Download Dorsolateral Prefrontal Association Cortex

How You Do What You Do
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The sensorimotor system is
hierachically organized
Motor output is guided by sensory
input
Learning can change the nature and
locus of sensorimotor control
1. Hierarchical organization
◦ Association cortex at the highest level, muscles at the lowest i.e
from general goals (cortical level) to specific details of action
(lower levels).
◦ Parallel structure – signals flow between levels over multiple paths
◦ Information flow is down, while in the Sensory system informtion
flows through the hierarchy.
2. Motor output guided by sensory input
◦ Sensory feedback plays an important role in the control of
movement (exception: ballistic movements).
3. Learning (experience) changes the nature and locus of
sensorimotor control.
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E.g. from Conscious behavior to automatic. From conscious
control (cortical level) to "Automatic Pilot" (lower levels).
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Sensory information is integrated
in Association cortex.
Two Major areas of Sensorimotor
Association Cortex are:◦ Posterior parietal association
cortex
◦ Dorsolateral prefrontal
association cortex
Each composed of several
different areas with different
functions
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This cortex receives input from
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The somatosensory system,
The visual system and
The auditory system.
This information specifies the initial conditions for the programming of
action:
The original position of the body parts to be moved.
The position of external objects
Damage to the posterior parietal cortex causes Apraxia and
Contralateral neglect.
Apraxia: difficulty in executing a movement when ordered to do so, but
able to do it when not thinking about it. Lesion is often on the left side.
Contralateral Neglect: Patient does not respond to sensory stimulation
from the side opposite to the lesion of the parietal cortex (usually on the
right side).
The output of the Posterior Parietal Association Cortex goes to the
Dorsolateral Prefrontal Association Cortex, and to the Frontal Eye Field
(Fig. 8.2).
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This cortex receives input from
The somatosensory system,
The visual system and
The auditory system.
This information specifies the initial conditions for the programming of action:
The original position of the body parts to be moved.
The position of external objects
Damage to the posterior parietal cortex causes Apraxia and Contralateral
neglect.
Apraxia: difficulty in executing a movement when ordered to do so, but able to do
it when not thinking about it. Lesion is often on the left side.
Contralateral Neglect: Patient does not respond to sensory stimulation from the
side opposite to the lesion of the parietal cortex (usually on the right side).
The output of the Posterior Parietal Association Cortex goes to the Dorsolateral
Prefrontal Association Cortex, and to the Frontal Eye Field (Fig. 8.2).
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Integrates information about
◦ Body part location
◦ External objects
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Receives visual, auditory, and somatosensory
information
Outputs to motor cortex
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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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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
Input mainly from association cortex
 Output mainly to primary motor cortex
 At least 7 different areas
◦ 2 supplementary motor areas
 SMA and preSMA
◦ 2 premotor areas
 dorsal and ventral
◦ 3 cingulate motor areas
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Subject of ongoing research
May be involved in programming movements
in response to input from dorsolateral
prefrontal cortex
Many premotor neurons are bimodal –
responding to 2 different types of stimuli
Precentral gyrus of the frontal lobe
 Major point of convergence of cortical
sensorimotor signals
 Major point of departure of signals
from cortex
 Somatotopic – more cortex devoted to
body parts which make many
movements
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Motor
homunculus
Control of hands involves a network of
widely distributed neurons
 Stereognosis – recognizing by touch –
requires interplay of sensory and
motor systems
 Some neurons are direction specific –
firing maximally when movement is
made in one direction
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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
10% of brain mass, > 50% of its
neurons
 Input from 1° and 2° motor cortex
 Input from brain stem motor nuclei
 Feedback from motor responses
 Involved in fine-tuning and motor
learning
 May also do the same for cognitive
responses
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A collection of nuclei
Part of neural loops that receive cortical input
and send output back via the thalamus
Modulate motor output and cognitive
functions
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2 dorsolateral
◦ Corticospinal
◦ Corticorubrospinal
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2 ventromedial
◦ Corticospinal
◦ Cortico-brainstem-spinal tract
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Both corticospinal tracts are direct
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Most synapse on interneurons of spinal gray
matter
Corticospinal - descend through the medullary
pyramids, then cross
◦ Betz cells – synapse on motor neurons
projecting to leg muscles
◦ 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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Corticospinal
◦ 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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Dorsolateral
one direct tract,
one that synapses
in the brain stem
Terminate in one
contralateral spinal
segment
Distal muscles
Limb movements
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Ventromedial
one direct tract,
one that synapses
in the brain stem
More diffuse
Bilateral innervation
Proximal muscles
Posture and whole
body movement
Motor units – a motor neuron +
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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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
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Flexors – bend or flex a joint
Extensors – straighten or extend
Synergistic muscles – any 2 muscles whose
contraction produces the same movement
Antagonistic muscles – any 2 muscles that act
in opposition
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Golgi tendon organs
◦ Embedded in tendons
◦ Tendons connect muscle to bone
◦ Detect muscle tension
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Muscle spindles
◦ Embedded in muscle tissue
◦ Detect changes in muscle length
Knee-jerk
reflex
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Stretch reflex – monosynaptic, serves to
maintain limb stability
Withdrawal reflex – multisynaptic
Reciprocal innervation – antagonistic muscles
interact so that movements are smooth –
flexors are excited while extensors are
inhibited, etc.
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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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
2° motor cortex
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Programs for many species-specific
behaviors established without practice
 Fentress (1973) – mice without
forelimbs still make coordinated
grooming motions
 Practice can also generate and modify
programs
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◦ Response chunking
◦ Shifting control to lower levels
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Response chunking
◦ Practice combines the central programs controlling
individual response
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Shifting control to lower levels
◦ Frees up higher levels to do more complex tasks
◦ Permits greater speed