recruitment modulate force production by –recruitment: changing the number of active mus size...
TRANSCRIPT
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– 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
• 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
Tot
al f
orce
Excitation Contraction Coupling
1. Axon2. Motor
Endplate3. Cell Membrane4. T-Tubule/Triad5. 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
Time
For
ce
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
• Action potential currents (nA, mV)– Inward (sodium)– Outward (potassium)– Nerve or muscle
1234
ReferenceMeasure
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 toneural-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
Ordered pairings by force
First-recruited unit has lower CV and smaller axon
Ordered pairings by conduction velocity
First-recruited unit produces less force
Line of unity(ie, later unit same
as earlier unit)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– 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– 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