Motor Units, Muscle Contraction, and ATP

Question 1

Whole muscle contraction

Answer 1

• 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

Question 2

Motor unit

Answer 2

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

Question 3

Motor units continued

Answer 3

• # 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

Question 4

Muscle twitch

Answer 4

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

Question 5

Myogram

Answer 5

graphical recording of muscle activity

Question 6

3 Phases of Muscle twitch

Answer 6

• latent period
• period of contraction
• period of relaxation

Question 7

latent period

Answer 7

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

Question 8

period of contraction

Answer 8

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

Question 9

period of relaxation

Answer 9

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

Question 10

the muscle twitch

Answer 10

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

Question 11

graded muscle responses

Answer 11

• 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

Question 12

changes in stimulus frequency

Answer 12

• 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

Question 13

frequency of stimuli continues to increase

Answer 13

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

Question 14

unfused (incomplete) tetanus

Answer 14

sustained, quivering contraction

Question 15

fused (complete) tetanus

Answer 15

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

Question 16

recruitment

Answer 16

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)

Question 17

3 types of stimuli involved in recruitment

Answer 17

• 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

Question 18

size principle in recruitment

Answer 18

• 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

Question 19

muscle tone

Answer 19

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

Question 20

isotonic muscle contractions

Answer 20

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

Question 21

concentric contractions

Answer 21

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

Question 22

eccentric contractions

Answer 22

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

Question 23

isometric contractions

Answer 23

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

Question 24

ATP

Answer 24

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

Question 25

3 mechanisms of atp regeneration

Answer 25

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

Question 26

creatine phosphate (CP)

Answer 26

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

Question 27

creatine kinase

Answer 27

enzyme that carries out the transfer of phosphate

Question 28

direct phosphorylation of ADP by CP

Answer 28

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

Question 29

glycolysis - sugar splitting

Answer 29

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

Question 30

anaerobic pathway

Answer 30

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

Question 31

anaerobic pathway part 2

Answer 31

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

Question 32

aerobic respiration

Answer 32

- 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

Question 33

aerobic respiration part 2

Answer 33

- 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

Question 34

energy systems for exercise

Answer 34

- 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

Question 35

aerobic endurance

Answer 35

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

Question 36

anaerobic endurance

Answer 36

the point at which muscle metabolism converts to anaerobic glycolysis

Question 37

energy systems used during exercise part 2

Answer 37

- 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

Question 38

muscle fatigue

Answer 38

- 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

Question 39

excess post exercise oxygen consumption

Answer 39

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

Question 40

excess postexercise oxygen consumption (EPOC)

Answer 40

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