Muscle contraction

CONTRACTION OF MUSCLE

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MUSCLE TISSUE

Three types of Muscle tissues in our body: Voluntary: Skeletal muscles Involuntary: Smooth muscles Cardiac muscles

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VOLUNTARY: SKELETAL MUSCLES

Under voluntary control Supplied by motor neurons Functions: Locomotion

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INVOLUNTARY: SMOOTH MUSCLES

Under involuntary control Spontaneous contraction Under autonomic control Are found in the wall of blood vessels Structurally and functionally different from

voluntary muscles.

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CARDIAC MUSCLES

Forms the heart muscles ( Myocardium) Are involuntary muscles Are regulated by Autonomic nervous system Structurally similar to Voluntary muscles

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SKELETAL MUSCLE6

SKELETAL MUSCLE

General Features Endomysium-

Connective tissue (CT) that surrounds individual muscle fibers

Perimysium- CT that surrounds groups (fascicles) of muscle fibers

Epimysium- CT that surrounds the entire muscle

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A GROSS VIEW OF SKELETAL MUSCLE AND THE CONNECTIVE TISSUE (CT) INVESTMENTS

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BAND STRUCTURE OF STRIATED MUSCLE

A band: I band: Z line:Lies in the I

band H band: M line:

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SKELETAL MUSCLEFibers Skeletal muscle fibers consist of long

cylindrical fibers with multiple ovoid nuclei located peripherally beneath the sarcolemma (plasma membrane) and with striations composed of alternating dark and light bands.A bands: The dark bands H band : In the center of the A band a paler

region, seen in relaxed muscle.I bands (isotropic): The light bands Z line: a dark transverse line, bisects each I

band.M line

These bands and the Z lines are well demonstrated in electron micrographs of skeletal muscle.

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SKELETAL MUSCLEELECTRON MMICROSCOPY

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SKELETAL MUSCLE

Myofibrils Skeletal muscle fibers contain 1- to 2-

mm myofibrils that lie in the sarcoplasm (cytoplasm) parallel to the long axis of the muscle fiber.

Myofibrils are composed of a series of sarcomeres

Sarcomeres consist of inter-digitating Thin filaments ( Actin) Thick filaments ( Myosin)

The sarcomeres are the basic units of contraction of striated muscle 15

THICK AND THIN FILAMENTS

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SARCOMERE STRUCTURE

The banding pattern seen in striated muscle is caused by the arrangement of thin and thick myofilaments.

Thick filaments occupy the central portions of the sarcomere.

Thin filaments attach at one end to the Z line and run parallel to, and between, the thick filaments.I bands are composed of thin filaments only.A bands are composed mostly of thick

filaments and the thin filaments between them.

H bands are composed of thick filaments only.

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SARCOMERE STRUCTUREBAND PATTERN

I bands are composed of thin filaments only.

A bands are composed mostly of thick filaments and the thin filaments between them.

H bands are composed of thick filaments only

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SARCOMERE STRUCTURE: THIN FILAMENTS

Thin filaments are composed of the proteins1. Actin, 2. Tropomyosin, and3. Troponin.

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SARCOMERE STRUCTURE: THIN FILAMENTS

1. Actin:Long fibrous

structure (F-actin) composed of two strands of spherical or globular G-actin monomers twisted in a double helix:

The filament is polar and contains myosin-binding sites on the G-actin monomers.

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THIN FILAMENTS

2. Tropomyosin is a polar molecule containing two polypeptide chains in the form of an a-helix. The tropomyosin molecules lie head-to-tail to form filaments that lie in the grooves of the actin helix:

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THIN FILAMENTS

3. Troponin (Tn) is composed of three polypeptides: TnT binds to tropomyosin at intervals along the thin filament, TnC binds calcium ions, and Tnl inhibits actin-myosin interaction

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THICK FILAMENTS

Thick filaments are composed of myosin.

Myosin is a molecule that contains a tail and two heads. . The tail fiber is formed from portions of two heavy chains, which are wound in a coil.

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THICK FILAMENTS

The heads are globular regions formed by the association of part of one heavy chain with two light chains.

Myosin heads function as active sites for ATPase activity and as actin-binding sites 30

THICK FILAMENTS: PARTS

tail of the myosin molecule

myosin head an arm that extends

the head outward from the body,

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The protruding arms and heads together are called cross-bridges.

Each cross-bridge is flexible at two points called hinges—one where the arm leaves the body of the myosin filament, and the other where the head attaches to the arm

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.

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THICK FILAMENTS: PARTS ATPase Activity of the Myosin Head.

the myosin head that is essential for muscle contraction

is that it functions as an ATPase enzyme. As explained later, this property allows the head

to cleave ATP and to use the energy derived from the ATP’s high-energy phosphate bond to energize the contraction process.

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SARCOMERE STRUCTURE

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SARCOMERE STRUCTURE TRANSVERSE TUBULAR SYSTEM

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Skeletal muscle fibers contain fingerlike invaginations of the sarcolemma that surround each myofibril.

These invaginations constitute the transverse (T) tubule system .

Note the following: Each T tubule lies between

the two cisternae of the sarcoplasmic reticulum (SR) to form a triad 41

SARCOMERE STRUCTURE TRANSVERSE TUBULAR SYSTEM There are two triads in each sarcomere,

which are present at the junction between the A and I bands.

These units serve to couple excitation of muscle cells to their contraction (excitation-contraction coupling).

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SARCOMERE STRUCTURE TRANSVERSE TUBULAR SYSTEM Function of ER:

Acts as a store of calcium and release Ca++ after action potential reaches the adjacent T tubule.

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CARDIAC MUSCLE

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CARDIAC MUSCLE

Cardiac muscle has an arrangement of sarcomeres similar to that in skeletal muscle as well as a T tubule system associated with the SR (near the Z line).

However, unlike skeletal muscle fibers, the fibers are electrically coupled through gap junctions.

Cardiac muscle fibers are joined together by junctional complexes called intercalated discs..

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CARDIAC MUSCLE

Striated pattern Fibres divide and join at intercalated discs There are many gap junctions between fibres

Provides easy spread of impulses Large T tubules are present SR is present, but less than in skeletal

muscle

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CARDIAC MUSCLE ACTION POTENTIAL

Action potentials are prolonged in the heart, 0.2 to 0.3 seconds

i.e. there is a ‘plateau’ in the AP This is due to an additional slow Ca++ channel Much of the Ca++ entry comes from the T

tubules

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Top: Phases of the action potential of a cardiac muscle fiber. 0, depolarization; 1,initial rapid repolarization; 2, plateau phase; 3, late rapid repolarization; 4, baseline. Bottom:Diagrammatic summary of Na+, Ca2+, and cumulative K+ currents during the action potential.Inward current down, outward current up.

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•Transient outward (ph 1; early incomplete repolarization) – ITO•Inward rectifying ( ph 2; plateau influx of K)- IKr•Delayed rectifying(ph 3; repolarization, slowly outflux of K)- IKsIKr + IKs = repolarization

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SMOOTH MUSCLE

Smooth muscle is found in the walls of blood vessels and hollow viscera.

Bands of smooth muscle cells can be found in the erector pili muscles of the skin

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SMOOTHMUSCLE

Gap Junctions Gap junctions electrically couple smooth

muscle cells.

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SMOOTHMUSCLE

Filaments Smooth muscles contain actin and myosin

filaments, but the filaments are not arranged in orderly arrays as in skeletal muscle.

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SMOOTH MUSCLE

Contraction Smooth muscle contraction may be triggered by

various stimuli such as autonomic nerves or hormones.

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SMOOTH MUSCLE. DIFFERENCES

Does not show striations Does contain actin and myosin, but in a

different arrangement Does not have troponin

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SMOOTH MUSCLE. DIFFERENCES 2

No sarcoplasmic reticulum (or very little) Ca++ enters from ECF, not from SR

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CONTROL OF SMOOTH MUSCLE CONTRACTION Spontaneous contraction occurs Contraction may be due to release of local

chemicals Stretching leads to contraction Autonomic system modifies contraction Parasympathetic and sympathetic

stimulation generally have opposite effects

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Muscle contraction

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