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The human muscles, seen from the front. 19th century illustration.
|This article may be weighted too heavily toward only one aspect of its subject. (December 2011)|
The human muscles, seen from the front. 19th century illustration.
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.
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.
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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. 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. 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. 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.
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 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.
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 and their basic features:
|gastrocnemius||femur||calcaneus||sural arteries||tibial nerve||plantarflexion, flexion of knee (minor)key||Tibialis anterior muscle|
|tibialis posterior||tibia, fibula||Foot||posterior tibial artery||tibial nerve||inversion of the foot, plantar flexion of the foot at the ankle||Tibialis anterior muscle|
|soleus||tibia, medial border of fibula||calcaneus||sural arteries||tibial nerve||plantarflexion||Tibialis anterior muscle|
|tibialis anterior||tibia||foot||anterior tibial artery||Fibular nerve||dorsiflex and invert the foot||Fibularis longus, Gastrocnemius, Soleus, Plantaris, Tibialis posterior|
|gluteus maximus muscle||ilium, sacrum, sacrotuberous ligament||Gluteal tuberosity of the femur||gluteal arteries||inferior gluteal nerve||external rotation and extension of the hip joint||Iliacus, Psoas major, Psoas minor|
|biceps femoris||ischium, femur||fibula||inferior gluteal artery, popliteal artery||tibial nerve, common peroneal nerve||flexes and laterally rotates knee joint, extends hip joint||Quadriceps muscle|
|semitendinosus||ischium||tibia||inferior gluteal artery||sciatic||flex knee, extend hip joint||Quadriceps muscle|
|semimembranosus||ischium||tibia||profunda femoris, [Gluteal artery (disambiguation)|[gluteal artery]]||sciatic nerve||Hip extension, Knee flexion||Quadriceps muscle|
|iliopsoas||ilium||femur||medial femoral circumflex artery, iliolumbar artery||femoral nerve, lumbar nerves||flexion of hip||Gluteus maximus, posterior compartment of thigh|
|quadriceps femoris||combined rectus femoris and vastus muscles||femoral artery||Femoral nerve||Knee extension; Hip flexion||Hamstring|
|adductor muscles of the hip||pubis||femur, tibia||obturator nerve||adduction of hip|
|trapezius||the rear of the skull, vertebral column||clavicle, scapula||cranial nerve XI, cervical nerves||retraction of scapula||Serratus anterior muscle|
|rectus abdominis||pubis||Costal cartilage of ribs 5-7, sternum||inferior epigastric artery||segmentally by thoraco-abdominal nerves||flexion of trunk/lumbar vertebrae||Erector spinae|
|transversus abdominis||ribs, ilium||pubic tubercle||lower intercostal nerves, iliohypogastric nerve and the ilioinguinal nerve||compress the ribs and viscera, thoracic and pelvic stability|
|Abdominal external oblique muscle||Lower 8 costae||Crista iliaca, ligamentum inguinale||lower 6 intercostal nerve, subcostal nerve||Rotates torso|
|Abdominal internal oblique muscle||Inguinal ligament, Iliac crest and the Lumbodorsal fascia||Linea alba, sternum and the inferior ribs.||Compresses abdomen and rotates vertebral column.|
|erector spinae||on the spines of the last four thoracic vertebræ||both the spines of the most cranial thoracic vertebrae and the cervical vertebrae||lateral sacral artery||posterior branch of spinal nerve||extends the vertebral column||Rectus abdominis muscle|
|pectoralis major||clavicle, sternum, costal cartilages||humerus||thoracoacromial trunk||lateral pectoral nerve and medial pectoral nerve||Clavicular 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 brachii||scapula||radius||brachial artery||Musculocutaneous nerve||flexes elbow and supinates forearm||Triceps brachii muscle|
|triceps brachii||scapula and humerus||ulna||deep brachial artery||radial nerve||extends forearm, caput longum adducts shoulder||Biceps brachii muscle|
|deltoid||clavicle, acromion, scapula||deltoid tuberosity of humerus||primarily posterior circumflex humeral artery||Axillary nerve||shoulder abduction, flexion and extension||Latissimus dorsi|
|latissimus dorsi||vertebral column, ilium and inferior 3 or 4 ribs||humerus||subscapular artery, dorsal scapular artery||thoracodorsal nerve||pulls the forelimb dorsally and caudally||deltoid, trapezius|
Medical and Health Encyclopedia, chapter 1
|The Wikibook Human Physiology has a page on the topic of: The Muscular System|
|The Wikibook Anatomy and Physiology of Animals has a page on the topic of: Muscles|
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