Exam 2 week 9 ppt 1 Motor Units & Muscle Physiology Flashcards
what is a motor unit?
•A single alpha motor neuron (lower motor neuron, or LMN) and the group of muscle fibers it innervates
WHat is a Motor Neuron Pool?
- The Motor Neuron Pool are all the Alpha motor neurons that innervate the same muscle and are grouped together in the same nucleus.
- A motor neuron pool is an assemblage of motor units which innervate the same muscle.
MUAP
Motor Unit Action Potential
what is a motor unit action potential?
Motor unit action potentials (MUAPs) are Electrically recorded intramuscular potentials which are recorded with needle electrode.
What is normal for electrical activity (MUAP) in a muscle when at rest?
no activity
What kind of electrical recording (MUAP) will a muscle produce when it is voluntarily recruted (contracted)?
–Normally biphasic or triphasic recording when voluntary recruitment of muscle
what happens to the MUAPs when muscle fibers are maixmallly recruited?
what is it called?
A great number of Motor unit action potentials (MUAPs) are seen when the muscle is Maximally recruited, so many that you cannot see individual MUAPs and this is referred to as an“interference pattern”.
do MUAPs stay the same with various peripheral or central disease?
no, changes in recruitment can be seen with various peripheral or central disease
What is damage ot the LMN cell body or axon characterized by? (5)
- –Hyporeflexia
- –Hypotonia
- –Paralysis or paresis
- –Atrophy
- –Denervation pattern of the EMG
What are some canges in EMG when a LMN syndrome is present? (2 main)
- –Spontaneous EMG activity at rest
- §fibrillations (fibs)
- §positive sharp waves (PSWs)
- –Abnormal MUAPs
- often polyphasic instead of bi or triphasic
Denervation changes in EMG which include
Spontaneous EMG activity at rest which is seen as Fibrillations seen as fibs in diagram & positive sharp waves (PSW in diagram) and Abnormal MUAPs, often polyphasic instead of bi or triphasic
how many fiber types are usually discussed?
There are Generally 3 muscle fiber types discussed even though a continuum of these characteristics that designate each type
- Slow Twitch - S or Type I
- Fast Twitch Oxidative-Glycolytic - FOG or Type IIa
- Fast Twitch Glycolytic - FG or Type IIb
what are the three fiber types? (all their namess)
- –Slow Twitch - S , SO, or Type I
- –Fast Twitch Oxidative-Glycolytic - FOG, FR, or Type IIa
- –Fast Twitch Glycolytic - FG, FF, or Type IIb
what are the alternate names for type I fibers?
- –Slow Twitch
- S
- SO
what are the altermante names for type IIa fibers?
- –Fast Twitch (IIa)
- Oxidative-Glycolytic
- FOG,
- FR,
what are the alternate names for type IIb fibers?
- –Fast Twitch Glycolytic -
- FG,
- FF,
- I ahve also heard them called type IIx
what is the oldest terminology used for fiber types?
The Type I, IIa & IIb seems to be the oldest terminology. It apparently appears to go back to Shakespeare’s time. Remember Hamlet “IIb or not IIb, that is the question” was probably discussing muscle fiber types – or perhaps not
Slow-twitch oxidative (SO) fibers details:
contractile force & myosin ATPase activity
metabolism
capillary density
mitochondrial density
myoglobin content
- –Less contractile force with low myosin ATPase activity
- –Oxidative metabolism with low glycolytic capacity
- –Dense capillaries networks & high blood flow
- –High mitochondrial density
- –High myoglobin content
Fast-twitch, glycolytic fibers (FG - IIb) details:
contractile force & myosin ATPase activity
metabolism
capillary density
mitochondrial density
myoglobin content
- –More contractile force with high myosin ATPase activity
- –High glycolytic & low oxidative capacity
- –Poor capillaries networks & low blood flow
- –Low mitochondrial density
- –Low myoglobin content
Fast-twitch, oxidative-glycolytic fibers (FOG - IIa) details:
contractile force & myosin ATPase activity
metabolism
capillary density
mitochondrial density
myoglobin content
- –Intermediate contractile force & myosin ATPase activity
- –Both oxidative & glycolytic activity
- –Intermediate capillaries density & blood flow
- –Intermediate mitochondrial density
- –Intermediate myoglobin content
descrbe the fatigabilitiy of each muscle fiber type:
also explain why
- –Slow Twitch - S or Type I –
- fatigue resistant
- because of low energy demands (slow twitch low myosin ATPase activity) but high energy supply (oxidative metabolism)
- –Fast Twitch Oxidative-Glycolytic - FOG or Type IIa –
- intermediate fatigue resistance
- because of moderate energy demands (fast twitch higher myosin ATPase activity) but also moderate energy supply (oxidative + glycolytic metabolism)
- –Fast Twitch Glycolytic - FG or Type Iib –
- highly fatiguability
- because of high energy demands (fast twitch high myosin ATPase activity) but low energy supply (glycolytic metabolism)
Describe teh functional characteristics of type I fibers:
(4 things)
- Prolonged twitches so fewer twitches need for tetany and sustained contractions
- Low fatigability
- Lower force production
- Postural, prolonged low force/speed activities
Describe teh functional characteristics of type IIa fibers:
(4 things)
- §Shorter twitches so more twitches need for tetany and sustained contractions
- §Higher fatigability
- §Higher force production
- §Moderate forces and speeds
Describe teh functional characteristics of type IIb fibers:
(4 things)
- §Very short twitches so very many twitches need for tetany and sustained contractions
- §Highest fatigability
- §Highest production
- §Short duration high force or speed activities
Do muscle units contain all of the same type of muscle fiber, or do they contain a mix of the diferent muscle fiber typs?
Motor units contain all of the same type of muxcle fiber type
How are motor units named?
the same as muscle fibers:
- Slow twitch units – S with only type 1 or SO fibers
- Fast twitch fatigue resistant units – FR with only Type Iia or FOG fibers
- Fast twitch fatigable units – FF with only Tybe Iib or FG fibers
do muscle units vary in size?
yes
•These units also vary in size with FF larger than FR which are larger than S
Explian how muscle units vary in size and what makes them bigger and smaller
- –FF - larger lower motor neurons and greater axonal diameter
- –FR - intermediate sized lower motor neurons and axonal diameter
- –S – smallest lower motor neurons and least axonal diameter
Explain recruitment of motor units
- –When more force is needed more and larger motor units are recuited - more muscle fibers = more force
- –Motor unit types are recruited in a specific order:
- §Lowest force development to greatest force development
- §S ——> FR ——> FF
what is Heinnerman size principle?
you recruit S type followed by FR and finally FF at highest forces. This pattern is sometimes referred to as the Henneman size principle.
DRaw and explain the motor recruitment chart (with threshold numbers)
–Lowest force (or speed) development to greatest force (or speed) development
So as you recruit from lowest forces to higher forces you recruit ever bigger and more powerful but more fatiguing motor units. At forces less than 40% or less Maximal voluntary contraction (MVC) you may be recruiting only Type S motor units and these represent about 40-50% of the motor unit pool. Beginning at about 40% MVC you start recruiting Type FR motor unit. Up to about 60-70% of MVC you continue to use both Type S and FR motor units. At 60-70% MVC you start recruiting FF motor units and until your maximal force production you continue to use all three types of motor units
How does NMES differ from voluntary recruitment?
- •Voluntary recruitment
- –S type followed by FR and finally FF at highest forces
- –Henneman size principle
- •NMES
- –FF axons lowest resistance so first recruited followed by FR and finally S
- –Reverse size principle
- –This is why so much force and rapid fatigue
Remember in Voluntary recruitment (normal physiological recruitment on this diagram) you recruit S type followed by FR and finally FF at highest forces. This pattern is sometimes referred to as the Henneman size principle.
But when using neuromuscular electrical stimulation (NMES) you first recruit the FF axons because they have the lowest electrical input resistance so first recruited followed by FR and finally S. This is sometimes referred to as the Reverse size principle. This is why so much force is developed with relatively little electrical stimulation but rapid fatigue so you have to have long rests between contractions
