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Muscular system

The muscular system is the biological system of an organism that allows it 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.

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1 Muscles
2 Control of muscle contraction
3 See also
4 Notes
5 References

Muscles

Main article: Muscle

There are three distinct types of muscles: skeletal muscles, cardiac muscles and smooth muscles.

Skeletal muscle

Main article: Skeletal muscle

Skeletal muscle fibers are multinucleated, with the cell's nuclei located just beneath the plasma membrane. The cell comprises a series of striped or striated, thread-like myofibrils. Within each myofibril there are protein filaments that are anchored by dark Z lines. The fibre is one long continuous 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 of the sarcomere 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 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.

Anatomy

Main article: Table of muscles of the human body

There are approximately 639 skeletal muscles in the human body.

The following are some major muscles^[1] and their basic features:

Muscle	Origin	Insertion	Artery	Nerve	Action	Antagonist
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	fibula, medial border of tibia	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
longus	fibula	Foot	fibular artery	Superficial fibular nerve	plantarflexion, eversion	Tibialis anterior muscle
brevis	fibula	Foot, eversion	peroneal artery	superficial peroneal nerve
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	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 femoriss	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
levator scapulae	vertebral column	scapula	dorsal scapular artery	cervical nerve, dorsal scapular nerve	Elevates scapula, tilts its glenoid cavity inferiorly
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
brachialis	humerus	ulna	radial recurrent artery	musculocutaneous nerve	flexion at elbow joint
pronator teres	humerus, ulna	radius	ulnar artery and radial artery	median nerve	pronation of forearm, flexes elbow	Supinator muscle
brachioradialis	humerus	radius	radial recurrent artery	radial nerve	Flexion of forearm
rhomboids	nuchal ligaments, spinous processes of the C7 to T5 vertebrae	scapula	dorsal scapular artery	dorsal scapular nerve	Retracts the scapula and rotates it to depress the glenoid cavity. fixes the scapula to the thoracic wall.	Serratus anterior 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
Rotator cuff	scapula	humerus			lateral rotation, medial rotation, abduction

Aerobic and anerobic muscle activity

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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.^[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 being more quickly.^[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]

Heart muscle

Main article: Heart muscle

Heart muscles are distinct from skeletal muscles because the muscle fibres are laterally connected to each other. Furthermore, just as with smooth muscles, they are not controlled by will. Heart muscles are controlled by the sinus node, which, in turn, is infuenced by the autonomic nervous system.

Smooth muscle

Main article: Smooth muscle

Smooth muscles are controlled directly by the autonomic nervous system.

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.

Notes

^ List of major muscles of the human body
^ ^a ^b ^c ^d St Paul’s College Stage 2 EXERCISE PHYSIOLOGY Energy Systems Part 5 (ppt). Retrieved on 2007-10-16.

References

Online Muscle Tutorial

v • d • e Muscular system
Topics	Muscular tissue • Muscle contraction • Muscles of the human body
Types of muscles	Cardiac muscle • Skeletal muscle • Smooth muscle

v • d • e Human organ systems
Cardiovascular system • Digestive system • Endocrine system • Immune system • Integumentary system • Lymphatic system • Muscular system • Nervous system • Reproductive system • Respiratory system • Skeletal system • Urinary system

Category: Muscular system

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