Muscular System

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The Muscular System

The bodily system that is composed of skeletal, smooth, and cardiac muscle tissue and functions in movement of the body or of materials through the body, maintenance of posture, and heat production.

The muscular system consists of muscular cells, the contractile elements with the specialized property of exerting tension during contraction, and associated connective tissues. The three morphologic types of muscles are voluntary muscle, involuntary muscle, and cardiac muscle. The voluntary, striated, or skeletal muscles are involved with general posture and movements of the head, body, and limbs. The involuntary, nonstriated, or smooth muscles are the muscles of the walls of hollow organs of the digestive, circulatory, respiratory, and reproductive systems, and other visceral structures. Cardiac muscle is the intrinsic muscle tissue of the heart. See also Muscle.


Anatomy

Muscle groups are particularly distinct in elasmobranchs and other primitive fishes, and they are generally defined on the basis of their embryonic origin in these animals. Two major groups of skeletal muscles are recognized, somatic (parietal) muscles, which develop from the myotomes, and branchiomeric muscles, which develop in the pharyngeal wall from lateral plate mesoderm. The somatic musculature is subdivided into axial muscles, which develop directly from the myotomes and lie along the longitudinal axis of the body, and appendicular muscles, which develop within the limb bud from mesoderm derived phylogenetically as buds from the myotomes.

The vertebrate muscular system is the largest of the organ systems, making up 35–40% of the body weight in humans. The movement of vertebrates is accomplished exclusively by muscular action, and muscles play the major role in transporting materials within the body. Muscles also help to tie the bones of the skeleton together and supplement the skeleton in supporting the body against gravity. See also Skeletal system.

Most of the axial musculature is located along the back and flanks of the body, and this part is referred to as trunk musculature. But anteriorly the axial musculature is modified and assigned to other subgroups. Certain of the occipital and neck myotomes form the hypobranchial muscles, and the most anterior myotomes form the extrinsic ocular muscles.

The hypaxial musculature of tetrapods can be subdivided into three groups: (1) a subvertebral (hyposkeletal) group located ventral to the transverse processes and lateral to the centra of the vertebrae, (2) the flank muscles forming the lateral part of the body wall, and (3) the ventral abdominal muscles located on each side of the midventral line. The subvertebral musculature assists the epaxial muscles in the support and movement of the vertebral column. Most of the flank musculature takes the form of broad, thin sheets of muscle that form much of the body wall and support the viscera. The midventral hypaxial musculature in all tetrapods consists of the rectus abdominis, a longitudinal muscle on each side of the midline that extends from the pelvic region to the anterior part of the trunk.

The hypobranchial musculature extends from the pectoral girdle forward along the ventral surface of the neck and pharynx to the hyoid arch, chin, and into the tongue. It is regarded as a continuation of part of the hypaxial trunk musculature.

Limb muscles are often classified as intrinsic if they lie entirely within the confines of the appendage and girdle, and extrinsic if they extend from the girdle or appendage to other parts of the body. In fishes, movements of the paired fins are not complex or powerful and the appendicular muscles in the strictest sense are morphologically simple. In terrestrial vertebrates, the limbs become the main organs for support and locomotion, and the appendicular muscles become correspondingly powerful and complex. The muscles are too numerous to describe individually, but they can be sorted into dorsal and ventral groups, because tetrapod muscles originate embryonically in piscine fashion from a dorsal and a ventral premuscular mass within the limb bud. In general, the ventral muscles, which also spread onto the anterior surface of the girdle and appendage, act to protract and adduct the limb and to flex its distal segments; the dorsal muscles, which also extend onto the posterior surface of the girdle and appendage, have the opposite effects (retraction, abduction, and extension). The limb muscles also serve as flexible ties or braces that can fix the bones at a joint and support the body.

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Flight in birds has entailed a considerable modification of the musculature of the pectoral region. As one example, the ventral adductor muscles are exceedingly large and powerful, and the area from which they arise is increased by the enlargement of the sternum and the evolution of a large sternal keel. Not only does a ventral muscle, the pectoralis, play a major role in the downstroke of the humerus, but a ventral muscle, the supracoracoideus, is active in the upstroke as well.

In a number of terrestrial vertebrates, particularly amniotes, certain of the more superficial skeletal muscles of the body have spread out beneath the skin and inserted into it. These may be described as integumentary muscles. Integumentary muscles are particularly well developed in mammals and include the facial muscles and platysma, derived from the hyoid musculature, and often a large cutaneous trunci. The last is derived from the pectoralis and latissimus dorsi and fans out beneath the skin of the trunk. The twitching of the skin of an ungulate is caused by this muscle.

Muscle mechanics

Many of the bones serve as lever arms, and the contractions of muscles are forces acting on these arms (Fig. 2). The joint, of course, is the fulcrum and it is at one end of the lever. The length of the force arm is the perpendicular distance from the fulcrum to the line of action of the muscle; the length of the work arm is the perpendicular distance from the fulcrum to the point of application of the power generated in the lever. Compactness of the body and physiological properties of the muscle necessitates that a muscle attach close to the fulcrum; therefore, the force arm is considerably shorter than the work arm. Most muscles are at a mechanical disadvantage, for they must generate forces greater than the work to be done, but an advantage of this is that a small muscular excursion can induce a much greater movement at the end of the lever.

Slight shifts in the attachments of a muscle that bring it toward or away from the fulcrum, and changes in the length of the work arm, can alter the relationship between force and amount or speed of movement.

In general, the force of a muscle is inversely related to the amount and speed of movement that it can cause. Certain patterns of the skeleton and muscles are adapted for extensive, fast movement at the expense of force, whereas others are adapted for force at the expense of speed. In the limb of a horse, which is adapted for long strides and speed, the muscles that move the limb insert close to the fulcrum and the appendage is long. This provides a short force arm but a very long work arm to the lever system (Fig. 2). In the front leg of a mole, which is adapted for powerful digging, the distance from the fulcrum to the insertion of the muscles is relatively greater and the length of the appendage is less, with the result that the length of the force arm is increased relative to the length of the work arm.

Wikipedia: 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.

Muscles

There are distincts types of muscles: skeletal muscles, heart muscles and smooth muscles.

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.

Aerobic and anerobic muscle activity

At rest, the body produces the majority of its ATP aerobically in the mitochondria without producing lactic acid or other fatiguing byproducs.[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.

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

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.

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