The Motor Unit

Question 1

Describe the morphological characteristics of the different types of motor units

Answer 1

SOMA SIZE: IIx > IIa> I
AXON DIAMETER: IIx > IIa> I
FIBER NUMBER: IIx > IIa> I
FIBER SIZE: IIa > IIb > I (smaller difference in II in females compared to males)

Question 2

Characteristics of type I muscle fibers

Answer 2

-slow ATP breakdown
-smaller
-more mitochondria
-better blood supply
-fatigue resistant

Question 3

Characteristics of type IIa

Answer 3

-fast ATP breakdown
-larger than type I
-less mitochondria
-less blood supply
-less aerobic capacity

Question 4

Characteristics of IIx fibers

Answer 4

-larger than type I
-fewest mitochondria
-least blood supply
-lowest aerobic capacity
-fatigable

Question 5

Which MU have greater absolute force and why

Answer 5

type II
-more and larger fibers per MU

Question 6

Which MU have greater relative force and why

Answer 6

type II
-posses myosin heavy chain isoform meaning more force per CB and greater number of cross bridges attached

Question 7

What factors affect the MU type contraction speed

Answer 7

Myosin ATPase
CB power stroke speed
Ca2+ release and reuptake

Question 8

Which type of MU has the greatest activation speed and why

Answer 8

type II
- increased diameter of axon leading to less resistance to current flow

Question 9

The use a PCr as a form of metabolic power

Answer 9

PCr is broken down into Pi + Cr and is coupled with ADP + P to form ATP
-only enough PCr stored locally for about 10s worth of fuel

Question 10

The use of glucose or glycogen as a form of metabolic power

Answer 10

breakdown of glucose through glycolysis produces ATP and pyruvate
-Only enough glucose stored for about 1 min

Question 11

The use of oxidative phosphorylation as a form of metabolic power

Answer 11

Oxygen moved through mitochondria during the breakdown of fats, proteins and pyruvate into energy to make ATP
-slow but able to use indefinitely

Question 12

Determinants of fatigue resistance

Answer 12

Mitochondrial size
myoglobin concentration - movement of O2
muscle fiber diameter - diffusion
capillarization

Question 13

Explain the size principle

Answer 13

Motor units are recruited in order according to the size of the motoneuron’s cell body
-larger body’s are more difficult to activate

Question 14

Coordination of recruitment and firing rate

Answer 14

-recruitment is the predominant mechanism in the low force range
-firing rate is the predominant mechanism in the high force range
-50% contraction = 90% of MU recruited

Question 15

Compare the after-hyperpolarization model to the onion skin model of motor unit recruitment

Answer 15

AFTER-HYPERPOLARIZATION: increasing threshold MU start at an increased firing rate and immediately increase firing rate
ONION SKIN: earlier-recruited MU maintain higher firing rates than later recruited ones

Question 16

Def: motor unit activation

Answer 16

refers to the # of MU activated and/or the firing rates of those motor units

Question 17

What factors impact the magnitude of EMG

Answer 17

number of fibers activated
firing rate
size of fiber

Question 18

Effect of exercise intensity on MU activation

Answer 18

-During brief, max effort exercise all fibers contribute
-during submaximal effort exercise as the intensity decreases the fiber types decrease from type IIx -type IIa - type I
-opposite occurs during progressive increment exercise

Question 19

Effect of contraction type on MU activation

Answer 19

-at same MUA but different absolute force there is a greater force produced in ECC
-ECC will not reach 100% max EMG at relative force because of reflex inhibition
-at same absolute force there will be less MUA in ECC due to greater force/CB

Question 20

Effect of MUA on rate of force development

Answer 20

increased firing rate produces maximal force faster

Question 21

If one has an adaptation that causes there type I fibers to release Ca2+ at an increased rate, how may this effect the relative contraction speed of fiber types

Answer 21

type 1 would now have a faster relative contraction speed

Question 22

If you were to decrease the size of type II fibers, how would that effect contraction speed

Answer 22

there would be a slower release and reuptake of Ca2+ causing a decreased contraction speed

Question 23

If all fiber types had the same myosin ATPase, would they have the same contraction speed

Answer 23

No, the larger type II fibers would still allow for a faster Ca2+ release and reuptake

Question 24

If both type I and Type II fibers had the same myosin isoforms, would there still be a difference in relative or absolute contractile force per MU

Answer 24

There would be no difference seen in relative force, but there would still be a difference in absolute due to the greater number and large fibers seen in type II MU

Question 25

If all fiber types exhibited the same number of mitochondria would Type II fibers become more resistant to fatigue

Answer 25

Maybe slightly but still not great blood supply, limiting oxidative capacity
-size also limits defusion

Question 26

Explain how the characteristics of type II fibers cause them to have a smaller aerobic capacity

Answer 26

-low blood supply to transport oxygen
-few mitochondria to facilitate production
-large diameter decreases diffusability

Question 27

If an individual had a disease that caused them to have a larger diameter in type I MU axons how would this affect the MU characteristics

Answer 27

this would cause an increase in the speed of activation of type I fibers in the individual

Question 28

If an individual had an inability to produce PCr how would this affect the speed of their contraction

Answer 28

It would decrease because the individual would have to rely on glycolysis for there anaerobic system which is a slower way to make ATP

Question 29

In order to produce a contraction at 75% of your maximal contraction what is the primary mechanism used and why

Answer 29

to achieve the final 25% the primary mechanism would be increased firing rate to increase activation of large type IIx MU with high thresholds

Question 30

If an individual were to preform progressive incremental exercise, how would this look on an EMG

Answer 30

start with low MUA and then slowly increase which each increment