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Copyright © 2010 Pearson Education, Inc. Chapter 9 Muscle Tissue
Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 1.Skeletal muscle tissue: Attached to bones and skin Striated Voluntary (i.e., conscious control) Powerful Primary topic of this chapter
Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 2.Cardiac muscle tissue: Only in the heart Striated
Copyright © 2010 Pearson Education, Inc. Three Types of Muscle Tissue 3.Smooth muscle tissue: In the walls of hollow organs, e.g., stomach, urinary bladder, and airways Not striated Involuntary More details later in this chapter
Copyright © 2010 Pearson Education, Inc. Special Characteristics of Muscle Tissue Excitability (responsiveness or irritability): ability to receive and respond to stimuli Contractility: ability to shorten when stimulated Extensibility: ability to be stretched Elasticity: ability to recoil to resting length
Copyright © 2010 Pearson Education, Inc. Muscle Functions 1.Movement of bones or fluids (e.g., blood) 2.Maintaining posture and body position 3.Stabilizing joints 4.Heat generation (especially skeletal muscle)
Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Each muscle is served by one artery, one nerve, and one or more veins All enter or exit near the central part of muscle Rich blood supply Give off large amounts of waste
Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Connective tissue sheaths of skeletal muscle: Epimysium: dense regular connective tissue surrounding entire muscle Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) Endomysium: fine areolar connective tissue surrounding each muscle fiber
Copyright © 2010 Pearson Education, Inc. Myofibrils Densely packed, rodlike elements ~80% of cell volume Exhibit striations: perfectly aligned repeating series of dark A bands and light I bands Contain the contractile elements of skeletal muscle
Copyright © 2010 Pearson Education, Inc. Sarcomere Smallest contractile unit (functional unit) of a muscle fiber The region of a myofibril between two successive Z discs Composed of thick and thin myofilaments made of contractile proteins
Copyright © 2010 Pearson Education, Inc. Features of a Sarcomere Thick filaments (myosin): run the entire length of an A band Thin filaments (actin): run the length of the I band and partway into the A band Z disc: coin-shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another H zone: lighter midregion where filaments do not overlap M line: line of protein myomesin that holds adjacent thick filaments together
Copyright © 2010 Pearson Education, Inc. Skeletal Muscle Fiber
Copyright © 2010 Pearson Education, Inc. Ultrastructure of Thick Filament Composed of the protein myosin Myosin tails contain: 2 interwoven chains Myosin heads contain: 2 smaller chains that act as cross bridges during contraction Link the thick and thin filaments together Binding sites for ATP ATPase enzymes-split ATP to generate energy
Copyright © 2010 Pearson Education, Inc. Ultrastructure of Thin Filament Composed of actin Actin bears active sites for myosin head attachment during contraction Tropomyosin and troponin: regulatory proteins bound to actin Both help control the myosin-actin interactions involved in contractions
Copyright © 2010 Pearson Education, Inc. Thick (myosin) and Thin (actin) Filaments
Copyright © 2010 Pearson Education, Inc. Sarcoplasmic Reticulum (SR) Network of smooth endoplasmic reticulum surrounding each myofibril Pairs of terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular Ca 2+ levels Release Ca 2+ when muscle contracts
Copyright © 2010 Pearson Education, Inc. T Tubules Continuous with the sarcolemma Penetrate the cell’s interior at each A band–I band junction Associate with the paired terminal cisternae to form triads that encircle each sarcomere
Copyright © 2010 Pearson Education, Inc. Contraction The generation of force Does not necessarily cause shortening of the fiber Shortening occurs when tension generated by cross bridges on the thin filaments exceeds forces opposing shortening
Copyright © 2010 Pearson Education, Inc. Sliding Filament Model of Contraction In the relaxed state, thin and thick filaments overlap only slightly During contraction, myosin heads bind to actin, detach, and bind again, to propel the thin filaments toward the M line As H zones shorten and disappear, sarcomeres shorten, muscle cells shorten, and the whole muscle shortens
Copyright © 2010 Pearson Education, Inc. Figure 9.6 I Fully relaxed sarcomere of a muscle fiber Fully contracted sarcomere of a muscle fiber I A ZZ H IIA ZZ 1 2
Copyright © 2010 Pearson Education, Inc. Requirements for Skeletal Muscle Contraction 1.Activation: neural stimulation at a neuromuscular junction 2.Excitation-contraction coupling: Generation and propagation of an action potential along the sarcolemma 3.Final trigger: a brief rise in intracellular Ca 2+ levels
Copyright © 2010 Pearson Education, Inc. Events at the Neuromuscular Junction Axons of motor neurons travel from the central nervous system via nerves to skeletal muscles Each axon forms several branches as it enters a muscle Each axon ending forms a neuromuscular junction with a single muscle fiber
Copyright © 2010 Pearson Education, Inc. Events at the Neuromuscular Junction Nerve impulse arrives at axon terminal ACh is released and binds with receptors on the sarcolemma Electrical events lead to the generation of an action potential
Copyright © 2010 Pearson Education, Inc. Destruction of Acetylcholine ACh effects are quickly terminated by the enzyme acetylcholinesterase Prevents continued muscle fiber contraction in the absence of additional stimulation Myasthenia gravis – shortage of Ach receptors; autoimmune disease
Copyright © 2010 Pearson Education, Inc. Events in Generation of an Action Potential 1.Local depolarization (end plate potential) 2.Generation and propagation of an action potential 3.Repolarization
Copyright © 2010 Pearson Education, Inc. Figure 9.10 Na + channels close, K + channels open K + channels close Repolarization due to K + exit Threshold Na + channels open Depolarization due to Na+ entry
Copyright © 2010 Pearson Education, Inc. Excitation-Contraction (E-C) Coupling Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments Latent period: Time when E-C coupling events occur Time between AP initiation and the beginning of contraction
Copyright © 2010 Pearson Education, Inc. Events of Excitation-Contraction (E-C) Coupling AP is propagated along sarcomere to T tubules Voltage-sensitive proteins stimulate Ca 2+ release from SR Ca 2+ is necessary for contraction
Copyright © 2010 Pearson Education, Inc. Role of Calcium (Ca 2+ ) in Contraction At low intracellular Ca 2+ concentration: Tropomyosin blocks the active sites on actin Myosin heads cannot attach to actin Muscle fiber relaxes
Copyright © 2010 Pearson Education, Inc. Role of Calcium (Ca 2+ ) in Contraction At higher intracellular Ca 2+ concentrations: Ca 2+ binds to troponin Troponin changes shape and moves tropomyosin away from active sites Events of the cross bridge cycle occur When nervous stimulation ceases, Ca 2+ is pumped back into the SR and contraction ends
Copyright © 2010 Pearson Education, Inc. Cross Bridge Cycle Continues as long as the Ca 2+ signal and adequate ATP are present Cross bridge formation—high-energy myosin head attaches to thin filament Working (power) stroke—myosin head pivots and pulls thin filament toward M line
Copyright © 2010 Pearson Education, Inc. Cross Bridge Cycle Rigor mortis Dying cells can not remove calcium This promotes myosin cross bridging After breathing stops, ATP synthesis stops but it is still used Cross bridging detachment is impossible Only thing that stops it is muscle protein breakdown
Copyright © 2010 Pearson Education, Inc. Review Principles of Muscle Mechanics 1.Same principles apply to contraction of a single fiber and a whole muscle 2.Contraction produces tension, the force exerted on the load or object to be moved
Copyright © 2010 Pearson Education, Inc. Review Principles of Muscle Mechanics 3.Contraction does not always shorten a muscle: Isometric contraction: no shortening; muscle tension increases but does not exceed the load Isotonic contraction: muscle shortens because muscle tension exceeds the load 4.Force and duration of contraction vary in response to stimuli of different frequencies and intensities
Copyright © 2010 Pearson Education, Inc. Motor Unit: The Nerve-Muscle Functional Unit Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies
Copyright © 2010 Pearson Education, Inc. Motor Unit Muscle fibers from a motor unit are spread throughout the muscle so that a single motor unit causes weak contraction of entire muscle Motor units in a muscle usually contract asynchronously; helps prevent fatigue
Copyright © 2010 Pearson Education, Inc. Graded Muscle Responses Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: 1.Changing the frequency of stimulation 2.Changing the strength of the stimulus
Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Frequency If stimuli are given quickly enough, fused (complete) tetany results
Copyright © 2010 Pearson Education, Inc. Response to Change in Stimulus Strength Threshold stimulus: stimulus strength at which the first observable muscle contraction occurs Muscle contracts more vigorously as stimulus strength is increased above threshold Motor unit summation – the more motor units recruited, the stronger the contraction
Copyright © 2010 Pearson Education, Inc. Isotonic Contractions Muscle changes in length and moves the load Isotonic contractions are either concentric or eccentric: Concentric contractions—the muscle shortens and does work Eccentric contractions—the muscle contracts as it lengthens
Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is the only source used directly for contractile activities Supplies the energy needed cross bridge movement Also operates the calcium pump Available stores of ATP are depleted in 4–6 seconds
Copyright © 2010 Pearson Education, Inc. Muscle Metabolism: Energy for Contraction ATP is regenerated by: 1.Direct phosphorylation of ADP by creatine phosphate (CP) 2.Anaerobic pathway (glycolysis) 3.Aerobic respiration
Copyright © 2010 Pearson Education, Inc. Muscle Fatigue Physiological inability to contract Occurs when: Ionic imbalances (K +, Ca 2+, P i ) interfere with E- C coupling Prolonged exercise damages the SR and interferes with Ca 2+ regulation and release Total depletion of ATP rarely occurs If it does, contractures occur and cross bridging detachment can not occur
Copyright © 2010 Pearson Education, Inc. Oxygen Deficit Extra O 2 needed after exercise for: Replenishment of Oxygen reserves Glycogen stores must be replenished ATP and CP reserves must be resynthesized Conversion of lactic acid to pyruvic acid, glucose, and glycogen
Copyright © 2010 Pearson Education, Inc. Heat Production During Muscle Activity ~ 40% of the energy released in muscle activity is useful as work Remaining energy (60%) given off as heat Dangerous heat levels are prevented by radiation of heat from the skin and sweating
Copyright © 2010 Pearson Education, Inc. Force of Muscle Contraction The force of contraction is affected by: 1.Number of muscle fibers stimulated (recruitment) 2.Relative size of the fibers—hypertrophy of cells increases strength 3.Frequency of stimulation 4.Length-tension relationship
Copyright © 2010 Pearson Education, Inc. Velocity and Duration of Contraction Influenced by: 1.Muscle fiber type 2.Load 3.Recruitment
Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type Classified according to two characteristics: 1.Speed of contraction: slow twitch or fast twitch, according to: Speed at which myosin ATPases split ATP Pattern of electrical activity of the motor neurons
Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type 2.Metabolic pathways for ATP synthesis: Oxidative (slow) fibers—use aerobic pathways Glycolytic (fast) fibers—use anaerobic glycolysis
Copyright © 2010 Pearson Education, Inc. Muscle Fiber Type Three types: Slow oxidative fibers Fast oxidative fibers Fast glycolytic fibers
Copyright © 2010 Pearson Education, Inc. Effects of Exercise Aerobic (endurance) exercise: Leads to increased: Muscle capillaries Number of mitochondria Myoglobin synthesis Results in greater endurance, strength, and resistance to fatigue May convert fast glycolytic fibers into fast oxidative fibers
Copyright © 2010 Pearson Education, Inc. Effects of Resistance Exercise Resistance exercise (typically anaerobic) results in: Muscle hypertrophy (due to increase in fiber size) Increased mitochondria, myofilaments, glycogen stores, and connective tissue
Copyright © 2010 Pearson Education, Inc. The Overload Principle Forcing a muscle to work hard promotes increased muscle strength and endurance Muscles adapt to increased demands Muscles must be overloaded to produce further gains
Copyright © 2010 Pearson Education, Inc. Table 9.3.
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