Muscle is a specialized tissue of mesodermal origin.About 40-50 percent of the body weight of a human adult is contributed by muscles.Muscular tissue is characterized by the property of shortening called contractility.It also has the properties of extensibility, elasticity, flexibility, conductivity, etc.The muscular tissue consists of highly, elongated modified cells called muscle fibers.There are three kinds of muscular tissue in the body of vertebrates: striated, un-striatedand cardiac muscles.The striated muscles perform voluntary movementsThe un-striated and cardiac muscles perform involuntary movements dr. aarif
TYPES OF MUSCLESSTRIATED CARDIAC NON-STRIATED dr. aarif
STRIATED MUSCLES:1. Striated muscle cells are elongated, cylindrical un-branched, multi-nucleated and withcross striations.2. The striated muscle cells are covered by modified cell membrane called sarcolemmar –basement membrane and reticular connective tissue.3. Sarcolemma is an electrically charged membrane of the body.4. Each muscle fiber is formed by many myofibrils containing myofilaments made up ofproteins actin and myosin in filaments.5. The contractile units of muscle are sarcomeres formed by thin actin filaments and thickmyosin filaments.6. Unlike the other two muscular tissues, the striated muscle fibers are packed together inconnective tissue into distinct bundles, called muscle bundles.7. In the human body, about 640 muscles are present.8. Each muscle contains many fasciculi and each fasciculus contains bundle of musclefibers.9. Striated muscles are found attached to skeleton by means of tendons.10. Tendons are inelastic thick bands of white fibrous connective tissue, which give firmattachment to muscle with bone dr. aarif
LOCATION AND STRUCTURE OF STRIATED MUSCLE:The muscle which moves a body part usually does not lie in that part but is located in theupper part.E.g. biceps and triceps that move forearm are located in the upper arm.At any joint two types of bones are present i.e. stationary and movable.The end of muscle attached to stationary bone is called origin while the opposite endattached to movable bone is called insertion. The middle thick part of muscle is calledbelly.All the fibers in a muscle do not extend from end to end and there is a maximumconcentration in the middle. Thus, large muscles are most often fusiform in shape dr. aarif
TYPES OF STRIATED MUSCLES: On the basis of movements, striated muscles are of three types:Prime movers (agonist): They bring initial movement of part.E.g. Biceps.Antagonists: These bring the action opposite to that of prime movers.E.g. Triceps.Synergists: These assist prime movers.E.g. Brachialis assists Biceps. dr. aarif
WORKING OF SKELETAL MUSCLES:Generally muscles work in pair and produce opposite action.E.g. biceps (flexors) bring flexion and triceps (extensors) bring extension of elbow joint.The muscles, which bring opposite action, are called antagonistic.If one member of a pair is capable of bending the joint by pulling of bones, the othermember is capable of straightening the same joint also by pulling.E.g. Biceps and triceps of upper arm are antagonistic to each other.In antagonistic pair of muscles, one member is much stronger than the other, e.g. thebiceps, which flex the arm are stronger than the triceps which extend it.The fundamental characteristic of muscle is contraction. Therefore, muscle can only pull andnot push the bone.The response or contraction of striated muscles is quick and for short duration. Therefore,these muscles are prone to fatigue.These, muscles are neurogenic i.e. need repeated stimulus from Central Nervous System(CNS) dr. aarif
Some important antagonistic muscles:Flexor: On contraction results in bending or flexionof a joint e.g. Biceps.Extensor: The contraction results in straighteningor extension of a joint e.g. Triceps.Abductor: It moves body part away from the bodyaxis e.g. deltoid muscle of shoulder moves the armaway from the body.Adductor: It moves body part towards the bodyaxis e.g. Latissimus dorsi of shoulder moves the armnear to the body.Pronator: To turn the palm downward or backward.Supinator: To turn the palm upward or forward.Levator: Raises a body part Depressor: Lowers the body partProtractor: To move forward.Retractor: To move backward.
STRUCTURE OF SKELETAL MUSCLEEach organized skeletal muscle in our body is made of a number of muscle bundles orfascicles held together by a common collagenous connective tissue layer called fascia.Each muscle bundle contains a number of muscle fibers.Each muscle fiber is lined by the plasma membrane called sarcolemma enclosing thesarcoplasm.Muscle fiber is a syncitium (a mass of cytoplasm having many nuclei but no internal cellboundaries) as the sarcoplasm contains many nuclei.The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibers is the storehouse of calcium ions.A characteristic feature of the muscle fiber is the presence of a large number of parallelarranged filaments in the sarcoplasm called myofilaments or myofibrils.Each myofibril has alternate dark and light bands on it. dr. aarif
MYOFIBRIL:A detailed study of the myofibril has established that the striated appearance is due to thedistribution pattern of two important proteins - Actin and Myosin.The light bands contain actin and is called I-band or Isotropic band, whereas the dark bandcalled A-band or Anisotropic band contains myosin.Both the proteins are arranged as rod-like structures, parallel to each other and also to thelongitudinal axis of the myofibrils.Actin filaments are thinner as compared to the myosin filaments, hence are commonlycalled thin and thick filaments respectively.In the center of each I-band is an elastic fiber called ‘Z’-line which bisects it. The thinfilaments are firmly attached to the Z-line.The thick filaments in the A- band are also held together in the middle of this band by athin fibrous membrane called M line.The A and I bands are arranged alternately throughout the length of the myofibrils.The portion of the myofibril between two successive Z lines is considered as the functionalunit of contraction and is called a sarcomere. dr. aarif
In a resting state, the edges of thin filaments on either side of the thick filamentspartially overlap the free ends of the thick filaments leaving the central part of thethick filaments.This central part of thick filament, not overlapped by thin filaments is called the H-zone. dr. aarif
STRUCTURE OF CONTRACTILE PROTEINS ACTIN:Each actin (thin) filament is made of twoF(filamentous) actins helically wound to eachother.Each F’ actin is a polymer of monomeric G(Globular) actins.Two filaments of another protein,tropomyosin also run close to the F actinsthroughout its length.A complex protein Troponin is distributed atregular intervals on the tropomyosin.In the resting state, a subunit of troponinmasks the active binding sites for myosin onthe actin filaments. dr. aarif
MYOSIN:Each myosin (thick) filament is also a polymerizedprotein.Many monomeric proteins called Meromyosins constitute one thickfilament.Each meromyosin has two importantparts: a globular head with a shortarm and a tailThe head with a short arm is called the heavymeromyosin (HMM) and the tail is called the lightmeromyosin (LMM).The HMM component, i.e. the head and short armprojects outwards at regular distance and anglefrom each other from the surface of a polymerizedmyosin filament and is known as cross arm.The globular head is an active ATPase enzyme andhas binding sites for ATP and active sites for actin. dr. aarif
MECHANISM OF MUSCLE CONTRACTIONMuscle contraction is initiated by a signal sent by the central nervous system (CNS) via amotor neuron.A motor neuron along with the muscle fibers connected to it constitutes a motor unit. Thejunction between a motor neuron and the sarcolemma of the muscle fiber is called theneuromuscular junction or motor-end plate.A neural signal reaching this junction releases a neurotransmitter (Acetylcholine) whichgenerates an action potential in the sarcolemma. This spreads through the muscle fibre andcauses the release of calcium ions into the sarcoplasm. Increase in Ca++ level leads to thebinding of calcium with a subunit of troponin on actin filaments and thereby remove themasking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin headnow binds to the exposed active sites on actin to form a cross bridge This pulls the attachedactin filaments towards the centre of ‘A’ band.The myosin releasing the ADP and Pi goes backto its relaxed state. A new ATP binds and the cross-bridge is brokenThe Z-Line attached to these actins are also pulled inwards thereby causing a shortening ofthe sarcomere, i.e. contraction. In this stage, the I-bands get reduced whereas the A bandsretain the length.The ATP is again hydrolyzed by the myosin head and the cycle of cross bridge formation andbreakage is repeated causing further sliding dr. aarif
The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternaeresulting in the masking of actin filaments. This causes the return of Z lines back to theiroriginal position, i.e., relaxation.Repeated activation of the muscles can lead to the accumulation of lactic acid due toanaerobic breakdown of glycogen in them, causing fatigue.Muscle contains a red colored oxygen storing pigment called myoglobinMyoglobin content is high in some of the muscles which give a reddish appearance. Suchmuscles are called the Red fibers. These muscles also contain plenty of mitochondria whichcan utilize the large amount of oxygen stored in them for ATP production. These muscles,therefore, can also be called aerobic muscles.On the other hand, some of the muscles possess very less quantity of myoglobin andtherefore, appear pale or whitish. These are the White fibers. Number of mitochondria arealso few in them, but the amount of sarcoplasmic reticulum is high. They depend onanaerobic process for energy. dr. aarif
RIGOR MORTIS:It is strange that muscles require ATP to relax as well as contract.Myosin heads cannot detach from the actin myofilaments until ATP molecules jointhem.On an animal’s death, its muscles soon exhaust ATP and lose the ability to contract orrelax. They become rigidly locked in whatever position they were when ATP was used up.This postmortem (after death) stiffening of the body from hardening of muscle tissue iscalled rigor mortis dr. aarif
SKELETAL DISORDERS MUSCULAR DYSTROPHYIt is inherited muscle destroying disease (i.e. it is a geneticdefect).It is characterized by degeneration of individual musclefiber, which leads to progressive atrophy of the skeletalmuscle.Usually the voluntary skeletal muscles are weakenedequally on both sides of the body, whereas the internalmuscles, such as the diaphragm, are not affected.The most common form of muscular dystrophy isDuchenne type dr. aarif
OSTEOPOROSIS:It is reduction in bone tissue mass causingweakness of skeletal strength. (Gr. Osteon = bone,poros = pore, osis = condition)It is an age dependent disease.In women, after menopause, the estrogensecretion becomes less causing loss of calcium.The bones become porous due to low bone mass.Skeleton fails to withstand the stress of body.It is also caused by deficiency of vitamin D,calcium, sex hormones and thyrocalcitocin. dr. aarif
GOUTY ARTHRITIS (GOUT) It is an inherited disorder of purine metabolism. It is caused due to excessive accumulation of uric acid in the body due to excessive production or inability to excrete. It gets deposited in joints and leads to severe pain.A Affects men predominantlyR OSTEOARTHRITIS Secretion of the lubricating synovial fluid between the bones at the joint stops.T It is characterized by degeneration of the cartilage pad. The joint becomes inflammed, it movement becomes painful, and its function isH diminished. Such stiffness or fixation of a joint is also called ankylosis.R Joints of knees, hands, and spine are usually affected by this disease. Usually occurs in old persons as it is a result of wear and tear due to years of use.IT RHEUMATOID ARTHRITIS It is an auto-immune disease in which chronic painful inflammation of the synovialI membranes of many joints simultaneously. It is characterized by the inflammation of synovial membrane.S The membrane thickens and synovial fluid increases, exerting pressure that causes severe pain. The membrane then starts secreting abnormal granules, called pannus, which cause erosion of cartilage. It usually starts in the small joints in the hand and it progresses in centripetal and symmetrical manner. Affects the women more often than men
MYASTHENIA GRAVIS: Autoimmune disorder affectingneuromuscular junction leadingto fatigue, weakening andparalysis of skeletal muscle.TETANY : Rapid spasms (wildcontractions) in muscle due tolow Ca++ in body fluid. dr. aarif
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