Chapter 9: Muscles part 2

Tropomyosin

(help to stiffen & stabilize; block myosin heads from binding during rest) • Troponin (binds to tropomyosin, actin, & calcium ions

Skeletal muscle fibers contain two sets of intracellular tubules that help regulate muscle contraction:

1) the sarcoplasmic reticulum (2) T tubules.

Steps of Muscle Contraction at the Microscopic level

1. I bands shorten 2. Distance between successive Z disc shorts 3. Actin slides centrally 4. Z discs are pulled toward the M line 5. H zone disappears 6. A bands move closer together, but length remains the same

In a relaxed muscle fiber, the thin and thick filaments overlap only at the ends of the

A band

Titin

A series elastic component protein responsible for allowing the sarcomere to stretch and recoil -extends from the Z disc to the thick filament, and then runs within the thick filament (forming its core) to attach to the M line. -It holds the thick filaments in place, maintaining the organization of the A band, and helps the muscle cell spring back into shape after stretching. ---- Titin does not resist stretching in the ordinary range of extension, but it stiffens as it uncoils helping the muscle resist excessive stretching, which might pull the sarcomeres apart.

Muscle contraction depends on the myosin- and actin-containing myofilaments.

As noted earlier, thick filaments are composed primarily of the protein myosin. Each myosin molecule consists of six polypeptide chains: two heavy (high-molecular-weight) chains and four light chains. The heavy chains twist together to form myosin's rodlike tail, and each heavy chain ends in a globular head that is attached to the tail via a flexible hinge

The roles of the T tubules and SR in providing signals for contraction are tightly linked.

At the triads, membrane-spanning proteins from the T tubules and SR link together across the gap between the two membranes. The protruding integral proteins of the T tubule act as voltage sensors. The integral proteins of the SR form gated channels through which the terminal cisterns release Ca+2

Dark bands= A bands•

Contain a lighter region known as an H zone• H zone is bisected form a dark line known as a M line. The dark areas are called A bands, which is fairly easy to remember because "a" is the second letter in "dark.

Sarcomere

Contractile unit of muscle

Both troponin and tropomyosin help control the myosin-actin interactions involved in _____

Contraction

Light bands= I bands•

Midline interruption or a darker area knownas a Z disc

Molecular Composition of Myofilaments

Muscular contraction is dependent on myosin & actin myofilaments

sliding filament model

Nervous system stimulates muscle fibers Myosin heads attach to myosin binding site on actin to initiate the sliding component Cross bridge attachments continuously form & break during contraction(generate tension & propel forward) Process occurs simultaneously in sarcomeres causing the muscle cells to shorten

Tropomyosin

Polypeptide strands of tropomyosin a rod-shaped protein, spiral about the actin core and help stiffen and stabilize it. Successive tropomyosin molecules are arranged end to end along the actin filaments, and in a relaxed muscle fiber, they block myosin-binding sites on actin so that myosin heads on the thick filaments cannot bind to the thin filaments.

Thin filaments also contain several regulatory proteins.

Polypeptide strands of tropomyosin a rod-shaped protein, spiral about the actin core and help stiffen and stabilize it. Troponin, the other major protein in thin filaments, is a globular protein with three polypeptide subunits.

The lumen (cavity) of the T tubule is continuous with the extracellular space. As a result:

T tubules tremendously increase the muscle fiber's surface area. This allows changes in the membrane potential to rapidly penetrate deep into the muscle fiber.

The term muscular dystrophy refers to a group of inherited muscle-destroying diseases that generally appear during childhood.

The affected muscles initially enlarge due to deposits of fat and connective tissue, but the muscle fibers atrophy and degenerate.

Myosin

The central thick filaments containing myosin (red) extend the entire length of the A band They are connected in the middle of the sarcomere at the M line.

Troponin

Troponin, the other major protein in thin filaments, is a globular protein with three polypeptide subunits . -One subunit attaches troponin to actin. -Another subunit binds tropomyosin and helps position it on actin. The third subunit binds calcium ions.

The more lateral thin filaments containing

actin (blue) extend across the I band and partway into the A band. The Z disc, a protein sheet, anchors the thin filaments.

Myosin has binding sites for _______.

actin and ATP/ATPase

Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing

actin and myosin to overlap more • Actin and myosin themselves do not change length

exitation-contraction coupling

connection between muscle fiber stimulation and muscle contraction during muscle relaxation: - Ca+2 ions are stored in SR upon muscle stimulation: - muscle impulses cause SR to release Ca+2 ions into cytosol

Actin The more lateral thin filaments

containing actin (blue) extend across the I band and partway into the A band. The Z disc, a protein sheet, anchors the thin filaments. Actin• Myosin heads attach to • Contain regulatory proteins • Tropomyosin (help to stiffen & stabilize; block myosin heads from binding during rest) • Troponin (binds to tropomyosin, actin, & calcium ions

terminal cisternae

enlarged areas of the sarcoplasmic reticulum surrounding the transverse tubules. form larger, perpendicular cross channels at the A band-I band junctions, and they always occur in pairs.

The thin filaments are composed chiefly of the protein actin . Actin has kidney-shaped polypeptide subunits, called

globular actin or G actin. Each G actin has a myosin-binding site (or active site) to which the myosin heads attach during contraction. G actin subunits polymerize into long actin filaments called filamentous, or F, actin. Two intertwined actin filaments, resembling a twisted double strand of pearls, form the backbone of each thin filament.

sarcoplasmic reticulum (SR) is an elaborate smooth endoplasmic reticulum. The SR regulates

intracellular levels of ionic calcium. It stores calcium and releases it on demand when the muscle fiber is stimulated to contract. As you will see, calcium provides the final "go" signal for contraction.

As you will recall, the proteins actin and myosin play a role in

motility and shape change in virtually every cell in the body. This property reaches its highest development in the contractile muscle fibers.

Sarcoplasm, the cytoplasm of a

muscle cell is similar to the cytoplasm of other cells, but it contains unusually large amounts of glycosomes (granules of stored glycogen that provide glucose during muscle cell activity for ATP production) and myoglobin, a red pigment that stores oxygen. Myoglobin is similar to hemoglobin, the pigment that transports oxygen in blood.

Striations, a repeating series of dark and light bands, are evident along the length of each

myofibril. In an intact muscle fiber, the dark A bands and light I bands are nearly perfectly aligned, giving the cell its striated appearance.

In addition to the usual organelles, a muscle cell contains three specialized structures:

myofibrils, sarcoplasmic reticulum, and T tubules.

Myofibrils are made up of a chain of sarcomeres linked end to end. Sarcomeres contain even smaller rodlike structures called

myofilaments.

The central thick filaments containing

myosin (red) extend the entire length of the A band . -- They are connected in the middle of the sarcomere at the M line.

Other proteins that bind filaments or sarcomeres together and maintain their alignment include

nebulin, myomesin, and C proteins. intermediate (desmin) filaments extend from the Z disc and connect each myofibril to the next throughout the width of the muscle cell.

Muscle contraction is ultimately controlled by

nerve-initiated electrical impulses that travel along the sarcolemma. Because T tubules are continuations of the sarcolemma, they conduct impulses to the deepest regions of the muscle cell and every sarcomere. These impulses trigger the release of calcium from the adjacent terminal cisternae. Think of the T tubules as a rapid communication or messaging system that ensures that every myofibril in the muscle fiber contracts at virtually the same time.

Myofilaments we see that it arises from orderly arrangement of even smaller structures within the sarcomeres. These smaller structures, the myofilaments

or filaments, are the muscle equivalents of the actin-containing microfilaments and myosin motor proteins As you will recall, the proteins actin and myosin play a role in motility and shape change in virtually every cell in the body. This property reaches its highest development in the contractile muscle fibers. There are two types of contractile myofilaments in a sarcomere:

Along its length, each T tubule runs between the

paired terminal cisterns of the SR, forming triads successive groupings of the three membranous structures (terminal cistern, T tubule, and terminal cistern). ---As they pass from one myofibril to the next, the T tubules also encircle each sarcomere.

Think of the T tubules as a

rapid communication or messaging system that ensures that every myofibril in the muscle fiber contracts at virtually the same time.

Myoglobin:

red pigment that stores oxygen

A single muscle fiber contains hundreds to thousands of

rodlike myofibrils that run parallel to its length. -The myofibrils, each 1-2 μm in diameter, are so densely packed in the fiber that mitochondria and other organelles appear to be squeezed between them. They account for about 80% of cellular volume.

The region of a myofibril between two successive Z discs is a

sarcomere (muscle segment))))) a sarcomere is the smallest contractile unit of a muscle fiber—the functional unit of skeletal muscle. It contains an A band flanked by half an I band at each end. Within each myofibril, the sarcomeres align end to end like boxcars in a train.

if we examine the banding pattern of a myofibril at the molecular level, we see that it arises from orderly arrangement of even smaller structures within the

sarcomeres. These smaller structures, the myofilaments or filaments, are the muscle equivalents of the actin-containing microfilaments and myosin motor proteins

Molecular Composition of Myofilaments Each myosin molecule consists of

six polypeptide chains: two heavy (high-molecular-weight) chains and four light chains. --The heavy chains twist together to form myosin's rodlike tail, and each heavy chain ends in a globular head that is attached to the tail via a flexible hinge

Interconnecting tubules of SR surround each myofibril the way the

sleeve of a loosely knitted sweater surrounds your arm. Most SR tubules run longitudinally along the myofibril, communicating with each other at the H zone. Closely associated with the SR are large numbers of mitochondria and glycogen granules, both involved in producing the energy used during contraction.

Glycosomes:

stored glycogen that produces glucose during muscle activity for ATP production•

At each A band-I band junction, the sarcolemma of the muscle cell protrudes deep into the cell interior, forming an elongated tube called

the T tubule (T for "transverse")

Another important structural protein is dystrophin

which links the thin filaments to the integral proteins of the sarcolemma (which in turn are anchored to the extracellular matrix).

Each thick filament contains about 300 myosin molecules bundled together,

with their tails forming the central part of the thick filament and their heads facing outward at the end of each molecule As a result, the central portion of a thick filament (in the H zone) is smooth, but its ends are studded with a staggered array of myosin heads.

Contraction: the activation of myosin cross bridges to generate force

• Contraction ends when cross bridges become inactive • Tension declines, muscle fiber relaxes

3 specialized structures within the muscle cell:

• Myofibrils • Sarcoplasmic Reticulum • T tubules

There are two types of contractile myofilaments in a sarcomere:

• Myosin= thick filament • Actin= Thin filament,

Muscle myofibrils Contain contractile elements:

• Sarcomeres• Myofilaments

skeletal muscle facts

• Skeletal muscle fibers are HUGE CELLS! • Each skeletal muscle fiber is a long cylindricalcell • Multiple nuclei below its sarcolemma or plasmamembrane • Hundreds of embryonic cells fuse together toproduce each fiber

Functions in regulation of intracellular Ca2+ levels in Sarcoplasmic reticulum

• Stores and releases Ca2+ when the muscle fiber is stimulated to contract • Ca2+ allows for the final signal for contraction

Myosin has two

• Two heads • Form cross bridges by linking thick & thin filaments together during contraction • Uses ATP to drive movement/contraction

The globular heads, each associated with two light chains, are the "business end" of myosin.

During contraction, they link the thick and thin filaments together, forming cross bridges, and swivel around their point of attachment, acting as motors to generate force. Myosin itself splits ATP (acts as an ATPase) and uses the released energy to drive movement.