Final Exam Flashcards
A motor unit consists of …
A motoneuron and all the muscle fibres it innervates
Compare fibres per motor unit for gross movement and fine movement
Gross movement = many fibres per MU
Fine/Complex movement = few fibres per MU
Is a muscle limited to 1 fibre type?
No. Fibres of different MUs intermingle with each other
Why is MU territory spread over a larger area?
- Mus’ Force distributed over a larger area
a. “smoother” contraction
b. More equal force on tendon
i. If not equal it might tear- 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
- May help delay fatigue
All fibers of one motor unit are the _____ fiber type
SAME
Neuron cell bodies are larger the more _______ the fiber type is
anaerobic
List the fibre types in order of decreasing fibre size in animals and humans
IIB > IIA > I (ANIMALS)
IIA > IIB > I (HUMANS)
List the fiber types in order of decreasing motoneuron soma size
IIb/x > IIA > I
List the fibre types in order of decreasing fibre number
IIB/X > IIA > I
List the fiber types in order of decreasing axon diameter
IIb/x > IIa > I
What is the limitation of fibre sizes across different fiber types?
Fibre size was based on animal studies (frog muscles)
What are the functional characteristics of MU types?
Contraction force (strength) Contraction speed (velocity) Speed of activation Metabolic power Fatigue resistance (endurance) Recruitment threshold
Compare MU Contraction Force (strength) between fiber types
Fiber type II MU’s have a greater absolute force (force of 1 MU; N) because they have more and larger muscle fibers
Why do fiber type II muscle fibers have greater specific (relative) force than fiber type I?
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
Why is there greater specific force in type II fibers
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
Compare MU contraction speed across fiber types
Type II fibers contract with a higher velocity than type I fibers. Type IIx are fastest.
Compare force produced at velocities in the fiber types
Type II fibers produce greater force at higher velocities ; type IIx produces most force at highest velocity
Which fiber type produces the greatest power ?
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
Determinants of Contraction Speed
- Myosin ATPase Activity
- CB Power Stroke Speed
- Ca2+ release and reuptake
How does myosin ATPase activity impact contraction speed and which fiber type has greatest activity?
Myosin ATPase activity determines the cross-bridge cycle speed
Activity is greatest in type IIx fibers > IIa > I
Compare CB power stroke speed between fibre types. Explain
CB power stroke in type II is faster because of myosin heavy chain in type II fibres are different from type I
How does calcium release and reuptake determine contraction speed of fiber types?
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
What is speed of activation and what factor does it depend on?
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)
Do muscles fibres with a big diameter (100um) conduct faster than nerve axons (10um)? why or why not?
No because nerve axons are myelinated and specially designed for fast conduction, allowing for saltatory conduction
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
Stored ATP = 1-2 seconds
PCr = 10 seconds
Glycolysis = 1 minute
Ox. Phosphorylation = indefinite
Fast twitch muscle fibres (type II) have more…?
Myosin ATPase Activity Storage of PCr CK Activity Glycogen Storage Glycolytic Enzyme Activity
Why are slow oxidative fibers the most fatigue resistant ?
Because SO have an enhanced ability to perform oxidative phosphorylation
What are the determinants of fatigue resistance?
Mitochondrial size and number
Myoglobin concentration
Muscle fiber diameter
Capillarization
What does myoglobin do and which fiber type is it found most in?
Myoglobin facilitates diffusion of oxygen from the capillary to the mitochondria. It stores oxygen.
Type I fibers have more myoglobin.
How does muscle fiber diameter make type I fibers more fatigue resistant?
Type I fibers have a smaller muscle fiber diameter so there is a smaller diffusion distance and more capillary/area of fiber
How does Capillarization affect fatigue resistance?
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
What are recruitment thresholds?
% 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
What is the size principle?
Motor units are recruited in order according to the size of the motoneuron’s cell body
Larger size = larger threshold
Why does the size principle work?
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
What is Motor Unit Activation?
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
How do we measure Motor Unit Activation?
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)
What Does EMG Measure?
It records Wave of Depolarization from Motor Unit AP
What are the factors that impact the magnitude of EMG?
- number of fibres active (MUs recruited)
- MAPs per fibre (firing rate)
- size of fibre
EMG shows absolute or relative motor unit activation?
relative
Why does an eccentric contraction have less MUA than a concentric contraction?
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
Why is MUA less for eccentric contractions at the same absolute load/force?
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).
Why is there a dip in Eccentric Contraction EMG
- Fear of Max Eccentric Contraction?
- Unfamiliar with max Eccentric Contraction
- Reflex inhibition by Golgi Tendon Organ?
At the same relative force, why is there same MUA in eccentric and concentric contractions?
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)
How does the GTO contribute to the maximal eccentric EMG being lower than the maximal concentric EMG (at the same relative force)?
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.
How do higher firing rates result in greater rate of force development?
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
What is fatigue?
A decrease in force/power generating capacity
What does fatigue look like during a concentric and eccentric contraction?
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)
What Does Fatigue Look like in an Isometric Contraction?
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
describe neuromuscular junction failure
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
