Motor Units, Muscle Contractions, and ATP

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 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

Muscles that exert fine control

Question 5

What types of movements are performed by large motor units?

Answer 5

Muscle that create large, less precise movements

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

The 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 7

What is a muscle twitch?

Answer 7

simplest form of contraction - a muscle fiber’s response to a
single action potential

Question 8

What is a myogram?

Answer 8

graphical recording of muscle activity

Question 9

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

Answer 9

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

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

Period of Relaxation - final phase lasting 10-100ms; Ca2+ is being pumped back into the SR; number of active cross bridges is declining;
muscle tension declines to 0

Question 10

All muscles ________ faster then they_________.

Answer 10

contratc, relax

Question 11

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

Answer 11

Twitch contractions in some muscles are rapid and brief
Ex: extraocular muscles (eyes)

Twitch contractions in other muscles are slow and long lasting
Ex: gastrocnemius, soleus (calf)

Question 12

Muscle responses are graded by __________ and/or ___________.

Answer 12

changing frequency of stimulation, changing strength of stimulation

Question 13

Define wave/temporal summation.
Summation occurs in response to changes in
stimulus ____________. Physiologically, what
causes the increase in the strength of the
contraction?

Question 14

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

Answer 14

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

Question 15

Define unfused and fused tetany.

Answer 15

Eventually, the muscle can reach a sustained, quivering contraction called unfused (incomplete) tetanus and then potentially fused (complete) tetanus

Question 16

Define recruitment. Physiologically, what causes recruitment?

Answer 16

Recruitment - 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 contraction more precisely

Question 17

Define subthreshold, threshold, and maximal stimuli.

Answer 17

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 18

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 18

• 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

Question 19

What is asynchronous activation? Why is it
important?

Answer 19

All motor units may be recruited simultaneously to produce an exceptionally
strong contraction, but typically, motor units are activated asynchronously

Asynchronous activation helps to prolong a strong contraction and delay/
prevent muscle fatigue

Question 20

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

Answer 20

• the constant, slightly contracted state of all muscles

Muscle tone is due to spinal reflexes – groups of motor units are alternately
activated in response to input from stretch receptors within the muscles

Keeps muscles firm, healthy, and ready to respond

Question 21

Define hypotonia and hypertonia.

Answer 21

Hyptonia reduced muscle tone | Floppy

Hypertonia, Excess muscle tone | too rigid

Question 22

Fully define the following classifications of muscle contraction: isotonic, isometric, eccentric, and concentric. Think of examples of each type of contraction to help you
remember what they look like!

Answer 22

Isotonic Contractions - muscle changes in length and moves load
“iso” = same and “ton” = tension
Once enough tension is generated to move the load, tension remains
relatively constant
- Isotonic contractions can either be concentric or eccentric

Concentric Contractions - muscle shortens and does work
Ex: biceps brachii contraction to pick up a book

Eccentric Contractions - muscle lengthens and generates force – 50%
stronger contractions than concentric!
Ex: quadriceps contractions while walking downstairs
Many activities involve both types of isotonic contractions!

Isometric Contraction - 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 with 1 hand

Question 23

Why can eccentric contractions be stronger
than concentric?

Answer 23

Muscle Lengthens

Stretch Refelx

Muscle activates more muscle fibers

Some are optimized foeccentrics

Question 24

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

Answer 24

As a muscle contracts, 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

Question 25

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

Answer 25

1. Direct phosphorylation of ADP by creatine phosphate (CP) - fastest
2. Glycolysis/Anaerobic pathway: glycolysis and lactic acid formation
3. Aerobic respiration - most ATP

Question 26

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 26

CP + ADP creatine + ATP

Creatine Phosphate ( CP ) - unique high energy molecule stored in muscle
fibers

CP donates a phosphate to ADP to instantly form ATP

Creatine Kinase - enzyme that carries out the transfer of phosphate

Muscle fibers have enough ATP + CP reserves to power the muscle cell for
15s

CP reserves are replenished during rest or inactivity

Question 27

Define glycolysis. Write out the steps of glycolysis. Does this process require
Oxygen? How much ATP is produced? Define lactic acid. How long can we power a muscle cell using this method?

Answer 27

Glycolysis (“Sugar-Splitting”) = the 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

Lactic acid defuses into blood stream

• Used as fuel by the liver, kidneys and heart
• It is converted back to pyruvic acid or glucose by the liver
  Responsible for post-activity muscle soreness
  Less efficient (5) but is 2.5x faster

Question 28

Define aerobic respiration. Where does it
take place? What is the chemical equation
for the process? In this process, what are
the ingredients for ATP? What happens to
the CO2 produced? List the fuel sources for aerobic respiration in the order that they are used.

Answer 28

Aerobic respiration is the process by which cells convert oxygen and nutrients, such as glucose, into energy (in the form of ATP) in the presence of oxygen, producing carbon dioxide and water as byproducts.

Takes place in mitochondria

Glucose + O2 -> CO2 + H2O + ATP

Ingrediants for ATP is ADP and Pi (Inorganic Phosphate)

CO2 is breathed out

Fuel is Carbs, Fatty Acids and Amino Acids

Question 29

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

Answer 29

Aerobic Pathway

Question 30

Define aerobic endurance. Define anaerobic
threshold.

Answer 30

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

Anaerobic Threshold - the point at which muscle metabolism converts to
anaerobic glycolysis

Question 31

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

Answer 31

Exercises that require a surge of power but last only a few seconds rely entirely
on ATP and CP stores (Vault)

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” (Triathlon)

Question 32

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

Question 33

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

Answer 33

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