Ch 9: Muscle and Muscle Tissue Flashcards


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1

How many types of Muscle Tissue?

Three:

1. Skeletal
2. Cardiac
3. Smooth

2

Skeletal Muscle

  • Attach to bones/skin
  • striated
  • voluntary (conscious control)
  • powerful
3

Cardiac Muscle

  • only in heart
  • striated
  • involuntary
4

Smooth Muscle

  • in hollow organs (stomach, bladder, airways)
  • not striated
  • involuntary
5

What enables muscle tissue to perform its duties

1. Excitability (responsiveness/irritability) : ability to receive and respond to stimuli (usually a chemical-neutotransmitter or pH changes)
2. Contractility: ability to SHORTEN when stimulated
3. Extensibility: ability to EXTEND/STRETCH
4. Elasticity: ability to RECOIL and resume resting length

6

Muscle Functions

1. Movement of bones or fluids (ie blood)
2. Maintaining posture and body position
3. Stabilizing joints
4. Heat generation (especially skeletal muscle)
5. Protect internal organs (ie viscera)

7

What kinds of tissues constitued in skeletal muscle?

1. Fibers (predominant)
2. Blood vessels
3. Nerves
4. connective tissues

8

Gross Anatomy

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

9

Epimysium

dense regular tissue surrounding entire muscle (externally)

10

Perimysium

fibrous connective tissue surround fascicles (groups of muscle fibers)

11

Endomysium

fine areolar connective tissue surrounding each muscle fiber

12

How Muscles attach to bones------ DIRECTLY

epimysium of muscle is fused to periosteum of bone or perichondrium of cartilage

13

How muscles attach to bones ----- INDIRECTLY

connective tissue wrappings extend beyond the muscle as a ropelike tendon or sheet-like aponeurosis

14

Microscopic anatomy of skeletal muscle fiber

-cylindrial cell (10 to 100 um in diameter, up to 30 cm long)
-multinucleated
-many mitchondria
-glycosomes for glycogen storage
-myoglobin for O2 storage
-contains myofibrils, sarcoplasmic reticulum, and T tubules where Ca is stored

15

Myofibrils

densley packed, rodlike elements

80% of cell volume

exhibit striations: perfectly aligned repeating series of dark A bands and light I bands

16

Sarcomere

smallest contractile unit (functional unit) of a muscle fiber

the region of a myofibril between two successive Zdiscs

composed of thick and thin myofilaments made of contractile proteins

17

Myosin filaments

-Thick filaments: run the entire length of an A band

18

Actin Filaments

Thin filaments: run the length of the I band and partway into the A band

19

Z disc

coin shaped sheet of proteins that anchors the thin filaments and connects myofibrils to one another

20

H zone

lighter midregion where filaments do not overlap

21

M line

line of protein myomesin that holds adjacent thick filaments together

22

Ultrastructure of the thick filaments

compose of myosin
-myosin tail: 2 interwoven, heavy polypeptide chains

-myosin head: 2 smaller, light polypeptide chains that act as cross bridges during contraction, binding sites for actin, binding sites for ATP, ATPase enzymes

23

Ultrastructure of thick filaments

-twisted double strand of fibrous protein F Actin
-Factin consists of G (globular) actin subunits
-G actin bears active sites for myosin head attachment during contraction
-Tropomyosin and troponin: regulatory proteins bound to actin

24

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 levels

25

Transverse Tubules (T Tubules)

-continuous with the sarcolemma
-penetrate the cell's interior at each A band-Iband junction
-associate with the paired terminal cisternae to form triads that encircle each sarcomere

26

Triad Relationship

-T Tubules and SR are tightly linked and come into close contact and provide signals for contraction
-T tubules conduct impulses deep into muscle fiber
-Integral proteins protrude into the intermembrane space from T tubule and SR cisternae membranes
-T Tubule proteins: voltage sensors
-SR foot proteins: gated channels that regulate Ca releaase from the SR cisternae

27

Contraction

-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

28

Sliding Filament Model of Contraction

-States during contraction thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree.
-in 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

29

Requirements for Skeletal Muscle Contraction

1. Activation: neural stimulation at a neuromuscular junction: impulse => action potential

2. Excitation-contraction coupling: generation and propagation of an action potention along the sarcolemma; Final trigger: a brief rise in intracellular Ca levels

30

Neuromuscular Junction Concept

-Neurons are nerve fibers that conduct electrical or chemical impulses
-Neurons are not continuous throughout body
--tiny gaps called synapses
-Synapses bridge neuron to neuron or muscles to glands
-situated midway along the length of a muscle fiber
-axon terminal and muscle fiber are separated by a gel filled spaced = synaptic cleft
-synaptic vesicles of axon terminal contain the Acetylcholine(ACh)
-Junctional folds of the sarcolemma contain ACh receptors

31

Neuromuscular Junction ----how they bridge

-by diffusion, neurotransmitters diffuse

32

Neuromuscular junction

junction that connect nervous system to muscular system via synapses

33

Events at the Neuromuscular

-skeletal muscles are stimulated by somatic motor neurons
-axons of motor neurons travel from the CNS via nerves to skeletal muscles
-each axon forms several branches as it enters a muscles
-each axon ending forms a neuromuscular junction w/ a single muscle fiber
-nerve impulse arrives at axon terminal (motor neuron)
-Calcium ions enter the axon terminal and activate synaptic vesicles
-Vesicles release neurotransmitters, ACh
-ACh bind w/ receptors on the sarcolemma
-Electrical evens lead to membrane potentional change: Na influx

34

Depolarization

muscle contraction

ACh is destroyed by acetylcholinesterase

35

Repolarization

muscle relaxtion

36

Destruction or elimination of ACh

-ACh effects are quickly terminated by the enzyme acetylcholinesterase, by diffusing the synaptic cleft and up taken

-prevents continued muscle fiber contraction in the absence of additional stimulation

37

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

38

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 reach, an AP is generated
-local depolarization waves continues to spread, changing the permeability of sarcolemma
-Voltage-regulated Na channels open in the adjacent patch, causing it to depolarize to threshold

39

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

40

Excitation-Contraction (E-C) Coupling

-Sequence of events by which transmission of an AP along the sarcolemma leads to sliding of the myofilaments

41

Latent Period

-Time when E-C coupling events occurs
-Time between AP initiation and the beginning of contraction

42

Events of E-c Coupling

-AP is propagated along sarcomere to T tubules
-voltage-sensitive proteins stimulate Ca release from SR
>Ca is necessary for contraction

43

Roles of Calcium at LOW intracellular concentration

-tropomyosin blocks the active sites on actin
-myosin heads cannot attach to actin
-muscle fiber relaxes

44

Role of Calcium at HIGHER intracellular Ca concentration

-Ca 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 is pumped back into the SR and contraction ends

45

Cross Bridge Cycle

-continues as long as the Ca 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
-Cross bridge detachment: ATP attaches to myosin head and the cross bridge detaches
-"Cocking" of the myosin head: energy from hydrolysis of ATP cocks the myosin head into the high energy state

46

Principes 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
3. Contraction does not always shorten a muscle
4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities

47

Isometric Contraction

No shortening; muscle tension increases but does not exceed the load

48

Isotonic Contraction

Muscle shorten because muscle tension exceeds the load

49

Concentric Contraction

muscle actively shortening

50

Eccentric Contraction

muscle actively lengthening (ie during walking

51

Motor Unit

a motor neuron and all (4 to several hundred) muscle fibers it supplies
-small motor units in muscles that control fine movements (fingers, eyes)
-large motor units in large weight-bearing muscles ( thighs, hips)
-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

52

What is a muscle twitch?

-response of a muscle to a single, brief threshold stimulus
-simplest contraction observable in the lab (recorded as a myogram)

53

Three phases of a twitch

1. latent period: events of excitation-contraction coupling
2. period of contraction: cross bridge formation; tension increases
3. Period of relaxation: Ca reentry into the SR; tension declines to zero

54

Muscle twitch comparisons

-different stregnth and duration of twitches are due to variations in metabolic properties and enzymes between muscles

55

Graded muscle responses

-variations in the degree of muscle contraction
-required for proper control of skeletal movement

56

Responses are Graded by

1. Changing the frequency of stimulation

2. Changing the strength of the stimulus

57

Response to change in stimulus frequency

-a single stimulus results in a single contractile response---a muscle twitch
-increase frequency of stimulus (muscle doesn't have time to completely relax between stimuli)
-Ca release stimulates further contraction-->temporal (wave) summation
-further increase in stimulus frequency --> unfused (incomplete) tetanus
-if stimuli are given quickly enough, fused (complete) tetany results

58

Response to change in stimulus strength

-threshold stimulus: stimulus strength at which the first observable muscle contraction occurs
-muscle contracts more vigorously as stimulus strenth is increased above threshold
-contraction force is precsely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
-size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases

59

Muscle Tone

-Constant, slightly contracted state of all muscles
-due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptor in musces
-keeps muscles firm, healthy, and ready to respond

60

Isotonic Contraction

-muscle changes in length and moves the load
-isotonic contractions are either concentric or eccentric:
>Concentric: muscle shortens and does work (ie picking up a book
>Eccentric: muscle contract as it lengthens (ie walking up a hill)
-load is greater than the tension the muscle is able to develop
-tension increases to the muscles's capacity, but the muscle neither shortens nor lengthens

61

Muscle metabolism: Energy for Contraction

-ATP is the only source used directly for contractile activities
-Available stores of ATP are depleted in 4-6 seconds
-ATP is regenerated by:

  • Direct phosphorylation of ADP by creatine phosphate (CP)
  • Anaerobic pathway (glycolysis)
  • Aerobic respiration
62

Anaerobic Pathway

-at 70% of maximum contractile activity:

  • bulging muscles compress blood vessels
  • oxygen delivery is impaired
  • pyruvic acid is converted into lactic acid
63

Lactic Acid

-diffuses into the bloodstream
-used as fuel by the liver, kidneys, and heart
-converted back into pyruvic acid by the liver

64

Aerobic Pathway

-Produces 95% of ATP during rest and light to moderate exercise
-Fuels: stored glycogen, then bloodborne glucose, puruvic and acid from glycolysis, and free fatty acids

65

Muscle Fatigue

-Physiological inability to contract
-occurs when:
>Ionic imbalances (K, Ca, P) interfere with E-C coupling
>Prolonged exercise damages the SR and interferes with Ca regulation and release
-Total lack of ATP occurs rarely, during states of continous contraction, and causes contractures (continuous contractions)

66

Oxygen Deficit

-shortage of oxygen
-extra O2 needed after exercise for replenishment of:
1) oxygen reserves
2) glycogen stores
3) ATP and CP reserves
-Conversion of lactic acid to pyruvic acid, glucose, and glycogen

67

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

68

What affects force of muscle contraction?

1. # of muscle fibers stimulated (recruitment)
2. Relative size of the fibers--hypertropy of cells increases strength (size)
3. Frequency of stimulation--increase frequency allows time for more effective transfer of tension to noncontractile components
4. length-tension relationship---muscles contract most strongly when muscle fibers are 80-120% of their normal resting length

69

What influences the velocity and duration of contraction?

1. Types of the muscle fiber
2. Load
3. Recruitment

70

HOw do Muscle fibers Classified?

1. Speed of contraction: slow or fast according to :
>Speed at which myosin ATPases split ATP
>Pattern of electrical activity of the motor neurons

2. Metabolic pathways for ATP synthesis:
>oxidative fibers --use aerobic pathways
>glycolytic fibers--use anaerobic glycolysis

71

Oxidative Fibers divided in three types

1. Slow oxidative fibers

2. fast oxidative fibers

3. fast glycolytic fibers

72

Influences of load on muscle contraction

increase load ---> increased latent period, decreased contraction, and decreased duration of contraction

73

Influence of recruitment on muscle contraction

Recruitment: faster contraction and increased duration of contraction

74

Effects of exercise on muscle contraction

Aerobic (endurance) exercise:
1. Leads to increased:
-Muscle capillaries
- # of mitochondria
-Myoglobin synthesis
2. Results in greater endurance, strength, and resistance to fatigue
3. May convert fast glycolytic fibers into fast oxidative fibers

75

Effects of Resistance exercise

Resistance exercise (typically anaerobic) results in:
1. muscle hypertrophy (due to increase in fiber size)
2. increased mitochondria, myofilaments, glycogen stores, and connective tissue

76

The Overload Principle

1. forcing a muscle to work hard promotes increased muscle strength and endurance
2. Muscles adapt to increased demands
3. muscles must be overloaded to produce further gains

77

Smooth Muscle

1. Non-striated
2. involuntary
3. found in walls of most hollow organs
4. usually in two layers (longitudinal and circular)

78

Peristalsis

-alternating contractions and relaxations of smooth muscle layers that mix and squeeze substance through the lumen of hollow organs
>longitundinal layer contracts; organ dilates and shortens
>Circular layer contracts; organ constricts and elongates

79

Microscopic structure of smooth muscles

-spindle-shaped fibers: thin and short compared with skeletal muscle fibers
-connective tissue: endomysium only
-SR: less developed than in skeletal muscle
-Pouch-like infolding (caveolae) of sarcolemma sequester Ca
-No sarcomeres, myofibrils, or T tubules

80

Inneration of smooth muscles:

-autonomic nerve fibers innervate smooth muscle at diffuse junctions
-varicosities (bublous swellings) of nerve fibers store and release neurotransmitters

81

Myofilaments in smooth muscle

-ration of thick to thin filaments (1:13) is much lower than in skeletal muscle (1:2)
-Thick filaments have heads along their entire length
-no troponin complex; protein calmodulin binds Ca
-myofilaments are spirally arranged, causing smooth muscle to contract in a corkscrew manner
-dense bodies: proteins that anchor noncontractile intermediate filaments to sarcolemma at regular intervals

82

Smooth Muscle Contractions

-slow, synchronized contractions
-cells are electrically coupled by gap junctions
-some cells are self-excitatory (depolarize without external stimuli); act as pacemakers for sheets of muscle
-rate and intensity of contraction may be modified by neural and chemical stimuli
-sliding filament mechanism
-final tripper is increased intracelllular Ca
-Ca is obtained from the SR and extracellular space

83

How is the contraction of smooth muscle?

-very energy efficient (slow ATPases)
-myofilaments may maintain a latch state for prolonged contractions

84

Relaxation requires

-Ca detachment from calmodulin
-active transport of Ca into SR and ECF
-dephosphorylation of myosin to reduce myosin ATPase activity

85

Role of Calcium

-Ca binds to and activate calmodulin
-activated calmodulin activates myosin (light chain) kinase
-Activated kinase phosphorylates and activates myosin
-cross bridges interact with actin

86

Regulation of smooth muscle contraction

1. Neural regulation
>neurotransmitter binding --->Increased Ca in sarcoplasm; either graded (local) potential or action potential
> response depends on neurotransmitter released and type of receptor molecules
2. Hormones and local chemicals:
>may bind to G protein-linked receptors
>may either enhance or inhibit Ca entry

87

Stress-Relaxation response

  • responds to stretch only briefly, then adapts to new length
  • retains ability to contract on demand
  • enables organs such as the stomach and bladder to temporarily store contentents
88

Length and tension Changes

can contract when between half and twice its resting length

89

hyperplasia

smooth muscle cells can divide and increase their number (ie estrogen effects on uterus at puberty and during pregnancy

90

Single-unit (visceral) smooth muscle

  • sheets contract rhythmically as a unit (gap junctions)
  • often exhibit spontaneous action potentials
  • arranged in opposing sheets and exhibit stress-relaxation response
91

Multiunit smooth muscle

  • located in large airways, large arteries, arrector pili muscles, and iris of eye
  • gap junctions are rare
  • arranged in motor units
  • graded contractions occur in response to neural stimuli
92

Developmental Aspects of Muscle Tissue

  • All muscle tissues develope from embryonic myoblasts
  • multinucleated skeletal muscle cells form by fusion
  • growth factor agrin stimulated clustering of ACh receptor at neuromuscular junctions
  • cardiac and smooth muscle myoblasts develop gap junctions
93

Developmental Aspects of Muscle Tissue Continued

  • Cardiact and skeletal muscle become amitotic, but can lengthen and thicken
  • myoblast-like skeletal muscle satellite cells have limited regenerative ability
  • injured heart muscle is mostly replaced by connective tissue
  • smooth muscle regenerates throughout life
  • muscular development reflects neuromuscular coordination
94

Developmental Aspects of Muscle Tissue Continued

  • Development occurs head to toe/proximal to distal
  • peak natural neural control occurs by midadolescence
  • athletics and training can improve neuromuscular control
  • female skeletal muscle makes up 36% of body mass
  • male skeletal muscle makes up 42% of body mass, primarily due to testosterone
95

Developmental Aspects of Muscle Tissue continued

  • Body strength per unit muscle mass is the same in both sexes
  • with age, connective tissue increases and muscle fibers decrease
  • by age 30, loss of muscle mass (sarcopenia) begins
  • regular exercise reverses sarcopenia
  • atherosclerosis may block distal ateries, leading to intermittent claudication and severe pain in leg muscles
96

Muscular Dystrophy (MD)

  • group of inherited muscle-destroying diseases
  • muscles enlarge due to fat and connective tissue deposits
  • muscle fibers atrophy
97

Duchenne muscular dystrophy (DMD)

  • most common and severe type
  • inherited, sex-linked, carried by females and expressed in males (1/3500) as lack of dystrophin
  • victims become clumsy and fall frequently; usually die of respiratory failure in their 20s
  • no cure, but viral gene therapy or infusion of stem cells with correct dystrophin genes show promise