Motor systems1 ACTIVE SENSING Lecture 2: Motor systems

Motor systems2 Basic muscle operation and control

Motor systems3 Muscles have one trick: contraction joint extension flexion

Motor systems4 Muscles have one trick: contraction joint extension

Motor systems5 Muscles have one trick: contraction joint extension flexion

Motor systems6 Muscles have one trick: contraction joint flexion

Motor systems7 Muscles have one trick: contraction joint extension flexion Torque net = torque flex - torque ext F flex F ext d flex d ext = F flex d flex - F ext d ext Flexor Extensor

Motor systems8 Elbow rotation

Motor systems9 Muscles Muscles consist of arrays of individual fibers, muscle fibers, covered by a membrane. Fibers are constituted from smaller contractile units called myofibrils. Muscles have active state, when they contract (shorten) and inactive state when they lengthen. Fibers that work together are synergistic. Antagonistic fibers work in opposition to each other. Muscle fibers (motor units) differ in dynamic range: slow: slow, long duration, low tension fast fatigue-resistant: fast, medium duration, medium tension fast fatigable: very fast, short duration, high tension Inactive state active state

Motor systems10 Twitch, tetanic force, fatigability SlowFast fatigue-resistantFast fatigable Twitch Tetanic force Fatigability

Motor systems11 Each muscle fiber is innervated by only a single axon, but a single axon innervates several muscle fibers. The motor neuron and the muscle fibers it innervates constitute a motor unit. The size of motor units varies with the precision of movement control: Leg: >1000 muscle fibers per unit Eye: <100 muscle fibers per unit Constituents of Motor Units Ventral horn of spinal cord Motor neurons Muscle fibers Motor Units

Motor systems12 Excitation Contraction Coupling Phase 1: Firing of Motor Neuron Phase 2: Release of Neurotransmitter Input to motorneuron

Motor systems13 Excitation Contraction Coupling Phase 1: Firing of Motor Neuron Phase 2: Release of Neurotransmitter Input to motorneuron Phase 3: Muscle contraction

Motor systems14 Input to motorneuron Open-loop system Information flows in one direction (from neurons to muscles

Motor systems15 Closed-loop system Input to motorneuron Information flows in a closed loop: from neurons to muscles and from muscles to neurons What kind of information ? Open-loop system Information flows in one direction (from neurons to muscles

Motor systems16 Closed-loop system The direct feedback from muscles and joints is mediated by proprioceptive signals muscle length Sensitive to: muscle tension Flexion, extension Proprioceptive receptor types Muscle spindle receptors Name: Golgi tendon organs Joint receptors

Motor systems17 Proprioceptive receptor types Muscle spindle receptors muscle length Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension Location: Fleshy part of the muscle Between muscle and tendon Joint capsule Parallel to muscle fibers Serial to muscle fibers Between bones Sensitive to:

Motor systems18 Proprioceptive receptor types Muscle spindle receptors muscle length Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension Sensitive to:

Motor systems19 What proprioceptors encode?

Motor systems20 Proprioceptive receptor types Muscle spindle receptors muscle length Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension From Arthur Prochazka, University of Alberta Sensitive to:

Motor systems21 Proprioceptive receptor types Muscle spindle receptors muscle length Sensitive to: Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension Encode: force f = k 1 F

Motor systems22 Proprioceptive receptor types Muscle spindle receptors muscle length Sensitive to: Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension Length + velocity f = k 1 L + k 2 V 0.6 Encode: force f = k 1 F angle f = k 1 

Motor systems23 Proprioceptive receptor types Muscle spindle receptors muscle length Sensitive to: Name: Golgi tendon organs muscle tension Joint receptors Flexion, extension Length + velocity f = k 1 L + k 2 V 0.6 Encode: force f = k 1 F angle f = k 1  

Motor systems24 PID control  Proportional (to the controlled variable)  Integral (of the controlled variable)  Derivative (of the controlled variable) Present Past Future  

Motor systems25 Negative feedback loop  Characteristic: The effect of a perturbation is in opposite direction  Requirement: The cumulative sign along the loop is negative  Function: Can keep stable fixed points

Motor systems26 Positive feedback loop  Characteristic: The effect of a perturbation is in the same direction  Requirement: The cumulative sign along the loop is positive  Function: amplifies perturbations

Motor systems27 Reflexes probing closed-loop control

Motor systems28 The stretch reflex probes the control function of muscle spindles

Motor systems29 Is the loop positive or negative?  The stroke stretches the spindle  As a result the muscle contracts  The result opposes the perturbation => negative FB loop => negative FB loop

Motor systems30 Is the anatomical loop negative?  Muscle spindle excites the motor neuron  Motor neuron excites muscle fibers  Muscle contraction suppresses spindle response

Motor systems31 What about the flexor muscles? Positive or negative loop? What is the underlying circuit?

Motor systems32 Pain reflex Positive or negative? What is the underlying circuit?