Skeletal Muscles and Contraction Mechanisms

1.         Function of Skeletal muscles:

a.         Skeletal muscles make up the bulk of muscle in the body, about 40% of total body weight.

b.         They are responsible for positioning and movement of the skeleton and are attached to bones by collagenous tendons.

c.         The origin of a muscle is the end of the muscle that is attached closest to the trunk or to the more stationary bone.

d.         The insertion of the muscle is the more distal or mobile attachment.

e.         Contraction of the muscle results in movement of bones connected to each other at a flexible joint.

f.          If the centers of the connected bones are brought closer together when the muscle contracts, it is called a flexor.

g.         If the bones move away from each other when the muscle contracts, it is called an extensor.

h.         Movement is brought about by flexor-extensor pairs called antagonistic muscle groups – flexion of the protagonist muscle and extension of the antagonistic muscle causes the bending of arm.

2.         Composition of Skeletal Muscle:

a.         A skeletal muscle is a collection of muscle cells, or muscle fibers which enable it to function as a unit:

b.         The number of muscle fibers in a muscle is dependent on its size:

i.          a small muscle in the ear may have several hundred muscle fibers.

ii.          larger muscles like those in the quadriceps may have several thousand fibers.

c.         The individual fibers of a skeletal muscle are arranged with their long axes in parallel and bound with connective tissue.

d.         Collagen, elastic fibers, nerves, and blood vessels are found between the bundles of muscle fibers.

e.         The entire muscle is enclosed in a connective tissue sheath that is continuous with the connective tissue around the muscle fibers and with the tendons holding the muscle to underlying bones.

f.          Appearance:

i.          each skeletal muscle fiber is a long, cylindrical cell with up to several hundred nuclei on the surface of the fiber.

ii.          the fibers have a striated appearance due to the presence of two types of protein filaments which make up the fiber.

g.         Muscle fibers are the largest cell in the body, created by the fusion of many individual embryonic muscle cells.

h.         A muscle fiber contains little cytosol; the main intracellular structures being the myofibrils which are bundles of contractile and elastic proteins that carry out the work of contraction.

i.          The cytosol between the myofibrils contains many glycogen granules and mitochondria.

j.          Glycogen is the storage form of glucose and is a reserve source of energy while mitochondria provide much of the ATP for muscle contraction through oxidative phosphorylation of glucose and other biomolecules.

3.         Sarcolemma:

a.         The sarcolemma is the cell membrane of a muscle fiber and it consists of:

i.          a plasma membrane

ii.          an outer coat made up of a thin layer of polysaccharide material that contains numerous thin collagen fiber.

b.         At the end of the muscle fiber, this surface layer of sarcolemma fuses with a tendon fiber, and the tendon fibers in turn collect into bundles to form the muscle tendons and insert into the bones.

c.         The sarcolemma invaginates into the muscle fiber at many sites along its length forming transverse tubules, or T-tubules which penetrate all the way from one side of the muscle fiber to the other end.

d.         Structure of T-tubules:

i.          they are very small and run transverse to the myofibrils.

ii.          they branch among themselves so that they form entire planes of T tubules interlacing among all the separate myofibrils.

iii.         the membranes of the T-tubules are a continuation of the surface membranes of the muscle fiber, making the lumen of the t-tubules continuous with the extracellular fluid.

iv.         therefore, they contain extracellular fluid in their lumen.

v.         T-tubules are internal extensions of cell membrane.

e.         T-tubules allow action potentials that originate on the cell surface at the neuromuscular junction to move rapidly into the interior of the fiber.

f.          The action potential currents surrounding these T-tubules then elicit the muscle contraction.

4.         Sarcoplasmic Reticulum:

a.         Sarcoplasmic reticulum is a form of modified endoplasmic reticulum found in skeletal muscle fibers that wraps around each myofibril like a piece of lace.

b.         It is composed of two major parts:

i.          long longitudinal tubules that run parallel to the myofibrils and terminate in

ii.          large chambers called terminal cisternae which abut the T tubules.

c.         The arrangement of two cisternae abutting the central T-tubule on either side forms a triad.

d.         In skeletal muscle there are two T-tubule networks for each sarcomere located near the two ends of the myosin filaments, which are points where the actual mechanical forces of muscle contraction are created.

e.         Hence, the skeletal muscle is optimally organized for rapid excitation of muscle contraction.  

5.         Myofibrils: Actin and Myosin Filaments:

a.         Each muscle fiber contains a thousand or more myofibrils that occupy most of the intracellular volume, leaving little space for cytosol and organelles.

b.         Each myofibril is composed of several types of proteins:

i.          contractile proteins: myosin and actin

ii.          regulatory proteins: tropomyosin and troponin

iii.         giant accessory proteins: titin, nebulin.

c.         Each myofibril has, lying side by side, about 1500 myosin filaments and 3000 actin filaments, which are large polymerized molecules responsible for muscle contraction.

d.         Myosin:

i.          makes up the thick filaments of the myofibril.

ii.          various isoforms of myosin occurs in different types of muscle, accounting for the difference in contraction mechanics.

iii.         the myosin filament is composed of multiple myosin molecules, each having a molecular weight of about 480,000.

iv.         the myosin molecule is composed of six polypeptide chains:

            - two heavy chains each with a molecular weight of 200,000

            - four light chains with molecular weights of 20,000 each

v.         the two heavy chains wrap spirally around each other to form a double helix.

vi.         one end of each of these chains is folded into a globular polypeptide structure called the myosin head and they are two free heads lying next to each other.

vii.        the four light chains are also parts of the myosin heads with two to each head and they help control the function of the head during muscle contraction.

viii.       the elongation portion of the coiled helix is called the tail.

e.         Arrangement of myosin molecules in filaments:

i.          the tails of the myosin molecules are bundled together to form the body of the filaments which is stiff; this is the thick filament.

ii.          the heads of the molecule hangs outward to the sides of the body.

iii.         part of the helix portion of each myosin molecule extends to the side along with the head, thus providing an arm that extends the head outward from the body.

iv.         the protruding head and arm are called cross-bridges as a whole.

v.         each cross-bridge is flexible at two points called hinges:

            - one where the arm leaves the body of the myosin filament

            - the other where the two heads attach to the arm.

vi.         the hinged arms allow the heads either to be extended far outward from the body of the myosin filament or to be brought close to the body.

vii.        the myosin filament is itself twisted so that each successive set of cross-bridges is axially displaced from the previous set by 120 degrees which ensures that the cross-bridges extend in all directions around the filament.

viii.       the myosin head functions as an ATPase enzyme which allows it to cleave ATP and use the energy to energize the contraction process.

f.          Actin filaments:

i.          actin filaments form the thin filaments of the myofibril.

ii.          composed of three protein components: actin, tropomyosin and troponin.

iii.         are stranded F-actin protein molecule being wound in a helix.

iv.         each strand of double F-actin helix is composed of polymerized G-actin molecules, each having a molecular weight of 42,000.

v.         attached to each one of the G-actin molecule is one molecule of ATP which are the active sites on the actin filaments with which the cross-bridges of the myosin filaments interact to cause muscle contraction.

vi.         the active sites on the two F-actin strands of the double helix are staggered, giving one active site on the overall actin filament every 2.7nm.

vii         each actin filament is 1 micrometer long, with their bases inserted strongly into the Z discs, whereas the other ends protrude in both directions into the adjacent sarcomeres to lie in the space between the myosin molecules.

g.         Tropomyosin:

i.          the tropomyosin molecules are connected loosely with the F-actin strands, wrapped spirally around the sides of the F-actin helix.

ii.          in the resting stage, the tropomyosin molecules lie on top of the active sites of the actin strands, preventing contraction from taking place.

iii.         each tropomyosin molecule covers about seven of these active sites.

h.         Troponin:

i.          they are attached near one end of each tropomyosin molecule.

ii.          it is a complex of three loosely bound protein subunits.

iii.         troponin I has strong affinity for actin, troponin T for tropomyosin and troponin C for calcium ions.

iv.         this complex attach the tropomyosin to the actin.

6.         Composition and arrangement of a Sarcomere:

a.         A sarcomere is the smallest repeating unit of a myofibril and they lie between two successive Z discs.

b.         Z discs:

i.          these zigzag structures are made of proteins that serve as the attachment site for the thin filament.

ii.          it is composed of filamentous proteins that passes crosswise across the myofibril and also crosswise from myofibril to myofibril, attaching the myofibrils to one another all the way across the muscle fiber.

iii.         one sarcomere is composed of 2 Z discs and the filaments between them.

c.         I band:

i.          the lightest color bands of the sarcomere and they contain only actin filaments.

ii.          it is so called as it is isotropic to polarized light.

iii.         a Z disc runs through the middle of an I band, so each half of the I band belongs to a different sarcomere.

d.         A band:

i.          so called as they are anisotropic to polarized light.

ii.          encompasses the entire length of thick filaments.

iii.         at the outer edges of the A band, the thick and thin filaments overlap

iv.         the center of the A band is occupied by thick filaments only.

e.         H zone: the central region of the A band which is lighter than the outer edges of the A bands as it is occupied by thick filaments only.

f.          M line: lies in the center of the  sarcomere and contains attachment site for the thick filaments.

g.         In three-dimensional array, the actin and myosin molecules form a lattice of parallel, overlapping thin and thick filaments.

h.         When viewed end-on, each thin filament is surrounded by three thick filaments and six thin filaments encircle each thick filament.

I.          The proper alignment of filaments within a sarcomere is ensured by two accessory proteins: titin and nebulin.

j.          Titin and Nebulin:

i.          huge elastic protein with 25,000 amino acids.

ii.          a single titin molecule stretches from one Z disk to the next M line.

iii.         it stabilizes the position of contractile filaments, and its elasticity returns stretched muscles to their resting length.

iv.         titin is helped by nebulin, an inelastic giant protein that iles alongside thin filaments and attaches to the Z disc.

v.         nebulin helps align the actin filaments of the sarcomere.

7.         Molecular mechanism of Muscle Contraction:

a.         The contraction of skeletal muscles allow us to create force to move or resist a load.

b.         The force created by a contracting muscle is called a tension while the force that opposes contraction of a muscle is the load.

c.         The following changes are observed during a muscle contraction:

i.          the sarcomere shortens in length.

ii.          the Z disc moves towards each other.

iii.         the I band and H zone almost disappear.

iv.         the length of the A band remains constant.

d.         These observations lead to the proposal of the sliding filament theory of contraction in which overlapping muscle fibers of fixed length slide past each other in an energy-requiring process, resulting in muscle contraction.

8.         Sliding filament theory of contraction:

a.         In muscle contraction, the thin actin filaments slide along the thick myosin filaments as they move toward the M line, shortening the sarcomere in the process.

b.         The sliding force that moves the actin filament is the movement of myosin cross-bridges that link actin and myosin – the actin filaments serve as ‘roads’ along which the myosin heads walk, causing muscle contraction.

c.         When a muscle contracts, work is performed and energy is required:

i.          large amounts of ATP are cleaved to form ADP during the contraction process.

ii.          fenn effect: the greater amount of work produced by the muscle, the greater amount of ATP is cleaved.

c.         Each myosin head has two binding sites on it:

i.          one for an ATP molecule.

ii.          one that binds to actin.

d.         For contraction to occur, the myosin must bind to the actin filament at its active site.

e.         In a muscle at rest, these sites are physically covered by the tropomyosin-troponin complex which prevents contraction from taking place.

9.         The Contractile Cycle:

a.         Binding of ATP to myosin head:

i.          before contraction begins, the myosin heads of the cross-bridges bind with ATP.

ii.          the ATPase activity of the myosin head cleaves the ATP but leaves ADP and inorganic phosphate bound to the head.

iii.         in this state, the head extends perpendicularly toward the actin filament but is not yet attached to the actin.

b.         Binding of Ca²⁺ uncovers active site:     

i.          the sarcoplasmic reticulum contains high concentration of Ca²⁺  and are released into the sarcoplasm surrounding the myofibrils when an action potential reaches the adjacent T-tubule.

ii.          the binding of Ca²⁺ to troponin C causes a conformation change in the tropomyosin-troponin complex that moves the tropomyosin protein strand deeper into the groove between the two actin strands.

iii.         this uncovers the active sites of actin, enabling the myosin head to bind to it.

c.         Generating the Power Stroke:

i.          the bond between the head of the myosin and the active site of the actin filament causes a conformational change in the head, prompting the head to tilt toward the arm of the cross-bridge.

ii.          this rotation of the myosin head on its flexible neck creates the power stroke for pulling the actin filament toward the M line.

iii.         the power stroke begins when the inorganic phosphate is released from the myosin binding site following the binding of the head to actin.

iv.         the energy that activates the power stroke is the energy already stored in the head by the conformational change in the head when ATP is cleaved.

v.         the power stroke is also called cross-bridge tilting since the myosin head tilts from a 90 degree angle relative to the two filaments to a 45 degree angle.

d.         ADP is released from the active site on myosin, leaving it empty while the head remains tightly bind to the actin.

e.         Rigor state:

i.          in this state, no nucleotide occupies the binding site on the myosin head and this tight binding, known as the rigor state.

ii.          it is normally brief because the muscle fiber has a sufficient supply of ATP that bind to myosin when ADP is released.

iii.         however, after death, when metabolism stops and ATP supplies are exhausted, muscles are unable to bind more ATP, so they remain in a tightly bound state known as rigor mortis due to immovable cross bridges.

iv.         the tight binding of actin and myosin persists for a day or so after death, until enzymes within the decaying fiber begin to break down the muscle proteins.

f.          The binding of ATP to the nucleotide-binding site on the myosin head changes the affinity of the actin-binding site so that the myosin releases from actin.

g.         Following its detachment from the actin binding site, the myosin head lies at a right angle to it as in (a) and a new cycle begins again.

h.         This process repeats many times as a muscle fiber contracts, with the myosin heads repeatedly bind, swing, and release the actin molecules as they push the thin filaments toward the M line in the center of the sarcomere.

10.       Relaxation of Skeletal Muscle:

a.         Once the Ca²⁺ ions have been released from the sarcoplasmic reticulum, and have diffused to the myofibrils, muscle contraction will continue as long the Ca²⁺ ions remain in high concentration in the myofibrillar fluid.

b.         However, a continually active calcium pump located in the walls of the sarcoplasmic reticulum pumps Ca²⁺ ions away from the myofibrils back into the sarcoplasmic reticulum.

c.         In addition, a protein called calsequestrin in the reticulum bind 40 times as much calcium in the ionic state, increasing the storage of calcium.

d.         Without Ca²⁺ , the troponin-tropomyosin complex returns to its original conformation, covering the active sites on the actin filament.

e.         The myosin head does not bind to the actin filament and another cycle of contraction is inhibited.

f.          During this relaxation phase when actin and myosin are not bound to each other, the filaments of the sarcomere slide back to their original positions with the aid of elastic connective tissue within the muscle.

g.         Contraction will begin again with the arrival of another action potential to release the Ca²⁺ ions from the sarcoplasmic reticulum.

h.         Hence, for muscles to sustain their contraction or to contract frequently as in rapid movements, there must be a continuous series of repetitive action potentials.

11.       Excitation-Contraction Coupling:

a.         Signals for muscle contraction come from the central nervous system to skeletal muscles by way of somatic motor neurons.

b.         Acetylcholine from the somatic motor neuron initiates an action potential in the muscle fiber that in turn triggers contraction.

c.         This combination of electrical and mechanical events in a muscle fiber is called excitation-contraction coupling.

d.         Excitation and Contraction of Skeletal Muscle:

i.          action potential in somatic motor neuron arrives at axon terminal, opening voltage gated calcium channels.

ii.          the influx of Ca²⁺ channels triggers the exocytosis of acetylcholine-containing vesicles which releases the acetylcholine into the synaptic cleft.

iii.         the acetylcholine diffuses to the motor end plate of the muscle and bind to the nicotinic receptors present on the sarcolemma.

iv.         the binding of acetlycholine opens a non-specific cation channel, causing an influx of Na and efflux of K.

v.         the influx of Na exceeds that of K due to a greater electrochemical driving force, and this causes a net influx of positive charge which depolarizes the muscle membrane, creating an end-plate potential.

vi.         the end-plate potential is always above threshold and a muscle action potential results, which spreads from the neuromuscular junction along the sarcolemma and into the interior of the fiber via the T-tubules.

vii.        the action potential in the t-tubule activates dihydropyridine receptors which open Ca²⁺ channels in the membrane of the sarcoplasmic reticulum.

viii.       Ca²⁺ diffuses out of the sarcoplasmic reticulum and binds to troponin, pulling tropomyosin away from the myosin-binding site and allows myosin to release inorganic phosphate from ATP hydrolysis and complete its power stroke.

ix.         at the end of the power stroke, the myosin cross-bridges releases ADP and remains tightly bound to actin.

x.         the muscle fiber relaxes when Ca²⁺ releases form troponin and tropomyosin again blocks the myosin-binding site.

xi.         Ca²⁺ is transported back into the sarcoplasmic reticulum via a Ca²⁺ - ATPase.

e.         Temporal sequence of events during excitation-contraction coupling.

i.          the somatic motor neuron action potential is followed by the skeletal muscle action potential, which is in turn followed by contraction.

ii.          a single contraction-relaxation cycle is called a twitch.

iii.         there is a short delay, the latent period, between the muscle action potential and the beginning of muscle tension development.

iv.         this interval represents the time required for excitation-contraction coupling to take place.

v.         once contraction begins, muscle tension increases steadily to a maximum value.

vi.         tension then decreases in the relaxation phase of the twitch.

vii.        a single action potential in a muscle fiber evokes a single twitch.

12.       Source of Energy for Muscle Contraction:

a.         Energy supplied by ATP is used by the muscle to:

i.          actuate the walk-along mechanism by which the cross-bridges pull the actin filaments.

ii.          pump Ca²⁺ from the sarcoplasm into the sarcoplasmic reticulum.

iii.         pump Na⁺ and K⁺ ions through the muscle fiber to maintain the concentration gradient across the membrane after each excitation-contraction coupling.

b.         The amount of ATP (about 4mM)within the muscle fiber at any one time is enough to maintain full contraction for only 1 to 2 seconds (about 8 twitches).

c.         Phosphocreatine:

i.          serves as a backup energy store.

ii.          first store of energy that is used to form ATP;

iii.         a molecule whose high-energy phosphate bonds (has slightly greater free energy than ATP) are created from creatine and ATP when muscles are at rest.

iv.         when muscles become active, the high energy phosphate group of phosphocreatine is transferred to ADP, creating ATP.

v.         the enzyme responsible for transferring the phosphate group is creatine kinase (CK), also known as creatine phosphokinase (CPK)

vi.         muscle cells contain large amounts of this enzyme and hence elevated blood levels of creatine kinase usually indicate damage to skeletal or cardiac muscle.

vii.        however, the concentration of phosphocreatine in the blood is only about 5 times that of ATP and a combined  energy of both stored ATP and phosphocreatine is only capable of sustaining a contraction for 5 to 8 seconds.

d.         Carbohydrates:

i.          in the presence of adequate oxygen, pyruvate enters the krebs cycle, generating 30 molecules of ATP.

ii.          this accounts for more than 95 per cent of the energy used by muscle in sustaining contraction.

iii.         when oxygen concentration drops too low for aerobic metabolism to take place, the muscle fiber shifts to anaerobic glycolysis in which glucose is metabolized to lactic acid, producing only 2 molecules of ATP.

iv.         this enable the muscle contraction to be sustained for a short period of time when oxygen is not available.

v.         the rate of formation of ATP by glycolysis is two and a half times faster than that of oxidative metabolism and is a quicker source of ATP during heavy exercise.

vi.         due to the accumulation of end products in the muscle cells, glycolysis alone can only sustain maximum contraction for only a minute.

e.         Fatty acids:

i.          during rest and light exercise, skeletal muscles use fatty acids as an energy source together with glucose.

ii.          however, beta-oxidation, the process by which fatty acids are converted to acetyl CoA, is a slow process and cannot produce ATP rapidly enough to meet the energy needs of muscle fibers during heavy exercise.