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