pinel basics ch06
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Copyright © 2007 by Allyn and Bacon
Chapter 6The Sensorimotor System
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Copyright © 2007 by Allyn and Bacon
3 Principles of Sensorimotor Function Hierarchical organization
Association cortex at the highest level, muscles at the lowest
Parallel structure – signals flow between levels over multiple paths
Motor output guided by sensory input Learning (experience) changes the nature
and locus of sensorimotor controlConscious to automatic, for example
Copyright © 2007 by Allyn and Bacon
Copyright © 2007 by Allyn and Bacon
2 Major Areas of Sensorimotor Association Cortex Each composed of several different
areas with different functions Some disagreement exists about how
to divide the areas up Posterior parietal association cortex Dorsolateral prefrontal association
cortex
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Posterior Parietal Association Cortex
Integrates information aboutBody part locationExternal objects
Receives visual, auditory, and somatosensory information
Outputs to motor cortex
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What affect does damage to the posterior parietal area have? 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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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 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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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
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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 Somatotopic – more cortex devoted to
body parts which make many movements
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Motor homunculus
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The 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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Cerebellum and Basal Ganglia
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
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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Basal Ganglia
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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4 Descending Motor Pathways
2 dorsolateralCorticospinal Corticorubrospinal
2 ventromedialCorticospinalCortico-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 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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Copyright © 2007 by Allyn and Bacon
Ventromedial Tracts
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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Copyright © 2007 by Allyn and Bacon
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
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Motor Units and Muscles
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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Muscles
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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Muscles
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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Receptor Organs of Tendons and Muscles Golgi tendon organs
Embedded in tendonsTendons connect muscle to boneDetect muscle tension
Muscle spindlesEmbedded in muscle tissueDetect changes in muscle length
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Knee-jerk (patellartendon) reflex
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Reflexes
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.
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Copyright © 2007 by Allyn and Bacon
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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Motor equivalence
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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The Development of Central Sensorimotor Programs 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
programsResponse chunkingShifting control to lower levels
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The Development of Central Sensorimotor Programs Response chunking
Practice combines the central programs controlling individual response
Shifting control to lower levelsFrees up higher levels to do more
complex tasksPermits greater speed