Recruitment
• Modulate force production by
– Recruitment: changing the number of active MUs
• Size Principle: recruitment threshold is proportional to MU force
• Proportional control
– Rate coding: changing the firing rate of active MUs
• Force-frequency relationship
• Experimental models
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Henneman & al 1965, decerebrate cat
Jones, Lyons, et al., 1994, human FDI
De Luca & Contessa, 2012, human massive signal analysis
Yue & Cole, 1992, human training
Motor unit
• Motor unit
– 1 motor neuron
– 10-1000 muscle fibers
• Large variation in size
• Consistent fiber phenotype
• Electrical stimulation
– Input resistance inversely
proportional to CSA
– Large MNs activated at low
voltage
Recruitment: proportional control
Total force
• Motor units are recruited in size ranked order
• Smaller MN, slower contraction time, lower
threshold
• Force of next available MU increases with total
force
Recruitment Level
Excitation Contraction Coupling
1. Axon
2. Motor
Endplate
3. Cell Membrane
4. T-Tubule/Triad
5. Sarcoplasmic
Reticulum
Twitch & Tetanus
• Signal processing
– Delay
– Amplification
• Summation
– Multiple processes
– Saturation
Rate coding: force summation
Action potential 1-2 ms (500-1000 Hz)
Ca2+ elevation 100-200 ms (5-10 Hz)
Force 200-300 ms (3-5 Hz)
Additional action potentials increase force by
limiting relaxation and increasing saturation
Force
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Time
How can you study voluntary recruitment?
• Identify and characterize specific neurons
– Distinguish among 10s-100s of MUs
– Estimate of force contribution/size
• Produce graded (or at least different) forces
– Find relationship between “intensity” and MU pool
– Synaptic (chemical) activation, not electrical
Extracellular potentials
• Measure electrical potential by induced current
(i=V/R)
• Current changes potential
(dV/dt = i/C)
– Including intracellular current
Measure
1234
Reference
• Action potential currents (nA, mV)
– Inward (sodium)
– Outward (potassium)
– Nerve or muscle
Single fiber 1
Single fiber 2
Net signal
Flexion and crossed extension reflexes
• Spinal reflex for pain avoidance
– Cutaneous nocioceptor
– 2 spinal interneurons
– Motor neuron
• Ipsilateral: flexion
– Activate flexor MNs
– Inhibit extensor MNs
• Contralateral: extension
– Inhibit flexor
– Activate extensors
• Controllable interface to
neural-organized pools
Kandel & Schwartz
Elwood Henneman 1957
• Decerebrate cat
– No perception of pain
– No anesthetic suppression of neural activity
• Spinal root stimulation/recording
– Dorsal root (sensory) stimulation
– Ventral root (motor) recording
• Two-phase responses
– Initial, synchronous burst
– Persistent rhythmic but
asynchronous firing
• EMG vs ENG amplitude
Dorsal root
simulation strength
Graded intensity dorsal root stimulation
• Increasing cutaneous/DR
stimulus increases intensity
of withdrawal
• Recruited MNs fire more
action potentials
– ie: red amplitude MN gives 3
discharges at 7.5 V, 6 at 12.5 V
and 9 at 25 V
• More MNs are recruited
– Blue at 12.5
– Green at 25
• New MNs at higher
frequency
Size Principle
• Motor neurons are recruited in an orderly fashion
from smallest to largest
Distribution of available
MU forces
Line of unity
(ie, later unit same
as earlier unit)
Ordered pairings by
conduction velocity
First-recruited unit has lower
CV and smaller axon
Ordered pairings by force
First-recruited unit
produces less force
Cope & Clark, 1991
Jones & al., 1994
• Human First Dorsal Interosseus
– Take directions better than cats
– Truly voluntary behavior
• Electromyogram Decomposition
– Fine wire electrode
– Muscle signal,
filtered through tissue
Hudson & al., 2009
EMG decomposition
• Surface EMG is very coarse
– Cubic centimeters
– Thousands of fibers
• Fine wires record very small volume
– Few fibers, few MUs
– Identify discrete action potentials
• Amplitude
• Period
• Waveform
– No force/size
Individual MU waveforms
Three finger motions, consistent order
• Ab-duction of inceasing force to define pairing
order
• “Pincer” staple-remover
• “Rotation” unscrew a bolt
• Order of pairings is (mostly) preserved
De Luca & al., 2012
• Human FDI/VL
• Force Ramp-hold-release
– Improved signal analysis
– “Knowledge system” based, template identification
– SEMG
Conflicts with Henneman
• Order is preserved
• Firing rate is inverted
– Higher threshold units have lower frequency
– Individual MU firing rate increases with intensity
Decomposed MU firings with force
Firing rate for extracted MUs
Consequences of orderly recruitment
• Force
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Small MUs recruited at low force
Large MUs recruited at high force
Marginal force addition is proportional to current force
Proportional control
Signal-dependent noise
• Performance
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Small MUs are slow and oxidative
Large MUs are fast and glycolytic
Low intensity: high endurance
High intensity: low endurance
Ballistic: fast contraction dynamics
Yue & Cole, 1992
• 5th abductor digiti minimi
• 4 wks abduction strength training
– 1 set of 15 max, isometric
– “Imagined” contractions without force
Substantial strength gain, w/o force
• Actual training: +30%
• Imagined training: +22%
– Can’t statistically resolve difference
– All subjects in both groups increase “strength”
• Performance gains 0-3 weeks all in your head
Imagined training
Actual training
Summary
• Nervous system has a structure for grading force
– Recruitment: small MUs before large MUs
– Rate coding: frequency of recruited MUs increases with
effort
• Coordinated MU properties allows functional
optimization
– High-endurance units/fibers for ‘normal’ activities
– High-velocity units/fibers for ‘emergency’ activities
• Control strategy has a strong influence on function
– Completeness of recruitment
– Firing rate
– MU synchrony