Muscular system

The muscular system is a complex organ system essential for life. It consists of three distinct types of muscle: skeletal, smooth, and cardiac. These muscles permit the movement of the body and maintain posture. They also circulate blood throughout the body and generate heat to keep the body warm.

In humans, the muscular system works with the skeletal system. Together, they form the musculoskeletal system, which is responsible for all bodily movement. An adult male human possesses more than 600 muscles. Skeletal muscle alone accounts for approximately 40% of total human mass.

Each muscle is composed of elastic tissue containing thousands of small muscle fibers. These fibers contain even smaller strands called myofibrils. Within these myofibrils are the sarcomeres, which are the basic building blocks of striated muscle tissue. Skeletal muscle is a type of striated muscle because of its organized structure.

The process of contraction begins at the neuromuscular junction. This is the focal point where a motor neuron attaches to a muscle. When an action potential reaches the synapse, a neurotransmitter called acetylcholine is released. This chemical stimulates electrical changes in the muscle cell, triggering the release of calcium ions from the sarcoplasmic reticulum.

The actual movement within the muscle follows the sliding filament model. Inside the sarcomere, two main proteins called actin and myosin overlap. Myosin filaments feature club-shaped heads that project toward the actin filaments. These heads attach to specific binding sites on the actin. The myosin heads then move in a coordinated, ratchet-type drive system. They swivel toward the center of the sarcomere, detach, and reattach to the nearest active site. This process causes the sarcomere to shorten, resulting in a coordinated muscle contraction.

Muscle contraction requires a constant supply of energy called adenosine triphosphate, or ATP. ATP binds to the cross-bridges between the myosin heads and actin filaments. The release of energy from ATP powers the swiveling motion of the myosin heads. When ATP is used, it becomes adenosine diphosphate, also known as ADP. Because muscles store very little ATP, they must constantly replace ADP with new ATP. To assist with this, muscles store a fast-acting recharge chemical called creatine phosphate. This chemical helps the rapid regeneration of ADP into ATP when needed.

There are three specific types of muscle tissue categorized by their function and control. Skeletal muscle is striated and is generally controlled by the nervous system. Cardiac muscle is also striated but is distinct because its fibers are laterally connected. Unlike skeletal muscle, cardiac muscle movement is involuntary. It is controlled by the sinus node, which is influenced by the autonomic nervous system.

Smooth muscle is non-striated and is also involuntary. Its contraction is regulated by the autonomic nervous system, hormones, and local chemical signals. This allows for gradual, sustained contractions that help move materials through the digestive system.

The body produces ATP through two different pathways: aerobic and anaerobic. At rest, most ATP is produced aerobically in the mitochondria. This process uses oxygen to combine with stored carbohydrates and fats. Aerobic production is biochemically slower but produces a much greater number of ATP molecules. It also produces no fatiguing waste products. During high-intensity exercise, the body may switch to anaerobic pathways. The phosphagen system is an anaerobic method that uses creatine phosphate. It provides the highest intensity of energy but only lasts for about ten seconds.

Other anaerobic activities, such as anaerobic glycolysis, produce ATP much faster than aerobic methods. However, this process produces significant amounts of lactic acid. This buildup makes high-intensity exercise unsustainable for more than a few minutes. To move the skeleton, muscles use tendons, which are pieces of connective tissue. A tendon connects a muscle to a bone. When a muscle contracts, it pulls against the skeleton via the tendon to create movement. Coordination of these movements is often managed by the cerebellum in the brain.

Sometimes, genetic mutations or autoimmune issues affect the muscular system. Muscular dystrophy is a group of more than 30 disorders that cause progressive muscle weakness. These conditions affect between 19.8 and 25.1 per 100,000 person-years globally. Common types include Duchenne muscular dystrophy and Becker muscular dystrophy.

Some forms can impact a patient's heart and lungs or their ability to walk. Additionally, problems at the neuromuscular junction can lead to serious issues. A loss of function at this junction can produce paralysis, while a problem there can cause prolonged contractions known as tetanus.