{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

Muscle Coloring - 22 NERVE MUSCLE AND SYNAPSE Beating of...

Info iconThis preview shows pages 1–12. Sign up to view the full content.

View Full Document Right Arrow Icon
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 2
Background image of page 3

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 4
Background image of page 5

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 6
Background image of page 7

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 8
Background image of page 9

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 10
Background image of page 11

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Background image of page 12
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 22 NERVE, MUSCLE AND SYNAPSE Beating of the heart, blinking an eye, breathing fresh air — these obvious signs of life are all brought about by muscular contraction. How do muscles shorten? Something ”inside" must move, but what? Years ago, many physiologists believed that muscles contract because the proteins of which they are made actually shorten, either by folding or by changes in the pitch or diameter of helical molecules. In the 19503, they Were startled to discover that this is not the case at all. True, the contractile machinery is made of protein, but contraction does not occur by protein folding. Rather than changing their dimensions, the proteins simply slide past each other and change their relative positions. CONTRACTION: A BAND SHORTENS, I BAND DOES NOT An important clue came from early studies of the striped pattern of living skeletal muscle that could be seen under the light microscope. The stripes are localized in long fibrous cylinders called myofibrils that run the length of the muScle cell. The muscle cell contracts because the myofibrils contract; they contain the contractile machinery. Each myofibril is punctuated with alternating light and dark bands called A and I bands. These bands are "lined up” so that an A band on one myofibril is closest to an A band on its neighbor. When you look at the whole cell, you see stripe-s instead of a check— erboard. When a muscle contracts, the I band shortens, but the A band does not change size. The mystery of contraction seemed to reside in the 1 band. Soon after the electron micro- scope became available, however, a new picture emerged. Aan AND Myosm FlLAMENTS ARE THE CON‘I’RACI‘ILE MACHINE Examination with an electron microscope reveals that each myofibril contains many fibers, called filaments, which run parallel to the myofibri] axis. Thicker filaments are confined to the A band; the other, thinner, ones seem to arise in the middle of the Iband, at the Z line (a structure that runs per— pendicular to the myofibril through the I hand, connecting neighboring myofibrils). The thin filaments run the course of the I band and part way into the A band, where they CN: Use dark colors for G and H. 1. Begin at the top with skeletal muscle (A) and work your way down the right side of the page to its molecular components. Note that the end sur- face of each cylindrical example receives the color of its components. In the case of the myo- fibril (D), only the title and the myofibrils making up the end of the cell (C) receive the color D. The length of the myofibril receives the colors of the various bands of contractile elements. 2. Color the diagrams of the contractile elements mussels @‘ETGQWWM €33: @@[email protected]@ WMMWQ overlap (interdigitate) with the thick filaments. The next step is to identify the filaments and to determine their role in contraction. The chemical identity of the filaments can be determined by using concentrated salt solutions that selectively extract muscle proteins. When the protein called actin is extracted, the thin filaments disappear, and when the protein called myosin is extracted, the thick filaments disappear. Moreover, when the cell membrane is destroyed and substances other than these two proteins are leached out, the thick and thin fil— aments remain intact, and the muscle can still contract (if it is provided with ATP as an energy source). These results imply that the thick and thin filaments are the contractile machinery and that the thick filaments are made of myosin, the thin ones of actin. FILAMENTS SLIDE DURING Com-limos: A thick band-consists of a lighter middle region (the H zone) with denser regions on each side. The denser edges are where thick myosin and thin actin filaments overlap; the middle (H zone) contains only myosin. The 1 bands contain only actin. Whenever a muscle or myofibril changes length, either ‘by contracting or stretching, neither myosin nor actin filaments change length, yet they are the contractile machine! It follows that they must slide past each other, increasing their area of overlap during contraction and decreasing it during stretch- ing. During contraction, the I band decreases as more and more of the actin filaments are "buried” in the region of over- lap with myosin. The A band cannot change because it repre’ sents the length of the myosin filaments, which are invariant. However, if this picture is correct, you might expect the H zone to decrease upon contraction and lengthen on stretching. And it does! Because the motive force for contraction is provided by actin and myosin filaments sliding together, there must be some "connecting” elements that allow them to interact. These are the cross bridges, made of globular myosin heads (see figure) taken up in the next plate. A on the left side of the page. Note that the first dia- gram attempts to show how the two kinds of fila- ments actually make up the bands you previously colored. Note that the thin filaments (E) actually penetrate the A band. This wasn't shown in the drawing of the myofibril on right. Note, too, that the lower two diagrams represent a vertical enlarge— ment (in order to show cross-bridge activity) of the upper diagram, but not a horizontal enlargement. The Z lines (H) still coincide with the upper diagram. GOWTHFJCTI-L In.” 0 “@a mac WM??? 00 aura. 8 20% 31396030835“ Myofibrils are composed of repeating darkA and light 1 bands, which are responsible for the striations (stripes). Electron micr‘osccipy shows finer detail,- as illustrated in the lower two” diagrams, each fibril is com- pcised of thick and thin filaments. Thick filaments run the length of the A band; thin filaments run through the I hand and peripheral portions, but not the central H zone, of the A band. Thin filaments are anchored in the center of the I band by the Z Line. That portion of the myofibril (2.5a long} between the two Z lines is called a sarcomere. Thick and thin filaments interact through cross bridges which are bud-like ex- tensions of thick filaments. The cross bridges are given a separate color for identification purposes, When living muscle contracts, the l band shortens and the 1"] zone shortens, but the length of the A hand does not change. Thus, neither thick nor thin filaments change length; they simply slide past each other, increasing the area ”of overlap. meets exam 4' Whole muscles are macle of bundles of cylindrical striated cells called fibers. Cells (muscle fibers) range frOrn 5 to TOOL; in diameter but may be several thousand times longer, as they extend from one bone to another. WMWGJ Hundreds of banded cylindrical myofibrils run the length of each cell; they ar the contractile elements of the cell. W (macaw. Thick filaments are highly ordered assemblies of protein molecules called myosin. Thin filamentsare highly ordered assemblies of protein molecules called actin. new» tag“ 3 may ab? woman Actin molecules are pear—shaped (approx. 4am in diameter). In thin filaments they are foinecl together like two strings of beads inter twined at regular intervals. (Note: Thin filaments also contain other proteins in addition to actin). messages- Myusin molecules have long (lfiOnrn) rod-shaped tails with globular heads. The heads form cross bridges between thick and thin filaments. 23 N save, MUSCLE AND Synapse. In relaxed muscle, the myosin cross tiridges are detached from actin filaments. During contraction, they attach and provide the contractile force. How does this come about? Thick filo: ments are ordered assemblies of myosin molecules; each mole~ cule contains along rod-shaped tail, a shorter rodshaped neck, and two globular heads, which form the cross bridges. Only one head 15 shown in the drawings. The head attaches to the actin filament, forming a cross bridge betWeen actin and myosin filaments. The head then undergoes a conformational change (changes its shape) which propels the actin a distance of about 10 run. Following this movement, the head detaches and then repeats the cycle farther upstream. Each myosin fila— mentcontains about 300 heads, and each head can cycle about 5 times per second, moving the filaments at velocities up to 151.1111 per sec. This speed can move a muscle from its fully extended to fully contracted state well within 0.1 sec. The cycles of individual head attachments are not synchro— nized as show. They are out of phase, some attaching while others are detaching. Thus, at each moment, some of. the heads are entering the motive ”power-stroke” while others leave. The movement is not jerky, and there is no tendency for the filaments to slip backward. ATP SUPPLIES THE ENERGr FDR CONTRACTION Gross muscle mOVements are brought about by a cyclic reac— tion of the cross bridges: attachment (to actin) —> tilting (pro- ducing movement) —> release, attachment (to the next site) —> etc. By repeating the cycle many times the small movements add up to-the smooth, coordinated, macroscopic motions we all enjoy. But cyclic reactions cannot occur without an energy- source (if they could, We would be able to build perpetual motion machines). Further, muscle can do physical work (i.e., lift-a weight), and work requires energy. The immediate source of this energy is ATP. When we incorporate ATP in our scheme, the details of each cycle become more complex as we are able to distinguish more steps. These are shown 1n the set of diagrams' to the plate. Attachment of ATP to the myosin head groups allows the myosin heads to release the actin Further, a “lrigl1.—energy" phosphate is transferred from the ATP to the myosin, which becomes "energized," while the original ATP, having lost a phosphate, becomes ADP. The energized mass bridge is now ready for action. lithe muscle is stimu— lated, the cross bridge will attach to the actin, tilt, and move the actin along (the power stroke). Following the power stroke, the myosin and actin remain attached until the beginning of the next cycle, when ATP once again binds, releases the attachment, and energizes the myosin cross. bridge. Note that if ATP has been used up, the myosin heads will remain locked to the actin filaments, and no sliding can take place. The mus- cle will become rigid, resisting both contraction and stretch- ing. This is the condition known as rigor mortis, which is common after death when ATP has degenerated. Also note that ATP splitting is not directly involved in the power stroke. CN: Use same colors. for A, E, C and Dthat were used for those structures on the previous page. ‘1. Begin with upper diagrams that demonstrate how the @VCdédflfilD HECTORQ norm? masochism Its energy is used to ”prime” the myosin head so that it can attach to the actin and repeat the cycle. Ca++ Is REQUIRED FOR COmAcnON- .If ATP 15 present, why doesri’ tthe muscle continue to contract until all the ATP 15 used up? The answer involvgs an addi— tional Substance, Ca++ ,which is required for the attaclunent phase of the cycle. If sufficient CaH is present, attachment can Occur; at lower levels, it cannot. The action of Ca“+ as a trigger for contraction and its removal for relaxation are taken up in plate 24. Mroer Morons Muscle myosin is designated as myosin II; it is one member of a family of myosin molecular motors. Just as kinesinsand dyneins ”walk" on microtubules, myosins walk on actin fila— ments. During muscle contraction m-yosin 11 walks on actin, but the myosin filament does not move because the two ends of the filament are walking in opposite directions. Instead the actin filaments move. (Think about walking' in a boat while you hold on to the dock; you don t move, the boat does!) The secret of contraction appears to lie within the myosin heads. (Isolated myosin heads whose tails have been digested away are capable of walking on actin with unimpaired veloci- ties ) Detailed studios of the molecular structure of myosin shows a prominent cleft m the head region that has been iden— tified with the ATP binding si..te Another cleft 3.5 nm (a large molecular distance) away is thought to be the actin binding site. These clefts may provide the malleable spaces needed to initiate conformational changes involvod in binding and movement. Actin and myosin II as Well as the other two prominent myosins, 'rnyosin T and myosin V, are found in most cells One impinrtant difference between the myosins is that myosin l and V have shorter tails that contain binding sites for mem- branes. In many cells, these motors often transport vesicles on actin filaments (plate 21). On the other hand. the longer myo- sin tails are especially suited for interacting with other tails to form the filaments Wesee in muscle. Myosin II is prominent in driving cytokinesis, the final stage of cell division (plate 3), in addition to muscle contraction. ACTIN FILAMENTS Actin filaments are found in all cells. They are linear poly- mers — i.e., they are formed by linking many identical smaller units (G-actin) in a repetitive-fashion. Like microtubules they grow primarily at one end called the (+) end, and myosin only will walk tovirard that end (which is located at the 2 line in muscle). By interacting with other proteins a‘ctin filaments become instrumental in a number of diverse cell functions, such as the formation of the cell cortex, a meshwork of actin filaments just under the _-plasma membrane that gives the cell shape and mechanical strength. Rearrangements of actin within the cortex are responsible for the ability of some cells— like white blood cells—to crawl (Also see plate 2, microvilli. ) contraction of myosin draws actin filaments inward. 2. Completely color each step of the contraction cycle before going on to the next. meat-am waver? (3012,5631 MCé’CiD‘EJA MWOQUQJ (arenas Miami mawr (mm hemmed?» e 910110151 nee.- m; muecaeflei 1—2 Relaxed: Following a contractile maverhent, the myo'sin binds an ATP degrees of freedom to wiggle about. The bound ATP' 15 short lived because myosirl, Itself is an ATI‘ase (ATP splitt'mgenzyme) Myosin splits the ATP and (2) the products ADP and Pi remain bound to the: myosin. There isstill freedom to wiggle 3—4 Attachment: Myos'm makes mutant-with the actin. At first it is weak (3).- but as the ”attachment gets stronger (flthe mobility of myosin diminishes. 4—5 Force Generation: With the mleaeenf Pi' thereffinity of actin _ . “Natl, me ' it: flmzmms ti?— ' ln’r'n 5' LIKI!.\\ 1" @[email protected]@B allowing it ta enter a relaxed state where it .15 detached from act-in and has some 11-1 the relaxed muscle the cross bridges are detached from acfin filaments. During contraction they attach and provide contractile force. Thick filaments are madeiof myesin molecules; each molecule consists of; king rod shaped tail. a Shbrter rod shaped meek and and 2 globular heads which form the cross bridge (only me is shown). During connection the heads attach tn-actin. tilt. m1éasE‘afid then attach lathe-next 1111111111111 as theugh thiey were walking cm the filament. But, the acti'z'i filam'e11t5-aiie-ian'ehored with their (-1-) ends in the Z line and thy-min heads can only "walk" tOWai-d the (+) end.‘1he myosifi head's ciri the right "walk" toward the" Z line 1111 the right, while heads on the left walk tgw-ard the left 2 line. As a result the thick myc'ssin filaments 11111.net moue, but the actin filmen‘t‘s are pulled 111. arid myo'fim increases. The binding becames stronger and the mye'sin bec‘ome's. more rigid, as stress is applied to the neck eegihn.1hepawer stmke isintiated. 5-6 Sliding Filament: The myqs‘m head tilts propelling the assq'ciated 11121111 forward 6-? Rigor. Fblluwing the sliding motion, ADP 15 released and the myosin is stuck to the actin— la—ut 11111;; momentarily. 7— 1 Release: ATP binds to: the myosin head releasing it frqm adin'andmaking the muscle pliable. if no ATP is available. myosin heads remain stuck to'actjrl and the muscle becomes stiff. This is the rigidity 11f ”rigor mortis which fellows-death. 24 NERVE, MUSCLE AND Smarse If, in the presence of ATP, the cross bridges can enter repeated cycles of attachment, propulsion (tilting), and release, how does this process stop? How do muscles relax? Two key dis- coveries provided important clues. One was the realization that the presence of minute quantities office (311++ ions was esaential for contraction. This fact had escaped detection for many years because it was virtually impossible to remove small traces of Ca++ from laboratory chemicals or even from distilled water. Apparently, these traces were sufficient for the contractile process. After learning to control traces of Ca”, We now know that raising the cytoplasmic Ca++ (inside the muscle cell) to concentrations as low as .0001 mM is sufficient to support contraction. (This is twenty thousand times more dilute than the free Ca“ level in the plasma!) When Ca” is at this level or above, contraction ensues. When Ca'H is some— what belo'w this level, contraction cannot take place and the muscle relaxes. TROPOMYOSIN COVERS MYOSIN BINDING SITES How does CaH exert its influence? An important clue was the discovery that the thin filaments contain other proteins. besides actin. In particular, they contain tropomyosin and troponin. These proteins can be removed from the actin in highly purified artificial systems. When this is done, the requirement for Ca” disappears! The system contracts in the presence of ATP and in the absence of Ca”. In order for muscle to contract, the energized cross bridges must first attach to the actin filaments. During relaxation, this does not occur because the sites for myosln' attachment on the actin filaments are covered by tropomyosin molecules,- in this state, the sites are masked and not available for the cross bridges. Another protein, troponin, is bound to and serves as a ”handle" on the tropom osin. Troponin can bind Ca++ and change shape. When car is bound, the troponin moves the tropomyosin out of the way. The sites are now exposed, attachment of cross bridges can occur, and contraction ensues. When Ca“ is absent, the tropomyosin reverts back to its orig- inal position and blocks attachment; relaxation follDWS. But what controls Ca“? How does its concentration rise to trigger contraction and fall to allow relaxation? SR Stones CA“, RELEASES IT FOR CONTRACIION Although the free Ca++ concentration in relaxed musele is extremely low in the cytoplasm, other vesicular structures within the cell may contain an abundance. This is particularly true of the sarcoplasmic reticulum (SR), a compartment contain- ing Ca++ ions that are separated from the cytoplasm by the CN: Use same colors as on previous page for actin (C) and myosin (D). ‘1. Begin with the muscle cell. in the Upper right corner. Note that the titles for the axon (G) and axon terminal (H) are listed below. The myofibrils within the cell are left uncolored here and below. 2. Complete the enlarged strand of act'ln filament and the stages of cross-bridge activation. " next. It is the movement of Ca++ from the SR interior to the At this point, tropomyosin returns to mask the actin binding esteem releases Warren! membranes forming the compartment Walls. Each myofibril is surrounded by a sheath of sarcoplasmic reticulum, which resembles a net stocking stretching from one 2 line to the cytoplasm and back that controls contraction ahd relaxation. T Townes CARRY ACTION POTENTIALS To INTERIOR When nerve impulses activate muscles, the excitation is trans- mitted through the motor en-dpla‘te, and a muscle action potential quickly spreads over the surface of the muscle cell. Contrac- tion of all myofibrils, including those in the cell interior, fol« lows within milliseconds. This all-or-ncine response is possible because a system of tiny tubes, the T tubules (trans— verse tubules), extends from the surface membrane deep into the interior of the muscle and encircles the perimeter of each myofibril at the level of the Z line in some muscles (frog skele- tal muscle, mammalian heart) o...
View Full Document

{[ snackBarMessage ]}