Transcript Modeling and Imagery

Section 4
Motor Control & Learning
General intro
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Motor = movement
• A very diverse area
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An area of many questions
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How do we move?
Why do we move the way we do?
How do movement skills improve?
How does memory help learning?
Are experts “born” or “made”?
Should we tell learners what they ought to do?
General intro
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Inference
• We can’t just look into the CNS
• Stroke, injury, observation…a slow accumulation
of knowledge
• The most diverse of our sub-disciplines. See fig.
1.1…
General intro
General intro
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Sub-areas
• Neurophysiology – the wiring (ch. 14)
• Cognitive science – the behavior
• Together, they describe the development of movement
control in the growing human (ch. 16), the means by
which movements are eventually controlled (ch. 14 &
15), and give insight into the complex ideas underlying
how movement skills are acquired in the child and adult
(ch. 17)
• First, a quick look at the basic structure of the
problem…
Ch. 14
Neurophysiological
Approaches to Motor Control
Intro
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What is the problem of control?
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792 muscles, 100 joints.
degrees of freedom problem (Bernstein, 1967)
Motor equivalence
We never quite repeat ourselves (Bartlett, 1932)
We are not aware of what we do well (James,
1890)
Components of the nervous system
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Basic components (neurophysiologically
speaking):
• Sensory receptors
• Motor units
• Neurons (nerves) & synapses
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How complex?
• 1,000,000,000,000-100,000,000,000,000 neurons
• UK: 1 billion to 100 billion
• US: 1 trillion to 100 trillion
• Each neuron has up to 10,000 connections
• “More connections than there are particles in the known
universe”
Components of the nervous system
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Basic function
• CNS, PNS
• Afference, efference
Efferent
information
PNS
CNS
Afferent
information
Neurons and Synapses
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Structure and function of neurons
Neurons and Synapses
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Structure and function of neurons
Neurons and Synapses
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Structure and function of neurons
• Vary by # of dendrites, length of axon
• Structure determines role – many types (bipolar,
multipolar, golgi I, golgi II…etc.)
Purkinje cell
(in cerebellum)
Basic reflex arc,
with
interneuron
Neurons and Synapses
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Structure and function of neurons
• Neural communication:
• Dendrites pick up electrical signal
• If total signal strength exceeds a threshold, neuron
fires…pulse sent along axon
• Pulse is carried more efficiently if axon is myelinated
(myelin is fatty stuff that insulates)
• So signal transmission is complex, determined by total
strength of arriving signal and subsequent strength of
descending signal
Neurons and Synapses
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Structure & function of
synapses
• Joints of the nervous
system
• Electrical or chemical
communication across
synaptic gap
• Chemical most common,
via neurotransmitter
• Can be either inhibitory
or excitatory
synapse
Neurons and Synapses
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Structure & function of
synapses
• So, typical signal
transmission is:
• Electrical activity 
• Chemical activity 
• Electrical activity 
• Excitation, inhibition
necessary for complex
patterns of
communication
synapse
Sensory Receptor Systems for Movement
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Primarily from vision and proprioception
• Purpose is to communicate information
• Achieved by converting signals through several
energy types
Sensory Receptor Systems for Movement
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The visual system
• Light  retina  rods,
cones  optic nerve 
LGN (70%)  visual
cortex…focal vision
• Light  retina  rods,
cones  optic nerve 
SC…ambient vision
• Focal vision pathway =
“what” pathway
• Ambient vision pathway
= “vision for action”
pathway
Sensory Receptor Systems for Movement
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The visual system
• Rods, cones
• All over retina but more
concentrated in fovea
• Cones sense color,
require high light
• Rods only sense B&W,
only require low light
• Hence seeing at night in B
&W
Sensory Receptor Systems for Movement
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The kinesthetic system
• Sensory receptors are distributed through muscle,
tendon, joints and skin
Sensory Receptor Systems for Movement
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Muscle receptors
• Muscle spindles
• In all muscles – esp. small ones, used for fine control (think
about rate control…see open & closed loop control later)
• Made up of intrafusal muscle fibers and sensory receptors
• Transmits info about amount and rate of stretch in muscle
Sensory Receptor Systems for Movement
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Muscle receptors
• Muscle spindles
• Basically, they lie alongside the main muscle fibers, and are
stretched by them(at endpoints)
Sensory Receptor Systems for Movement
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Muscle receptors
• Muscle spindles
• Intrafusal fibers:
• Innervated by gamma motor neuron activity
• Contract at end points only
• Loaded with sensory connections returning signals to the spinal
cord
Fires when entire muscle
stretches
Type Ia afferent: connected at
the non-contractile center.
Fires on mismatch between
contraction due to alpha
motor neuron and
contraction due to gamma
motor neuron activation
Sensory Receptor Systems for Movement
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Tendon receptors
• Golgi tendon organs
• Impulse returns via type
Ib afferent fiber
• Respond to tension in the
tendon
• Fire when entire muscle
shortens (contrast
w/spindle)
• Function:
• protection – like a fuse
in a circuit
• Feedback to spinal cord
• Stiffness?
Tension? – fine
tuning
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Exact role of spindles &
GTO’s still debated
Sensory Receptor Systems for Movement
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Skin (cutaneous) receptors
Respond to
light vibration
Responds to
high frequency
vibration, &
compression
Sensory Receptor Systems for Movement
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Skin (cutaneous) receptors
• Density of cutaneous receptors varies around the
body
• Gives rise to “just noticeable differences” varying in
different parts of the skin
• Loss or damage to these receptors ruins fine control of
movements
• Problems w/robots lacking these senses
Sensory Receptor Systems for Movement
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Joint receptors
• Modified Ruffini corpuscles,
Modified Pacinian corpuscles
• In joint capsule
• Golgi organs
• In ligaments
Thought to signal
problems w/extreme
ranges of motion
Sensory Receptor Systems for Movement
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The vestibular system
• Signals body orientation in space
Highly integrated with
vision…mismatch leads to
motion sickness
• Semicircular canals, otolith organs (utricle, saccule)
Canals allow for both linear and
angular acceleration sense…like
internal accelerometers
Sensory Receptor Systems for Movement
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Intersensory integration and sensory dominance
• Overall sense of what is going on dependent on
information flowing from many receptors
simultaneously
• Occasionally they contradict each other
• Vision is dominant…can lead to some amusing
experiments (and experiences)
• The swinging room (see next 4 slides)
• The maze at navy pier
• Alcohol, helicopters, and keeping the lights on
• See, for example,
http://icbmp.uaeu.ac.ae/Proceedings/PDFPAPERS/59_ICBMP.p
df if you’re curious.
Sensory Receptor Systems for Movement
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The swinging room (Lee & Aronson, ‘74)
The swinging “room” was
(is?) a box hanging from the
ceiling of a large hall
Experimenter (Lee)
stands behind fake
room, holding a rod
connecting the back
walls, so that he can
swing the room back
and forth an inch or 2
Participant stands in center of
fake room, looking at front
wall (field of view filled with
fake room)
Sensory Receptor Systems for Movement
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The swinging room (Lee & Aronson, ‘74)
Direction of motion of
swinging room
Sensory Receptor Systems for Movement
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So which way would you sway (relative to
the room)?
Direction of motion of
swinging room
?
Sensory Receptor Systems for Movement
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Other applications of  (tau - time to contact
- used to guide movement)…
• Echolocation in bats
• Diving gannets
• Driving, feeding (hand to mouth), kicking, running,
steering, catching, etc., etc., etc.
Effector Systems for Movement
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The motor unit
• Covered in Chapter 2.
Motor units containing
smaller muscle fibers
are recruited first –
size principle
Motor Control Functions of the Spinal Cord
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Structure of the spinal cord
• 2 basic functions:
Surprisingly
functional
• Two-way communications route
• Maintains movements in progress (&
prevents bad things getting worse)
• Mesencephalic cat
• 31 pairs of nerves attach – see figure
• It’s protected by bone along it’s
route, and is a massive information
superhighway
Motor Control Functions of the Spinal Cord
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Spinal reflexes
• 4 components:
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Sensory receptor
Afferent neuron
Efferent neuron
Effector
• The stretch reflex
• The simplest example of the above 4 components
• Knee-jerk reflex is an example
• Muscle is stretched, reflex makes muscle contract in
response to the stretch
Motor Control Functions of the Spinal Cord
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Spinal reflexes
• The flexion reflex
• Typical response to pain or threat
• Limbs flex as a consequence of reflexive contraction of
flexor muscles & inhibition of contraction of extensor
muscles
Motor Control Functions of the Spinal Cord
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Spinal reflexes
• The crossed
extensor reflex
• Works with flexion
reflex to get away
from the
pain/threat source
• Opposite pattern of
contraction
Motor Control Functions of the Spinal Cord
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Spinal reflexes
• The extensor thrust reflex
• Responds to pressure on sole of foot by contracting
extensor muscles in leg
• Aids balance, walking
• Spinal reflexes for gait control
• Overall, these types of reflex show patterns of
reciprocal inhibition that set limbs to work in cycles and
initiate rhythm…leads to spine as…
• Central pattern generator (mes. cat again)
Motor Control Functions of the Spinal Cord
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The role of reflexes in voluntary movement
control
• All paths lead to α–motor neuron activation
• α-γ coactivation
• The γ activation of the intrafusal fibers serves as a
reflexive check on the α activated extrafusal fibers
• If there’s a match, all is well
• If there’s a mismatch, the α–motor neuron fires some
more
• Basic idea: reflexes are incorporated in voluntary
movement control
Motor Control Functions of the Brain
Basal ganglia
(deep)
brainstem
Motor Control Functions of the Brain
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The motor cortex
• Proportional
representation
• Mostly opposite to side
of body
• As you move to the left,
planned movements are
more gross, and involve
more muscles
• Takes in signals from a #
of sources
• Sends out final plan to
spine
Motor Control Functions of the Brain
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The motor cortex
• Pyramidal tract:
• primary path to muscles (via α–motor neurons and
interneurons)
• Damage results in paralysis
• Extrapyramidal tract:
• Secondary pathway, via basal ganglia, cerebellum,
thalamus, brainstem
• Primarily inhibitory function (required for coordination,
of course)
• Damage results in spasticity
Motor Control Functions of the Brain
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The cerebellum
• “the seat of
coordination”
• Receives input from all
over…cortex, brainstem,
vestibular apparatus,
sensory receptors from
spine
• Output to thalamus and
brainstem
Motor Control Functions of the Brain
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The basal ganglia
• Input from motor cortex
& brainstem
• Output to thalamus &
brainstem
• No direct link to α-motor
neurons, but still
important in regulation
of movements
• Parkinson’s disease
• Huntington’s disease
Motor Control Functions of the Brain
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The brainstem
• Pons, medulla, reticular
formation
• Integration of both
afferent & efferent
information
• Regulates many long
loop reflexes (righting
reflex, tonic reflexes)
• Also tunes lower
reflexes, regulates
arousal
Integrative Brain Mechanisms for Movement
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Understanding how all systems work
together in movement regulation is
incredibly difficult…final understanding is a
long way off
So there are a number of theories that deal
instead with a different level of analysis
• Cognitive science – see next chapter