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Neuromuscular aspects of Motor Control (1 (Evaluating Excitability of…
Neuromuscular aspects of Motor Control
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Motor Unit - a motor neuron and muscle fibres innervate
Motor Neuron Pool - all alpha motor neurons innervating a muscle
Input from cortico-spinal neurons can trigger AP in axon of motor neuron if postsynaptic potential reaches threshold
Synaptic Integration - Post-synaptic potentials
Inhibitory - hyperpolarises motor neuron membrane = AP less likely
Excitatory - depolarises motor neuron membrane = AP more likely
Spatial and temporal summation of post synaptic potentials influence AP triggering
Facilitation and Inhibition can be pre-synaptic
Pre and post excitatory and inhibitory processes occur via:
Ionotropic receptors - directly gate ion channels to rapidly influence membrane potentials
Metabotropic receptors - indirectly gate ion channels through 2nd messengers for slower neuromodulation
Example of Neuromodulation - Persistant Inward Current
Combined activation of ionotropic channels and release of monoanimes facilitate persistant inward currents and self-sustained firing of AP
Why Modulate the Excitability of Motor Neurons?
State of high arousal = want motor neurons to be highly responsive
When balancing we want less excitable motor neurons = less disruption
muscle neurons synapse with muscle fibres at neuromuscular junction
When an endplate potential reaches synapse then an AP may be generated in muscle fibre = contraction
Measuring Signal
Electromyography (EMG) - records AP of all/parts of muscle, uses 2 electrodes on skin (surface) or inserted in muscle (intramuscular)
Limitations of surface:
Muscles close together = record other muscles
If muscle is deep = only records top muscles
Strengths of intramuscular:
Can identify single neuron
Size Principle Innervation Number
Higher recruitment threshold = higher innovation no. fibres being recruited (type 2) - not consistent/linear relationship
Motor Unit Number
No. motor units within a muscle:
varies between muscles
Muscle force can be increased through increased stimulation frequency
Involuntary contractions - discharge rate
Motor Unit Number Estimation (MUNE)
Compound muscle AP - M-wave response to max stimulation of nerve
Single motor unit AP - single motor unit response to stimulation
MUNE = CMAP/S-MUP
A more inhibited motor neuron pool:
Fewer neurons likely to discharge less frequently
muscle activation less for given amount of voluntary central drive
A more excited motor neuron pool:
More motor neurons likely to discharge and frequently
Muscle activation greater for given amount of voluntary central drive
Evaluating Excitability of Neuron Pool
H-Reflex
Only in limited no.muscles more when relaxed
Influenced by presynaptic inhibition from afferent neurons
CMEPs
Cervicomedullary motor evoked potentials - stimulates cortico-spinal neurons directly
Possible during contractions at rest
Painful
Hard to evoke response in some p's
Neural Response to Fatigue - Fatigue and Force Output
Repeated contraction reduce the force output for a given level of central drive
Neural Response to Sustained Submax Contraction (EMG)
Additional recruitment to maintain force output as other motor units fatigue
Motor units recruits at lower force threshold as fatigue sets
Discharge rate coding and fatigue
Motor units recruited initially reduce their discharge rate with fatigue
Relaxation rates of muscle slow with fatigue
'muscle wisdom' - decreasing discharge rate takes advanatge of slower relaxation rates to maintain force output economically (Enoka)
Reduced Discharge Rates Controlled by Inhibition of Motor Neurons
Reduction in CMEPs
Indicates inhibition of motor neuron pool
likely driving reduction in discharge rate
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Excitation Contraction Coupling
Nerve AP
Neuromuscular junction transmission
AP transmitted along fibre and down T-tuble
Ca released from sarcoplasmic reticulum and binds to tropin
Cross-bridge generate force
Ca pumped back to SR
Cross-bridges detach (relax)
Each step in excitation-contraction coupling is rate limited.
e.g. excitation (nerve impulse and Ca release) takes ~5-10milliseconds
e.g. contraction and relaxation ~100 milliseconds
Therefore, mechanical force generation lags behind excitation process
Time Delay Occurs:
muscle excitation - the proportion of muscle fibres in a muscle that are receiving AP from motor neurons.
Muscle activation - the proportion of available actin and myosin cross bridges that are bound (generating tension)
Tdeact = time constant describing lag between excitation of a muscle turning off and deactivation of muscle
-Tact = time constant describing lag between excitation of a muscle and activation
Muscle Twitch vs Tetanus
Higher rates of stimulation cause summation of force and hence a tetanus (fused force)
Active Tension in Skeletal Muscle
The number of active actin and myosin cross-bridges determines the amount of active force production.
Sarcomeres sit in series and distribute strain (length change) across muscle fibres
Resting fibre lengths of a muscle relates to how many sarcomeres are in series in muscle fibres
Muscle Architecture
Muscle Architecture - fibres in series vs in parallel
In series: length changes sum, force equal
In parallel: length changes equal, forces sum
Muscle Architecture - resting length
Resting fibre length influences active force production by the whole muscle
Active and passive force-length relations vary between muscles
Muscles with Long Resting Fibre Lengths:
Have more sarcomeres
maintain greater force production throughout their ROM
Well suited to producing large length changes and mechanical work
work = force x displacement
Muscles with long fibres and large PCSA
Metabolically expensive
A shorter muscle with same no. fibres in parallel (PCSA) can produce same force by activating a smaller vol of muscle = economical.
Muscle Architecture - pennation angle
Pennation allows a greater no. fibres to be packed into a vol of muscle.
Fibre length shortens
Results in greater PCSA without having to increase muscle vol (+mass)
Muscle structure and FV relationships
Shorter fibres have lower absolute max shortening speed than longer fibres.
However, they may produce more force (greater PCSA)
Force-length-vel and tension
length and vel influence force
Tendon Architecture influences stiffness
Increasing tendon stiffness/width increases stiffness
Increase tendon length will decrease stiffness/increase compliance
Structure-function relationship (benefits of a long compliant achillies tendon)
Energy storage and return- reduces work muscle fibres have to do
Allows muscles to work on a economical part of their force-vel relationship
can protect muscle fibres from rapid stretch
Cons of long compliant achillies tendon
hard to generate external work
Structure-function relationship - proximal - to - distal leg muscle
proximal muscles have long muscle fibres that facilitate external work production
Distal muscles have longer compliant tendons that help improve steady running economy.
Hip extensors have large flat tendons, making them stiff.
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Length-tension relationship
Muscle fibre length changes with joint angle
length-tension predicts max force generated
Muscle Moment Arms
May change with joint angle
Torque Output and Muscle Design
Muscle force output and moment arm relationships both contribute to resultant output by a muscle at a given joint angle.
Muscle Moment arms - linear to ang kinematics
change in muscle length (k) = theta x r1
Moment arm vs Muscle length
Longer fibres required for longer moment arm
Longer muscle fibres permit greater ROM
Combination allows large ang displacements over which forces can be produce
-Long fibred muscles must have a large vol to have PCSA
Summary of Short Moment Arms
small moment arms help muscle remain on a favourable part of the force-vel relationship, while producing fast joint rotations.
Require more force for given torque
Narrows force-angle relationship
Moment Arms and Effective Mechanical Advantage (EMA)
Mechanical advantage - ratio of input to output moment arms
EMA = External load (output force)/muscle moment arm (input force)
Bi-Articular Muscle Function
Bi-articular muscles act across 2 joint
This can be seen as counterintuitive for some movements
Using bi-articular muscles, power can be transferred from large hip muscle to the knee and ankle
Elastic Power amp and stretch-shortening cycles
Jump height = take off vel
squared
/2g
Increased take off vel requires a large impulse force in a short time (force x time)
Power Amplification
Muscle shortens to stretch tendon while joint at low vel
Joint movement is then powered by rapid release of energy stored in tendon
Tendons aren't constrained by force-vel relationship = release energy faster than muscle can generate work
Power Amplification: where's the 'catch'?
Some insects have a physical 'catch' mechanism - a latch, it can be used to block motion
Frogs manipulate the muscles mechanical advantage - R/r
High value early = tendon stretch
Low value late = tendon recoil
What is a stretch-shortening cycle?
Muscle actively lengthen then shortens
eccentric-concentric sequence