The muscular system is an organ system consisting of skeletal, smooth, and cardiac muscle. It permits movement of the body, maintains posture, and circulates blood throughout the body. The muscular systems in vertebrates are controlled through the nervous system although some muscles (such as the cardiac muscle) can be completely autonomous. Together with the skeletal system in the human, it forms the musculoskeletal system, which is responsible for the movement of the body.

Types

thumb|258x258px|Three distinct types of muscle (L to R): Smooth (non-striated) muscle in internal organs, cardiac or heart muscle, and skeletal muscle.

There are three distinct types of muscle: skeletal muscle, cardiac or heart muscle, and smooth (non-striated) muscle. Muscles provide strength, balance, posture, movement, and heat for the body to keep warm.

There are more than 600 muscles in an adult male human body. A kind of elastic tissue makes up each muscle, which consists of thousands, or tens of thousands, of small muscle fibers. Each fiber comprises many tiny strands called fibrils, impulses from nerve cells control the contraction of each muscle fiber.

Skeletal

Skeletal muscle, is a type of striated muscle, composed of muscle cells, called muscle fibers, which are in turn composed of myofibrils. Myofibrils are composed of sarcomeres, the basic building blocks of striated muscle tissue. 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. Within the sarcomere, actin and myosin fibers overlap in a contractile motion towards each other. Myosin filaments have club-shaped myosin heads that project toward the actin filaments, and 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 ratchet-type drive system.

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.

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Image: Muscles anterior labeled.png|Skeletal muscles, viewed from the front

Image: Muscle posterior labeled.png|Skeletal muscles, viewed from the back

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Cardiac

Heart muscle is striated muscle but is distinct from skeletal muscle because the muscle fibers are laterally connected. Furthermore, just as with smooth muscles, their movement is involuntary. Heart muscle is controlled by the sinus node influenced by the autonomic nervous system.

Tendon

A tendon is a piece of connective tissue that connects a muscle to a bone. When a muscle intercepts, it pulls against the skeleton to create movement. A tendon connects this muscle to a bone, making this function possible.

Aerobic and anaerobic muscle activity

At rest, the body produces the majority of its ATP aerobically in the mitochondria without producing lactic acid or other fatiguing byproducts. During exercise, the method of ATP production varies depending on the fitness of the individual as well as the duration and intensity of exercise. At lower activity levels, when exercise continues for a long duration (several minutes or longer), energy is produced aerobically by combining oxygen with carbohydrates and fats stored in the body.

During activity that is higher in intensity, with possible duration decreasing as intensity increases, ATP production can switch to anaerobic pathways, such as the use of the creatine phosphate and the phosphagen system or anaerobic glycolysis. Aerobic ATP production is biochemically much slower and can only be used for long-duration, low-intensity exercise, but produces no fatiguing waste products that cannot be removed immediately from the sarcomere and the body, and it results in a much greater number of ATP molecules per fat or carbohydrate molecule. Aerobic training allows the oxygen delivery system to be more efficient, allowing aerobic metabolism to begin quicker. Anaerobic ATP production produces ATP much faster and allows near-maximal intensity exercise, but also produces significant amounts of lactic acid which render high-intensity exercise unsustainable for more than several minutes. The phosphagen system is also anaerobic. It allows for the highest levels of exercise intensity, but intramuscular stores of phosphocreatine are very limited and can only provide energy for exercises lasting up to ten seconds. Recovery is very quick, with full creatine stores regenerated within five minutes.

Clinical significance

Multiple diseases can affect the muscular system.

Muscular Dystrophy

alt=five body outlines, muscle areas outlines|thumb|Main areas of muscle weakness in different types of dystrophy

Muscular dystrophy is a group of disorders associated with progressive muscle weakness and loss of muscle mass. These disorders are caused by mutations in a person's genes. The disease affects between 19.8 and 25.1 per 100,000 person-years globally.

There are more than 30 types of muscular dystrophy. Depending on the type, muscular dystrophy can affect the patient's heart and lungs, and/or their ability to move, walk, and perform daily activities. The most common types include:

  • Duchenne muscular dystrophy (DMD) and Becker muscular dystrophy (BMD)
  • Myotonic dystrophy
  • Limb-Girdle (LGMD)
  • Facioscapulohumeral dystrophy (FSHD)
  • Congenital dystrophy (CMD)
  • Distal (DD)
  • Oculopharyngeal dystrophy (OPMD)
  • Emery-Dreifuss (EDMD)

Autoimmune Conditions

There are 2 main autoimmune conditions that impact the muscular system. They are Myositis (including polymyositis and dermatomyositis) and Myasthenia Gravis.

See also

  • Major systems of the human body
  • Intramuscular coordination

References

Further reading

  • Online Muscle Tutorial
  • GetBody Smart Muscle system tutorials and quizzes
  • MedBio.info Use and formation of ATP in muscle