Muscle Study Guide PDF

Chapter 12 Study Guide: 1. Skeletal muscle cellular and molecular structure – fascicles, muscle fibers, sarcolemma, myofibrils, sarcoplasmic reticulum, transverse tubules, thin filaments, thick filaments, sarcomere structure (all bands and zones), G-actin, Factin, myosin, troponin, tropomyosin, cros...

Chapter 12 Study Guide: 1. Skeletal muscle cellular and molecular structure – fascicles, muscle fibers, sarcolemma, myofibrils, sarcoplasmic reticulum, transverse tubules, thin filaments, thick filaments, sarcomere structure (all bands and zones), G-actin, Factin, myosin, troponin, tropomyosin, crossbridges, titin, actin binding site, ATPase binding site (pg. 323-325; Figure 12.3, 12.4, 12.5) Structure at the Cellular Level - Skeletal muscles: o These are connected to two or more bones by tendons. ▪ Fascicles: It is a bundle of muscle fibers surrounded by connective tissue known as perimysium. - Components of a muscle fiber: o Muscle fibers: ▪ Also known as muscle cells. ▪ Surrounded by connective tissue (endomysium). ▪ These are multinucleated. ▪ It contains many myofibrils. ▪ It contains many mitochondria, meaning it has high energy. o Sarcolemma: ▪ It is the muscle fiber’s plasma membrane. o Sarcoplasm: ▪ The semifluid cytoplasm of muscle fiber. o Sarcoplasmic reticulum: ▪ A saclike membranous network (Smooth Endoplasmic Reticulum). o Transverse Tubules (T Tubules): ▪ These are continuous with the sarcolemma and penetrate into the cell’s interior. o Multinucleated o It contains many myofibrils. o It contains many mitochondria. o Lateral sacs (terminal cisternae) ▪ Ca2+ o Triad: T tubule + two lateral sacs Structure at the Molecular Level - Myofibrils: o It gives skeletal and cardiac muscles the striated appearance. o It is an orderly arrangement of thick and thin filaments: ▪ Thick filaments: These are filaments composed of myosin that form part of the contractile machinery of a muscle cell. Each thick filament is made of hundreds of myosin molecules, each of which looks a bit like two golf clubs wrapped around each other. ▪ ▪ ▪ Thin filaments: These are filaments composed of actin that form part of the contractile machinery of a muscle cell; also contain troponin and tropomyosin in striated muscle cells. Actin: the most common microfilament; found in thin filaments in muscle fibers; provides structural support for microvilli. It is also a contractile protein. Myosin: The contractile protein found in thick filaments in striated muscle. o Actin & Myosin: ▪ They are also protein fibers. ▪ These filaments form sarcomere. ▪ It gives striations due to thick and thin filaments that run parallel to the long axis. - Sarcomeres: o The fundamental repeating units that make up myofibrils - Structure of Sarcomere: o A band: ▪ The dark striation observed under the microscope is due to the presence of thick filaments. o H zone: ▪ In the center of the A band is a region that is lighter than the sides because only thick filaments are present; that is, there are no thin filaments overlapping the thick filaments here. o M line: ▪ It connects with thick filaments in a sarcomere which runs perpendicular to the long axis. o I band: ▪ This region is the light striation that comprises areas where there are thin filaments with no overlap with thick filaments. o Z line: ▪ It connects thin filaments is located in the center of the I band. o Sarcomere: ▪ A functional unit. ▪ Z line to Z line. Figure 12.3: - Actin: o It is a contractile protein. o G actin: ▪ An actin monomer that has the basic components of each thin filament. ▪ Each G actin has a binding site for myosin. o F actin: ▪ These are strands that forms when G actins are linked together end to end, like pears in a necklace. ▪ Two F actins are arranged in a double helix to form the actin strands found in thin filaments. - - Tropomyosin: o It is a regulatory protein. o It is a long fibrous molecule that extends over numerous actin monomers in such a way that it blocks the myosin-binding sites in muscles at rest. ▪ It overlaps binding sites on actin for myosin. Troponin: o It is a regulatory protein. o It contains a complex of three proteins: ▪ It attaches to actin. ▪ It attaches to tropomyosin. ▪ It binds Ca2+ reversibly. o The binding of Ca2+ to troponin regulates the skeletal muscle contraction. Figure 12.4: structure of thin filament: - - (a) The backbone of a thin filament consists of two strands of polymerized actin molecules wound together to form a double helix. Myosin-binding sites on individual actin molecules (G actin) are represented by dark dots. (b) A portion of a thin filament showing troponin and tropomyosin in their normal resting positions on the actin strands. Notice that actin’s myosin-binding sites are covered by tropomyosin when a muscle cell is at rest. - Crossbridges: o Each myosin molecule is a dimer consisting of two intertwined subunits, each having a long tail and a fat, protruding head. o Under certain conditions, they bridge the gap between the thick and think filaments. - Titin: o o o o - Is a very elastic protein. It supports protein in muscle. It anchors thick filaments between the M line and the Z line. It provides structural support and elasticity. Figure 12.5: Structure of a thick filament: o (a) A myosin molecule. Note the actin-binding and ATPase sites in the head region. o (b) Two myosin molecules joined tail to tail. o (c) A portion of a thick filament showing myosin heads (crossbridges) protruding at either end but not in the middle region (the bare zone). o (d) A detailed view of a sarcomere showing the relative positions of thick and thin filaments and the protein titin, which anchors the thick filaments in place. o (e) An electron micrograph of a sarcomere showing thick and thin filaments and crossbridges. - Actin binding site: o A specific region on the actin filament where the myosin protein binds during muscle contraction. o It is located on the globular head region of the myosin molecule. o It is a critical component of the molecular level that is responsible for muscle contraction, enabling the interaction between actin and myosin that powers movement in the body. - ATPase binding site: o A specific region on the myosin molecule where adenosine triphosphate (ATP) is bound and hydrolyzed to provide energy for muscle contraction. o It is located on the globular head region of the myosin protein, which contains an enzyme called myosin ATPase. o It plays a critical role in muscle contraction by facilitating the hydrolysis of ATP and providing the energy necessary for the cyclic interaction between actin and myosin that generates force and movement. 2. Sarcomere and the sliding filament theory - A band, H zone, I band, M line, Z line (pg. 324, 326) a. What happens to the sarcomere during contraction? - Sliding-Filament Model: o It is a muscle contraction which means the shortening of muscle. o Thick and thin filaments overlap. o Neither thick nor thin filaments shorten. o Filaments slide past each other. o Within a sarcomere during contraction: ▪ A band stays the same length. ▪ I band shortens. ▪ H zone shortens ▪ Sarcomere shortens o Sliding is due to cyclical formation and breaking of cross bridges = crossbridge cycle 3. 5 steps of the crossbridge cycle, power stroke (pg. 326-328; Figure 12.7) - 5 Steps of the Crossbridge Cycle: o (1) Binding of myosin to actin: ▪ It starts with myosin in its energized form; that is, ADP and Pi (inorganic phosphate) are bound to the ATPase site of the myosin head. ▪ In this state, myosin has a high affinity for actin, and the myosin head binds to an actin monomer in the adjacent thin filament. o (2) Power stroke: ▪ The binding of myosin to actin triggers the release of the Pi from the ATPase site. During this process, the myosin head pivots toward the middle of the sarcomere, pulling the thin filament along with it. o (3) Rigor: ▪ As the power stroke ends, ADP is released from the myosin head and the myosin molecule goes into its low-energy state. ▪ In this state, myosin and actin are tightly bound together, a condition called rigor. (Rigor mortis—the stiffening of the body that occurs after death—occurs because the crossbridge cycle gets stuck at this step due to: (1) an excess of calcium when cell membranes are damaged. (2) a lack of ATP due to the termination of energy production. ▪ Rigor mortis continues until enzymes leaked by disintegrating cellular components begin to break down the myofibrils.) o (4) Unbinding of myosin and actin: ▪ This is when a new ATP enters the ATPase site on the myosin head, triggering a conformational change in the head, which decreases the affinity of myosin for actin, so the myosin detaches from the actin. o (5) Cocking of the myosin head: ▪ Soon after it binds to myosin’s ATPase site, ATP is split by hydrolysis into ADP and Pi, which releases energy. ▪ Some of the energy is captured by the myosin molecule as it goes into its high-energy conformation. Although ATP has been hydrolyzed at this point, the end-products of the reaction (ADP and Pi) remain bound to the ATPase site. ▪ If calcium is present, the cycle will continue by revisiting step 1. 4. Excitation-contraction coupling in skeletal muscle (pg. 329-331; Figure 12.8, 12.9) - Excitation-contraction coupling: o It is the sequence of events whereby an action potential in the sarcolemma causes contraction: ▪ It is dependent on neural input from the motor neuron. ▪ It requires Ca2+ release from the sarcoplasmic reticulum. o Gating of sarcoplasmic reticulum Ca2+ channels: ▪ Voltage-gated opening Coupled to T tubules by ryanodine and DHP receptors. o Ca2+-induced opening o Ca2+-induced closing o Termination of contraction: ▪ Ca2+ must leave troponin, allowing tropomyosin to cover myosin binding sites on actin. ▪ To remove Ca2+ from cytosol: Ca2+-ATPase in the sarcoplasmic reticulum. Transports Ca2+ from cytosol into the sarcoplasmic reticulum. - Neuromuscular Junction: o It is the connection between a motor neuron and a muscle cell. - The role of Neuromuscular Junction in Excitation-Contraction Coupling: o Each motor neuron innervates several muscle cells. o Each muscle fiber receives input from a single motor neuron. o Similar to ordinary synapse. o Acetylcholine released. o Motor end plate: ▪ High density of acetylcholine receptors ▪ Highly folded o End-plate potential. o Motor neuron AP always creates a muscle cell AP. Figure 12.8: - o (1) Acetylcholine (ACh) is released from the axon terminal of a motor neuron and binds to receptors in the motor end plate. This binding elicits an end-plate potential, which triggers an action potential in the muscle cell. o (2) Action potential propagates along the sarcolemma and down T tubules. o (3) The action potential triggers Ca2+ release from SR. o (4) Ca2+ binds to troponin, exposing myosin-binding sites. o (5) Crossbridge cycle begins (muscle fiber contracts) o (6) Ca2+ is actively transported back into lumen of SR following the action potential o (7) Tropomyosin blocks myosin-binding sites (muscle fiber relaxes) - The Roles of Calcium, Troponin, and Tropomyosin in ExcitationContraction Coupling: o If no Ca2+ → troponin holds tropomyosin over myosin binding sites on actin: ▪ No crossbridges form between actin and myosin. ▪ Muscle relaxed. o If Ca2+ present → binds to troponin, causing movement of troponin, causing movement of tropomyosin, exposing binding sites for myosin on actin: ▪ Crossbridges form between actin and myosin. ▪ A cycle occurs, the muscle contracts. - Figure 12.9 Actions of troponin and tropomyosin in excitation-contraction coupling: o (a) Relaxed muscle o (b) Role of calcium in exposing the myosinbinding sites on actin. - Steps of Excitation-Contraction Coupling: o (1) Action potential in sarcolemma. o (2) Action potential down T tubules. o (3) DHP receptors of T tubules open Ca2+ channels (ryanodine receptors) in lateral sacs of SR. o (4) Ca2+ increases in cytosol o (5) Ca2+ binds to troponin, shifting tropomyosin. o (6) Crossbridge cycling occurs. 5. Excitation-contraction coupling in smooth muscle and neural regulation (pg. 351352; Figure 12.35) - Smooth Muscle: o It lacks striations. o It is found in internal organs and blood vessels. o It is under involuntary control by the autonomic nervous system. o It appears as spindle-shaped. o It is small—approximately 1/10 the size of skeletal muscle. o It contains actin and myosin. o It does not have sarcomeres. o It has dense bodies. - Sliding-filament mechanism of contraction: o Actin and myosin are longer in smooth muscle than in skeletal muscle. o Myosin heads are distributed along the entire length of the myosin filament, as opposed to being confined to specific regions as seen in skeletal muscle. o Longer range of contraction. - Smooth muscle steps of excitation-contraction coupling: o (1) Most Ca2+ comes from outside the cell. o (2) Voltage-gated Ca2+ channels in plasma membrane. o (3) Ca2+ triggers release of Ca2+ from sarcoplasmic reticulum. o (4) Ca2+ binds to calmodulin. o (5) Ca2+-calmodulin activates myosin light-chain kinase. o (6) Myosin light chain kinase (MLCK) phosphorylates myosin. o (7) Crossbridge cycling. Figure 12.35: Excitation-contraction coupling in smooth muscle - Relaxation of smooth muscle: o Phosphatase removes phosphate from myosin. o Ca2+ is removed from cytoplasm: ▪ Ca 2+-ATPase ▪ Ca 2+-Na+ countertransport - Contraction time in smooth muscle: o Myosin ATPase contraction is 10–100 times slower in smooth muscle than in skeletal muscle. - Neural regulation of smooth muscle contraction: o Neural regulation of smooth muscle contraction o Innervated by autonomic nervous system: ▪ Sympathetic and/or parasympathetic o May be excitatory or inhibitory. o Precise response depends on the receptor type. o Neurotransmitter is released from varicosities. o Diffuse binding of neurotransmitter to receptors 6. Study the cardiac muscle (pg. 354) a. Significance of cardiac action potential - Cardiac Muscle: o It is similar to skeletal muscle in that it is striated, has the same sarcomere structure, and has contractions that are regulated by the troponintropomyosin system. o Cardiac muscle cells: ▪ These are similar to smooth muscle cells in that they are extensively connected by gap junctions, such that an action potential, once initiated, travels throughout the entire cell network. o Gap junctions (within intercalated disks): ▪ Cardiac muscle cells are connected by specialized structures called intercalated discs. o Pacemaker Cells: ▪ Cardiac muscle contains specialized cells called pacemaker cells, which generate electrical impulses that initiate and regulate the heartbeat. ▪ These cells are primarily located in the sinoatrial (SA) node in the heart's right atrium. o Innervated by Autonomic Nervous System: ▪ Cardiac muscle is innervated by the autonomic nervous system, specifically the sympathetic and parasympathetic divisions. ▪ Sympathetic stimulation increases heart rate and contractility, while parasympathetic stimulation decreases heart rate. o Calcium Regulation: ▪ Calcium ions (Ca2+) required for cardiac muscle contraction come from both extracellular fluid and the sarcoplasmic reticulum (SR). ▪ Calcium influx from extracellular fluid triggers calcium release from the SR, leading to muscle contraction. o Action Potential Duration and Refractory Period: ▪ The action potential in cardiac muscle cells lasts almost as long as the tension generated during contraction. ▪ Cardiac muscle has a relatively long refractory period, which prevents summation of contractions and ensures proper relaxation between heartbeats. ▪ This characteristic helps prevent tetanic contractions and allows the heart to fill with blood before each contraction. - Significance of Cardiac Action Potential: o The relatively long duration of cardiac action potentials is significant: ▪ Because they last nearly as long as it takes for cardiac muscle cells to contract and relax, summation of cardiac muscle contractions cannot occur, even when the action potential frequency is high, and the heart is beating rapidly. ▪ This is good, because summation would be detrimental to the heart’s pumping action in that the heart would not be able to relax completely and fill with blood between contractions.