Human Anatomy and Physiology - Chapter 09 - Muscles and Muscle Tissue PDF

Human Anatomy and Physiology Eleventh Edition Chapter 09 Part A Muscles and Muscle Tissue PowerPoint® Lectures Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College...

Human Anatomy and Physiology Eleventh Edition Chapter 09 Part A Muscles and Muscle Tissue PowerPoint® Lectures Slides prepared by Karen Dunbar Kareiva, Ivy Tech Community College Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 9.1 Overview of Muscle Tissue Nearly half of body’s mass Can transform chemical energy (ATP) into directed mechanical energy, which is capable of exerting force To investigate muscle, we look at: – Types of muscle tissue – Characteristics of muscle tissue – Muscle functions Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Muscle Tissue (1 of 4) Terminologies: Myo, mys, and sarco are prefixes for muscle – Example: sarcoplasm: muscle cell cytoplasm Three types of muscle tissue – Skeletal – Cardiac – Smooth Only skeletal and smooth muscle cells are elongated and referred to as muscle fibers Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Muscle Tissue (2 of 4) Skeletal muscle – Skeletal muscle tissue is packaged into skeletal muscles: organs that are attached to bones and skin – Skeletal muscle fibers are longest of all muscle and have striations (stripes) – Also called voluntary muscle: can be consciously controlled – Contract rapidly; tire easily; powerful – Key words for skeletal muscle: skeletal, striated, and voluntary Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Muscle Tissue (3 of 4) Cardiac muscle – Cardiac muscle tissue is found only in heart  Makes up bulk of heart walls – Striated – Involuntary: cannot be controlled consciously  Contracts at steady rate due to heart’s own pacemaker, but nervous system can increase rate – Key words for cardiac muscle: cardiac, striated, and involuntary Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Types of Muscle Tissue (4 of 4) Smooth muscle – Smooth muscle tissue: found in walls of hollow organs  Examples: stomach, urinary bladder, and airways – Not striated – Involuntary: cannot be controlled consciously – Key words for smooth muscle: visceral, nonstriated and involuntary Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 9.3-1 Comparison of Skeletal, Cardiac, and Smooth Muscle Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Characteristics of Muscle Tissue All muscles share four main characteristics: – Excitability (responsiveness): ability to receive and respond to stimuli – Contractility: ability to shorten forcibly when stimulated – Extensibility: ability to be stretched – Elasticity: ability to recoil to resting length Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Muscle Functions Four important functions 1. Produce movement: responsible for all locomotion and manipulation  Example: walking, digesting, pumping blood 2. Maintain posture and body position 3. Stabilize joints 4. Generate heat as they contract Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 9.2 Skeletal Muscle Anatomy Skeletal muscle is an organ made up of different tissues with three features: nerve and blood supply, connective tissue sheaths, and attachments Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Nerve and Blood Supply Each muscle receives a nerve, artery, and veins – Consciously controlled skeletal muscle has nerves supplying every fiber to control activity Contracting muscle fibers require huge amounts of oxygen and nutrients – Also need waste products removed quickly Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Connective Tissue Sheaths Each skeletal muscle, as well as each muscle fiber, is covered in connective tissue Support cells and reinforce whole muscle Sheaths from external to internal: – Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia – Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) – Endomysium: fine areolar connective tissue surrounding each muscle fiber Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Connective Tissue Sheaths of Skeletal Muscle: Epimysium, Perimysium, and Endomysium (1 of 2) Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Attachments Muscles span joints and attach to bones Muscles attach to bone in at least two places – Insertion: attachment to movable bone – Origin: attachment to immovable or less movable bone Attachments can be direct or indirect – Direct (fleshy): epimysium fused to periosteum of bone or perichondrium of cartilage – Indirect: connective tissue wrappings extend beyond muscle as ropelike tendon or sheetlike aponeurosis Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Connective Tissue Sheaths of Skeletal Muscle: Epimysium, Perimysium, and Endomysium (2 of 2) Figure 9.1a Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Table 9.1-1 Structure and Organizational Levels of Skeletal Muscle Table 9.1 Structure and Organizational Levels of Skeletal Muscle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 9.3 Muscle Fiber Microanatomy and Sliding Filament Model Skeletal muscle fibers are long, cylindrical cells that contain multiple nuclei Sarcolemma: muscle fiber plasma membrane Sarcoplasm: muscle fiber cytoplasm Contains many glycosomes for glycogen storage, as well as myoglobin for O2 storage Modified organelles – Myofibrils – Sarcoplasmic reticulum – T tubules Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (1 of 7) Myofibrils are densely packed, rodlike elements – Single muscle fiber can contain 1000s – Accounts for ~80% of muscle cell volume Myofibril features – Striations – Sarcomeres – Myofilaments – Molecular composition of myofilaments Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of a Skeletal Muscle Fiber (1 of 4) Figure 9.2b Microscopic anatomy of a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (2 of 7) Striations: stripes formed from repeating series of dark and light bands along length of each myofibril – A bands: dark regions  H zone: lighter region in middle of dark A band – M line: line of protein (myomesin) that bisects H zone vertically – I bands: lighter regions  Z disc (line): coin-shaped sheet of proteins on midline of light I band Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of a Skeletal Muscle Fiber (2 of 4) Figure 9.2a Microscopic anatomy of a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (3 of 7) Sarcomere – Smallest contractile unit (functional unit) of muscle fiber – Contains A band with half of an I band at each end  Consists of area between Z discs – Individual sarcomeres align end to end along myofibril, like boxcars of train Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of a Skeletal Muscle Fiber (3 of 4) Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (4 of 7) Myofilaments – Orderly arrangement of actin and myosin myofilaments within sarcomere – Actin myofilaments: thin filaments  Extend across I band and partway in A band  Anchored to Z discs – Myosin myofilaments: thick filaments  Extend length of A band  Connected at M line – Sarcomere cross section shows hexagonal arrangement of one thick filament surrounded by six thin filaments Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Microscopic Anatomy of a Skeletal Muscle Fiber (4 of 4) Figure 9.2d, e Microscopic anatomy of a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (5 of 7) Molecular composition of myofilaments – Thick filaments: composed of protein myosin that contains two heavy and four light polypeptide chains  Heavy chains intertwine to form myosin tail  Light chains form myosin globular head – During contraction, heads link thick and thin filaments together, forming cross bridges  Myosins are offset from each other, resulting in staggered array of heads at different points along thick filament Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (6 of 7) Molecular composition of myofilaments (cont.) – Thin filaments: composed of fibrous protein actin  Actin is polypeptide made up of kidney-shaped G actin (globular) subunits – G actin subunits bears active sites for myosin head attachment during contraction  G actin subunits link together to form long, fibrous F actin (filamentous)  Two F actin strands twist together to form a thin filament – Tropomyosin and troponin: regulatory proteins bound to actin Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Thick and Thin Filaments (1 of 4) Figure 9.3 Composition of thick and thin filaments. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Composition of Thick and Thin Filaments (2 of 4) Figure 9.3 Composition of thick and thin filaments. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Myofibrils (7 of 7) Molecular composition of myofilaments (cont.) – Other proteins help form the structure of the myofibril  Elastic filament: composed of protein titin – Holds thick filaments in place; helps recoil after stretch; resists excessive stretching  Dystrophin – Links thin filaments to proteins of sarcolemma  Nebulin, myomesin, C proteins bind filaments or sarcomeres together – Maintain alignment of sarcomere Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 9.1 (1 of 3) Duchenne muscular dystrophy (DMD) is most common and serious form of muscular dystrophies, muscle-destroying diseases that generally appear during childhood Inherited as a sex-linked recessive disease, so almost exclusively in males (1 in 3600 births) Appears between 2 and 7 years old when boy becomes clumsy and falls frequently Disease progresses from extremities upward, finally affecting head, chest muscles, and cardiac muscle. With supportive care, people with DMD can live into 30s and beyond Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 9.1 (2 of 3) Caused by defective gene for dystrophin, a protein that links thin filaments to extracellular matrix and helps stabilize sarcolemma Sarcolemma of DMD patients tear easily, allowing entry of excess calcium which damages contractile fibers Inflammation follows and regenerative capacity is lost resulting in increased apoptosis of muscle cells and drop in muscle mass Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved A Boy with Duchenne Muscular Dystrophy (DMD) Figure 9.4 A boy with Duchenne muscular dystrophy (DMD). Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 9.1 (3 of 3) chest muscles, and cardiac muscle. The weakness continues to progress, but with supportive care, DMD patients are living into their 30s and beyond. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sarcoplasmic Reticulum and T Tubules (1 of 4) Sarcoplasmic reticulum: network of smooth endoplasmic reticulum tubules surrounding each myofibril – Most run longitudinally – Terminal cisterns form perpendicular cross channels at the A–I band junction – SR functions in regulation of intracellular Ca2+ levels – Stores and releases Ca2+ Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sarcoplasmic Reticulum and T Tubules (2 of 4) T tubules – Tube formed by protrusion of sarcolemma deep into cell interior  Increase muscle fiber’s surface area greatly  Lumen continuous with extracellular space  Allow electrical nerve transmissions to reach deep into interior of each muscle fiber – Tubules penetrate cell’s interior at each A–I band junction between terminal cisterns  Triad: area formed from terminal cistern of one sarcomere, T tubule, and terminal cistern of neighboring sarcomere Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sarcoplasmic Reticulum and T Tubules (3 of 4) Triad relationships – T tubule contains integral membrane proteins that protrude into intermembrane space (space between tubule and muscle fiber sarcolemma)  Tubule proteins act as voltage sensors that change shape in response to an electrical current – SR cistern membranes also have integral membrane proteins that protrude into intermembrane space  SR integral proteins control opening of calcium channels in SR cisterns Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sarcoplasmic Reticulum and T Tubules (4 of 4) Triad relationships (cont.) – When an electrical impulse passes by, T tubule proteins change shape, causing SR proteins to change shape, causing release of calcium into cytoplasm Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Relationship of the Sarcoplasmic Reticulum and T Tubules to Myofibrils of Skeletal Muscle Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sliding Filament Model of Contraction (1 of 3) Contraction: the activation of cross bridges to generate force Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening Contraction ends when cross bridges become inactive Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sliding Filament Model of Contraction (2 of 3) In the relaxed state, thin and thick filaments overlap only slightly at ends of A band Sliding filament model of contraction states that during contraction, thin filaments slide past thick filaments, causing actin and myosin to overlap more – Neither thick nor thin filaments change length, just overlap more When nervous system stimulates muscle fiber, myosin heads are allowed to bind to actin, forming cross bridges, which cause sliding (contraction) process to begin Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sliding Filament Model of Contraction (3 of 3) Cross bridge attachments form and break several times, each time pulling thin filaments a little closer toward center of sarcome in a ratcheting action – Causes shortening of muscle fiber Z discs are pulled toward M line I bands shorten Z discs become closer H zones disappear A bands move closer to each other Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sliding Filament Model of Contraction (1 of 2) Figure 9.6-1 Sliding filament model of contraction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Sliding Filament Model of Contraction (2 of 2) Figure 9.6-2 Sliding filament model of contraction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved 9.4 Muscle Fiber Contraction (2 of 2) Background and Overview Decision to move is activated by brain, signal is transmitted down spinal cord to motor neurons which then activate muscle fibers Neurons and muscle cells are excitable cells capable of action potentials – Excitable cells are capable of changing resting membrane potential voltages AP crosses from neuron to muscle cell via the neurotransmitter acetylcholine (ACh) Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Background and Overview (1 of 4) Ion Channels – Play the major role in changing of membrane potentials – Two classes of ion channels:  Chemically gated ion channels – opened by chemical messengers such as neurotransmitters – Example: ACh receptors on muscle cells  Voltage-gated ion channels – open or close in response to voltage changes in membrane potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved “Chemically Gated Ion Channel” and “Voltage-Gated ion Channel” Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Background and Overview (2 of 4) Anatomy of Motor Neurons and the Neuromuscular Junction – Skeletal muscles are stimulated by somatic motor neurons – Axons (long, threadlike extensions of motor neurons) travel from central nervous system to skeletal muscle – Each axon divides into many branches as it enters muscle – Axon branches end on muscle fiber, forming neuromuscular junction or motor end plate  Each muscle fiber has one neuromuscular junction with one motor neuron Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Skeletal Muscle Contraction (1 of 3) Figure 9.7 Overview of skeletal muscle contraction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Background and Overview (3 of 4) – Axon terminal (end of axon) and muscle fiber are separated by gel-filled space called synaptic cleft – Stored within axon terminals are membrane-bound synaptic vesicles  Synaptic vesicles contain neurotransmitter acetylcholine (ACh) – Infoldings of sarcolemma, called junctional folds, contain millions of ACh receptors – NMJ consists of axon terminals, synaptic cleft, and junctional folds Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Skeletal Muscle Contraction (2 of 3) Figure 9.7 Overview of skeletal muscle contraction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Background and Overview (4 of 4) The Big Picture - Four steps must occur for skeletal muscle to contract: 1. Events at neuromuscular junction 2. Muscle fiber excitation 3. Excitation-contraction coupling 4. Cross bridge cycling Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Overview of Skeletal Muscle Contraction (3 of 3) Figure 9.7 Overview of skeletal muscle contraction. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Events at the Neuromuscular Junction 1. AP arrives at axon terminal 2. Voltage-gated calcium channels open, calcium enters motor neuron 3. Calcium entry causes release of ACh neurotransmitter into synpatic cleft 4. ACh diffuses across to ACh receptors (Na+ chemical gates) on sarcolemma 5. ACh binding to receptors, opens gates, allowing Na+ to enter resulting in end plate potential 6. Acetylcholinesterase degrades ACh Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved When a Nerve Impulse Reaches a Neuromuscular Junction, Acetylcholine (ACh) is Released (6 of 6) FOCUS FIGURE 9.1 Events at the Neuromuscular Junction Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 9.2 Many toxins, drugs, and diseases interfere with events at the neuromuscular junction – Example: myasthenia gravis: disease characterized by drooping upper eyelids, difficulty swallowing and talking, and generalized muscle weakness – Involves shortage of Ach receptors because person’s ACh receptors are attacked by own antibodies – Suggests this is an autoimmune disease Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generation of an Action Potential Across the Sarcolemma (1 of 4) Resting sarcolemma is polarized, meaning a voltage exists across membrane – Inside of cell is negative compared to outside Action potential is caused by changes in electrical charges Occurs in three steps 1. Generation of end plate potential 2. Depolarization 3. Repolarization Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generation of an Action Potential Across the Sarcolemma (2 of 4) 1. End plate potential – ACh released from motor neuron binds to ACh receptors on sarcolemma – Causes chemically gated ion channels (ligands) on sarcolemma to open – Na+ diffuses into muscle fiber  Some K+ diffuses outward, but not much – Because Na+ diffuses in, interior of sarcolemma becomes less negative (more positive) – Results in local depolarization called end plate potential Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Summary of Events in the Generation and Propagation of an Action Potential in a Skeletal Muscle Fiber (1 of 3) Figure 9.8 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generation of an Action Potential Across the Sarcolemma (3 of 4) 2. Depolarization: generation and propagation of an action potential (AP) – If end plate potential causes enough change in membrane voltage to reach critical level called threshold, voltage-gated Na+ channels in membrane will open – Large influx of Na+ through channels into cell triggers AP that is unstoppable and will lead to muscle fiber contraction – AP spreads across sarcolemma from one voltage-gated Na+ channel to next one in adjacent areas, causing that area to depolarize Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Summary of Events in the Generation and Propagation of an Action Potential in a Skeletal Muscle Fiber (2 of 3) Figure 9.8 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Generation of an Action Potential Across the Sarcolemma (4 of 4) 3. Repolarization: restoration of resting conditions – Na+ voltage-gated channels close, and voltage-gated K+ channels open – K+ efflux out of cell rapidly brings cell back to initial resting membrane voltage – Refractory period: muscle fiber cannot be stimulated for a specific amount of time, until repolarization is complete – Ionic conditions of resting state are restored by Na+-K+ pump  Na+ that came into cell is pumped back out, and K+ that flowed outside is pumped back into cell Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Summary of Events in the Generation and Propagation of an Action Potential in a Skeletal Muscle Fiber (3 of 3) Figure 9.8 Summary of events in the generation and propagation of an action potential in a skeletal muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Action Potential Tracing Indicates Changes in Na+ and K+ ion Channels Figure 9.9 Recording of an action potential (AP) in a muscle fiber. Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Excitation-Contraction (E-C) Coupling Excitation-contraction (E-C) coupling: events that transmit AP along sarcolemma (excitation) are coupled to sliding of myofilaments (contraction) AP is propagated along sarcolemma and down into T tubules, where voltage-sensitive proteins in tubules stimulate Ca2+ release from SR – Ca2+ release leads to contraction AP is brief and ends before contraction is seen Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Excitation-Contraction (E-C) Coupling is the Sequence of Events by Which Transmission of an Action Potential Along the Sarcolemma Leads to the Sliding of Myofilaments (1 of 3) FOCUS FIGURE 9.2 Excitation-Contraction Coupling Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Excitation-Contraction (E-C) Coupling is the Sequence of Events by Which Transmission of an Action Potential Along the Sarcolemma Leads to the Sliding of Myofilaments (2 of 3) FOCUS FIGURE 9.2 Excitation-Contraction Coupling Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Excitation-Contraction (E-C) Coupling is the Sequence of Events by Which Transmission of an Action Potential Along the Sarcolemma Leads to the Sliding of Myofilaments (3 of 3) FOCUS FIGURE 9.2 Excitation-Contraction Coupling Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Muscle Fiber Contraction: Cross Bridge Cycling (1 of 3) At low intracellular Ca2+ concentration: – Tropomyosin blocks active sites on actin – Myosin heads cannot attach to actin – Muscle fiber remains relaxed Voltage-sensitive proteins in T tubules change shape, causing sarcoplasmic reticulum (SR) to release Ca2+ to cytosol Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Muscle Fiber Contraction: Cross Bridge Cycling (2 of 3) At higher intracellular Ca2+ concentrations, Ca2+ binds to troponin Troponin changes shape and moves tropomyosin away from myosin-binding sites Myosin heads is then allowed to bind to actin, forming cross bridge Cycling is initiated, causing sarcomere shortening and muscle contraction When nervous stimulation ceases, Ca2+ is pumped back into SR, and contraction ends Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Muscle Fiber Contraction: Cross Bridge Cycling (3 of 3) Four steps of the cross bridge cycle 1. Cross bridge formation: high-energy myosin head attaches to actin thin filament active site 2. Working (power) stroke: myosin head pivots and pulls thin filament toward M line 3. Cross bridge detachment: ATP attaches to myosin head, causing cross bridge to detach 4. Cocking of myosin head: energy from hydrolysis of ATP “cocks” myosin head into high-energy state  This energy will be used for power stroke in next cross bridge cycle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved The Cross Bridge Cycle is the Series of Events During Which Myosin Heads Pull Thin Filaments Toward the Center of the Sarcomere (4 of 4) FOCUS FIGURE 9.3 Cross Bridge Cycle Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Clinical – Homeostatic Imbalance 9.3 Rigor mortis – 3–4 hours after death, muscles begin to stiffen  Peak rigidity occurs about 12 hours postmortem – Intracellular calcium levels increase because ATP is no longer being synthesized, so calcium cannot be pumped back into SR  Results in cross bridge formation – ATP is also needed for cross bridge detachment  Results in myosin head staying bound to actin, causing constant state of contraction – Muscles stay contracted until muscle proteins break down, causing myosin to release Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved Copyright Copyright © 2019, 2016, 2013 Pearson Education, Inc. All Rights Reserved