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 –1 motor neuron – 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 Total force

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 ( Hz) Ca 2+ elevation ms (5-10 Hz) Force ms (3-5 Hz) Additional action potentials increase force by limiting relaxation and increasing saturation Time Force

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 Reference Measure 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 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 forceFiring 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, th 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 trainingActual 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