The Motor Unit Flashcards
Describe the morphological characteristics of the different types of motor units
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)
Characteristics of type I muscle fibers
-slow ATP breakdown
-smaller
-more mitochondria
-better blood supply
-fatigue resistant
Characteristics of type IIa
-fast ATP breakdown
-larger than type I
-less mitochondria
-less blood supply
-less aerobic capacity
Characteristics of IIx fibers
-larger than type I
-fewest mitochondria
-least blood supply
-lowest aerobic capacity
-fatigable
Which MU have greater absolute force and why
type II
-more and larger fibers per MU
Which MU have greater relative force and why
type II
-posses myosin heavy chain isoform meaning more force per CB and greater number of cross bridges attached
What factors affect the MU type contraction speed
Myosin ATPase
CB power stroke speed
Ca2+ release and reuptake
Which type of MU has the greatest activation speed and why
type II
- increased diameter of axon leading to less resistance to current flow
The use a PCr as a form of metabolic power
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
The use of glucose or glycogen as a form of metabolic power
breakdown of glucose through glycolysis produces ATP and pyruvate
-Only enough glucose stored for about 1 min
The use of oxidative phosphorylation as a form of metabolic power
Oxygen moved through mitochondria during the breakdown of fats, proteins and pyruvate into energy to make ATP
-slow but able to use indefinitely
Determinants of fatigue resistance
Mitochondrial size
myoglobin concentration - movement of O2
muscle fiber diameter - diffusion
capillarization
Explain the size principle
Motor units are recruited in order according to the size of the motoneuron’s cell body
-larger body’s are more difficult to activate
Coordination of recruitment and firing rate
-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
Compare the after-hyperpolarization model to the onion skin model of motor unit recruitment
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
Def: motor unit activation
refers to the # of MU activated and/or the firing rates of those motor units
What factors impact the magnitude of EMG
number of fibers activated
firing rate
size of fiber
Effect of exercise intensity on MU activation
-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
Effect of contraction type on MU activation
-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
Effect of MUA on rate of force development
increased firing rate produces maximal force faster
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
type 1 would now have a faster relative contraction speed
If you were to decrease the size of type II fibers, how would that effect contraction speed
there would be a slower release and reuptake of Ca2+ causing a decreased contraction speed
If all fiber types had the same myosin ATPase, would they have the same contraction speed
No, the larger type II fibers would still allow for a faster Ca2+ release and reuptake
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
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
If all fiber types exhibited the same number of mitochondria would Type II fibers become more resistant to fatigue
Maybe slightly but still not great blood supply, limiting oxidative capacity
-size also limits defusion
Explain how the characteristics of type II fibers cause them to have a smaller aerobic capacity
-low blood supply to transport oxygen
-few mitochondria to facilitate production
-large diameter decreases diffusability
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
this would cause an increase in the speed of activation of type I fibers in the individual
If an individual had an inability to produce PCr how would this affect the speed of their contraction
It would decrease because the individual would have to rely on glycolysis for there anaerobic system which is a slower way to make ATP
In order to produce a contraction at 75% of your maximal contraction what is the primary mechanism used and why
to achieve the final 25% the primary mechanism would be increased firing rate to increase activation of large type IIx MU with high thresholds
If an individual were to preform progressive incremental exercise, how would this look on an EMG
start with low MUA and then slowly increase which each increment
