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Bio153 > Exam 6 PART 2 > Flashcards

Exam 6 PART 2 Flashcards

1
Q

Whole Muscle Contraction

A
  • Same principles apply to contraction of both single fibers and whole muscles
  • Contraction produces muscle tension, the force exerted on load or object to be moved
  • Contraction may/may not shorten muscle

–Isometric contraction: no shortening; muscle tension increases but does not exceed load

–Isotonic contraction: muscle changes length because muscle tension changes relative to load

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2
Q

Whole Muscle Contraction

A
  • Force and duration of contraction vary in response to stimuli of different frequencies and intensities
  • Each muscle is served by at least one motor nerve

–Motor nerve contains axons of up to hundreds of motor neurons

–Axons branch into terminals, each of which forms NMJ with single muscle fiber

•Motor unit is the nerve-muscle functional unit

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3
Q

Motor unit

A

•consists of the motor neuron and all muscle fibers (four to several hundred) it supplies

–Smaller the fiber number, the greater the fine control

•Muscle fibers from a motor unit are spread throughout the whole muscle, so stimulation of a single motor unit causes only weak contraction of entire muscle

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4
Q

Muscle twitch:

A

•simplest contraction resulting from a muscle fiber’s response to a single action potential from motor neuron

–Muscle fiber contracts quickly, then relaxes

•Twitch can be observed and recorded as a myogram

–Tracing: line recording contraction activity

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5
Q

•Three phases of muscle twitch

A

–Latent period: excitation-contraction coupling is occurring but no muscle tension seen yet

–Period of contraction: cross bridge formation

•Tension increases

–Period of relaxation: initiated by Ca2+ reentry into SR

  • Tension declines to zero
  • Muscle contracts faster than it relaxes
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6
Q

Differences in strength and duration of twitches are due to

A

•variations in metabolic properties and enzymes between muscles

–Example: eye muscles contraction are rapid and brief, whereas larger, fleshy muscles (calf muscles) contract more slowly and hold it longer

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7
Q

Graded Muscle Responses

A

•Normal muscle contraction is relatively smooth, and strength varies with needs

–A muscle twitch is seen only in lab setting or with neuromuscular problems, but not in normal muscle

•Graded muscle responses vary strength of contraction for different demands

–Required for proper control of skeletal movement

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8
Q

•Responses are graded by:

A

Changing frequency of stimulation

Changing strength of stimulation

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9
Q

•Muscle response to changes in stimulus frequency

A

–Single stimulus results in single contractile response (i.e., muscle twitch)

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10
Q

Wave (temporal) summation

A

–results if two stimuli are received by a muscle in rapid succession

  • Muscle fibers do not have time to completely relax between stimuli, so twitches increase in force (tension) with each stimulus
  • Additional Ca2+ that is released with second stimulus stimulates more shortening
  • Produces smooth, continuous contractions that add up (summation)
  • Further increase in stimulus frequency causes muscle to progress to sustained, quivering contraction referred to as unfused (incomplete) tetanus
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11
Q

–If stimuli frequency increases, muscle tension reaches maximum

A
  • Referred to as fused (complete) tetanus because contractions “fuse” into one smooth sustained contraction plateau
  • Prolonged muscle contractions lead to muscle fatigue
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12
Q

•Muscle response to changes in stimulus strength

A

–Recruitment (or multiple motor unit summation): stimulus is sent to more muscle fibers, leading to more precise control of contraction force

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13
Q

–Types of stimulus involved in recruitment:

A
  • Subthreshold stimulus: stimulus not strong enough, so no contractions seen
  • Threshold stimulus: stimulus is strong enough to cause first observable contraction
  • Maximal stimulus: strongest stimulus that increases to maximum contractile force

–All motor units have been recruited

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14
Q

Recruitment works on

A

–size principle

  • Motor units with smallest muscle fibers are recruited first
  • Motor units with larger and larger fibers are recruited as stimulus intensity increases
  • Largest motor units are activated only for most powerful contractions
  • Motor units in muscle usually contract asynchronously

–Some fibers contract while others restHelps prevent fatigue

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15
Q

Muscle Tone

A
  • Constant, slightly contracted state of all muscles
  • Due to spinal reflexes

–Groups of motor units are alternately activated in response to input from stretch receptors in muscles

•Keeps muscles firm, healthy, and ready to respond

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16
Q

Isotonic contractions:

A

muscle changes in length and moves load

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17
Q

Concentric contractions:

A

•muscle shortens and does work

–Example: biceps contract to pick up a book

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18
Q

Eccentric contractions

A

•muscle lengthens and generates force

–Example: laying a book down causes biceps to lengthen while generating a force

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19
Q

•Isometric contractions

A

–Load is greater than (or equal to) the maximum tension the muscle can generate, so the muscle neither shortens nor lengthens

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20
Q

•Electrochemical and mechanical events are same in isotonic or isometric contractions, but results are different

A

–In isotonic contractions, actin filaments slide and cause movement

–In isometric contractions, cross bridges generate force, but actin filaments do not slide

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21
Q

Providing Energy for Contraction

A

•ATP supplies the energy needed for the muscle fiber to:

–Move and detach cross bridges

–Pump calcium back into SR

–Pump Na+ out of and K+ back into cell after excitation-contraction coupling

  • Available stores of ATP depleted in 4–6 seconds
  • ATP is the only source of energy for contractile activities; therefore it must be regenerated quickly
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22
Q

•ATP is regenerated quickly by three mechanisms:

A

–Direct phosphorylation of ADP by creatine phosphate (CP)

–Anaerobic pathway: glycolysis and lactic acid formation

–Aerobic respiration (mitochondria)

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23
Q

•Direct phosphorylation of ADP by creatine phosphate (CP)

A

–Creatine phosphate is a unique molecule located in muscle fibers that donates a phosphate to ADP to instantly form ATP

  • Creatine kinase is enzyme that carries out transfer of phosphate
  • Muscle fibers have enough ATP and CP reserves to power cell for about 15 seconds

Creatine phosphate + ADP ® creatine + ATP

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24
Q

•Anaerobic pathway: glycolysis and lactic acid formation

A

–ATP can also be generated by breaking down and using energy stored in glucose

•Glycolysis: first step in glucose breakdown

–Does not require oxygen

–Glucose is broken into 2 pyruvic acid molecules

–2 ATPs are generated for each glucose broken down

•Low oxygen levels prevent pyruvic acid from entering aerobic respiration phase

–Lactic acid

  • Diffuses into bloodstream
  • Used as fuel by liver, kidneys, and heart
  • Converted back into pyruvic acid or glucose by liver

–Anaerobic respiration yields only 5% as much ATP as aerobic respiration, but produces ATP 2½ times faster

–Normally, pyruvic acid enters mitochondria to start aerobic respiration phase; however, at high intensity activity, oxygen is not available

•Bulging muscles compress blood vessels, impairing oxygen delivery

–In the absence of oxygen, referred to as anaerobic glycolysis, pyruvic acid is converted to lactic acid

25
Q

•Aerobic respiration

A

–Produces 95% of ATP during rest and light-to-moderate exercise

•Slower than anaerobic pathway

–Consists of series of chemical reactions that occur in mitochondria and require oxygen

•Breaks glucose into CO2, H2O, and large amount ATP (32 can be produced)

–Fuels used include glucose from glycogen stored in muscle fiber, then bloodborne glucose, pyruvic acid from glycolysis and free fatty acids

•Fatty acids are main fuel after 30 minutes of exercise

26
Q

•Energy systems used during sports

A

–Aerobic endurance

•Length of time muscle contracts using aerobic pathways

–Light-to-moderate activity, which can continue for hours

–Anaerobic threshold

•Point at which muscle metabolism converts to anaerobic pathway

27
Q

Muscle Fatigue

A
  • Physiological inability to contract despite continued stimulation
  • Usually occurs when there are ionic imbalances

–Levels of K+, Ca2+, Pi can interfere with E‑C coupling

–Prolonged exercise may also damage SR and interferes with Ca2+ regulation and release

•Lack of ATP is rarely a reason for fatigue, except in severely stressed muscles

28
Q

Excess Postexercise Oxygen Consumption

A

•For a muscle to return to its pre-exercise state:

–Oxygen reserves are replenished

–Lactic acid is reconverted to pyruvic acid

–Glycogen stores are replaced

–ATP and creatine phosphate reserves are resynthesized

•All replenishing steps require extra oxygen, so this is referred to as excess postexercise oxygen consumption (EPOC)

Formerly referred to as “oxygen debt”

29
Q

Force of Muscle Contractions

A

•Force of contraction depends on number of cross bridges attached, which is affected by four factors:

  1. Number of muscle fibers stimulated (recruitment): the more motor units recruited, the greater the force.
  2. Relative size of fibers: the bulkier the muscle, the more tension it can develop

•Muscle cells can increase in size (hypertrophy) with regular exercise

3.Frequency of stimulation: the higher the
frequency, the greater the force

•Stimuli are added together

  1. Degree of muscle stretch: muscle fibers with
    sarcomeres that are 80–120% their normal
    resting length generate more force
  • If sarcomere is less than 80% resting length, filaments overlap too much, and force decreases
  • If sarcomere is greater than 120% of resting length, filaments do not overlap enough so force decreases
30
Q

•How fast a muscle contracts and how long it can stay contracted is influenced by:

A

–Muscle fiber type

–Load

–Recruitment

31
Q

•Muscle fiber type

–Classified according to two characteristics

A

1.Speed of contraction – slow or fast fibers
according to:

–Speed at which myosin ATPases split ATP

–Pattern of electrical activity of motor neurons

2.Metabolic pathways used for ATP synthesis

–Oxidative fibers: use aerobic pathways

–Glycolytic fibers: use anaerobic glycolysis

32
Q

–Based on these two criteria, skeletal muscle fibers can be classified into three types:

A

•Slow oxidative fibers, fast oxidative fibers, or fast glycolytic fibers

33
Q

–Most muscles contain mixture of fiber types, resulting in a range of contractile speed and fatigue resistance

A
  • All fibers in one motor unit are the same type
  • Genetics dictate individual’s percentage of each
34
Q

–Different muscle types are better suited for different jobs

A

•Slow oxidative fibers: low-intensity, endurance activities

–Example: maintaining posture

•Fast oxidative fibers: medium-intensity activities

–Example: sprinting or walking

•Fast glycolytic fibers: short-term intense or powerful movements

–Example: hitting a baseball

35
Q

Load:

A

–muscles contract fastest when no load is added

  • The greater the load, the shorter the duration of contraction
  • The greater the load, the slower the contraction
36
Q

Recruitment:

A

the more motor units contracting, the faster and more prolonged the contraction

37
Q

Aerobic (endurance) exercise

A
  • such as jogging, swimming, biking 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

38
Q

Resistance exercise (typically anaerobic)

A

•such as weight lifting or isometric exercises, leads to

–Muscle hypertrophy

•Due primarily to increase in fiber size

–Increased mitochondria, myofilaments, glycogen stores, and connective tissue

–Increased muscle strength and size

39
Q

Disuse atrophy

A

•(degeneration and loss of mass)

–Due to immobilization or loss of neural stimulation

–Can begin almost immediately

  • Muscle strength can decline 5% per day
  • Paralyzed muscles may atrophy to one-fourth initial size
  • Fibrous connective tissue replaces lost muscle tissueRehabilitation is impossible at this point

Muscles must be active to remain healthy

40
Q

Smooth Muscle

A
  • Found in walls of most hollow organs, except heart
  • Microscopic structure:

–Spindle-shaped fibers: thin and short compared with skeletal muscle fibers

•Only one nucleus, no striations

–Lacks connective tissue sheaths

•Contains endomysium only

41
Q

Microscopic Structure of Smooth Muscles

A

•All but smallest blood vessels contain smooth muscle organized into two layers of opposing sheets of fibers

–Longitudinal layer: fibers run parallel to long axis of organ

•Contraction causes organ to shorten

–Circular layer: fibers run around circumference of organ

  • Contraction causes lumen of organ to constrict
  • Allows peristalsis: alternating contractions and relaxations of layers mix and squeeze substances through lumen of hollow organs
42
Q

•No neuromuscular junction, as in skeletal muscle

A

•Instead, autonomic nerve fibers innervate smooth muscle

–Contain varicosities (bulbous swellings) of nerve fibers

–Varicosities store and release neurotransmitters into a wide synaptic cleft referred to as a diffuse junction

43
Q

Smooth muscle does not contain

A
  • sarcomeres, myofibrils, or T tubules
  • SR is less developed than in skeletal muscle

–SR does store intracellular Ca2+, but most calcium used for contraction has extracellular origins

•Sarcolemma contains pouchlike infoldings called caveolae

–Caveolae contain numerous Ca2+ channels that open to allow rapid influx of extracellular Ca2+

44
Q

•Smooth muscle also differs from skeletal muscle in following ways:

A

–Thick filaments are fewer and have myosin heads along entire length

–No troponin complex

•Protein calmodulin binds Ca2+

45
Q

–Thick and thin filaments arranged diagonally

A

•Myofilaments are spirally arranged, causing smooth muscle to contract in corkscrew manner

46
Q

–Intermediate filament–dense body network

A
  • Contain lattice-like arrangement of noncontractile intermediate filaments that resist tension
  • Dense bodies: proteins that anchor filaments to sarcolemma at regular intervals

–Correspond to Z discs of skeletal muscle

47
Q

•Mechanism of contraction

A

–Slow, synchronized contractions

–Cells electrically coupled by gap junctions

•Action potentials transmitted from fiber to fiber

–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
48
Q

–Contraction in smooth muscle is similar to skeletal muscle contraction in following ways:

A
  • Actin and myosin interact by sliding filament mechanism
  • Final trigger is increased intracellular Ca2+ level
  • ATP energizes sliding process
  • Contraction stops when Ca2+ is no longer available
49
Q

–Contraction in smooth muscle is different from skeletal muscle in following ways:

A
  • Some Ca2+ still obtained from SR, but mostly comes from extracellular space
  • Ca2+ binds to calmodulin, not troponin
  • Activated calmodulin then activates myosin kinase (myosin light chain kinase)
  • Activated myosin kinase phosphorylates myosin head, activating it

–Leads to crossbridge formation with actin

50
Q

–Stopping smooth muscle contraction requires more steps than skeletal muscle

A

•Relaxation requires:

–Ca2+ detachment from calmodulin

–Active transport of Ca2+ into SR and extracellularly

–Dephosphorylation of myosin to inactive myosin

51
Q

•Energy efficiency of smooth muscle contraction

A

–Slower to contract and relax but maintains contraction for prolonged periods with little energy cost

  • Slower ATPases
  • Myofilaments may latch together to save energy

–Makes ATP via aerobic respiration pathways

52
Q

•Regulation of contraction

A

–Controlled by nerves, hormones, or local chemical changes (hormones, CO2, pH)

53
Q

•Special features of smooth muscle contraction

A

–Response to stretch

•Stress-relaxation response: responds to stretch only briefly, then adapts to new length

–Retains ability to contract on demand

–Enables organs such as stomach and bladder to temporarily store contents

54
Q

•All smooth muscle is categorized as either:

A

–Unitary or multiunit

55
Q

•Unitary (visceral) smooth muscle

A

–Electrically coupled by gap junctions (so work together as a unit)

–In all hollow organs except heart

56
Q

•Multiunit smooth muscle

A

–Located in large airways, large arteries, arrector pili muscles, and iris of eye

–Few Gap junctions

–Independent muscle fibers; innervated by autonomic NS; graded contractions occur in response to neural stimuli

–Has motor units; responds to hormones

57
Q

•Difference in muscle mass between sexes:

A

–Female skeletal muscle makes up 36% of body mass

–Male skeletal muscle makes up 42% of body mass, primarily as a result of testosterone

•Males have greater ability to enlarge muscle fibers, also because of testosterone

–Body strength per unit muscle mass is the same in both sexes

58
Q

Muscular dystrophy:

A

•group of inherited muscle-destroying diseases

–Generally appear in childhood

•Muscles enlarge as a result of fat and connective tissue deposits, but then atrophy and degenerate

59
Q

•Duchenne muscular dystrophy (DMD) is the most common and severe type

A

–Caused by defective gene for dystrophin

–Inherited, sex-linked trait, carried by females and expressed in males (1/3600)

–Dystrophin is a cytoplasmic protein that links the cytoskeleton to the extracellular matrix, stabilizing the sarcolemma

•Fragile sarcolemma tears during contractions, causing entry of excess Ca2+

–Leads to damaged contractile fibers

  • Inflammatory cells accumulate
  • Muscle mass declines
  • Victims become clumsy and fall frequently

–Usually appears between ages 2 and 7

–Currently no cure is known

–Prednisone can improve muscle strength and function

–Myoblast transfer therapy has been disappointing

–Coaxing dystrophic muscles to produce more utrophin (protein similar to dystrophin) has been successful in mice

–Viral gene therapy and infusion of stem cells with correct dystrophin genes show promise

•Patients usually die of respiratory failure in their early 20s

Bio153 (9 decks)

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