The Motor System

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The Motor System
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Objectives

• To understand the organisation of motor systems
• and the problems of controlling movement
• To understand that behaviours reflect the
  integrated activity of multiple components.
• That the mechanisms that regulate behaviour are
  subject to plasticity
• That defects in motor performance reflect
  functional deficits in motor components

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Sources

• Kandel, Schwartz Jessel, Principles of Neural
  Science. Detailed account (many of the figures I
  use are from here).
• Bear, Connors, and Paradiso, Neuroscience
  exploring the brain. Shorter summary.
• Shepherd, Neurobiology. Comparative analysis of
  different motor systems in different animals.

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Motor control

• Considered understood, or trivial
• Limited focus of decade of the brain
• Largely unconscious and invariant development
• Reflects highly sophisticated system
• Cognitive simulators vs robotic systems

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• To move is all mankind can do.. whether in
  whispering a syllable, or in felling a forest
• Charles Sherrington, The Integrative action of
  the Nervous System.

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Functions of the motor system

• Move through the environment
• Manipulate the external world
• Maintain posture/balance
• Autonomic e.g. respiration
• Speech/ gestures/ writing
• Sensation-saccades, haptics

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Types of movement

• Reflex - cough, knee-jerk
• Few muscle groups, highly stereotyped, graded
  with stimulus
• Rhythmic - walking, swimming, chewing
• Several muscle groups around limb/joint,
  relatively stereotyped, not necessarily graded
  with stimulus intensity
• Voluntary speech, manipulating objects
• Goal directed, highly modifiable, not need
  external input

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Movement is difficult to study

• It is the end point of a series of commands
• Depend on history of movement- thixotropy
• Requires insight into neural network function
• Motor equivalence- every movement potentially
  unique

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Topics

• Lecture 1 principles of motor control
• Lectures 2 and 3 spinal systems and
  proprioceptive feedback
• Lecture 4 descending pathways
• Lecture 5 motor cortex
• Lecture 6 cerebellum
• Lecture 7 basal ganglia

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General features of motor systems

• Interaction of activity in multiple centres
  (networks)
• Continuous flow of feedforward inputs and
  feedback loops- internal connections and external
  sensory feedback

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Spinal cord

• Lectures 2 and 3
• Assumed to be home of hard-wired reflexes
• Assumed to relay inputs from brain
• Actually contains neural machinery (CPG) for
  independently coordinating motor outputs

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Brainstem

• Lecture 4
• Relay for inputs from brain to spinal cord
• Complicated system of motor (and non-motor areas)
• Integrates inputs from various sources
• Signals to spinal cord the type of motor output
  required

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Motor cortex

• Lecture 5
• Primary cortex provides motor output to spinal
  cord
• Contains motor synergies (commands for sequences
  of muscle activation)
• Association areas involved in planning and
  preparing for movements

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Cerebellum

• Lecture 6
• Literally little brain
• 20 of brains volume- 50 of its neurons
• Highly ordered structure
• Motor and non-motor areas
• Probably compares planned outputs with actual
  motor performance to allow error correction

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Basal ganglia

• Lecture 7
• Collection of sub-cortical nuclei
• Little organisation compared to cerebellum
• Selection of motor outputs
• Ensures motor outputs are appropriate with
  ongoing function

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Motor hierarchy
Cortical Cortex, Cerebellum, Basal ganglia
Hierarchy of Command systems
Brainstem- Fast pathways Slow pathways
Spinal cord
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Motor hierarchy?
Motor cortex
Basal ganglia
Thalamus
Cerebellum
Brainstem
Spinal cord
Sensory feedback
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Motor hierarchy

• Parallel processing in distributed centres
• No linear chain of command - ascending and
  descending inputs connect levels

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Functional hierarchy

• IDEA (what is my goal?)
• PLAN (how do I achieve it?)
• PROGRAM (which muscles contract? How much?)
• EXECUTION (send out motor commands)
• MOVEMENT

Cortical motor areas
Brainstem
Spinal cord
Muscles
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Functional hierarchy?

• Spinal cord- lowest level- can program and
  coordinate complex motor patterns
• Motor cortex-highest level- directly connect to
  motor neurons and so has a role in the execution
  of motor commands

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Sensory-motor integration

• Sensory inputs trigger and modify ongoing motor
  patterns. Information on position of body in
  space and the activity of muscles
  (proprioception).

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Motor-sensory interactions
Held and Hein
Bach-y-Rita
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Feedback mechanisms
Desired output

Comparator
Controller
Output
-
Sensory system
Feedback
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Effects of biological delays
Error first detected
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The Problem of Delays
Error signal First generated
Over-correction Leads to oscillation
Desired position
Correction starts here
TIME
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Feedback delays

• Fast movements are important
• Hunting
• Escape
• Speech
• A significant problem with feedback systems is
  that they introduce delays.

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Anticipation of Motor Commands

• Delays in sensory feedback systems limit their
  use to slow movements
• Fast movements estimate motor commands in advance
  learnt feedforward predictive systems.
• Ballistic- stored motor programs

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Feedforward mechanisms
Learning
Stored program

Feedforward controller
Generator
Desired output
Output
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Internal feedback-efferent copy
Feedforward controller
Desired output
Comparator for internal and external feedback
Generator
Output
Internal feedback
External feedback
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Feedforward and feedback loops
Brain
Feedback (efferent copy)
Feedforward input
Spinal cord
Feedback
Output
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Summary

• Motor systems fulfil diverse roles
• Motor control is computationally difficult
• Reflects activity in several structures
• Biological delays limit sensory feedback to slow
  movements.
• Fast movements use feedforward prediction.