Transcript The Sensorimotor System

Sensorimotor systems
Chapters 8
Three principles of sensorimotor
function
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hierarchical organization
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Two other organizing characteristics?
motor output is guided by sensory input
The case of G.O. – darts champion
 The exception?
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learning changes the nature and locus of
sensorimotor control
Posterior Parietal Association Cortex
Function: Integrates information on the
position of parts of the body and external
objects to direct voluntary movement and
attention.
Sensory system inputs: visual, auditory and
somatosensory.
Outputs: dorsolateral PFC, secondary motor
cortex and frontal eye fields.
Dorsolateral PFC
Frontal eye
field
Auditory
cortex
Visual
cortex
Inputs to Posterior Parietal
Association Cortex
Dorsolateral PFC
Frontal eye
field
Auditory
cortex
Visual
cortex
Outputs to Posterior Parietal
Association Cortex
Damage to the Posterior Parietal
Association Cortex
Can produce a variety of deficits
 Perception and memory of spatial
relationships
 Reaching and grasping
 Control of eye movements
 Attention
Damage to the Posterior Parietal
Association Cortex
Apraxia – a disorder of voluntary movement
not attributable to a simple motor deficit
(weakness or paralysis) or to a deficit in
comprehension or motivation.
Results from unilateral damage to the left
posterior parietal cortex.
Damage to the Posterior Parietal
Association Cortex
Contralateral neglect – a disturbance in a
patient’s ability to respond to stimuli on the
side of the body contralateral to a brain
lesion (not a simple sensory or motor
deficit).
Often associated with large lesions of the
right posterior parietal lobe.
Dorsolateral Prefrontal Cortex
Function: plays a role in the evaluation of
external stimuli and initiation of voluntary
responses to those stimuli.
Main input: posterior parietal cortex
Outputs: secondary motor cortex
primary motor cortex
frontal eye fields
Dorsolateral Prefrontal connectivity
Dorsolateral Prefrontal cortex
Neurons in this area respond to the
characteristics of objects (e.g.,
color/shape), the location of objects or to
both.
The activity of other neurons is related to the
response itself.
Secondary motor cortex
Input: most from association cortex
Output: primary motor cortex
Two classic areas:
1) SMA
2) Premotor cortex
Secondary Motor Cortex
Current classifications suggest
 At least 8 different areas
 3 supplementary motor areas
SMA, preSMA &
supplementary eye field
 2 premotor areas
PMd and PMv
 3 cingulate motor areas
CMAr, CMAv & CMAd
Mirror neurons
Discovered in the ventral premotor cortex
of the macaque (Rizzolatti et al., 2006)
 Social cognition – knowledge of the
perceptions, ideas and intentions of others
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http://video.pbs.org/video/1615173073/
Secondary Motor Cortex
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
(most common - somatosensory and visual)
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Primary Motor Cortex
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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
Motor
homunculus
Primary Motor Cortex
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Monkeys have two hand areas in each
hemisphere, one receives feedback from
receptors in skin.
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Stereognosis – recognizing by touch – requires
interplay of sensory and motor systems
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Damage to primary motor cortex
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Movement of independent body parts (e.g., 1 finger)
Astereognosia
Speed. accuracy and force of movement
Other sensorimotor structures
outside of the hierarchy
(sometimes called extrapyramidal systems)
 Cerebellum
 Basal ganglia
both modulate and coordinate the activity of
the pyramidal systems by interacting with
different levels of the hierarchy.
Cerebellum
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10% of brain mass, > 50% of its neurons
Converging signals from
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primary and secondary motor cortex
brain stem motor nuclei (descending motor signals)
Somatosensory and vestibular systems (motor
feedback)
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Involved in motor learning, particularly
sequences of movement
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Damage to cerebellum – disrupts direction, force,
velocity and amplitude of movements; causes tremor
and disturbances of balance, gait, speech, eye
movement and motor sequence learning .
Basal Ganglia
A collection of nuclei
 Part of neural loops that receive cortical
input and send output back via the
thalamus (cortical-basal ganglia-thalamocortical loops)
 Modulate motor output and cognitive
functions
 Cognitive functions of the basal ganglia
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Descending Motor Pathways
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Two dorsolateral
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Two ventromedial
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Corticospinal
Corticorubrospinal
Corticospinal
Cortico-brainstem-spinal tract
The corticospinal tracts are direct pathways
Dorsolateral Vs Ventromedial
Motor Pathways
Dorsolateral
 one direct tract, one
that synapses in the
brain stem
 Terminate in one
contralateral spinal
segment
 Distal muscles
 Limb movements
Ventromedial
 one direct tract, one
that synapses in the
brain stem
 More diffuse
 Bilateral innervation
 Proximal muscles
 Posture and whole
body movement
Experiments by Lawrence and
Kuypers (1968)
Experiment 1: bilateral transection of the
Dorsolateral (DL) corticospinal tract
Results:
1) monkeys could stand, walk and climb
2) difficulty reaching improved over time
3) could not move fingers independently of
each other or release objects from their
grasp.
Experiments by Lawrence and
Kuypers (1968)
Experiment 2:
The same monkeys with DL corticospinal tract
lesions received 1 of 2 additional lesions:
1) The other indirect DL tract was transected
2) Both ventromedial (VM) tracts were
transected
Experiments by Lawrence and
Kuypers (1968)
Experiment 2 Results:
• The DL group could stand, walk and climb
but limbs could only be used to ‘rake’ small
objects of interest along the floor
• VM group had severe postural abnormalities:
great difficulty walking or sitting. Although
they had some use of the arms they could
not control their shoulders.
Experiments by Lawrence and
Kuypers (1968)
Conclusions:
• the VM tracts are involved in the control of
posture and whole-body movements
• the DL tracts control limb movements (only
the direct tract controls independent
movements of the digits.