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University of Florida
PCB 4723C

Topic 16 Tuesday, November 12, 2013 1:53 PM • Skeletal Muscle • The smallest contractile unit of skeletal muscle is a multinucleated, elongated cell ○ Called a muscle fiber, muscle cell, or myofiber ○ Each single muscle fiber is surrounded by endomysium • A bundle of linearly aligned muscle fibers forms a fascicle ○ Each fascicle is envelopedby a sheath called the perimysium • Bundles of fascicles form a muscle ○ • The whole muscle is contained within an external sheath called the epimysium ○ • Beneath the endomysium surrounding each muscle fiber is the plasma membrane called the sarcolemma ○ • Each individual skeletal muscle contains a densely arranged parallel array of cylindrical elements called myofibrils ○ Each myofibril is essentially and end-to-end chain of regular repeating units, sarcomeres, that consist of smaller filaments called myofilaments,which contain both think and thin filaments ○ ○ • Skeletal muscle also contains sarcoplasmic reticulum, a form of modified ER that wraps around each myofibril ○ Consists of longitudinal tubules that surround all surfaces of the actual contracting myofibrils, which release Ca2+ ions, and terminal cisternae, enlarged regions at the ends of the tubules that concentrateand sequester Ca2+ • Muscle Action Potential ○ Skeletal muscle fiber is so large that Aps spreading along its surface membrane cause almost no current flow deep within the fiber no current flow deep within the fiber ○ In order to cause maximum muscle contraction, current needs to penetrate deep ○ Achieved by transmission of AP along transverse tubules, T-tubules, that penetrate all the way through the muscle fiber from one side to the other ○ ○ T-tubules associate with two cisternae, which are specialis regions of the sarcoplasmic reticulum  • Depolarizationof the T-tubule membrane results in Ca2+ release from the SR at the triad ○ Propagation of the AP into the t-tubules depolarizes the triad region, activating L-type Ca2+ channels ○ These voltagegated channels cluster in groups of four called tetrads ○ Once the AP hits these L-type channels, a conformationalchange allows Ca2+ to enter through the Ca2+ pores AND induces a conformationalchange in each of the four subunits of another channel, the Ca2+ release channel, located in the SR membrane  ○ Ca2+ stored in the SR rapidly leaves through the Ca2+ release channel   The resultant rapid increase in Ca2+ concentrationactivates troponin C (discussed later) ○ • Sliding filament mechanismof muscle contraction ○ One muscle fiber (cell)contains thousands or more myofibrilsthat occupy most of the intracellular volume ○ Each myofibril is composedof several types of proteins;  The contractile proteins myosin and actin, the regulatory proteins tropomyosin and troponin, and the giant accessory proteins titin and nebulin ○ Myosin is a motorprotein with the ability to create movement  Each myosin molecule is composedof protein chains that intertwine to form a long tail and a pair of tadpole heads  □ The tadpole heads contain ATPase enzyme, which can cleave ATP and use the energy  The rodlike tail is tuff, but the protruding myosinheads have an elastic hinge region where the heads join the rods ○ In skeletal muscle, about 250 myosin molecules join to make a thick filament  Each thick filament is arranged so that the myosin heads are clustered at each end of the filament, and the central region is just a bundle of myosin tails ○ Actin is a protein that makes up the thin filaments of the muscle fiber  One actin molecules is a globular protein, g-actin  Usually g-actin molecules polymerizeto form long chains or filaments, called f-actin, which consists of 13 individual actin monomersand is ~70nm long  F-actin polymers twist together like a double strand of beads, creating the thin filaments of myofibril   Individual tropomyosinmolecules consist of two identical alpha helices that coil around each other and sit near the two grooves that are formed by the two helical action strands □ In the resting state, tropomyosinmoleculeslie on top of the active sites of the active strand  Attached intermittentlyalong the sides of the tropomyosinmoleculesare protein moleculescalled troponin □ These are actually complexof three looselybound protein subunits, each of which plays a specific role in muscle contraction □ Subunit troponin I has a strong affinity for actin, troponin T for tropomyosin, troponin C for calcium ○ Most of the time, the parallel thick and thin filaments of the myofibril are connected by myosin cross bridges (the heads) that span the space between the filaments   Each G-actin molecule has a single myosin-binding site, and each myosinhead has one actin-binding site and one binding site for ATP actin-binding site and one binding site for ATP  Cross bridges have two states: low-force (relaxed muscle) and high-force (contracting muscle) ○ Isoforms of myosin determine functional properties such as rate of ATP hydrolysis ○ Myosin is not specific to muscle and controls intracellular movements in plant, protozoa, and fungi • Muscle contraction ○ Ca2+ exerts its effects by binding to regulatory proteins rather than directly interacting with contractile proteins ○ In the absence of Ca2+, these regulatory proteins act in concert to inhibit action-myosin interactions, thus inhibiting contractile process ○ When Ca2+ binds to to one or more of these proteins, a conformationalchange occurs, allowing contraction ○ Troponin C has two pairs of Ca2+ binding sites, two high affinity sites that are always occupied by Ca2+ or Mg2+, and two low affinity states that bind and release Ca2+ as Ca2+ concentrationrises and falls  ○ Binding of Ca2+ to these low affinity sites induces a conformationalchange in the troponin complex,causing Troponin I to move away from the action/tropomyosinfilaments, thereby permitting the tropomyosinmolecule to move  ○ During this cross bridge cycling, contractile proteins convertthe energy of ATP hydrolysis into mechanical energy  Initially, the myosin head is attached to an action filament after the power stroke from the previous cycle □  ATP binding to the head of the myosin heavy chain reduces the affinity of myosin for actin, causing the myosin head to release from the actin filament. If all cross-bridges in a muscle were in this state, the muscle would be fully relaxed □  The breakdown of ATP to ADP and inorganic phosphate occurs in the myosin head. As a result of hydrolysis, the myosin head pivots around the hinge into a cocked position, causing the tip of the myosin to move about 11 nm along the actin filament so that it now lines up with a new actin monomertwo monomersfarther along the actin filament] □  The cocked myosin head now binds to its new position on the actin filament □  Dissociationof inorganic phosphate form the myosanhead triggers the power stroke, causing the actin filament to be drawn along the myosin filament, generating force and motion □  Dissociationof ATP from myosin completesthe cycle □  If unrestrained, the cross-bridge cycling would continue until the cytoplasmis depleted of ATP  Contraction is controlled at the third step by preventing cross-bridge formation until the tropomyosinmoves out of the way in response to an increase in Ca2+ concentration  ○ Once ATP binds, the head releases. ATPase splits the ATP, cocking the head, which is then ready to bind ○ Once Ca2+ enters, P leaves, finishing the stroke  • Under a light microscope,the arrangement of thick and thin filaments in a myofibril creates a repeating pattern of alternating light and dark bands • One repeat of the pattern is called a sarcomere ○ ○ Z disks are zigzag protein structures that serve as the attachmentsite for thin filaments ○ I bands are the lightest color bands of the sarcomereand represent a region occupied by only thin filaments  A Z disk runs through the middle of every I band, so each half of an I band belongs to a different sarcomere ○ A bands are the darkest of the sarcomeresbands and encompassthe entire length of thick filament  At the outer edge of the A band, the thick and thin filaments overlap  The center of the A band is occupied by thick filaments only ○ H zone is the central region of the A band that is lighter than the outer edges of the A band because the H zone is occupied by thick filaments only ○ M line is the protein line for thick filament attachment ○ ○ The proper alignment of filaments within a sarcomereis ensure by titin and nebulin  Titin stabilizes the position of the contractile filaments and its elasticity returns stretched muscles to their resting length  Titin is helped by nebulin and helps align the actin filaments  • In a muscle fiber, the tension developed during a twitch is a direct reflection of the length of individual sarcomeresbefore contraction begins ○ The sliding filament theory predicts that the tension a muscle fiber can generate is directly proportional to the number of cross bridges formed between the thick and thin filaments ○ In order to increase the force of contractionthen, then the muscle must receive a high frequency of APs • Long and smoothcontractions can occur via temporal summationand tetanus, which can increase tension in a single fiber by as much as 3-4 fold (it is possible to initiate a second AP before a first contraction has fully subsided, thus stimulating a twitch that is superimposedon the residual tension of the first twitch) ○ A single twitch does not represent the max force a muscle fiber can develop ○ The force generated by the contractionof a single muscle fiber can be increased by increasing the rate at which muscle AP stimulate the muscle fiber  A typical muscle AP lasts between 1 and 3 msec, while muscle contraction lasts around 100 msec  If repeated AP are separated by long intervals of time, the muscle fiber has time to  If repeated AP are separated by long intervals of time, the muscle fiber has time to relax completelybetween stimulus □  If the interval of time between AP is shortened, the muscle fiber does not have time to relax completelybetween two stimuli, resulting in a more forceful contraction, known as summation □  If the AP continue to stimulate the muscle fiber repeatedly at short intervals, relaxation between contractions diminishes until the muscle fiber achieves a state of maximal contractionknown as tetanus □ In incompletetetanus, the stimulation rate of the muscle fiber is not at a maximum value and consequently the fiber relaxes slightly between stimuli  □ In completetetanus, the stimulation rate is fast enough that the muscle fiber does not have time to relax  • IN a whole skeletal muscle, the force developed may be increased by summing the contractionsof multiple fibers • All muscles contain elastic fibers in the tendons and other connective tissues that attach muscle to bine, and in the connective tissue between muscle fibers ○ In muscle fibers, elastic cytoskeletalproteins occur between the myofibrils and as a part of the sarcomere ○ When sarcomeresshorten during contractions,the elastic elements stretch  This stretching allows the fibers to maintain a relatively constant length even though the sarcomeresare shortening and creating tension the sarcomeresare shortening and creating tension  • Toadfish Sonic muscle ○ The male toadfish produces a mating call generated by rapid oscillatorycontractions of the muscles enciciling the fishes gas-filled swim bladder ○ The time it takes for a single muscle twitch to reach half its max is 500 ms (swimming),200 ms (escape), and 10 ms (sonic) ○ If a muscle is to activate and relax rapidly, two conditions must be met  First Ca2+, the trigger for muscle contraction,must enter the myoplasmrapidly and be removedrapidly  Second, myosin cross bridges must attach to ac
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