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Anatomy & Physiology 207 > Motor Units, Muscle Contraction, and ATP > Flashcards

Motor Units, Muscle Contraction, and ATP Flashcards

1
Q

Whole muscle contraction

A
  • muscle tension: the force exerted on an object by contracting a muscle
  • load: the opposing force exerted on the muscle by the weight of the object to be moved
  • the principles that apply to contraction of a single fiber apply to contraction of whole muscles
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2
Q

Motor unit

A

1 motor neuron and all the muscle fibers it innervates
- each skeletal muscle is served by at least 1 motor nerve; a nerve that contains the axons of 100s of motor neurons
- as an axon enters a muscle it branches into terminals; each terminal forms a neuromuscular junction with a single muscle fiber
- when a motor neuron fires, all the muscle fibers it innervates will contract

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3
Q

Motor units continued

A
  • # of muscle fibers per motor unit may be as high as several hundred or as low as four
  • muscles that exert fine control have small motor units
  • muscle that create large, less precise movements have larger motor units
  • muscle fibers within a particular motor unit are spread throughout the muscle - not clustered together
  • stimulation of a single motor unit causes a weak but uniform contraction throughout the muscle
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4
Q

Muscle twitch

A

the simplest form of contraction - a muscle fiber’s response to a single action potential
- muscle fiber contracts quickly than relaxes

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5
Q

Myogram

A

graphical recording of muscle activity

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6
Q

3 Phases of Muscle twitch

A
  • latent period
  • period of contraction
  • period of relaxation
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7
Q

latent period

A

1st few milliseconds following stimulation; excitation-contraction coupling is occurring; cross bridges begin to cycle, but muscle tension is not yet measurable

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8
Q

period of contraction

A

cross bridges are active; myogram tracing rises to a peak; period lasts 10-100ms

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9
Q

period of relaxation

A

final phase lasting 10-100ms; ca 2+ is being pumped back into the SR; # of active cross bridges is declining; muscle tension declines to 0

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10
Q

the muscle twitch

A
  • all muscles contract faster than they relax
  • some twitches are rapid and brief (extraocular muscles)
  • some twitches are slow and long (gastrocnemius and soleus)
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11
Q

graded muscle responses

A
  • normal muscle contractions are smooth
  • strength of muscle contraction varies by need - graded muscle responses
    responses are graded by
    • changing frequency of stimulation
    • changing strength of stimulation
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12
Q

changes in stimulus frequency

A
  • a single stimulus results in a single contractile response (muscle twitch)
  • wave temporal summation occurs when a second stimuli occurs before the first relaxation period is completed, which increases the strength of the contraction
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13
Q

frequency of stimuli continues to increase

A
  • relaxation between twitches gets shorter
  • concentration of ca2+ in the cytosol becomes greater
  • degree of summation becomes greater
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14
Q

unfused (incomplete) tetanus

A

sustained, quivering contraction

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15
Q

fused (complete) tetanus

A

contractions fuse into 1 smooth, sustained contraction plateau
-prolonged muscle contractions lead to muscle fatigue

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16
Q

recruitment

A

also called multiple motor unit summation; stimuli of increasing voltage are delivered, and more muscle fibers are called into play (this controls the force of the contraction more precisely)

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17
Q

3 types of stimuli involved in recruitment

A
  • subthreshold stiumulus: stimulus is not strong enough, no contractions are seen
  • threshold stimulus: stimulus is strong enough to cause 1st observable contraction
  • maximal stimulus: strongest stimulus that increases contractile force - all motor units are recruited
18
Q

size principle in recruitment

A
  • motor units w smallest muscle fibers are recruited first - they’re controlled by smallest + most excitable neurons
  • motor units with larger muscle fibers are recruited next and contractile strength increases
  • largest motor units, containing large & coarse muscle fibers are controlled by largest and least excitable (highest threshold) neurons - these units are only activated when the most powerful contraction is necessary
19
Q

muscle tone

A

the 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 the muscles
- keep muscles firm, healthy, ready to respond
* hypotonia - low muscle tone
* hypertonia - high muscle tone

20
Q

isotonic muscle contractions

A

muscle changes in length and moves load
- once enough tension is generated to move the load, tension remains relatively constant
- can be concentric or eccentric

21
Q

concentric contractions

A

muscle shortens and does work
- ex. biceps brachii contraction to pick up a book

22
Q

eccentric contractions

A

muscle lengthens and generates force - 50% stronger contractions than concentric
- ex. quadriceps contractions while walking downstairs

23
Q

isometric contractions

A

muscle tension develops, but the load is not moved - load is greater than the tension the muscle can develop
- ex. attempting to lift a piano w one hand
- takes place primarily to uphold posture or keep joints stationary
- ex. wall sits and planks
- cross bridges are formed and generate force, but they do not slide the thin filaments

24
Q

ATP

A

as a muscle contracts, atp supplies energy to:
- move / detach cross bridges
- operate the calcium pump in the SR
- operate the na+ - k+ pump in the plasma membrane

muscles store 4-6 seconds worth of atp
- atp is the only source of energy for contractile activities - must be regenerated quickly

25
Q

3 mechanisms of atp regeneration

A
  1. direct phosphorylation of ADP by creatine phosphate (cp)
  2. anaerobic pathway: glycolysis and lactic acid formation
  3. aerobic respiration
26
Q

creatine phosphate (CP)

A

unique high energy molecule stored in muscle fibers
- donates a phosphate to ADP to instantly form ATP

27
Q

creatine kinase

A

enzyme that carries out the transfer of phosphate

28
Q

direct phosphorylation of ADP by CP

A

muscle fibers have enough atp + cp reserves to power the muscle cell for 15 seconds
- cp reserves are replenished during rest or inactivity
CP+ADP–> creatine + ATP

29
Q

glycolysis - sugar splitting

A

1st step in glucose breakdown
- process does NOT require oxygen
- glucose is broken into 2 pyruvic acid molecules
- 2 ATPs are generated for each glucose broken down

30
Q

anaerobic pathway

A

ATP can be generated by breaking down glucose from the blood or the glycogen stored in the muscle
- glycolysis
- normally, pyruvic acid enters the mitochondria to start the aerobic respiration phase
- during high intensity activity, oxygen isn’t available because contracting muscles compress blood vessels and impair oxygen delivery
- in absence of oxygen, pyruvic acid is converted to lactic acid

31
Q

anaerobic pathway part 2

A

lactic acid diffuses into the bloodstream
- its used as fuel by liver, kidneys, heart
- its converted back to pyruvic acid or glucose by the liver
- lactic acid is responsible for post-activity muscle soreness
- anaerobic respiration harvests only about 5% as much atp from each glucose molecule as the aerobic pathway, but it produces atp about 2.5x faster
- glycolysis can provide most the atp needed for 30-40 seconds of the strenuous muscle activity
- together, stored atp + cp and glycolysis can support about a minute of strenuous activity

32
Q

aerobic respiration

A
  • during rest and light-moderate exercise, 95% of the atp used for muscle activity comes from aerobic respiration
  • aerobic respiration is slower than the anaerobic pathway, but creates more atp (up to 32 molecules)
  • aerobic respiration starts with glycolysis and then moves into multiple reactions that take place within the mitochondria
  • glucose + O2 –> CO2 + H2O + ATP
  • aerobic respiration requires consistent O2 and glucose
  • the CO2 produced diffuses out of the muscle and into blood - will be removed by lungs
33
Q

aerobic respiration part 2

A
  • as exercise begins, muscle glycogen provides most of the fuel
  • shortly after bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are major fuel sources
  • after abt 30 mins, fatty acids become the major energy fuels
  • aerobic respiration is slow but provides a high yield of atp
34
Q

energy systems for exercise

A
  • when atp demands are within the capacity of the aerobic pathway, light to moderate activity can continue for several hours
  • when atp demands exceed the ability to carry out the necessary reactions fast enough, anaerobic pathways contribute more
35
Q

aerobic endurance

A

the length of time a muscle can continue to contract using aerobic pathways

36
Q

anaerobic endurance

A

the point at which muscle metabolism converts to anaerobic glycolysis

37
Q

energy systems used during exercise part 2

A
  • exercises that require a surge of power but last only a few seconds rely entirely on atp and cp stores
  • exercises requiring slightly longer bursts of activity are fueled almost entirely by anaerobic glycolysis
  • prolonged endurance activities depend mainly on aerobic respiration - levels of atp and cp don’t change much - pay as you go
38
Q

muscle fatigue

A
  • fatigue: the physiological inability to contract despite continued stimulation
  • prevents total depletion of atp
  • possible causes
    • ionic imbalances that disrupt membrane potentials
    • increased organic phosphate (Pi) from CP and ATP breakdown may interfere with calcium release from the SR
    • decreased atp and increased mg2+ act on voltage-sensitive proteins in the t tubule and decrease ca2+ release from the SR
  • decreased glycogen
  • lack of atp and rise of lactic acid are rarely reasons for muscle fatigue
39
Q

excess post exercise oxygen consumption

A

for a muscle to return to its pre-exercise state:
- oxygen must be replenished
- accumulated lactic acid must be reconverted to pyruvic acids
- glycogen stores must be replaced
- atp and creatine phosphate reserves must be resynthesized

40
Q

excess postexercise oxygen consumption (EPOC)

A

the extra amount of oxygen that the body must consume for these restorative processes - formerly called the oxygen debt

Anatomy & Physiology 207 (44 decks)

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