Final Exam

Question 1

A motor unit consists of …

Answer 1

A motoneuron and all the muscle fibres it innervates

Question 2

Compare fibres per motor unit for gross movement and fine movement

Answer 2

Gross movement = many fibres per MU

Fine/Complex movement = few fibres per MU

Question 3

Is a muscle limited to 1 fibre type?

Answer 3

No. Fibres of different MUs intermingle with each other

Question 4

Why is MU territory spread over a larger area?

Answer 4

1. Mus’ Force distributed over a larger area
   a. “smoother” contraction
   b. More equal force on tendon
   i. If not equal it might tear
   
   2. May help delay fatigue
      a. Active and inactive fibres sharing metabolites and capillaries
      By spreading out we can share load of metabolites and have more oxygen per muscle fibre

Question 5

All fibers of one motor unit are the _____ fiber type

Answer 5

SAME

Question 6

Neuron cell bodies are larger the more _______ the fiber type is

Answer 6

anaerobic

Question 7

List the fibre types in order of decreasing fibre size in animals and humans

Answer 7

IIB > IIA > I (ANIMALS)

IIA > IIB > I (HUMANS)

Question 8

List the fiber types in order of decreasing motoneuron soma size

Answer 8

IIb/x > IIA > I

Question 9

List the fibre types in order of decreasing fibre number

Answer 9

IIB/X > IIA > I

Question 10

List the fiber types in order of decreasing axon diameter

Answer 10

IIb/x > IIa > I

Question 11

What is the limitation of fibre sizes across different fiber types?

Answer 11

Fibre size was based on animal studies (frog muscles)

Question 12

What are the functional characteristics of MU types?

Answer 12

Contraction force (strength)
Contraction speed (velocity) 
Speed of activation
Metabolic power
Fatigue resistance (endurance)
Recruitment threshold

Question 13

Compare MU Contraction Force (strength) between fiber types

Answer 13

Fiber type II MU’s have a greater absolute force (force of 1 MU; N) because they have more and larger muscle fibers

Question 14

Why do fiber type II muscle fibers have greater specific (relative) force than fiber type I?

Answer 14

Because fiber type II fibers have greater force per unit muscle CSA and because they have more actin and myosin filaments (more Sarcomeres) because of that, more cross bridges = more force

Question 15

Why is there greater specific force in type II fibers

Answer 15

Myosin heavy chains - fast twitch isoforms
- more force per CB
- more CB attached
(assumes maximal activation. saturating Ca)

BUT fast twitch have similar myofilament density and similar myofibrillar density

Question 16

Compare MU contraction speed across fiber types

Answer 16

Type II fibers contract with a higher velocity than type I fibers. Type IIx are fastest.

Question 17

Compare force produced at velocities in the fiber types

Answer 17

Type II fibers produce greater force at higher velocities ; type IIx produces most force at highest velocity

Question 18

Which fiber type produces the greatest power ?

Answer 18

Type IIx produce the greatest amount of power as they produce the most amount of force at the highest velocity

so more fast twitch fibers = higher max power and higher velocity

Question 19

Determinants of Contraction Speed

Answer 19

1. Myosin ATPase Activity
2. CB Power Stroke Speed
3. Ca2+ release and reuptake

Question 20

How does myosin ATPase activity impact contraction speed and which fiber type has greatest activity?

Answer 20

Myosin ATPase activity determines the cross-bridge cycle speed
Activity is greatest in type IIx fibers > IIa > I

Question 21

Compare CB power stroke speed between fibre types. Explain

Answer 21

CB power stroke in type II is faster because of myosin heavy chain in type II fibres are different from type I

Question 22

How does calcium release and reuptake determine contraction speed of fiber types?

Answer 22

Fiber type II have larger and more extensive sarcoplasmic reticulum. Thus, they have more rapid release and uptake of calcium, allowing for faster contraction and relaxation of type II fibers

Question 23

What is speed of activation and what factor does it depend on?

Answer 23

Speed of activation is how fast the action potential makes it down the neuron and across the muscle fibre
it depends on diameter of the axon: the larger the diameter, the faster the conduction velocity (due to less resistance)

Question 24

Do muscles fibres with a big diameter (100um) conduct faster than nerve axons (10um)? why or why not?

Answer 24

No because nerve axons are myelinated and specially designed for fast conduction, allowing for saltatory conduction

Question 25

Match the time to the process of energy production. Time: 10 seconds, 1-2 seconds, indefinite, 1 minute Process: Oxidative phosphorylation, PCr, Stored ATP, glycolysis

Answer 25

Stored ATP = 1-2 seconds PCr = 10 seconds Glycolysis = 1 minute Ox. Phosphorylation = indefinite

Question 26

Fast twitch muscle fibres (type II) have more...?

Answer 26

``` Myosin ATPase Activity Storage of PCr CK Activity Glycogen Storage Glycolytic Enzyme Activity ```

Question 27

Why are slow oxidative fibers the most fatigue resistant ?

Answer 27

Because SO have an enhanced ability to perform oxidative phosphorylation

Question 28

What are the determinants of fatigue resistance?

Answer 28

Mitochondrial size and number Myoglobin concentration Muscle fiber diameter Capillarization

Question 29

What does myoglobin do and which fiber type is it found most in?

Answer 29

Myoglobin facilitates diffusion of oxygen from the capillary to the mitochondria. It stores oxygen. Type I fibers have more myoglobin.

Question 30

How does muscle fiber diameter make type I fibers more fatigue resistant?

Answer 30

Type I fibers have a smaller muscle fiber diameter so there is a smaller diffusion distance and more capillary/area of fiber

Question 31

How does Capillarization affect fatigue resistance?

Answer 31

MU activity affects the number of capillaries arount its fibres, therefore fatigue resistance Type I has more capillary per muscle fibre so its more fatigue resistant

Question 32

What are recruitment thresholds?

Answer 32

% of maximum force at which a MU is recruited Low Threshold = recruited at low % of max force High Threshold = recruited at high % of max force

Question 33

What is the size principle?

Answer 33

Motor units are recruited in order according to the size of the motoneuron's cell body Larger size = larger threshold

Question 34

Why does the size principle work?

Answer 34

Larger cell bodies are more difficult to excite, so theyre more difficult to recruit Thus they need a higher activation from the nervous system to reach threshold

Question 35

What is Motor Unit Activation?

Answer 35

Motor unit activation is the number of motor units activated and their firing rates so 100% MUA means all motor units are firing and at their maximal firing rates

Question 36

How do we measure Motor Unit Activation?

Answer 36

Using EMG: Surface EMG & Indwelling EMG Indwelling: To measure for 1 specific muscle : indwelling EMG (just 5-6 muscle fibers) (can stick it into belly of muscle) Surface: Surface EMG : measuring for all the muscles in that nearby area (but they can do more movement and stuff while having them attached) (only picks up surface signals)

Question 37

What Does EMG Measure?

Answer 37

It records Wave of Depolarization from Motor Unit AP

Question 38

What are the factors that impact the magnitude of EMG?

Answer 38

- number of fibres active (MUs recruited) - MAPs per fibre (firing rate) - size of fibre

Question 39

EMG shows absolute or relative motor unit activation?

Answer 39

relative

Question 40

Why does an eccentric contraction have less MUA than a concentric contraction?

Answer 40

because an eccentric contraction produces more absolute force than a concentric contraction at the same relative force, so it requires less MUA to produce the same amount of force

Question 41

Why is MUA less for eccentric contractions at the same absolute load/force?

Answer 41

Because in an eccentric contraction, for the same absolute force, there is more force per CB. Thus less CBs are needed, so less muscle fibers are needed, so there is less MUA (smaller EMG signal).

Question 42

Why is there a dip in Eccentric Contraction EMG

Answer 42

1. Fear of Max Eccentric Contraction? 2. Unfamiliar with max Eccentric Contraction 3. Reflex inhibition by Golgi Tendon Organ?

Question 43

At the same relative force, why is there same MUA in eccentric and concentric contractions?

Answer 43

Because the same number of CBs are active, so the same number of muscle fibers are active, so same number of motor units are active, and same motor unit activation occurs (same EMG amplitude)

Question 44

How does the GTO contribute to the maximal eccentric EMG being lower than the maximal concentric EMG (at the same relative force)?

Answer 44

The GTO senses high force in the tendon and is stimulated. It inhibits motor output (reflex inhibition) to prevent too much tendon force and thus injury. And the GTO is more active at higher eccentric velocities.

Question 45

How do higher firing rates result in greater rate of force development?

Answer 45

At a higher firing rate, more MAPs are sent per second, so there's a faster release of Calcium from the SR, so more rapid onset of CB cycling as a result there is greater rate of force development

Question 46

What is fatigue?

Answer 46

A decrease in force/power generating capacity

Question 47

What does fatigue look like during a concentric and eccentric contraction?

Answer 47

Concentric: reduced force and velocity, so reduced power Eccentric: reduced force and reduced velocity? so reduced power (but also more likely to cause ultrastructural damage) - we may or may not be able to reduce velocity but we'll be able to produce less force at a given velocity and therefore be able to produce less eccentric power - eccentric contraction when there is fatigue, is more likely to cause structural damage to muscle (sarcolemma tearing)

Question 48

What Does Fatigue Look like in an Isometric Contraction?

Answer 48

Reduced Force Reduced Rate of Force Development (RFD): takes Longer for force to increase w/ Fatigue Reduced Rate of Force Relaxation (RFR): takes Longer for force to be fully gone after relaxation

Question 49

describe neuromuscular junction failure

Answer 49

There is only a finite amount of Ach and if APs are sent for too long or there are too many of them, eventually Ach will be depleted. The axon terminal cannot resynthesize APs fast enough so the AP will not be transmitted across to the motor end plate to cause a motor AP Additionally, there may be reduced excitability of the motor end plate and if AP's occur too frequently across the motor endplates then the concentration gradients of potassium and sodium might be messed up a bit and cause them to not be as easily excited The nerve axon itself is not fatiguable, it can transmit APs indefinitely