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MUSCULAR MOVEMENT

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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-striated and cardiac muscles.

The striated muscles perform voluntary movements

The un-striated and cardiac muscles perform involuntary movements

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TYPES OF MUSCLES

STRIATED

NON-STRIATED

CARDIAC

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STRIATED MUSCLE

STRIATED MUSCLES:

1. Striated muscle cells are elongated, cylindrical un-branched, multi-nucleated and with cross 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 of proteins actin and myosin in filaments.5. The contractile units of muscle are sarcomeres formed by thin actin filaments and thick myosin filaments.6. Unlike the other two muscular tissues, the striated muscle fibers are packed together in connective 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 muscle fibers.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 firm attachment to muscle with bone

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LOCATION AND STRUCTURE OF STRIATED MUSCLE: The muscle which moves a body part usually does not lie in that part but is located in the upper 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 end attached to movable bone is called insertion. The middle thick part of muscle is called belly.

All the fibers in a muscle do not extend from end to end and there is a maximum concentration in the middle. Thus, large muscles are most often fusiform in shape

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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.

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 other member 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. the biceps, which flex the arm are stronger than the triceps which extend it.

The fundamental characteristic of muscle is contraction. Therefore, muscle can only pull and not 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)

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Some important antagonistic muscles:

Flexor: On contraction results in bending or flexion of a joint e.g. Biceps.Extensor: The contraction results in straightening or extension of a joint e.g. Triceps. Abductor: It moves body part away from the body axis e.g. deltoid muscle of shoulder moves the arm away from the body.Adductor: It moves body part towards the body axis e.g. Latissimus dorsi of shoulder moves the arm near 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 part Protractor: To move forward. Retractor: To move backward.

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STRUCTURE OF SKELETAL MUSCLE

Each organized skeletal muscle in our body is made of a number of muscle bundles or fascicles 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 the sarcoplasm.

Muscle fiber is a syncitium (a mass of cytoplasm having many nuclei but no internal cell boundaries) as the sarcoplasm contains many nuclei.

The endoplasmic reticulum, i.e., sarcoplasmic reticulum of the muscle fibers is the store house of calcium ions.

A characteristic feature of the muscle fiber is the presence of a large number of parallel arranged filaments in the sarcoplasm called myofilaments or myofibrils.

Each myofibril has alternate dark and light bands on it.

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MYOFIBRIL:A detailed study of the myofibril has established that the striated appearance is due to the distribution pattern of two important proteins - Actin and Myosin.

The light bands contain actin and is called I-band or Isotropic band, whereas the dark band called A-band or Anisotropic band contains myosin.

Both the proteins are arranged as rod-like structures, parallel to each other and also to the longitudinal axis of the myofibrils.

Actin filaments are thinner as compared to the myosin filaments, hence are commonly called thin and thick filaments respectively.

In the center of each I-band is an elastic fiber called ‘Z’-line which bisects it. The thin filaments 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 a thin 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 functional unit of contraction and is called a sarcomere.

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In a resting state, the edges of thin filaments on either side of the thick filaments partially overlap the free ends of the thick filaments leaving the central part of the thick filaments.

This central part of thick filament, not overlapped by thin filaments is called the 'H'-zone.

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STRUCTURE OF CONTRACTILE PROTEINS

ACTIN: Each actin (thin) filament is made of two 'F'(filamentous) actins helically wound to each other.

Each 'F’ actin is a polymer of monomeric 'G' (Globular) actins.

Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length.

A complex protein Troponin is distributed at regular intervals on the tropomyosin.

In the resting state, a subunit of troponin masks the active binding sites for myosin on the actin filaments.

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MYOSIN:Each myosin (thick) filament is also a polymerized protein.

Many monomeric proteins called Meromyosins constitute one thick filament.

Each meromyosin has two important parts: a globular head with a short arm and a tail

The head with a short arm is called the heavy meromyosin (HMM) and the tail is called the light meromyosin (LMM).

The HMM component, i.e. the head and short arm projects outwards at regular distance and angle from each other from the surface of a polymerized myosin filament and is known as cross arm.

The globular head is an active ATPase enzyme and has binding sites for ATP and active sites for actin.

MECHANISM OF MUSCLE CONTRACTION

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MECHANISM OF MUSCLE CONTRACTIONMuscle contraction is initiated by a signal sent by the central nervous system (CNS) via a motor neuron.

A motor neuron along with the muscle fibers connected to it constitutes a motor unit. The junction between a motor neuron and the sarcolemma of the muscle fiber is called the neuromuscular junction or motor-end plate.

A neural signal reaching this junction releases a neurotransmitter (Acetylcholine) which generates an action potential in the sarcolemma. This spreads through the muscle fibre and causes the release of calcium ions into the sarcoplasm. Increase in Ca++ level leads to the binding of calcium with a subunit of troponin on actin filaments and thereby remove the masking of active sites for myosin. Utilising the energy from ATP hydrolysis, the myosin head now binds to the exposed active sites on actin to form a cross bridge This pulls the attached actin filaments towards the centre of ‘A’ band.The myosin releasing the ADP and Pi goes back to its relaxed state. A new ATP binds and the cross-bridge is broken

The Z-Line attached to these actins are also pulled inwards thereby causing a shortening of the sarcomere, i.e. contraction. In this stage, the I-bands get reduced whereas the A bands retain the length.

The ATP is again hydrolyzed by the myosin head and the cycle of cross bridge formation and breakage is repeated causing further sliding

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The process continues till the Ca++ ions are pumped back to the sarcoplasmic cisternae resulting in the masking of actin filaments. This causes the return of 'Z' lines back to their original position, i.e., relaxation.

Repeated activation of the muscles can lead to the accumulation of lactic acid due to anaerobic breakdown of glycogen in them, causing fatigue.

Muscle contains a red colored oxygen storing pigment called myoglobin

Myoglobin content is high in some of the muscles which give a reddish appearance. Such muscles are called the Red fibers. These muscles also contain plenty of mitochondria which can 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 and therefore, appear pale or whitish. These are the White fibers. Number of mitochondria are also few in them, but the amount of sarcoplasmic reticulum is high. They depend on anaerobic process for energy.

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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 join them.

On an animal’s death, its muscles soon exhaust ATP and lose the ability to contract or relax. 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 is called rigor mortis

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SKELETAL DISORDERS

MUSCULAR DYSTROPHY It is inherited muscle destroying disease (i.e. it is a genetic defect).

It is characterized by degeneration of individual muscle fiber, which leads to progressive atrophy of the skeletal muscle.

Usually the voluntary skeletal muscles are weakened equally on both sides of the body, whereas the internal muscles, such as the diaphragm, are not affected.

The most common form of muscular dystrophy is Duchenne type

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OSTEOPOROSIS: It is reduction in bone tissue mass causing weakness of skeletal strength. (Gr. Osteon = bone, poros = pore, osis = condition)

It is an age dependent disease.

In women, after menopause, the estrogen secretion 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.

ARTHRITIS

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.Affects men predominantly

OSTEOARTHRITISSecretion of the lubricating synovial fluid between the bones at the joint stops.It is characterized by degeneration of the cartilage pad. The joint becomes inflammed, it movement becomes painful, and its function is diminished. Such stiffness or fixation of a joint is also called ankylosis.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.

RHEUMATOID ARTHRITISIt is an auto-immune disease in which chronic painful inflammation of the synovial membranes of many joints simultaneously.It is characterized by the inflammation of synovial membrane.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

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MYASTHENIA GRAVIS: Auto immune disorder affecting neuromuscular junction leading to fatigue, weakening and paralysis of skeletal muscle.

TETANY : Rapid spasms (wild contractions) in muscle due to low Ca++ in body fluid.