Human Anatomy and Physiology: The Muscular System

Human Anatomy and Physiology Elaine N. Marieb and Suzanne M. Keller The Muscular System: Muscle Tissue This slide deck contains animations. Please disable animations if...

Human Anatomy and Physiology Elaine N. Marieb and Suzanne M. Keller The Muscular System: Muscle Tissue This slide deck contains animations. Please disable animations if they cause issues with your device. Copyright ©2021 John Wiley & Sons, Inc. Learning Objectives 1. Explaining the structural differences among the three types of muscular tissue. 2. Describe the microscopic anatomy of a skeletal muscle fiber and the functions of its proteins 3. Describe the sliding filament mechanism and how muscle action potentials arise at the neuromuscular junction 4. Describe the reactions by which muscle fibers produce ATP 5. Describe the factors contributing to muscle fatigue 6. Distinguish between isotonic and isometric contractions 7. Describe the main structural and functional characteristics of cardiac and smooth muscle tissues Copyright ©2021 John Wiley & Sons, Inc. 2 Types of Muscular Tissue 1. Skeletal muscle 2. Cardiac muscle 3. Smooth muscle Functions of Muscular Tissue 1. Producing body movements: whole body such as walking and running, and localized movements such as grasping a pencil, or typing. 2. Stabilizing body positions: Skeletal muscle contractions stabilize joints and help maintain body positions, such as standing or sitting. 3. Storing and moving substances within the body: ringlike smooth muscle sphincters control the movement of contents of hollow organs e.g. food in the stomach or urine in the urinary bladder. 4. Generating heat: As muscular tissue contracts, it produces heat e.g. during shivering. Properties of Muscular Tissue 1. Electrical excitability: ability to respond to certain stimuli by producing electrical signals called action potentials (impulses) 2. Contractility: ability of muscular tissue to contract forcefully when stimulated by a nerve impulse 3. Extensibility: ability of muscular tissue to stretch, within limits, without being damaged 4. Elasticity: ability of muscular tissue to return to its original length and shape after contraction or extension Copyright ©2021 John Wiley & Sons, Inc. 5 Skeletal Muscle Tissue Copyright ©2021 John Wiley & Sons, Inc. 6 Gross Anatomy of Skeletal Muscles Three layers of connective tissue protect and strengthen skeletal muscle – i) Epimysium, ii) Perimysium and iii) Endomysium. Epimysium - outer layer, encircling the entire muscle. Consists of dense irregular connective tissue.  Gross Anatomy of Skeletal Muscles Perimysium: a layer of dense irregular connective tissue that Surrounds groups of 10 to 100 or more muscle fibers/ bundles called muscle fascicles Endomysium: penetrates the interior of each muscle fascicle and separates individual muscle fibers from one another. The endomysium is mostly reticular fibers. Levels of Organization within a Skeletal Muscle  Cellular Components of a Muscle Fiber Sarcolemma Plasma membrane of a muscle fibre that forms T tubules Sarcoplasm Cytoplasm of the muscle fiber that contains myofibrils Myofibril Consists of bundles of myofilament and plays a key role in muscle contraction Myofilament Consists of thick and thin filaments that give muscle tissue its striated appearance and play a role in muscle contraction Myoglobin A reddish brown pigment (gives muscle tissue its dark-red colour) that stores oxygen for muscle contraction T tubule Permit rapid transmission of the action potential deep inside the cell, and also play an important role in regulating cellular calcium concentration. Sarcoplasmic Storage site for calcium ions reticulum Copyright ©2021 John Wiley & Sons, Inc. 10 Microanatomy of Skeletal Muscle Fibre Copyright ©2021 John Wiley & Sons, Inc. 11  The Arrangement of a Sarcomere Sarcomere: basic functional unit of a myofibril. Contains thin and thick filaments and other structural components/proteins. A sarcomere extends from one Z disc to the next Z disc. The components of a sarcomere are organized into a variety of bands and zones Components of a Sarcomere Copyright ©2021 John Wiley & Sons, Inc. 13 Muscle Proteins Contractile: Regulatory: Structural Proteins: Myosin Troponin Titin Actin Tropomyosin Nebulin Alpha-actin Myomesin Dystrophin Copyright ©2021 John Wiley & Sons, Inc. 14 Muscle Proteins Regulatory Proteins: help switch muscle contraction process on and off Tropomyosin -when skeletal muscle fiber is relaxed, tropomyosin covers myosin-binding sites on actin molecules, preventing myosin from binding to actin. Troponin - when calcium ions (Ca2+) bind to troponin, it changes shape; moves tropomyosin away from myosin binding sites on actin and myosin binds to actin. Copyright ©2021 John Wiley & Sons, Inc. 15  The Sliding Filament Mechanism Myosin pulls on actin, causing the thin filament to slide inward Consequently, Z discs move toward each other, and the sarcomere shortens Due to the structural proteins, there is a transmission of force throughout the entire muscle, resulting in whole muscle contraction Copyright ©2021 John Wiley & Sons, Inc. 16 The Contraction Cycle The Contraction Cycle Interactions Animation Contraction of Skeletal Muscle Cells The neuromuscular junction or neuromuscular synapse, parts of a muscle fiber, and the contraction cycle Copyright ©2021 John Wiley & Sons, Inc. 18 The Neuromuscular Junction (NMJ) or Neuromuscular Synapse (NMS) Events at the NMJ (or NMS) produce a muscle action potential: 1. Release of Acetylcholine (ACh): Voltage-gated calcium channels in a neuron’s synaptic end bulb open, resulting in calcium influx. This causes exocytosis of a neurotransmitter (ACh) into synaptic cleft 2. Activation of ACh Receptors: ACh binds to ligand-gated Na+ channels on the motor endplate, which causes an influx of Na+ into muscle 3. Production of muscle action potential- depolarizes the muscle and results in Ca2+ release from the sarcoplasmic reticulum 4. Termination of ACh Activity: ACh gets broken down by acetlycholinesterase The Neuromuscular Junction (NMJ)  Excitation-Contraction (EC) Coupling This concept connects the events of: a muscle action potential with the sliding filament mechanism Rigor Mortis (Rigidity of death) Is the stiffening of skeletal muscle several hours after death; This occurs because ATP levels decrease as nutrients and oxygen needed to form ATP are no longer supplied to cells from circulation. THEREFORE BREAKAGE OF LINK BETWEEN ACTIN AND MYOSIN DOES NOT OCCUR Stiffness of rigor mortis disappears after 2-3 days when muscle tissue disintegrates Copyright ©2021 John Wiley & Sons, Inc. 22 Muscle Metabolism Muscles have 3 ways to produce ATP: 1. Creatine phosphate (Direct phosphorylation of ADP) 2. Anaerobic glycolysis (Anaerobic glycolysis and lactic acid formation) 3. Aerobic respiration Copyright ©2021 John Wiley & Sons, Inc. 23 1. Creatine Phosphate (C P) Creatine kinase catalyzes the transfer of a phosphate group from Creatine Phosphate to ADP to rapidly yield ATP Produces 1 ATP 2. Anaerobic Glycolysis When CP stores are depleted, glucose is converted into pyruvic acid to generate ATP Produces 2 ATP Molecules  3. Aerobic Respiration Under aerobic conditions (requires oxygen) - pyruvic acid can enter the mitochondria and undergo reactions to generate large amounts of ATP Produces 30/32 ATP Molecules Muscle Fatigue Muscle fatigue is the inability to maintain force of contraction after prolonged activity The onset of fatigue is due to: 1. Inadequate release of Ca2+ from SR 2. Depletion of CP, oxygen, and nutrients 3. Build up of lactic acid and ADP 4. Insufficient release of Acetyl Choline (ACh) at the Neuromuscular Junction (NMJ)  Muscle Tone Even when at rest, a skeletal muscle exhibits a small amount of tension, called muscle tone Muscle tone keeps skeletal muscles firm, but it does not result in a force strong enough to produce movement. Tone is established by the alternating, weak involuntary activation of small groups of motor units in a muscle Flaccid is when muscle tone is lost Copyright ©2021 John Wiley & Sons, Inc. 28 Isotonic vs. Isometric Contractions (1 of 2) 1. Isotonic – tension is constant while muscle length changes. Isotonic contractions are used for body movements and for moving object Two types: Concentric - the muscle shortens during contraction Eccentric- the muscle lengthens during contraction 2. Isometric – muscle contracts but does not change length. Important for maintaining posture and for supporting objects in a fixed position Copyright ©2021 John Wiley & Sons, Inc. 29 Isotonic vs. Isometric Contractions (2 of 2) Copyright ©2021 John Wiley & Sons, Inc. 30 Skeletal Muscle Fiber Types Skeletal muscle fibers are classified into three main types: (1) slow oxidative fibers, (2) fast oxidative–glycolytic fibers, and (3) fast glycolytic fibers. Copyright ©2021 John Wiley & Sons, Inc. 31 Skeletal Muscle Fiber Types Copyright ©2021 John Wiley & Sons, Inc. 32 Exercise and Skeletal Muscle Tissue Questions to consider: What fiber type does a marathoner use primarily? What fiber type does a soccer player use primarily? Copyright ©2021 John Wiley & Sons, Inc. 33 Copyright ©2021 John Wiley & Sons, Inc. 34  Cardiac Muscle Cardiac muscle has the same arrangement as skeletal muscle, but also has intercalated discs Intercalated discs contain desmosomes and gap junctions that allow muscle action potentials to spread from one muscle fiber to another Cardiac muscle cells have more mitochondria and their contractions last 10 to 15 times longer than skeletal muscle contractions Copyright ©2021 John Wiley & Sons, Inc. 35  Smooth Muscle Smooth muscle looks quite different than cardiac and skeletal muscle. It is thick in the middle, tapered on the ends, and is not striated It can be arranged as either single-unit or multi-unit fibers Copyright ©2021 John Wiley & Sons, Inc. 36  Smooth Muscle Smooth muscle contractions start more slowly and last longer than skeletal and cardiac muscle contractions. Smooth muscle can shorten and stretch to a greater extent than skeletal and cardiac muscle. Smooth muscle fibers shorten in response to stretch. Smooth muscle can regenerate compared to the other types of tissues Copyright ©2021 John Wiley & Sons, Inc. 37 Summary of Major Features of Three Types of Muscular Tissue Copyright ©2021 John Wiley & Sons, Inc. 38 Aging and Muscle Tissue Between 30–50 years of age, about 10% of our muscle tissue is replaced by fibrous connective tissue and adipose tissue. Between 50–80 years of age another 40% of our muscle tissue is replaced. Consequences are: Muscle strength and flexibility decreases Reflexes slow down Slow oxidative fiber numbers increase Copyright ©2021 John Wiley & Sons, Inc. 39