ACTIVE SENSING
Lecture 2: Motor systems
Motor systems
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Basic muscle operation and control
Motor systems
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Muscles have one trick: contraction
flexion
joint
extension
Motor systems
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Muscles have one trick: contraction
joint
extension
Motor systems
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Muscles have one trick: contraction
flexion
joint
extension
Motor systems
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Muscles have one trick: contraction
flexion
joint
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Muscles have one trick: contraction
Extensor
Fext
Flexor
Fflex
flexion
joint
Torquenet = torqueflex - torqueext
extension
= Fflex dflex - Fext dext
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Elbow rotation
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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
Motor systems
Inactive
state
active
state
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Twitch, tetanic force, fatigability
Slow
Fast fatigue-resistant
Fast fatigable
Twitch
Tetanic force
Fatigability
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Motor Units
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.
Constituents of Motor Units
Motor neurons
Ventral horn
of spinal cord
The size of motor units varies with
the precision of movement control:
Leg: >1000 muscle fibers per unit
Eye: <100 muscle fibers per unit
Muscle fibers
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Excitation Contraction Coupling
Phase 1:
Firing of Motor Neuron
Input to motorneuron
Phase 2:
Release of Neurotransmitter
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Excitation Contraction Coupling
Phase 1:
Firing of Motor Neuron
Input to motorneuron
Phase 2:
Release of Neurotransmitter
Phase 3:
Muscle contraction
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Open-loop system
Information flows in one
direction (from neurons
to muscles
Input to motorneuron
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Open-loop system
Information flows in one
direction (from neurons
to muscles
Input to motorneuron
Closed-loop system
Information flows in a
closed loop: from
neurons to muscles and
from muscles to
neurons
What kind of information ?
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Closed-loop system
The direct feedback from
muscles and joints is mediated
by proprioceptive signals
Proprioceptive receptor types
Name:
Muscle spindle
receptors
Sensitive to: muscle length
Motor systems
Golgi tendon
organs
Joint receptors
muscle tension
Flexion, extension
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Proprioceptive receptor types
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
Sensitive to:
muscle length
muscle tension
Flexion, extension
Location:
Fleshy part of
the muscle
Between muscle
and tendon
Joint capsule
Parallel to
muscle fibers
Serial to
muscle fibers
Between bones
Name:
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Proprioceptive receptor types
Name:
Sensitive to:
Motor systems
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
muscle length
muscle tension
Flexion, extension
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What proprioceptors encode?
Motor systems
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Proprioceptive receptor types
Name:
Sensitive to:
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
muscle length
muscle tension
Flexion, extension
From
Arthur Prochazka,
University of Alberta
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Proprioceptive receptor types
Name:
Sensitive to:
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
muscle length
muscle tension
Flexion, extension
force
Encode:
Motor systems
f = k1 F
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Proprioceptive receptor types
Name:
Sensitive to:
Encode:
Motor systems
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
muscle length
muscle tension
Flexion, extension
Length + velocity
force
angle
f = k1L + k2 V 0.6
f = k1 F
f = k1 q
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Proprioceptive receptor types
Name:
Sensitive to:
Encode:
Muscle spindle
receptors
Golgi tendon
organs
Joint receptors
muscle length
muscle tension
Flexion, extension
Length + velocity
force
angle
f = k1L + k2 V 0.6
f = k1 F
f = k1 q
q
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q
q
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PID control

Proportional

Integral

Derivative
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(to the controlled variable)
(of the controlled variable)
(of the controlled variable)
Present
q
Past
q
Future
q
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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
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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
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Reflexes
probing closed-loop control
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The stretch reflex probes the control function of
muscle spindles
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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
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Is the anatomical loop negative?

Muscle spindle excites the motor neuron

Motor neuron excites muscle fibers

Muscle contraction suppresses spindle response
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What about the flexor muscles?
Positive or negative loop?
What is the underlying circuit?
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Pain reflex
Positive or negative?
What is the underlying circuit?
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