerobic ATP production by producing increases in respiratory rate and heart rate. This allows for a greater supply of oxygen to stimulate aerobic production of ATP which occurs in the mitochondria. Once the activity levels decrease, such as occurs at the end of a race, the body will continue to maintain an extended elevated respiratory and heart rate while the energy borrowed from the muscle cells during the transformation to the aerobic mode is restored. This physiological process is called repayment of oxygen debt. Once all borrowed substances have been repaid to the muscle cells, the body will return to anaerobic metabolism.

There are about 650 skeletal muscles in the human body; see List of muscles of the human body.

Reference:
 * Online Muscle Tutorial