Nerves and Muscle Flashcards
Extrafusal fibres
main fibres of muscles, respnsible for muslcular force porduction with skeletal muscles
intrafusal fibres
muscle fibres contained within sensory organs called muscle spindles (inside) but not force producing
three types of motor neurons that innervate different fibre types
Alpha motor neurons: innervate only extrafusal fibres
beta - extrafusal and intrafusal
gamma - only intrafusal
single motor neuron innervation rule
in a mature animal, any given skeletal muscle fibre is innnervated by one and only one motor neuron
motor unit - what is it?
2 characteristics
the whole alpha motor neuron together with all the extrafusal muscle fibres that it innervates
- smallest functional unit of the neuromotor system
- produces synchronized chontraction of innervated muscle fibres - to reduce computational load
neuromuscular junction
synaptic connection between motor neuron axon terminal and muslce fibre (AP down axon to terminal - synchonized contraction of all the muscles that this alpha connects to)
motor end plate
region of muslce fibre that receives neurotransmitter (ACh) from neuromuscular junction, produces EPSP at motor end plate that radiates along muscle fibre as motor unit action potential (MUAP)
twitch contraction (2)
propagation of single AP along muscle fibre
muscle fibre generates contractile force for short time and then relaxes
tetanic contraction (2)
propagation of train of APs along muscle fibr at relatively high frequency
force developed by muscle fibre is continuously above zero but may show variation due to individual twitches - no time to relax
innervation ratio - what is it?
2 example
average number of fibres in a motor unit for a given muscle
fine control: small ratio - 5-100 fibres (extraocular, hand)
gross muscles: high ratio - 1000 fibres (back, thighs)
Are all muscle fibres innervated are?
of the same fibre type
ratio of action potential to motor unit action potential
1:1 - alpha motor neuron to muscle cell (all fibres)
As age increases, what happens to your motor neurons?
innervate more muscle fibres which loses the accuracy
physical differences in motor units (2) both are related to
size of the cell body
diameter of axon
alpha MN
Muscle fibre types innervated are differentiated by (3)
physiological - vascularization, mitochondrial density etc.
contractile - capacity for force production
- fatigibility - low force/fatigue
mid force/fatigue (postural muscles)
high force/fatigue
fast fatigue, fast twitch (3) - name - cell body - axon diameter - muscle fibres innervated (2) NCV
FF [MU1] largest cell body thickest axon diameter muscle fibres innervated: - highest force capacity - quickest fatigue nerve conduction velocity - ~100ms
fast resistant, fast twitch (3) - name - cell body - axon diameter - muscle fibres innervated (2) NCV
FR[MU2] mid-size cell body mid-size axon diameter muscle fibres innervated - mid-level force capacity - fatigue resistant nerve conduction velocity - less than FF [MU1], immediate properties
fatigue-resistant, slow twitch (3) - name - cell body - axon diameter - muscle fibres innervated (2) NCV
S [MU3] smallest cell body thinnest axon diameter muscle fibres innervated - lowest force capacity - high fatigue - resistant nerve conduction velocity = ~40m/s
Motor units and force production principle
muscle utilize neural input for purpose of creating muscle contraction which produces force
How can forces be graded?
Magnitude of force generated depends on
- number of active motor units
- firing/discharge rates of active units
same force output can be achieved by
more MUs firing at lower frequency OR fewer MUs firing at higher frequency
how to maximize force output
More MUs and high frequency
Henneman size principle
the order in which the motor units of a muslce are recruited into force production is determined by the size of associated motor neuron - smaller MU to larger MU
- [S]->[FR]->[FF]
low grade contractions achieved by
fatigue resistant, slow twitch [s] - can last over an hour
Larger MUs are recruited as
contraction force increases, but [s] does not decrease
increases in muscle tension (force) achieved by
orderly recruitment of MU
derecruitment
as force output declines - derecruitment follows inverse order FF, FR, S
Reasons for force decline? (2)
voluntary decline (muscle relaxation) Fatigue (sustained contraction) - MU drop out
How to record MU activity in a muscle
electromyography
EMG: what is it and ow does it work?
electromyography - algebraic sum of MUAP train - MUAP firing over time
graphical rep of the electrical acivity of a skeletal muscle
“electrode” records MUAPs as they pass beneath the site of recording
How does MUAP get detected by the recording electrodes?
AP crosses neuromuscular junction to depolorize all muscle fibres within a MU - release of Ach then postsynaptic neurons receive the signals
In what direction do MUAPs propagate?
both directions along the muscle fibre
what are the 2 types of electrodes?
intramuscular (fine wire) EMG
Surface (inferential) EMG
Intramuscular (fine-wire) EMG (3)
Records MUAP within targeted muscle only
small, muscle specific receptive zone
precise and accurate recording from receptive zone
suface/inferential EMG (3)
extramuscular
records MUAP of all MU in proximity to electrodes - multiple MU within/across muscle
precision requires knowledgeable application
2 principles of recording MUAP
- distance from recording site affects amplitude of recorded MUAP - alpha motor neuron synapses with muscle fibre(s) which get detected by the electrodes on the muscle fibre - as we get further away the amplitude gets smaller
- recorded sighal h(t) represents the sum of all MUAP occurring within the vicinity of recording electrodes at that point in time
what do we do to raw EMG data?
we rectify it - half way rectified is to turn all the negative ones into positives, after rectifying both - full way - we can use it to infer force
appearance of EMG signal depends on 3
anatomy, neurophysiology, instrumentation
how does anatomy construct EMG signal? 1-3
MU
- alpha MN and associated muslce fibres
- # of muscle fibres innervated
- contractile properties of innervated fibres
how does neurophysiology construct EMG signals? 1-2
MUAPs
- # MU recruited - depends of force
- firing frequency of MUs - depends on force
3 ways to use EMG
Clinical, sport, industrial
clinical applications of EMG - 3
diagnosis - duration and amplitude
surgical intervention - effects intended
rehab - muscle activity - after a stroke
purpose of ground electrode
factors affecting signal quality
take out background activity/system noise by taking out the common activity between it and the electrodes
cable sway, light, electrical appliances
sport application of EMG
performance - pro vs amateur
industrial application of EMG
fatigue - of different muscle groups - ergonomics/work injuries
Semmler - EMG activity for elbow flexor groups before and after 4 weeks of casting - 2 -3
no change in force production, however, neural strategy is altered
- conpensatory recruitment of synergistic muslces at onset
- brachialis dropout (MU fatigue)
- intermittent firing in synergistic muscles following dropout
CNS strategy to compensate for fatigue
