LE Goossens: Motor units, spinal reflexes and muscle coordination
Muscle cell: multiple nuclei.
Active and passive muscle forces varies as a function of muscle length.
Length increases (less and less overlap) → more force.
When there is no overlap, active force goes down, passive force goes up.
- Active force: generated by actin/myosin fibres
- Passive force: generated by connective tissue in muscle
Motor unit: motoneuron, its axon and all the muscle fibres it
innervates.
A single motoneuron innervates many muscle fibres.
- Each muscle fibre is innervated by a single motor neuron
- Each motor neuron innervates 10-500 muscle fibres
- All muscle fibres in a given motor unit contract together (not
independently)
Increasing activation frequency elevates force of contraction:
1
,3 types of motor units:
Type S = weak and slow but fatigue resistant; respiratory muscles, core muscles. Lower forces with
slower build-up.
Type FR = strong/intermediate strength, fast and fatigue resistant
Type FF = strong and fast but fatigue quickly
Tetanic tension is an inverse function of contraction time.
Types of motor units
Recruitment threshold Twitch amplitude Twitch duration Fatigable Muscle fibres
Low Small Long No Red
High Large Short Yes White
The size principle: the recruitment order occurs in order of contraction strength.
Cat example; recruitment of motor units under different behavioural conditions:
slow is recruited first, then FR, then FF.
2 mechanisms to regulate muscle force:
- Recruitment of motor units according to the size principle
- Summation of force as a function of firing frequency; temporal summation
Coordination of movement
- Redundancy (‘abundancy’) of muscles
Multiple muscles act across each joint. Each muscle may act across multiple
joints or contribute to multiple degrees of freedom.
Recruitment of motor units
Individual motor units can be identified in the needle EMG by their waveform.
Each motor unit is recruited at its own specific torque level.
Recruitment if a biceps motor unit
depends on the combination of
flexion-extension and supination-
pronation torques.
2
,Several groups of motor unit behaviour in the biceps:
The biceps contribute to flexion and supination, and contains
separate motor-unit populations, that are activated for these tasks.
Motor units with different behaviour are located in different parts of
the biceps.
- Motor units in 1 muscle can reveal a different recruitment pattern
due to a different activation.
- The different activation of motor units in a muscle reflect a
neuronal component, not differences in mechanical effects.
- Motor units with the same recruitment behaviour are localized in
the same muscle part: compartmentalization
- Within each group of motor units, there are motor units with a low
and high recruitment threshold.
- Not muscles, but groups of motor units are the basic elements of
motor activation patterns.
- The activation of a muscle cannot be understood from the
anatomy of that single muscle. The anatomy of the complete motor
system acting across a joint should be considered.
Tricep: extension muscle. Bicep: flexion muscle.
Motor-unit behaviour during reflexes; reflexes are coordinated.
Important for: walking through loose sand, walking on uneven
terrain, moving of subjects that have an unknown weight.
Reflex components:
- Short-latency reflex or mono-synaptic reflex (<50ms)
- Long-loop reflex (50-100ms)
- Triggered reactions
The mono-synaptic reflex comes first, then the long-loop reflex.
The mono-synaptic reflex only occurs during muscle lengthening. The ‘long-latency reflex’ also occurs
in muscles that do not change in length, but that are needed to produce compensatory forces in the
correct direction. Long-loop reflexes take care of coordinated responses.
A good understanding of the activation of a muscle can only be obtained by measuring motor-unit
activity (surface EMG is not sufficient).
Insight in the role of motor units during coordination requires knowledge about the anatomy of all
muscles that act across a joint.
LE Saris: Concepts and techniques in clinical EMG
EDx for neuromuscular disorders
Purpose: to help a clinician determine what part of the PNS is involved, to what extent and with
which pattern, and with which severity. This is part of the clinical work-up for a suspected NMD.
Necessary for correct interpretation is knowledge of: anatomy, a priori chance of abnormal finding,
test performance (sensitivity, specificity, validity, etc)
EDx clinical methods for NMD: nerve conduction studies, needle electromyography (EMG), TMS, etc.
3
, Electromyography (EMG)
This is actually a combination of 2 techniques, nerve conduction studies & needle examination of
muscle.
Time for examination: 45-90 min., contraindications: measuring nerves in open wounds, blood
coagulation disorders, limb not accessible.
Nerve conduction studies (NCS)
Measuring:
- Motor nerves with electrodes on the target muscle → compound muscle action potential (CMAP,
~2mV)
- Sensory nerves with electrodes on the skin → sensory nerve action potential (SNAP, ~10microV)
- Proximal nerve segments with: H-reflexes (electrical version of tendon reflexes) & F-responses
- Neuromuscular junction (NMJ): repetitive nerve stimulation & stimulated single fibre EMG
NCS parameters: amplitude, (distal)
latency, nerve conduction velocity,
dispersion.
CMAP and conduction velocity; the time of latency also includes
time that is needed to get the signal through the synapse and for
the muscle to contract, so this is not only conduction velocity.
When a muscle fibre is stimulated, the current travels to both
sides: to the side of the muscle and to the side of the neuronal
soma.
Nerve conduction velocity: time from onset latency 1 to onset
latency 2 (ms) / distance from stimulation point 1 to stimulation
point 2 on limb surface.
Normal nerve conduction velocities: from 61m/s in the fastest myelinated fibres, to 1.6 in
unmyelinated fibres.
Dispersion: when responses do not arrive together; they arrive at different times; phase cancellation.
Proximal segments of nerve conduction velocity:
- H-reflex: measures both motor and sensory part
When you stimulate harder, collision occurs: M
4
Muscle cell: multiple nuclei.
Active and passive muscle forces varies as a function of muscle length.
Length increases (less and less overlap) → more force.
When there is no overlap, active force goes down, passive force goes up.
- Active force: generated by actin/myosin fibres
- Passive force: generated by connective tissue in muscle
Motor unit: motoneuron, its axon and all the muscle fibres it
innervates.
A single motoneuron innervates many muscle fibres.
- Each muscle fibre is innervated by a single motor neuron
- Each motor neuron innervates 10-500 muscle fibres
- All muscle fibres in a given motor unit contract together (not
independently)
Increasing activation frequency elevates force of contraction:
1
,3 types of motor units:
Type S = weak and slow but fatigue resistant; respiratory muscles, core muscles. Lower forces with
slower build-up.
Type FR = strong/intermediate strength, fast and fatigue resistant
Type FF = strong and fast but fatigue quickly
Tetanic tension is an inverse function of contraction time.
Types of motor units
Recruitment threshold Twitch amplitude Twitch duration Fatigable Muscle fibres
Low Small Long No Red
High Large Short Yes White
The size principle: the recruitment order occurs in order of contraction strength.
Cat example; recruitment of motor units under different behavioural conditions:
slow is recruited first, then FR, then FF.
2 mechanisms to regulate muscle force:
- Recruitment of motor units according to the size principle
- Summation of force as a function of firing frequency; temporal summation
Coordination of movement
- Redundancy (‘abundancy’) of muscles
Multiple muscles act across each joint. Each muscle may act across multiple
joints or contribute to multiple degrees of freedom.
Recruitment of motor units
Individual motor units can be identified in the needle EMG by their waveform.
Each motor unit is recruited at its own specific torque level.
Recruitment if a biceps motor unit
depends on the combination of
flexion-extension and supination-
pronation torques.
2
,Several groups of motor unit behaviour in the biceps:
The biceps contribute to flexion and supination, and contains
separate motor-unit populations, that are activated for these tasks.
Motor units with different behaviour are located in different parts of
the biceps.
- Motor units in 1 muscle can reveal a different recruitment pattern
due to a different activation.
- The different activation of motor units in a muscle reflect a
neuronal component, not differences in mechanical effects.
- Motor units with the same recruitment behaviour are localized in
the same muscle part: compartmentalization
- Within each group of motor units, there are motor units with a low
and high recruitment threshold.
- Not muscles, but groups of motor units are the basic elements of
motor activation patterns.
- The activation of a muscle cannot be understood from the
anatomy of that single muscle. The anatomy of the complete motor
system acting across a joint should be considered.
Tricep: extension muscle. Bicep: flexion muscle.
Motor-unit behaviour during reflexes; reflexes are coordinated.
Important for: walking through loose sand, walking on uneven
terrain, moving of subjects that have an unknown weight.
Reflex components:
- Short-latency reflex or mono-synaptic reflex (<50ms)
- Long-loop reflex (50-100ms)
- Triggered reactions
The mono-synaptic reflex comes first, then the long-loop reflex.
The mono-synaptic reflex only occurs during muscle lengthening. The ‘long-latency reflex’ also occurs
in muscles that do not change in length, but that are needed to produce compensatory forces in the
correct direction. Long-loop reflexes take care of coordinated responses.
A good understanding of the activation of a muscle can only be obtained by measuring motor-unit
activity (surface EMG is not sufficient).
Insight in the role of motor units during coordination requires knowledge about the anatomy of all
muscles that act across a joint.
LE Saris: Concepts and techniques in clinical EMG
EDx for neuromuscular disorders
Purpose: to help a clinician determine what part of the PNS is involved, to what extent and with
which pattern, and with which severity. This is part of the clinical work-up for a suspected NMD.
Necessary for correct interpretation is knowledge of: anatomy, a priori chance of abnormal finding,
test performance (sensitivity, specificity, validity, etc)
EDx clinical methods for NMD: nerve conduction studies, needle electromyography (EMG), TMS, etc.
3
, Electromyography (EMG)
This is actually a combination of 2 techniques, nerve conduction studies & needle examination of
muscle.
Time for examination: 45-90 min., contraindications: measuring nerves in open wounds, blood
coagulation disorders, limb not accessible.
Nerve conduction studies (NCS)
Measuring:
- Motor nerves with electrodes on the target muscle → compound muscle action potential (CMAP,
~2mV)
- Sensory nerves with electrodes on the skin → sensory nerve action potential (SNAP, ~10microV)
- Proximal nerve segments with: H-reflexes (electrical version of tendon reflexes) & F-responses
- Neuromuscular junction (NMJ): repetitive nerve stimulation & stimulated single fibre EMG
NCS parameters: amplitude, (distal)
latency, nerve conduction velocity,
dispersion.
CMAP and conduction velocity; the time of latency also includes
time that is needed to get the signal through the synapse and for
the muscle to contract, so this is not only conduction velocity.
When a muscle fibre is stimulated, the current travels to both
sides: to the side of the muscle and to the side of the neuronal
soma.
Nerve conduction velocity: time from onset latency 1 to onset
latency 2 (ms) / distance from stimulation point 1 to stimulation
point 2 on limb surface.
Normal nerve conduction velocities: from 61m/s in the fastest myelinated fibres, to 1.6 in
unmyelinated fibres.
Dispersion: when responses do not arrive together; they arrive at different times; phase cancellation.
Proximal segments of nerve conduction velocity:
- H-reflex: measures both motor and sensory part
When you stimulate harder, collision occurs: M
4
