Muscular system

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Muscular system
Bougle whole2.jpg
The human muscles, seen from the front. 19th century illustration.
LatinSystema musculare
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Muscular system
Bougle whole2.jpg
The human muscles, seen from the front. 19th century illustration.
LatinSystema musculare

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


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

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 continuously 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.

Aerobic and anaerobic muscle activity

At rest, the body produces the majority of its ATP aerobically in the mitochondria[1] without producing lactic acid or other fatiguing byproducts.[2] 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. 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 can not be removed immediately from sarcomere and body and 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.[2] 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 greater than several minutes.[2] The phosphagen system is also anaerobic, 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.[2]

Cardiac muscle

Heart muscles are distinct from skeletal muscles because the muscle fibers are laterally connected to each other. Furthermore, just as with smooth muscles, they are not controlling themselves. Heart muscles are controlled by the sinus node influenced by the autonomic nervous system.

Smooth muscle

Smooth muscles are controlled directly by the autonomic nervous system and are involuntary, meaning that they are incapable of being moved by conscious thought. Functions such as heart beat and lungs (which are capable of being willingly controlled, be it to a limited extent) are involuntary muscles but are not smooth muscles.

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 microscopic 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, balance, 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.


There are approximately 639 skeletal muscles in the human body.

The following are some major muscles[3] and their basic features:

gastrocnemiusfemurcalcaneussural arteriestibial nerveplantarflexion, flexion of knee (minor)keyTibialis anterior muscle
tibialis posteriortibia, fibulaFootposterior tibial arterytibial nerveinversion of the foot, plantar flexion of the foot at the ankleTibialis anterior muscle
soleustibia, medial border of fibulacalcaneussural arteriestibial nerveplantarflexionTibialis anterior muscle
tibialis anteriortibiafootanterior tibial arteryFibular nervedorsiflex and invert the footFibularis longus, Gastrocnemius, Soleus, Plantaris, Tibialis posterior
gluteus maximus muscleilium, sacrum, sacrotuberous ligamentGluteal tuberosity of the femurgluteal arteriesinferior gluteal nerveexternal rotation and extension of the hip jointIliacus, Psoas major, Psoas minor
biceps femorisischium, femurfibulainferior gluteal artery, popliteal arterytibial nerve, common peroneal nerveflexes and laterally rotates knee joint, extends hip jointQuadriceps muscle
semitendinosusischiumtibiainferior gluteal arterysciaticflex knee, extend hip jointQuadriceps muscle
semimembranosusischiumtibiaprofunda femoris, [Gluteal artery (disambiguation)|[gluteal artery]]sciatic nerveHip extension, Knee flexionQuadriceps muscle
iliopsoasiliumfemurmedial femoral circumflex artery, iliolumbar arteryfemoral nerve, lumbar nervesflexion of hipGluteus maximus, posterior compartment of thigh
quadriceps femoriscombined rectus femoris and vastus musclesfemoral arteryFemoral nerveKnee extension; Hip flexionHamstring
adductor muscles of the hippubisfemur, tibiaobturator nerveadduction of hip
trapeziusthe rear of the skull, vertebral columnclavicle, scapulacranial nerve XI, cervical nervesretraction of scapulaSerratus anterior muscle
rectus abdominispubisCostal cartilage of ribs 5-7, sternuminferior epigastric arterysegmentally by thoraco-abdominal nervesflexion of trunk/lumbar vertebraeErector spinae
transversus abdominisribs, iliumpubic tuberclelower intercostal nerves, iliohypogastric nerve and the ilioinguinal nervecompress the ribs and viscera, thoracic and pelvic stability
Abdominal external oblique muscleLower 8 costaeCrista iliaca, ligamentum inguinalelower 6 intercostal nerve, subcostal nerveRotates torso
Abdominal internal oblique muscleInguinal ligament, Iliac crest and the Lumbodorsal fasciaLinea alba, sternum and the inferior ribs.Compresses abdomen and rotates vertebral column.
erector spinaeon the spines of the last four thoracic vertebræboth the spines of the most cranial thoracic vertebrae and the cervical vertebraelateral sacral arteryposterior branch of spinal nerveextends the vertebral columnRectus abdominis muscle
pectoralis majorclavicle, sternum, costal cartilageshumerusthoracoacromial trunklateral pectoral nerve and medial pectoral nerveClavicular head: flexes the humerus
Sternocostal head: extends the humerus
As a whole, adducts and medially rotates the humerus. It also draws the scapula anteriorly and inferiorly.
biceps brachiiscapularadiusbrachial arteryMusculocutaneous nerveflexes elbow and supinates forearmTriceps brachii muscle
triceps brachiiscapula and humerusulnadeep brachial arteryradial nerveextends forearm, caput longum adducts shoulderBiceps brachii muscle
deltoidclavicle, acromion, scapuladeltoid tuberosity of humerusprimarily posterior circumflex humeral arteryAxillary nerveshoulder abduction, flexion and extensionLatissimus dorsi
latissimus dorsivertebral column, ilium and inferior 3 or 4 ribshumerussubscapular artery, dorsal scapular arterythoracodorsal nervepulls the forelimb dorsally and caudallydeltoid, trapezius

Additional images

See also


  1. ^ ABERCROMBIE M; HICKMAN C.J; JOHNSON M.L, 1973, A Dictionary of Biology, Page 179, Middlesex (England), Baltimore (U.S.A), Ringwood (Australia): Penguin Books
  2. ^ a b c d "St Paul’s College Stage 2 EXERCISE PHYSIOLOGY Energy Systems Part 5" (ppt). Retrieved 2007-10-16. 
  3. ^ List of major muscles of the human body


Medical and Health Encyclopedia, chapter 1

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