Motor Units, Muscle Contraction, and ATP Study Guide

Question 1

Define muscle tension

Answer 1

the force exerted on an object by a contracting muscle

Question 2

define muscle load

Answer 2

the opposing force exerted on the muscle by the weight of the object to be moved

Question 3

Define a motor unit.

Answer 3

1 motor neuron and all the muscle fibers it innervates

Question 4

What types of movements are performed by small motor units?

Answer 4

Fine control exerted by muscles (like in fingers)

Question 5

What types of movements are performed by large motor units?

Answer 5

Large, less precise movements (like the thigh muscle)

Question 6

How are the fibers of a particular large motor unit arranged within the muscle? What is the functional significance of this?

Answer 6

Muscle fibers in a motor unit are spread throughout muscle – not clustered together
Stimulation of a motor unit causes weak but uniform contraction throughout the muscle

Question 7

What is a muscle twitch

Answer 7

simplest form of contraction – a muscle fiber’s response to a single action potential
- Muscle fiber contracts quickly then relaxes

Question 8

All muscles ____ faster than they _____.

Answer 8

• contract
• relax

Question 8

What is a myogram

Answer 8

Myogram: graphical recording of muscle activity

Question 9

Define each part of a muscle twitch and state what is occurring.

Answer 9

1. Latent period: 1st few milliseconds following stimulation; excitation-contraction coupling is occurring; cross bridges begin to cycle, but muscle tension is not yet measurable
2. Period of contraction: cross bridges are active; myogram tracing rises to a peak; period lasts 10-100ms
3. Period of relaxation: final phase lasting 10-100ma; Ca2+ is being pumped back into the SR; number of active cross bridges is declining; muscle tension declines to 0

Question 10

Do all twitches occur at the same speed? List examples of twitches that occur at different speeds.

Answer 10

Twitches do not always occur at the same speed
Twitch contractions in some muscles are rapid and brief (extraocular muscles)
Twitch contractions in other muscles are slow and long lasting (gastrocnemius, soleus)

Question 11

Muscle responses are graded by ________ and/or ________.

Answer 11

• changing frequency of stimulation
• changing strength of stimulation

Question 12

Define wave/temporal summation.

Answer 12

2+ stimuli are received in rapid succession before relaxation is complete

Question 13

Summation occurs in response to changes in stimulus ________.

Answer 13

frequency

Question 14

Physiologically, what causes the increase in the strength of the contraction?

Answer 14

Muscle fibers do not have time to completely relax between stimuli
Additional calcium was squeezed into the cytosol of a partially contracted muscle
Contractions are added together

Question 15

What will happen if the frequency of stimuli continues to increase?

Answer 15

Relaxation time between twitches becomes shorter
Concentration of Ca2+ in the cytosol rises higher
Degree of summation becomes greater

Question 16

Define unfused and fused tetany.

Answer 16

Fused (complete tetany): contractions fuse into 1 smooth, sustained contraction plateau
Unfused (incomplete tetany): sustained, quivering contraction

Question 17

Define recruitment. Physiologically, what causes recruitment?

Answer 17

multiple motor unit summation; stimuli of increasing voltage are delivered, more muscle fibers are called into play
Controls the force of contraction more precisely
* Caused by changes in stimulus strength

Question 18

Define subthreshold, threshold, and maximal stimuli.

Answer 18

• Subthreshold stimulus: 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 19

Explain the size principle. Explain how the size principle helps us to do things like filling a pot with water and keeps us from looking like Elaine from Seinfeld.

Answer 19

• Motor units with the smallest muscle fibers are recruited first – they are controlled by the smallest, most excitable motor neurons
• Motor units with larger muscle fibers are recruited next and contractile strength increases
• The largest motor units, containing large, coarse muscle fibers, are controlled by the largest and least excitable (highest threshold) neurons – these units are only activated when the most powerful contraction is necessary
• typically motor units are activated asynchronously

Question 20

What is asynchronous activation? Why is it important?

Answer 20

When motor units aren’t all activated at once – helps to prolong a strong contraction and delay/prevent muscle fatigue

Question 21

What is muscle tone? What creates it? Why is it important?

Answer 21

• Muscle tone: the constant, slightly contracted state of all muscles
• Muscle tone is due to spinal reflexes – groups of motor units are alternately activates in response to input from stretch receptors within the muscles
• Muscle tone is important because it keeps muscles firm, healthy, and ready to respond

Question 22

hypotonia

Answer 22

very low muscle tone

Question 23

Hypertonia

Answer 23

very high muscle tone

Question 24

isotonic muscle contraction

Answer 24

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

Question 25

concentric muscle contractions

Answer 25

muscle shortens and does work
Biceps brachii contraction to pick up a book

Question 26

eccentric muscle contractions

Answer 26

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

Question 27

isometric muscle contractions

Answer 27

muscle tension develops, but the load is not moved – laid is greater than the tension the muscle can develop
Ex. attempting to lift a piano with one hand
Muscles contract isometrically when they act primarily to maintain upright posture or hold joints stationary while movement occurs elsewhere
Ex. holding a plank or wall sit
Cross bridges are formed and generate force, but they do not slide the thin filaments

Question 28

Why do we need ATP to contract muscle? What functions does it perform?

Answer 28

Atp supplies the energy to:
Move and detach cross bridges
Operate the calcium pump in the SR
Operate the Na+-K+ pump in the plasma membrane
Atp is the only source of energy for contractile activities

Question 29

Why can eccentric contractions be stronger than concentric?

Answer 29

Force is generated with the lengthening of the muscle
More cross bridges are formed
Energy and oxygen consumption is less

Question 30

List the 3 methods we use of regenerate ATP. Which of these methods is the fastest? Which creates the most ATP?

Answer 30

Direct phosphorylation of ADP by creatine phosphate (CP) FASTEST
Anaerobic pathway: glycolysis and lactic acids formation
Aerobic respiration: CREATES THE MOST ATP

Question 31

Write out the steps of direct phosphorylation of ADP. Know the roles of CP and CK. How long can we power a muscle cell using this method?

Answer 31

• Creatine Phosphate (CP): unique high energy molecule stored in muscle fibers
  1. CP donates a phosphate to ADP to instantly form ATP
• Creatine Kinase (CK): enzyme that carries out the transfer of phosphate
  2. Muscle fibers have enough ATP + CP reserves to power the muscle cell for 15 seconds
  3. CP reserves are replenished during rest of inactivity
• CP + ADP → creatine + ATP

Question 32

Define glycolysis. Write out the steps of glycolysis. Does this process require Oxygen? How much ATP is produced?

Answer 32

Glycolysis: “sugar splitting” – 1st step in glucose breakdown
- Doesn’t require oxygen
- Broken down into 2 pyruvic acid molecules
- 2 ATPs are generated for each glucose broken down
- Can provide most of the ATP needed for 30-40 seconds of strenuous muscle activity

Question 33

Define lactic acid. How long can we power a muscle cell using this method?

Answer 33

Lactic acid is diffused into the bloodstream
Lactic acid:
- used by fuel by the liver, kidneys, heart
- Converted back to pyruvic acid or glucose by the liver
- Responsible for post-activity soreness
- Stored ATP + CP and glycolysis can support about a minute of strenuous activity
- In absence of oxygen, pyruvic acid is converted to lactic acid

Question 34

Define aerobic respiration

Answer 34

During rest and light-moderate exercise, 95% of the ATP used for muscle activity comes from aerobic respiration. It is slower than anaerobic pathway, but creates more ATP – up to 32 molecules

Question 35

Where does aerobic respiration take place

Answer 35

starts with glycolysis and then moves into multiple reactions that place within the mitochondria

Question 36

chemical equation for aerobic respiration

Answer 36

Glucose(C6H12O6) + O2 → CO2 + H2O + ATP

Question 37

In aerobic respiration, what are the ingredients for ATP

Answer 37

requires consistent O2 and glucose

Question 38

What happens to the CO2 produced in aerobic respiration

Answer 38

diffuses out of the muscle and into the blood – it will be removed by the lungs

Question 39

List the fuel sources for aerobic respiration in the order that they are used.

Answer 39

• As exercise begins, muscle glycogen provides most of the fuel
• Secondly, bloodborne glucose, pyruvic acid from glycolysis, and free fatty acids are the major fuel sources
• Lastly, after about 30 mins, fatty acids become the major energy fuels

Question 40

When Oxygen is available, which ATP production pathway is used?

Answer 40

Aerobic pathway

Question 41

Define aerobic endurance

Answer 41

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

Question 42

Anaerobic threshold

Answer 42

the point at which muscle metabolism converts to anaerobic glycolysis

Question 43

Which pathway would power a gymnast performing a vault? How about a triathlete completing an Olympic distance triathlon?

Answer 43

• Gymnast performing a vault: ATP + CP stores – Direct phosphorylation of ADP by creatine phosphate (CP)
• Triathlete competing an olympic distance triathlon: aerobic respiration
• Sprinting, weight lifting: anaerobic (slightly longer bursts of energy)

Question 44

What must happen for a muscle to return to its pre-exercise state?

Answer 44

• Oxygen reserves must be replenished
• Accumulated lactic acid must be reconverted back into pyruvic acid
• Glycogen stores must be replaced
• ATP and creatine phosphate reserves must be resynthesized

Question 44

What is fatigue? What might cause it? What RARELY causes it?

Answer 44

• Fatigue: the physiological inability to contract despite continued stimulation, prevents total depletion of ATP
  Possible causes:
  
  • Ionic imbalances: changes in levels of K+, Na+, Ca2+ can disrupt membrane potentials
  • Increased inorganic 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 is highly correlated with fatigue
• RARELY a cause: lack of ATP and rise of lactic acid

Question 45

Define excess postexercise oxygen consumption (EPOC)

Answer 45

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

Question 46

How does an Oxygen deficit occur?

Answer 46

when the body needs more oxygen than is currently available – the difference between the oxygen needed by muscles and the amount present