Muscle

MuscleMuscle is composed of bundles of specialized cells capable of contraction and relaxation to create movement.

MuscleThere are three types of muscle in the body: Striped, or striated, skeletal muscles that move the bones.

MuscleThere are three types of muscle in the body: Smooth, involuntary muscles that line the blood vessels, stomach, digestive tract, and other internal organs. Cardiac muscles, which are a cross between the smooth and the striped muscles.

Muscleskeletal musclesSkeletal muscle is made up of thousands of cylindrical muscle fibres often running all the way from origin to insertion.

The fibres are bound together by connective tissue through which run blood vessels and nerves.

MuscleEach muscle fibre contains:An array of myofibrils

Endoplasmic reticulum

Many nuclei

Mitochondria

MuscleUltrastructural Organisation of Skeletal MuscleContractile cells of skeletal muscle are extremely long, multinucleated, and bound by an electrically excitable plasma membrane called the Sarcolemma. The myofibrils, the contractile elements, occupy a large volume of the muscle cell.

MuscleUltrastructural Organisation of Skeletal MuscleThe myofibrils are arranged in parallel bundles in the axis of contraction; each myofibril contains many myofilaments.

The myofibrils are surrounded and bathed by the sacroplasma.

It contains:

glycogen, glycolytic enzymes and intermediates, ATP, ADP, AMP, phosphate, phosphocreatine, creatine.

MuscleUltrastructural Organisation of Skeletal MuscleMuscle cells also contain highly differentiated endoplasmic reticulum, referred to as the sarcoplasmic reticulum.

Myofibrils, which are long, thin bundles of myofilaments, have along their length a structural pattern that repeats every 2.5 m.

These repeat units are called Sarcomeres, and are the contractile units of the myofibrils.

MuscleUltrastructural Organisation of Skeletal MuscleIn skeletal/striated muscle, the sarcomeres of many parallel myofibrils are in transverse register, yielding the characteristic cross striations across the muscle cell.

MuscleUltrastructural Organisation of Skeletal MuscleThe light bands are called Isotropic or I bands.

The dark bands are called Anisotropic or A bands.

MuscleUltrastructural Organisation of Skeletal MuscleIsotropic structures are those that have uniform physical properties regardless of the direction in which they are measured.

Anisotropic structures are those that have physical properties, which depend on the direction of measurement.

The A bands of muscle are optically anisotropic; i.e., their index of refraction is not uniform in all directions, giving them the property of double refraction, or birefringence.

MuscleUltrastructural Organisation of Skeletal MuscleThe I cross striations are bisected by a dense transverse line about 80 nm thick, the Z line. The central portion of the A band, the H zone, is less dense than the rest of the band; it is also bisected by a dense transverse line, the M line.

MuscleUltrastructural Organisation of Skeletal MuscleThere are two types of myofilaments:

thick thin

Only thin filaments are present in the transverse I band.

Thin filaments of the I band and a regular array of thick filaments are present in the dense portions of the A band.

MuscleUltrastructural Organisation of Skeletal MuscleThick and thin muscles are approximately 45 nm apart and are arranged in a hexagonal pattern.

Each thick filament is surrounded a six thin filaments, also in a hexagonal array.

MuscleUltrastructural Organisation of Skeletal MuscleThe thick filaments extend continuously from one end of the A band to the other.The thin filaments are not continuous through the entire A band.

MuscleUltrastructural Organisation of Skeletal MuscleThe thin filaments :

begin at the Z line

are continuous through the I bands

extend into the A bands, terminating at the end of the H zone of the A bands. Regularly disposed projections extending from the thick filaments toward adjacent thin filaments form cross bridges, which represent the only structural connection between the thick and the thin filaments.

MuscleThe Sarcomere during ContractionWhen maximally contracted, the Sarcomere may shorten between 20 and 50%. When passively stretched, they may extend to about 120% of their resting length.

The A bands, and thus the thick filaments always remain constant in length in the relaxed, contracted and stretched states.

The thin filaments likewise undergo no change in length.

HOW DOES THE SARCOMERE CONTRACT?

MuscleThe Sarcomere during ContractionWhen muscle undergo maximal shortening, the thin filaments may even slide pass each other to form a new, dense central band within the H zone.

MuscleThe Sarcomere during ContractionCross bridges are therefore rapidly formed and broken as filaments slide over each other.

The Sliding-Filament Model.

The actin (thin) filaments slide past the myosin (thick) filaments toward the middle of the sarcomere.

The result is shortening of the sarcomere without any change in filament length.

MuscleThe Protein Components of MuscleMuscle cells consist of various intracellular proteins:

The water-soluble proteins of the sarcoplasm, which make up about 20 to 25% of the total muscle protein.

water-insoluble filamentous proteins of the myofibrils.

Myogen is present in the water-soluble proteins and is rich in glycolytic enzymes, and from it the enzymes aldolase, glyceraldehydye-phosphate, etc.

MuscleThe Protein Components of MuscleMyosin as well as actin is present in the water-insoluble filamentous proteins. They make up approximately 80% of the proteins of the contractile apparatus.

In addition there are at least six other protein components:

TropomyosinTroponinC-proteinM-line protein - and -actinin

MuscleMyosinIt has been suggested that the myosin fraction is a major component of the anisotropic bands of skeletal muscle.

Myosin is also the major component of the A bands, thick muscle filaments.

Myosin is highly asymmetrical, molecules tend to associate tightly with each other and with other myofilament proteins, a property that gives the illusion of homogeneity.

MuscleMyosinIt has a molecular weight of 460 kDa, it is long (approx. 160 nm) and asymmetrical containing a globular head.

The molecule contains a globular head with two identical long polypeptide chains of approximately 200 kDa, called the heavy chain.

Myosin contains three unusual amino acids, 3-methylhistidine, -N-monomethyllysine, and -N-trimethyllysine.

MuscleMyosinThe head contains four smaller or light polypeptide chains.

Two of the light chains are identical and have a molecular weight of about 18 kDa.

The two other light chains are about 16 kDa and 21 kDa.

Through most of the length of the myosin molecule, each heavy chain is in -helical conformation and the two chains are wound around each other.

MuscleMyosinBoth the heavy polypeptide chains are folded into globular structures to form the head.

Highly purified myosin preparations can hydrolyse the terminal phosphate group of ATP. The ATPase activity of myosin is distinctive in that:

It is stimulated by Ca2+,It is inhibited by Mg2+, It is profoundly influenced by KCl concentration, It has two pH optima, one at pH 6.0 and the other at pH 9.5.

MuscleMyosinMyosin ATPase activity is characteristically dependent on two different sulfhydryl groups, which differ in their susceptibility to alkylation or to mercaptide formation.

When the more susceptible SH groups are blocked, the ATPase activity of the myosin molecule is increased.

When the second class of sulfhydryl groups is then blocked, the ATPase activity is abolished, suggesting that these SH groups are required for the hydrolytic process.

MuscleMyosinWhen myosin is exposed to trypsin and chymotrypsin heavy and light meromyosin are produced, HMM and LMM respectively.

MuscleMyosinLMM, like myosin, forms filaments.

HMM, catalyzes the hydrolysis of ATP and binds actin but does not form filaments.

HMM is the force generating unit in muscle contraction.

HMM can be further cleaved to two globular subfractions (S1) and one rod-shaped subfraction (S2).

MuscleMyosinEach S1 fragment contains an ATPase site and a binding site for actin.

MuscleMyosinThe ATPase activity of the myosin molecule resides entirely in its head and that there are two catalytic sites, one in each of the two S1 fragments.

Each contains an inhibitory and a catalytic sulfhydryl group.

The light chains in the myosin heads are concerned with the binding of ATP and in the ATPase activity.

Purified, intact myosin binds actin at two specific sites to form actomyosin, a crucial step in the contractile mechanism.

MuscleMyosinWhereas the ATPase activity of pure myosin requires Ca2+ and is inhibited by Mg2+, the ATPase activity of actomyosin is stimulated by Mg2+.

The entire actin-binding activity of myosin resides in the S1 fragments bearing the ATPase activity, and the two catalytic ATPase sites on the head of the myosin molecule are located at or near the actin binding sites.

MuscleThe Mechanism of Myosin ATPase ActivityThree different enzyme-substrate complexes, non-covalent in nature, are formed during the ATPase activity of myosin.

When equimolar amounts of ATP and myosin are mixed, there is a very rapid appearance of free H+ in the medium but a much slower appearance of free phosphate and ADP. The H+ arises as one of the products of hydrolysis of ATP at pH near 7.0:

ATP4- + H2O ADP3- + HPO42- + H+

MuscleThe Mechanism of Myosin ATPase ActivityVarious hypotheses have been proposed for the mechanism of myosin ATPase activity.

A simple version is (M- Myosin and M* - energized conformation of myosin):

M + ATP rev MATP

MATP + H2O rev M*ADPP + H+

M*ADPP MADPP

MADPP rev M + ADP + P

MuscleThe Mechanism of Myosin ATPase ActivityThe M*ADPP is postulated to be a high-energy complex, in which the free energy of hydrolysis of ATP is conserved in the form of an energized conformation of the myosin molecule.

The energy-releasing reaction has been proposed to be responsible for the power stroke in contraction.

The ATPase activity of myosin is markedly increased by actin. Actin has a capacity to activate ATP hydrolysis by myosin.

MuscleThe Mechanism of Myosin ATPase ActivityIt increases the turnover number of myosin 200-fold, from 0.05s-1 to 10s-1 by binding to the MADPP complex and accelerate the release of the products.

Actomyosin then binds ATP, which leads to the dissociation of actin and myosin.

MuscleOrganisation of the Thick Myofilaments

MuscleOrganisation of the Thick Myofilaments Each thick filament is about 1,500 nm long and 10 nm thick.

It consists of longitudinal bundled myosin molecules, each 160 nm long.

The myosin molecules are oriented with their heads away from the midpoint of the thick filaments.

The heads project laterally out of the bundle in a regular, helical fashion.

MuscleOrganisation of the Thick Myofilaments In a cross section of the thick filaments at any given point there are 18 myosin molecules in a regular packing arrangement. Altogether there are approximately 400 myosin molecules per complete thick filament.

The myosin heads, which resemble barbs, vary in the distance they project from the thick filament.

MuscleOrganisation of the Thick Myofilaments Each myosin head is located next to the hinge-like trypsin-sensitive point.

Thick filaments also contain two other proteins, namely, C-protein (140 kDa, 3.5% of thick filament protein, binds very strongly to myosin tail, 35 nm long and wound around the thick filament at regular intervals) and M-line protein.

Both may serve to hold the bundle of myosin molecules together.

MuscleActinEvolution has not changed the Actin genes much such that Actin from different species is found to be often interchangeable in in vitro assays.

This protein is the major component of thin filaments and occurs in two forms, G-actin (globular actin) and F-actin (fibrous actin), a polymer of G-actin.

MuscleActinThey are the principal components of the thin filaments in skeletal muscle.

They have a polar structure and this polarity from one end to the other is crucial for cell motility.

G-actin contains an unusual amino acid 3-methylhistidine, which is also present in myosin.

It also contains a large number of proline and cysteine residues.

MuscleActinEach molecule of G-actin binds one Ca2+ ion very tightly. It also binds one molecule of ATP or ADP with high affinity.

For each molecule of G-actin added to the F-actin chain, one molecule of ATP is split to ADP and phosphate.

The ADP formed remains bound to the G-actin subunit of the F-actin chain.

n{G-actin-ATP} {G-actin-ADP}n + nPi F-actin

MuscleOrganisation of the Thin FilamentsTwo strands of F-actin each composed of G-actin monomers are coiled about each other in the thin filament.

The thin filaments also contain two major accessory proteins, which serve a regulatory function in controlling the making and breaking of the cross bridges between the thick and thin myofilaments, as well as the generation of mechanical force.

MuscleOrganisation of the Thin FilamentsThe first is Tropomyosin (70 kDa with two helical chains of 33 kDa and 37 kDa, which form a two-chain twisted coil of about 40 nm long), which makes up 10 to 11% of the total contractile protein of muscle.

They are long, thin, and arranged end to end in the shallow grooves of the coiled F-actin filaments, in such a way that each tropomyosin molecule makes contact with only one of the two F-actin filaments.

MuscleOrganisation of the Thin FilamentsThe second is Troponin, which is a large globular protein that contains three polypeptide subunits:

The Ca2+ binding subunit of troponin (TN-C, or troponin A).

The inhibitory subunit of troponin (TN-I). It has a binding site specific for actin but it does not bind Ca2+. 3.Tropomyosin-binding subunit of troponin (TN-T).

MuscleOrganisation of the Thin FilamentsThe complete troponin molecule contains one each of the TN-C, TN-I, and TN-T subunits and has a globular shape.

Each troponin molecule is attached to the thin filament by two binding sites, one specific for an actin strand and the other for a Tropomyosin strand.

For every seven G-actin monomers there is one molecule of tropomyosin and one of troponin.

MuscleActomyosin complexesWhen pure myosin and actin are mixed, actomyosin complexes are formed, which results in increased viscosity and flow birefringence.

The ratio of myosin to actin and the particle weight of such actomyosin complexes depend on the experimental condition, such as:

pH, the KCl and MgCl2 concentrations, as well as the protein concentration.

MuscleActomyosin complexesSince an F-actin chain contains many G-actin monomers, each F-actin filament can bind many myosin molecules, however, only the heads of myosin molecules bind to the actin filaments to yield structures resembling barbs.

MuscleActomyosin complexesA significant property of actomyosin complexes is that they undergo dissociation in the presence of ATP and Mg2+; the dissociation is accompanied by a large and rapid decrease in the viscosity of the actomyosin solution.

When ATP is completely hydrolyzed to ADP, the actin and myosin re-aggregate.

MuscleCa2+ Triggers Thick-Filament-Thin-Filament Interaction and the Generation of ForceIn the relaxed state, the sarcoplasm has a high MgATP2- concentration, but the concentration of Ca2+ is below the threshold required for initiation of contraction.

The myosin heads are in a contracted state. Each head in this state contains two molecules of tightly bound ATP.

The ATP maybe present as ADP and phosphate, both tightly bound to the energized conformation of the myosin head.

MuscleCa2+ Triggers Thick-Filament-Thin-Filament Interaction and the Generation of ForceThe myosin head in this state is unable to react with actin of the thin filaments because in the absence of Ca2+ the Tropomyosin molecule masks the myosin binding site on the G-actin monomer or holds it in a conformation that is un-reactive, through the action of the TN-I subunit of troponin.

When free Ca2+ is now released into the sarcoplasm by the incoming nerve signal, Ca2+ is immediately bound to the Ca2+-binding sites of troponin.

MuscleCa2+ Triggers Thick-Filament-Thin-Filament Interaction and the Generation of ForceThrough a conformational change of the troponin molecule, the myosin-binding site of the G-actin monomer now becomes exposed and combines with the energized myosin head, with its bound ADP and phosphate, to form the force-generating complex, in which the myosin head is attached to a G-actin monomer.

MuscleCa2+ Triggers Thick-Filament-Thin-Filament Interaction and the Generation of ForceThe myosin head is now believed to undergo an energy-yielding conformational change, so that the cross bridge changes its angular relationship to the axis of the heavy filament, causing the thin filament to be moved along the thick filament; this is the power stroke.

MuscleThe Source of Energy for Muscular ContractionThe musculature of an adult man in the resting state utilizes approximately 30% of the total ATP energy generated by respiration.

During very intense muscle activity, the muscles consume 85% or more of the total ATP generated.

The high-energy Phosphocreatine is present in muscle in about 5 times the concentration of ATP.

MuscleThe Source of Energy for Muscular ContractionPhosphocreatine + ADP rev creatine + ATP

ATP formation is favoured at the expense of phosphocreatine. This explains why ATP concentration in muscle does not decline during a single contraction.

If the muscle is stimulated long enough in the absence of glycolysis or respiration, the phosphocreatine supply will eventually become depleted.

MuscleThe Source of Energy for Muscular ContractionThe ultimate source of metabolic energy for re-phosphorylation of ADP, and thus of phosphocreatine, is not identical in all muscles.

Although all muscles of vertebrates show both glycolytic and respiratory activity, the relative contribution of glycolysis and respiration to the regeneration of ATP from ADP may vary considerably.

MuscleThe Source of Energy for Muscular ContractionThere are two types of skeletal muscle fibres, red, or slow fibres and white, or fast fibres.

In red muscles, which owe their colour to their high content of myoglobin and cytochromes, respiration serves as the chief source of energy for re-phosphorylation of ADP via oxidative phosphorylation.

Red muscles contract more slowly than white muscles and normally function in regular periodic cycles.

MuscleThe Source of Energy for Muscular ContractionWhite muscles, on the other hand, contain little myoglobin and few mitochondria; in such muscles glycolysis is the chief source of energy for re-phosphorylation of ADP.

In general, red muscles use fatty acids as their major fuel, which they oxidize via the fatty acid oxidation cycle to acetyl-CoA; the latter is oxidized to CO2 via the TCA cycle.

White muscles, on the other hand, use glucose as major fuel.

MuscleThe Source of Energy for Muscular ContractionThe rates of glycolysis and respiration, and thus of ATP production, are adjusted to the rate of ATP consumption in muscle by a series of feedback controls.

In resting muscle the [ATP]/[ADP][Pi] ratio is high.

Here the rates of glycolysis and the TCA are low, because of allosteric inhibition by negative modular ATP.

MuscleThe Source of Energy for Muscular ContractionWhen the muscles are stimulated to maximal activity, the [ATP]/[ADP][Pi] ratio is on decline with an increase in fuel and oxygen consumption.

Oxygen uptake by skeletal muscles increased twenty fold.

Some of the ADP formed on contraction undergoes conversion to AMP, which stimulates glycolysis.

MuscleThe Source of Energy for Muscular ContractionDuring maximal muscular exertion lactate appears in the blood in large amounts resulting in the consumption of considerably extra oxygen.

This extra oxygen consumed is called the oxygen debt and corresponds to the oxidation of some or all the excess lactic acid.

Some of the lactic acid accumulated in the blood may be converted to glycogen by the liver.