Transcript Modeling and Imagery

Neurophysiological
Approaches to Motor Control
– with a slant to memory
Components of the nervous system

Basic components (neurophysiologically
speaking):
• Sensory receptors
• Motor units
• Neurons (nerves) & synapses

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

Basic function
• CNS, PNS
• Afference, efference
Efferent
information
PNS
CNS
Afferent
information
Neurons and Synapses

Structure and function of neurons
Neurons and Synapses

Structure and function of neurons
Neurons and Synapses

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

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

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

Structure & function of
synapses
• So, typical signal transmission is:
• Electrical activity 
• Chemical activity 
• Electrical activity 
• Excitation, inhibition
necessary for complex
patterns of communication
• Neurotransmitters necessary
for signals to travel
synapse
Sensory Receptor Systems for Movement

Primarily from vision and proprioception
• Purpose is to communicate information
• Achieved by converting signals through several
energy types
Sensory Receptor Systems for Movement

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

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

The kinesthetic system
• Sensory receptors are distributed through muscle,
tendon, joints and skin
Sensory Receptor Systems for Movement

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

Muscle receptors
• Muscle spindles
• Basically, they lie alongside the main muscle fibers, and are
stretched by them(at endpoints)
Sensory Receptor Systems for Movement

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

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

Exact role of spindles &
GTO’s still debated
Sensory Receptor Systems for Movement

Skin (cutaneous) receptors
Respond to
light vibration
Responds to
high frequency
vibration, &
compression
Sensory Receptor Systems for Movement

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

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

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

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)
Effector Systems for Movement

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

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

Spinal reflexes
• 4 components:
•
•
•
•
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

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

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

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

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

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

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

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

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

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


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
Memory – development and type
Memory types (From Squire and ZolaMorgan, 2003)
Memory types
Memory as Association

Answer these 2 questions:
• What continent is Kenya in?
• What are the two colors of the pieces in a game of
chess?
• Name any animal

Association, and spread