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Chapter 7 Lecture Slides Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Introduction • Muscle tissue is specialized for contraction • Contraction moves the body and body parts • Body has three types of muscle, differing in structure and function – Skeletal muscle – Smooth muscle – Cardiac muscle 7.1 Structure of Skeletal Muscle • Skeletal muscles is composed of mainly muscle fibers • Muscle fibers extend length of muscle • Muscle fibers are arranged in bundles called fascicles • Dense connective tissue surrounds each fiber, fascicle, and muscle – Establishes tendons to attach muscle to bone – Establishes aponeuroses to attach muscle to other muscles and connective tissues • Skeletal Muscle Fibers – Are multinucleated, long, thin cylinders with rounded ends that extend the length of the muscle – Sarcolemma is the plasma membrane – Sarcoplasm is the cytoplasm – Contain myofibrils, the contractile elements • Contain thin actin filaments and thick myosin filaments – Each myofibril consists of repeating contractile units called sarcomeres • The sacroplasmic reticulum is the name given to the smooth endoplasmic reticulum in a muscle cell. – Stores calcium (Ca2+) ions. • The transverse (T) tubule system – Invaginations of the sarcolemma that penetrate into the fiber so that they lie alongside and contact the sarcoplasmic reticulum. • The sacroplasmic reticulum is the name given to the smooth endoplasmic reticulum in a muscle cell. – Stores calcium (Ca2+) ions. • The transverse (T) tubule system – Extensions of the sarcolemma that penetrate into the fiber. – Carries impulses that causes the sarcoplasmic reticulum to release Ca2+. • Neuromuscular Interaction – A motor neuron sends impulses to a muscle fiber, which produces an action – Each muscle fiber is innervated and controlled by a motor neuron – Without nervous stimulation, a muscle fiber cannot contract • Motor Units – A motor neuron and all the muscle fibers it contacts – Precise control: motor unit with very few muscle fibers – No precise control: motor units contain 100s of muscle fibers – Motor neuron activation causes contraction of all associated muscle fibers • Neuromuscular Junction – Connection between axon of motor neuron and sarcolemma of muscle fiber – Space between axon and sarcolemma is synaptic cleft – Axon terminal has vesicles with neurotransmitter acetylcholine (ACh) – Nerve impulse reaches the axon terminal and ACh is released into the synaptic cleft – ACh attaches to receptors on the sarcolemma at the motor end plate – Begins series of reactions leading to contraction 7.2 Physiology of Muscle Contraction • Contraction involves a number of rapid structural and chemical changes within a muscle fiber • Explained by the Sliding Filament Model Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. • Mechanism of Contraction – ACh binds to receptors on the sarcolemma – Stimulates the release of Ca2+ from sarcoplasmic reticulum – Ca2+ binds to actin filaments, exposing active sites on actin filaments (2) – Myosin cross-bridges attach to actin active sites (3) – Using ATP energy, cross-bridges bend and pull actin filaments towards center of sarcomere (4) – Cross-bridges detach and reattach to active sites (5-6) – Cycle repeats as long as Ca2+ and ATP are present – Acetylcholinesterase from sarcolemma decomposes Ach in the synaptic cleft • Prevent continual stimulation of muscle fiber • Prepares fiber for next stimulus Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. Please note that due to differing operating systems, some animations will not appear until the presentation is viewed in Presentation Mode (Slide Show view). You may see blank slides in the “Normal” or “Slide Sorter” views. All animations will appear after viewing in Presentation Mode and playing each animation. Most animations will require the latest version of the Flash Player, which is available at http://get.adobe.com/flashplayer. • Energy for Contraction – Energy comes from ATP – Due to small amount of ATP in cells, more ATP must be formed to support contractions – Two possible pathways for making energy • Creatine phosphate • Cellular respiration – Creatine phosphate • Storage form of readily available energy – Stores energy from excess ATP • Energy is transferred back to ADP when ATP levels decrease • Depleted quickly in rapidly contracting muscle • Oxygen and Cellular Respiration – Cellular respiration is a 2 step process • Anaerobic phase • Aerobic phase – Requires oxygen to operate and produce ATP – Most ATP is produced during this phase – Oxygen for aerobic phase comes from • Hemoglobin in red blood cells • Myoglobin in muscle cells – Glucose is the main energy source – Sufficient oxygen levels allow for aerobic respiration to occur • Pyruvic acid from anaerobic phase to CO2 and H2O – Insufficient oxygen with strenuous exercise prevents aerobic respiration • Pyruvic acid from anaerobic phase converts to lactic acid – Causes discomfort and rapid, deep breathing – To remove lactic acid, it must be • Broken down by aerobic respiration • Converted back into glucose • Both require oxygen to occur – Oxygen debt: amount of oxygen required to metabolize lactic acid • Also restore normal ATP and creatine phosphate levels • Deep breathing occurs until debt is paid – Endurance training increases efficiency of aerobic cellular respiration by increasing • Number of mitochondria • Efficiency of obtaining oxygen from blood • Concentration of myoglobin • Fatigue: the reduced ability to do work – Occurs with continued nervous stimulation – Gradual decrease in contraction ending with an inability to contract with stimulation – Causes • Accumulated lactic acid and carbon dioxide • Depletion of ATP • Heat Production – Heat from muscle contraction is used to maintain normal body temperature • Decrease in body temperature results in shivering – Heat production in muscle is caused by • Cellular respiration • Other chemical reactions within the cell Types of Skeletal Muscle Fibers • Muscle fibers contract at different speeds, and vary in how quickly they fatigue • Muscle fibers are classified into three main types – Slow oxidative fibers (Type I) – Fast oxidative-glycolytic fibers (Type IIA) – Fast glycolytic fibers (Type IIB) Slow Oxidative Fibers (SO fibers) – Least powerful type of muscle fibers – Appear dark red (more myoglobin) – Generate ATP mainly by aerobic cellular respiration • Rich blood supply and many mitochondria – Have a slow speed of contraction – Very resistant to fatigue – Capable of prolonged, sustained contractions for many hours – Adapted for maintaining posture and for aerobic, endurance-type activities such as running a marathon Fast Oxidative–Glycolytic Fibers (FOG fibers) – Intermediate fibers – Generate considerable ATP by aerobic cellular respiration (still has decent blood supply, large amount of myglobin, and many mitochondria) – Resistance to fatigue is intermediate – Generate some ATP by anaerobic glycolysis – Speed of contraction faster – Contribute to activities such as walking Fast Glycolytic Fibers (FG fibers) – Generate the most powerful contractions – Have low myoglobin content – Relatively few blood capillaries – Few mitochondria – Appear white in color – Generate ATP mainly by glycolysis – Fibers contract strongly and quickly – Fatigue quickly – Adapted for intense anaerobic movements of short duration like weight lifting or sprinting Types of Skeletal Muscle Fibers • Distribution and Recruitment of Different Types of Fibers – Most muscles are a mixture of all three types of muscle fibers – Proportions vary, depending on the action of the muscle, the person’s training regimen, and genetic factors • Postural muscles of the neck, back, and legs have a high proportion of SO fibers • Muscles of the shoulders and arms have a high proportion of FG fibers • Leg muscles have large numbers of both SO and FOG fibers • Contraction Characteristics – Contraction of a Single Muscle Fiber • Threshold stimulus is the minimal stimulus that will cause a muscle fiber to contract • At threshold, contraction of a muscle fiber always follows the all-or-none response – Always contracts completely or not at all – Contraction is not proportional to stimulus strength – Contraction of Whole Muscles • Muscle contractions are recorded in myograms • A single contraction due to a threshold stimulus has three intervals – Latent period – Period of contraction – Period of relaxation • Graded Responses – Varying degrees of contraction in whole muscles • Due to presence of different motor units responding to different thresholds of stimulation • Recruitment – increasing the number of motor units to work – Maximal stimulus activates all motor units and produces maximal contraction • Further increases in stimulus strength does not produce stronger contractions • Muscle Tone – State of partial contraction in relaxed muscle – Keeps a muscle ready to respond – Due to alternating activation of different motor units – Loss of nervous innervation results in loss of muscle tone and atrophy (decrease in muscle size) 7.3 Actions of Skeletal Muscles • Origin and insertion – Insertion is the movable muscle attachment – Origin is the immovable muscle attachment – Isotonic contractions cause movement of a joint – Isometric contractions increase tension but do not cause movement – Effects of exercise on skeletal muscles • Strength training causes hypertrophy – Increase in myofibril number in muscle fibers • Endurance training improves the efficiency of muscle action but not hypertrophy – Increases number of mitochondria and blood vessels – Increases oxygen, nutrient, and ATP supply • Muscle Interactions – Muscles function in groups – Groups arranged to provide opposing movements • Agonists produce an action • Antagonists produce the opposite action – Agonists and antagonists contract alternately Muscle Disorders • Cramps – Involuntary, painful, sustained tetanic contractions – Possible causes • Chemical changes in the muscle • Physical blow to the muscle • Strains or “pulled muscles” – Due to excessive muscle stretching – Mild strains damage only a few muscle fibers – Severe strains tear both connective and muscle tissues • Severe impairment of muscle function • Spasms – Sudden, involuntary contractions of a muscle or group of muscles – Causes • Irritation of motor neurons • Emotional stress • Neurological disorders