Central Sensorimotor Programs

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PowerPoint Presentation for
Biopsychology, 8th Edition
by John P.J. Pinel
Prepared by Jeffrey W. Grimm
Western Washington University
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Inc. All rights reserved.
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Chapter 8
The Sensorimotor System
How You Move
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Three Principles of
Sensorimotor Function

Hierarchical Organization

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Association cortex at the highest level,
muscles at the lowest
Parallel structure – signals flow between levels
over multiple paths
Motor Output is Guided by Sensory Input
Learning Changes the Nature and Locus
of Sensorimotor Control

e.g. conscious to automatic
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FIGURE 8.1 A general model of the
sensorimotor system.
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Sensorimotor Association
Cortex
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Posterior parietal association cortex
Dorsolateral prefrontal association cortex
Each composed of several different areas
with different functions
Some disagreement exists about how to
divide the areas up
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Posterior Parietal Association
Cortex



Integrates information about
 Body part location
 External objects
Receives visual, auditory, and somatosensory
information
Outputs to motor cortex

Including dorsolateral prefrontal association cortex,
secondary motor cortex, and frontal eye field
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FIGURE 8.2 The major cortical input and output
pathways of the posterior parietal association cortex.
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Damage to the Posterior
Parietal Cortex


Apraxia – disorder of voluntary movement –
problem only evident when instructed to
perform an action – usually a consequence of
damage to the area on the left
Contralateral neglect – unable to respond to
stimuli contralateral to the side of the lesion –
usually seen with large lesions on the right
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FIGURE 8.3 Contralateral neglect is sometimes
manifested in terms of gravitational coordinates,
sometimes in terms of object-based coordinates.
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Dorsolateral Prefrontal
Association Cortex



Input from posterior parietal cortex
Output to secondary motor cortex, primary
motor cortex, and frontal eye field
Evaluates external stimuli and initiates
voluntary reactions – supported by neuronal
responses
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Secondary Motor Cortex


Input mainly from association cortex
Output mainly to primary motor cortex
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Identifying the Areas of
Secondary Motor Cortex

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At least eight different areas:
Three supplementary motor areas
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Two premotor areas
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SMA and preSMA, and supplementary eye
field
Dorsal and ventral
Three cingulate motor areas
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FIGURE 8.5 Four areas of secondary motor cortex—the
supplementary motor area, the premotor cortex, and two cingulate
motor areas—and their output to the primary motor cortex.
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Identifying the Areas of
Secondary Motor Cortex
Continued

Secondary motor cortex may be involved
in programming movements in response to
input from dorsolateral prefrontal cortex

Active during imagining or planning of
movements
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Mirror Neurons


Active when performing an action or watching
another perform the same action
In monkey studies, mirror neurons fired while

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grasping or watching another grasp a particular object but
not other objects
grasping or watching another grasp an object for a specific
purpose but not for another purpose
Possible neural basis of social cognition (knowledge
of others’ mental processes – e.g., intentions)
Likely to be found in humans

Indirect evidence from functional brain-imaging studies
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FIGURE 8.6 Responses of a
mirror neuron of a monkey.
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Primary Motor Cortex



Precentral gyrus of the frontal lobe
Major point of convergence of cortical
sensorimotor signals
Major point of departure of signals from
cortex
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Conventional View of Primary
Motor Cortex Function

Somatotopic – more cortex devoted to body
parts that make complex movements


Motor homunculus
Until recently, each neuron was thought to
encode the direction of movement
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FIGURE 8.7 The motor homunculus: the
somatotopic map of the human primary
motor cortex. (Adapted from Penfield &
Rasmussen, 1950.)
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Current View of Primary Motor
Cortex Function


Regions of primary motor cortex support
initiation of species-typical movements
Neurons direct to target of movement, rather
than simply a pre-coded direction
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Effects of Primary Motor
Cortex Lesions

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Small lesions often with minimal effects
Large lesions may disrupt a patient’s ability
to move one body part independently of
others
Large lesions may also produce
stereognosia

deficit in stereognosia (ability to identify an
object by touch)
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Cerebellum and Basal Ganglia


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Interact with different levels of the
sensorimotor hierarchy
Coordinate and modulate
May permit maintenance of visually guided
responses despite cortical damage
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Cerebellum

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10% of brain mass, but more than 50% of
its neurons
Input from primary and secondary motor
cortexes
Input from brain stem motor nuclei
Feedback from motor responses
Involved in timing, fine-tuning, and motor
learning
May also do the same for cognitive
responses
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Basal Ganglia


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A heterogenous collection of
interconnected nuclei
Part of neural loops that receive cortical
input and send output back via the
thalamus
Modulate motor output and cognitive
functions including learning
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Descending Motor Pathways

Two dorsolateral
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Two ventromedial
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Corticospinal
Corticorubrospinal
Corticospinal
Cortico-brainstem-spinal tract
Both corticospinal tracts are direct
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Dorsolateral Tracts


Most synapse on interneurons of spinal
gray matter
Corticospinal – descend through the
medullary pyramids, then decussate
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Betz cells – synapse on motor neurons projecting to leg
muscles
Control of wrist, hands, fingers, toes
Corticorubrospinal – synapse at red
nucleus and cross before the medulla


Some control muscles of the face
Distal muscles of arms and legs
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FIGURE 8.8 The two divisions of the
dorsolateral motor pathway: the
dorsolateral corticospinal tract and the
dorsolateral corticorubrospinal tract.
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Ventromedial Tracts
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Corticospinal
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Descends ipsilaterally
Axons branch and innervate interneuron circuits
bilaterally in multiple spinal segments
Cortico-brainstem-spinal


Interacts with various brain stem structures and
descends bilaterally carrying information from both
hemispheres
Synapse on interneurons of multiple spinal segments
controlling proximal trunk and limb muscles
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FIGURE 8.9 The two divisions of the
ventromedial motor pathway: the ventromedial
corticospinal tract and the ventromedial
cortico-brainstem-spinal tract.
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Comparison of the Two
Dorsolateral Motor Pathways
and the Two Ventromedial
Motor Pathways
Ventromedial
Dorsolateral
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One direct tract, one
that synapses in the
brain stem
Terminate in one
contralateral spinal
segment
Distal muscles
Limb movements
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One direct tract, one
that synapses in the
brain stem
More diffuse
Bilateral innervation
Proximal muscles
Posture and whole
body movement
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Sensorimotor Spinal Circuits

Motor circuits of the spinal cord show
considerable complexity

Independent of signals from the brain
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Muscles
Motor units – a motor neuron plus
muscle fibers; all fibers contract
when motor neuron fires
 Number of fibers per unit varies –
fine control, fewer fibers/neuron
 Muscle – muscle fibers bound
together by a tendon

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Muscles Continued
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Acetylcholine released by motor neurons at the
neuromuscular junction causes contraction
Motor pool – all motor neurons innervating the
fibers of a single muscle
Fast muscle fibers – fatigue quickly
Slow muscle fibers – capable of sustained
contraction due to vascularization
Muscles are a mix of slow and fast fibers
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Muscles Continued
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Flexors – bend or flex a joint
Extensors – straighten or extend
Synergistic muscles – any two muscles
whose contraction produces the same
movement
Antagonistic muscles – any two muscles
that act in opposition
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Receptor Organs of Tendons
and Muscles

Golgi tendon organs

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Embedded in tendons
Tendons connect muscle to bone
Detect muscle tension
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Receptor Organs of Tendons
and Muscles
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Muscle spindles
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Embedded in muscle tissue
Detect changes in muscle length
Intrafusal muscle within each muscle spindle
innervated by its own intrafusal motor neuron

Keeps tension on the middle, stretch-sensitive portion
of the muscle spindle to keep it responsive to changes
in the length of the extrafusal muscle
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FIGURE 8.13 The function of the intrafusal
motor neurons.
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Reflexes

Stretch Reflex: monosynaptic, serves to
maintain limb stability

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e.g. Patellar tendon reflex is monosynaptic
Withdrawal Reflex is NOT monosynaptic
Reciprocal Innervation – antagonistic muscles
interact so that movements are smooth – flexors
are excited while extensors are inhibited, etc.
Recurrent Collateral Inhibition – feedback
loop through Renshaw cells that gives muscle
fiber a rest after every contraction
Walking – a complex reflex in some animals
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FIGURE 8.14 The elicitation of a stretch
reflex.
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FIGURE 8.17 The
excitatory and inhibitory
signals that directly
influence the activity of a
motor neuron.
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Central Sensorimotor
Programs


Perhaps all but the highest levels of the
sensorimotor system have patterns of
activity programmed into them, and
complex movements are produced by
activating these programs
Cerebellum and basal ganglia then serve
to coordinate the various programs
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Central Sensorimotor
Programs Are Capable of Motor
Equivalence


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A given movement can be accomplished
various ways, using different muscles
Central sensorimotor programs must be
stored at a level higher than the muscle (as
different muscles can do the same task)
Sensorimotor programs may be stored in
secondary motor cortex
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Sensory Information That
Controls Central Sensorimotor
Programs Is Not Necessarily
Conscious



Evidence that patients could respond to visual
stimuli of which they had no conscious
awareness
Evidence that patients could not effectively
interact with objects that they consciously
perceived
Ebbinghaus Illusion: Conscious perception of
disk size differs from motor response
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FIGURE 8.18 The
Ebbinghaus illusion.
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The Development of Central
Sensorimotor Programs


Central sensorimotor programs may be
hierarchically organized and capable of
using sensory feedback without direct
control at higher levels
Programs for many species-specific
behaviors established without practice

Fentress (1973) – mice without forelimbs still
make coordinated grooming motions
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The Development of Central
Sensorimotor Programs
Continued

Practice can also generate and modify
programs
 Response Chunking


Practice combines the central programs controlling
individual response
Shifting Control to Lower Levels


Frees up higher levels to do more complex tasks
Permits greater speed
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Functional Brain Imaging of
Sensorimotor Learning

Functional brain-imaging studies in
humans have generally supported the
findings from more invasive studies of
non-human primates
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FIGURE 8.19 The activity recorded by PET scans
during the performance of newly learned and
well-practiced sequences of finger movements.
(Adapted from Jenkins et al., 1994.)
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