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Chapter 9 Muscle Tissue.

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Presentation on theme: "Chapter 9 Muscle Tissue."— Presentation transcript:

1 Chapter 9 Muscle Tissue

2 Three Types of Muscle Tissue
Skeletal muscle tissue: Attached to bones and skin Striated Voluntary (i.e., conscious control) Powerful Primary topic of this chapter

3 Three Types of Muscle Tissue
Cardiac muscle tissue: Only in the heart Striated

4 Three Types of Muscle Tissue
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

5 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

6 Muscle Functions Movement of bones or fluids (e.g., blood) Maintaining posture and body position Stabilizing joints Heat generation (especially skeletal muscle)

7 Each muscle is served by one artery, one nerve, and one or more veins
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

8 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

9 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

10 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

11 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

12 Skeletal Muscle Fiber

13 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 Each thick filament is about 300 myosin molecules bundled together with the tails forming the central part of the thick filament

14 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

15 Thick (myosin) and Thin (actin) Filaments

16 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 Ca2+ levels Release Ca2+ when muscle contracts

17 Continuous with the sarcolemma
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 Increase the muscle’s surface area Aid in conducting impulses to the deepest parts of the muscle and to every sarcomere

18 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

19 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

20 1 2 Figure 9.6 Z H Z I A I Fully relaxed sarcomere of a muscle fiber Z
Fully contracted sarcomere of a muscle fiber Figure 9.6

21 Requirements for Skeletal Muscle Contraction
Activation: neural stimulation at a neuromuscular junction Excitation-contraction coupling: Generation and propagation of an action potential along the sarcolemma Final trigger: a brief rise in intracellular Ca2+ levels

22 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

23 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 Situated midway along the length of a muscle fiber Muscle fiber and axon terminal (nerve ending) seperated by space – synaptic cleft Synaptic vesicles of axon terminal contain the neurotransmitter acetylcholine (ACh) Junctional folds of the sarcolemma contain ACh receptors

24 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 In people with myasthenia gravis the body produces antibodies that attack acetylcholine receptors on the muscle, preventing the signal from reaching the muscle properly and so weakening them.

25 Events in Generation of an Action Potential
Local depolarization (end plate potential) Generation and propagation of an action potential Repolarization Remember the resting membrane potential is -70 mV Local depolarization (end plate potential): ACh binding opens chemically (ligand) gated ion channels Simultaneous diffusion of Na+ (inward) and K+ (outward) More Na+ diffuses, so the interior of the sarcolemma becomes less negative Local depolarization – end plate potential Generation and propagation of an action potential: End plate potential spreads to adjacent membrane areas Voltage-gated Na+ channels open Na+ influx decreases the membrane voltage toward a critical threshold If threshold is reached, an action potential is generated (propogated) Repolarization: Na+ channels close and voltage-gated K+ channels open K+ efflux rapidly restores the resting polarity Fiber cannot be stimulated and is in a refractory period until repolarization is complete Ionic conditions of the resting state are restored by the Na+-K+ pump

26 Na+ channels close, K+ channels open Depolarization due to Na+ entry
Repolarization due to K+ exit Na+ channels open Threshold K+ channels close Figure 9.10

27 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 The action potential is short and gone long before any sign of contraction has taken place.

28 Events of Excitation-Contraction (E-C) Coupling
AP is propagated along sarcomere to T tubules Voltage-sensitive proteins stimulate Ca2+ release from SR Ca2+ is necessary for contraction

29 Role of Calcium (Ca2+) in Contraction
At low intracellular Ca2+ concentration: Tropomyosin blocks the active sites on actin Myosin heads cannot attach to actin Muscle fiber relaxes

30 Role of Calcium (Ca2+) in Contraction
At higher intracellular Ca2+ concentrations: Ca2+ binds to troponin Troponin changes shape and moves tropomyosin away from active sites Events of the cross bridge cycle occur When nervous stimulation ceases, Ca2+ is pumped back into the SR and contraction ends

31 Cross Bridge Cycle Continues as long as the Ca2+ 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 ATP is needed for the movement of the myosin cross bridge – new ATP is needed after each stroke

32 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 Rigor begins to settle in after 3-4 hours of death, peaks at 12 hours and then dissipates over the next hours

33 Review Principles of Muscle Mechanics
Same principles apply to contraction of a single fiber and a whole muscle Contraction produces tension, the force exerted on the load or object to be moved

34 Review Principles of Muscle Mechanics
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 Force and duration of contraction vary in response to stimuli of different frequencies and intensities

35 Motor Unit: The Nerve-Muscle Functional Unit
Motor unit = a motor neuron and all (four to several hundred) muscle fibers it supplies

36 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 Asynchronously means not all at once

37 Graded Muscle Responses
Variations in the degree of muscle contraction Required for proper control of skeletal movement Responses are graded by: Changing the frequency of stimulation Changing the strength of the stimulus Frequency A single stimulus results in a single contractile response—a muscle twitch Increase frequency of stimulus (muscle does not have time to completely relax between stimuli) Ca2+ release stimulates further contraction  temporal (wave) summation Further increase in stimulus frequency  unfused (incomplete) tetanus

38 Response to Change in Stimulus Frequency
If stimuli are given quickly enough, fused (complete) tetany results

39 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

40 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

41 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

42 Muscle Metabolism: Energy for Contraction
ATP is regenerated by: Direct phosphorylation of ADP by creatine phosphate (CP) Anaerobic pathway (glycolysis) Aerobic respiration Anaerobic metabolism At 70% of maximum contractile activity: Bulging muscles compress blood vessels Oxygen delivery is impaired Pyruvic acid is converted into lactic acid Lactic acid: Diffuses into the bloodstream Used as fuel by the liver, kidneys, and heart Converted back into pyruvic acid by the liver Aerobic Metabolism Produces 95% of ATP during rest and light to moderate exercise Fuels: stored glycogen, then bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids

43 Physiological inability to contract Occurs when:
Muscle Fatigue Physiological inability to contract Occurs when: Ionic imbalances (K+, Ca2+, Pi) interfere with E-C coupling Prolonged exercise damages the SR and interferes with Ca2+ regulation and release Total depletion of ATP rarely occurs If it does, contractures occur and cross bridging detachment can not occur E-C excitation contraction

44 Extra O2 needed after exercise for: Replenishment of
Oxygen Deficit Extra O2 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 Conversion of lactic acid in the blood to glucose or glycogen takes place in the liver

45 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

46 Force of Muscle Contraction
The force of contraction is affected by: Number of muscle fibers stimulated (recruitment) Relative size of the fibers—hypertrophy of cells increases strength Frequency of stimulation Length-tension relationship Number More fibers stimulated, greater the force of contraction Hypertrophy The larger the fiber, the greater the force Frequency  frequency allows time for more effective transfer of tension to noncontractile components Length-tension relationship—muscles contract most strongly when muscle fibers are 80–120% of their normal resting length

47 Velocity and Duration of Contraction
Influenced by: Muscle fiber type Load Recruitment Muscle vary in how fast and how long the can contract before fatigue

48 Classified according to two characteristics:
Muscle Fiber Type Classified according to two characteristics: 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 Duration of contraction in these fibers can vary also based on how quickly Ca2+ is moved from the cytosol to SR

49 Metabolic pathways for ATP synthesis:
Muscle Fiber Type Metabolic pathways for ATP synthesis: Oxidative (slow) fibers—use aerobic pathways Glycolytic (fast) fibers—use anaerobic glycolysis

50 Muscle Fiber Type Three types: Slow oxidative fibers
Fast oxidative fibers Fast glycolytic fibers

51 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

52 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

53 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

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