Central Sensorimotor Programs

PowerPoint Presentation for Biopsychology, 8th Edition by John P.J. Pinel

Prepared by Jeffrey W. Grimm Western Washington University

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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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 

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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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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  

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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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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 

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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) Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

Corticospinal  

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Descends ipsilaterally Axons branch and innervate interneuron circuits bilaterally in multiple spinal segments

Cortico-brainstem-spinal 

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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 

Muscle spindles   

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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

FIGURE 8.18 The Ebbinghaus illusion.

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The Development of Central Sensorimotor Programs 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

The Development of Central Sensorimotor Programs Continued 

Practice can also generate and modify programs  Response Chunking 

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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 Copyright © 2011 Pearson Education, Inc. All rights reserved.

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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