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Muscles and Muscle Tissue

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Title: Muscles and Muscle Tissue


1
Muscles and Muscle Tissue
  • Form and Function of Movement

2
Muscle Tissue
  • Anatomy and Histology

3
Functional Characteristics of Muscle Tissue
  • Excitability, or irritability the ability to
    receive and respond to stimuli
  • Contractility the ability to shorten forcibly
  • Extensibility the ability to be stretched or
    extended
  • Elasticity the ability to recoil and resume the
    original resting length

4
Muscle Overview
  • The three types of muscle tissue are skeletal,
    cardiac, and smooth
  • These types differ in structure, location,
    function, and means of activation

5
  • Cardiac muscle cells branch, are striated, are
    uninucleate (B) and have intercalated discs (A).
  • Locations heart
  • Function involuntary, rhythmic contraction

6
Skeletal Muscle
  • Skeletal muscle cells run the full length of a
    muscle. Line A show the width of one cell
    (fiber). Note the striations characteristics of
    this muscle type. These cells are multicellular,
    B marks one nucleus.
  • Location muscles associated with the skeleton
  • Function voluntary movement
  • Muscles are connected to bones by tendons. Bones
    are connected to other bones at their joints by
    ligaments.

7
Smooth Muscle
  • Smooth muscle cells are spindle shaped and
    uninucleate. (B).
  • Locations walls of hollow organs, i.e. stomach,
    intestine, uterus, ureter
  • Functions involuntary movement - i.e. churning
    of food, movement of urine from the kidney to the
    bladder, partuition

8
Skeletal Muscle
Figure 9.2 (a)
9
Myofibrils
Figure 9.3 (b)
10
Sarcomeres
Figure 9.3 (c)
11
Myofilaments Banding Pattern
12
Ultrastructure of Myofilaments Thick Filaments
Figure 9.4 (a)(b)
13
Ultrastructure of Myofilaments Thick Filaments
  • Thick filaments are composed of the protein
    myosin
  • Each myosin molecule has a rodlike tail and two
    globular heads
  • Tails two interwoven, heavy polypeptide chains
  • Heads two smaller, light polypeptide chains
    called cross bridges

14
Ultrastructure of Myofilaments Thin Filaments
Figure 9.4 (c)
15
Ultrastructure of Myofilaments Thin Filaments
  • Thin filaments are chiefly composed of the
    protein actin
  • Tropomyosin and troponin are regulatory subunits
    bound to actin

16
Arrangement of the Filaments in a Sarcomere
17
Sarcoplasmic Reticulum (SR)
Figure 9.5
18
Sarcoplasmic Reticulum (SR)
  • SR smooth endoplasmic reticulum that mostly runs
    longitudinally and surrounds each myofibril
  • Paired terminal cisternae form perpendicular
    cross channels
  • Functions in the regulation of intracellular
    calcium levels

19
T Tubules
  • T tubules are continuous with the sarcolemma
  • They conduct impulses to the deepest regions of
    the muscle
  • These impulses signal for the release of Ca2
    from adjacent terminal cisternae

20
Muscle Cell Contraction
  • Sliding Filament Theory

21
Excitation of a Muscle Fiber
22
Excitation (steps 1 2)
  • Nerve signal stimulates voltage-gated calcium
    channels that result in exocytosis of synaptic
    vesicles containing ACh ACh release

23
Excitation (steps 3 4)
  • Binding of ACh to the surface of muscle cells
    opens Na and K channels resulting in an
    end-plate potential (EPP)

24
Excitation (step 5)
  • Voltage change in end-plate region (EPP) opens
    nearby voltage-gated channels in plasma membrane
    producing an action potential

25
Excitation-Contraction Coupling
26
Excitation-Contraction Coupling(steps 67)
  • Action potential spreading over sarcolemma
    reaches and enters the T tubules -- voltage-gated
    channels open in T tubules causing calcium gates
    to open in SR

27
Excitation-Contraction Coupling(steps 89)
  • Calcium released by SR binds to troponin
  • Troponin-tropomyosin complex changes shape and
    exposes active sites on actin

28
Contraction (steps 10 11)
  • Myosin ATPase in myosin head hydrolyzes an ATP
    molecule, activating the head and cocking it
    in an extended position
  • It binds to an active site on actin

29
Contraction (steps 12 13)
  • Power stroke shows myosin head releasing the
    ADP phosphate as it flexes pulling the thin
    filament along
  • With the binding of more ATP, the myosin head
    releases the thin filament and extends to
    attach to a new active site further down the
    thin filament
  • at any given moment, half of the heads are bound
    to a thin filament, preventing slippage
  • thin and thick filaments do not become shorter,
    just slide past each other (sliding filament
    theory)

12. Power Stroke sliding of thin filament over
thick
30
Relaxation (steps 14 15)
  • Nerve stimulation ceases and acetylcholinesterase
    removes ACh from receptors so stimulation of the
    muscle cell ceases

31
Relaxation (step 16)
  • Active transport pumps calcium from sarcoplasm
    back into SR where it binds to calsequestrin
  • ATP is needed for muscle relaxation as well as
    muscle contraction

32
Relaxation (steps 17 18)
33
Muscle Tone
  • Muscle tone
  • Is the constant, slightly contracted state of all
    muscles, which does not produce active movements
  • Keeps the muscles firm, healthy, and ready to
    respond to stimulus
  • Spinal reflexes account for muscle tone by
  • Activating one motor unit and then another
  • Responding to activation of stretch receptors in
    muscles and tendons

34
Isotonic Contractions
  • In isotonic contractions, the muscle changes in
    length (decreasing the angle of the joint) and
    moves the load
  • The two types of isotonic contractions are
    concentric and eccentric
  • Concentric contractions the muscle shortens and
    does work
  • Eccentric contractions the muscle contracts as
    it lengthens

35
Isotonic Contractions
Figure 9.17 (a)
36
Isometric Contractions
  • Tension increases to the muscles capacity, but
    the muscle neither shortens nor lengthens
  • Occurs if the load is greater than the tension
    the muscle is able to develop

37
Isometric Contractions
Figure 9.17 (b)
38
Muscle Metabolism Energy for Contraction
  • ATP is the only source used directly for
    contractile activity
  • As soon as available stores of ATP are hydrolyzed
    (4-6 seconds), they are regenerated by
  • The interaction of ADP with creatine phosphate
    (CP)
  • Anaerobic glycolysis
  • Aerobic respiration

39
Muscle Metabolism Energy for Contraction
Figure 9.18
40
Muscle Metabolism Anaerobic Glycolysis
  • When muscle contractile activity reaches 70 of
    maximum
  • Bulging muscles compress blood vessels
  • Oxygen delivery is impaired
  • Pyruvic acid is converted into lactic acid

41
Muscle Metabolism Anaerobic Glycolysis
  • The lactic acid
  • Diffuses into the bloodstream
  • Is picked up and used as fuel by the liver,
    kidneys, and heart
  • Is converted back into pyruvic acid by the liver

42
Muscle Fatigue
  • Muscle fatigue the muscle is in a state of
    physiological inability to contract
  • Muscle fatigue occurs when
  • ATP production fails to keep pace with ATP use
  • There is a relative deficit of ATP, causing
    contractures
  • Lactic acid accumulates in the muscle
  • Ionic imbalances are present

43
Muscle Fatigue
  • Intense exercise produces rapid muscle fatigue
    (with rapid recovery)
  • Na-K pumps cannot restore ionic balances
    quickly enough
  • Low-intensity exercise produces slow-developing
    fatigue
  • SR is damaged and Ca2 regulation is disrupted

44
Oxygen Debt
  • Vigorous exercise causes dramatic changes in
    muscle chemistry
  • For a muscle to return to a resting state
  • Oxygen reserves must be replenished
  • Lactic acid must be converted to pyruvic acid
  • Glycogen stores must be replaced
  • ATP and CP reserves must be resynthesized
  • Oxygen debt the extra amount of O2 needed for
    the above restorative processes

45
Heat Production During Muscle Activity
  • Only 40 of the energy released in muscle
    activity is useful as work
  • The remaining 60 is 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
  • The number of muscle fibers contracting the
    more motor fibers in a muscle, the stronger the
    contraction
  • The relative size of the muscle the bulkier the
    muscle, the greater its strength
  • Degree of muscle stretch muscles contract
    strongest when muscle fibers are 80-120 of their
    normal resting length

47
Force of Muscle Contraction
Figure 9.20 (a)
48
Muscle Fiber Type Functional Characteristics
  • Speed of contraction determined by speed in
    which ATPases split ATP
  • The two types of fibers are slow and fast
  • ATP-forming pathways
  • Oxidative fibers use aerobic pathways
  • Glycolytic fibers use anaerobic glycolysis
  • These two criteria define three categories slow
    oxidative fibers, fast oxidative fibers, and fast
    glycolytic fibers

49
Muscle Fiber Type Speed of Contraction
  • Slow oxidative fibers contract slowly, have slow
    acting myosin ATPases, and are fatigue resistant
  • Fast oxidative fibers contract quickly, have fast
    myosin ATPases, and have moderate resistance to
    fatigue
  • Fast glycolytic fibers contract quickly, have
    fast myosin ATPases, and are easily fatigued

50
Smooth Muscle
  • Composed of spindle-shaped fibers with a diameter
    of 2-10 ?m and lengths of several hundred ?m
  • Lack the coarse connective tissue sheaths of
    skeletal muscle, but have fine endomysium
  • Organized into two layers (longitudinal and
    circular) of closely apposed fibers
  • Found in walls of hollow organs (except the
    heart)
  • Have essentially the same contractile mechanisms
    as skeletal muscle

51
Smooth Muscle
Figure 9.24
52
Peristalsis
  • When the longitudinal layer contracts, the organ
    dilates and contracts
  • When the circular layer contracts, the organ
    elongates
  • Peristalsis alternating contractions and
    relaxations of smooth muscles that mix and
    squeeze substances through the lumen of hollow
    organs

53
Innervation of Smooth Muscle
  • Smooth muscle lacks neuromuscular junctions
  • Innervating nerves have bulbous swellings called
    varicosities
  • Varicosities release neurotransmitters into wide
    synaptic clefts called diffuse junctions

54
Innervation of Smooth Muscle
Figure 9.25
55
Microscopic Anatomy of Smooth Muscle
  • SR is less developed than in skeletal muscle and
    lacks a specific pattern
  • T tubules are absent
  • Plasma membranes have pouchlike infoldings called
    caveoli
  • Ca2 is sequestered in the extracellular space
    near the caveoli, allowing rapid influx when
    channels are opened
  • There are no visible striations and no sarcomeres
  • Thin and thick filaments are present

56
Proportion and Organization of Myofilaments in
Smooth Muscle
  • Ratio of thick to thin filaments is much lower
    than in skeletal muscle
  • Thick filaments have heads along their entire
    length
  • There is no troponin complex
  • Thick and thin filaments are arranged diagonally,
    causing smooth muscle to contract in a corkscrew
    manner
  • Noncontractile intermediate filament bundles
    attach to dense bodies (analogous to Z discs) at
    regular intervals

57
Proportion and Organization of Myofilaments in
Smooth Muscle
Figure 9.26
58
Contraction of Smooth Muscle
  • Whole sheets of smooth muscle exhibit slow,
    synchronized contraction
  • They contract in unison, reflecting their
    electrical coupling with gap junctions
  • Action potentials are transmitted from cell to
    cell
  • Some smooth muscle cells
  • Act as pacemakers and set the contractile pace
    for whole sheets of muscle
  • Are self-excitatory and depolarize without
    external stimuli

59
Types of Smooth Muscle Single Unit
  • The cells of single-unit smooth muscle, commonly
    called visceral muscle
  • Contract rhythmically as a unit
  • Are electrically coupled to one another via gap
    junctions
  • Often exhibit spontaneous action potentials
  • Are arranged in opposing sheets and exhibit
    stress-relaxation response

60
Types of Smooth Muscle Multiunit
  • Multiunit smooth muscles are found
  • In large airways to the lungs
  • In large arteries
  • In arrector pili muscles
  • Attached to hair follicles
  • In the internal eye muscles

61
Types of Smooth Muscle Multiunit
  • Their characteristics include
  • Rare gap junctions
  • Infrequent spontaneous depolarizations
  • Structurally independent muscle fibers
  • A rich nerve supply, which, with a number of
    muscle fibers, forms motor units
  • Graded contractions in response to neural stimuli

62
Contraction Mechanism
  • Actin and myosin interact according to the
    sliding filament mechanism
  • The final trigger for contractions is a rise in
    intracellular Ca2
  • Ca2 is released from the SR and from the
    extracellular space
  • Ca2 interacts with calmodulin and myosin light
    chain kinase to activate myosin

63
Role of Calcium Ion
  • Ca2 binds to calmodulin and activates it
  • Activated calmodulin activates the kinase enzyme
  • Activated kinase transfers phosphate from ATP to
    myosin cross bridges
  • Phosphorylated cross bridges interact with actin
    to produce shortening
  • Smooth muscle relaxes when intracellular Ca2
    levels drop

64
Special Features of Smooth Muscle Contraction
  • Unique characteristics of smooth muscle include
  • Smooth muscle tone
  • Slow, prolonged contractile activity
  • Low energy requirements
  • Response to stretch

65
Response to Stretch
  • Smooth muscle exhibits a phenomenon called
    stress-relaxation response in which
  • Smooth muscle responds to stretch only briefly,
    and then adapts to its new length
  • The new length, however, retains its ability to
    contract
  • This enables organs such as the stomach and
    bladder to temporarily store contents
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