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TOPIC 6:The Sensorimotor System
How You Do What You Do
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Motor Systems Functions
MovementPosture & balanceCommunication
Guided by sensory systemsInternal representation of world & selfDetect changes in environment
external & internal ~
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3 Classes of Movement
1. VoluntaryComplex actions
reading, writing, playing pianoPurposeful, goal-orientedLearned
improve with practice ~
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Continue… 3 Classes of Movement2. Reflexes
Involuntary, rapid, stereotyped
eye-blink, coughing, knee jerkGraded control by eliciting stimulus
3. Rhythmic motor patternsCombines voluntary & reflexive acts
chewing, walking, runningInitiation & termination voluntaryonce initiated, repetitive & reflexive ~
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Control of movement by the brain What are the principles of
sensorimotor Function?
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3 Principles of Sensorimotor Control
The sensorimotor system is hierachically organized.
Motor output is guided by sensory input.
Learning can change the nature and locus of sensorimotor control.
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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
Like a company: President (association cortex) issues general commands and lower level (motor neurons and muscles) take care of details
Advantage: Higher levels are left free to focsu on complex fucntions.
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Continue… Motor output guided by sensory input.
Sensorimotor system monitors the external world (sensory input) and the consequences of its own actions
It acts accordingly Learning (experience) changes the nature
and locus of sensorimotor control With regards to new tasks, after much practice at the
lower levels – they mastered the task. These well-learned tasks need little involvement from
the higher level.
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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 cortexDorsolateral prefrontal association
cortex
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A. Posterior Parietal Association Cortex Before we respond to sensory input, we
integrates information aboutBody part location (Where are they?)External objects
Receives visual, auditory, and somatosensory information
Most outputs go to secondary 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 hemiphere.
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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B. Dorsolateral Prefrontal Association Cortex Input comes from posterior parietal cortex Projects output to secondary motor cortex,
primary motor cortex, and frontal eye field. Evaluates external stimuli (i.e.
characteristic, location, response fr object) Initiates voluntary reactions – supported
by neuronal responses.
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Secondary Motor Cortex (SMC)
Input mainly from association cortex Output mainly to primary motor cortex At least 7 different areas of SMC in each
hemisphere2 supplementary motor areas, 2 premotor areas
(i.e.dorsal and ventral); 3 cingulate motor areas All are interconnected, All send axons
to the motor circuits of the brainstem.
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Continue… SMC
Produces complex movements when stimulated
Activated before and during voluntary movements
Are active when either side of the body is involved in ta movement.
Premotor cortex: Respond → visual and touch stimuli
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Primary Motor Cortex (PMC)
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 is devoted to
body parts that make more movements (eg. face vs elbow)
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Motor homunculus
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The Motor Homunculus
Eg: Control of hands involves a network of widely distributed neurons on PMC → each area of PMC received feedback from muscles and joints it influences.
Stereognosis – ability of recognisation of object from touches – requires input from sensory (skin) and motor systems (feedback).
Some neurons are direction specific – firing maximally when movement is made in one direction (preferred direction)
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Subcortical sensorimotor structures: Cerebellum and Basal Ganglia
Interact with different levels of the sensorimotor hierarchy but neither participates directly in signal transmission to the spinal cord.
Coordinate and modulate activities at various level of the sensorimotor system.
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Cerebellum
10% of brain mass, > 50% of its neurons Input from 1° (PMC) and 2° (SMC) 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 information
from various part of cortex and send output back via the thalamus
Modulate motor output and cognitive functions
Eg: Sequencing of movements, expanded role in non-motor tasks (siognitive)
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SCL:
How do principle of sensorimotor system relate to our daily life?
What will happen when cerebellum is damaged?
How is Primary Motor Cortex organised and what is/are its main function(s)?
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4 Descending Motor Pathways:
2 dorsolateral areas of spinal cordCorticospinal tractCorticorubrospinal tract
2 ventromedial (each side of Spinal cord)Corticospinal tractCortico-brainstem-spinal tract tract
Both corticospinal tracts are direct
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Dorsolateral Tracts
DCorticospinalT = Axons from PMC descend through Medullary pyramids → then cross, continue to contralateral dorsolateral white matter of spinal cord Betz cells (in PMC) – synapse on motor neurons
projecting to leg muscles Wrist, hands, fingers, toes
DCorticorubrospinalT = Axons from PMC – 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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Ventromedial Tracts
Corticospinal Descends ipsilaterally (following the same side) Axons branch and stimulate 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 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/
stimulation Proximal muscles Posture and whole
body movement
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REFLECTIVE BEHAVIOUR: 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 requires fewer fibers/neuron
Muscle – fibers bound together by a tendon
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Muscles
Acetylcholine (NT) released by motor neurons at the neuromuscular junction causes contraction.
Motor pool – all motor neurons innervating the fibers of a single muscle
Types of muscles fibers: 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
Movement occurs at joints Flexors – bend or flex a jointExtensors – straighten or extend limbContraction & relaxation of opposing muscles
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Movement & Muscles Synergistic muscles – any 2 muscles
whose contraction produces the same movementAgonists: prime movers
Antagonistic muscles – any 2 muscles that act in oppositionAntagonists
counterbalance agonists decelerate movement ~
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Functions of muscles: Movement control more than contraction
& relaxationAccurately time control of many musclesMake postural adjustment during
movementAdjust for mechanical properties of joints &
muscles inertia, changing positions ~
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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 reflex
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Types of Reflexes:
Stretch reflex – monosynaptic, serves to maintain limb stability
Withdrawal reflex – multisynaptic, evoke by painful stimulus, before info reaches the brain
Reciprocal innervation – antagonistic muscles interact so that movements are smooth – flexors are excited while extensors are inhibited, etc.
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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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Motor equivalence
A given movement can be accomplished various ways, using different muscles.
Central sensorimotor programs is 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 (SMC)
Eg: You can sign your name with left or right hand. Signature is very similar and the SMC for preferred right hand is activated (even when left hand signs).
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The Development of Central Sensorimotor Programs Programs/controls: many species-specific
behaviors established without practice for central sensorimotor programs
Fentress (1973) – mice without forelimbs still make coordinated grooming motions
Practice can also generate and modify programs:Through response 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
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Hierarchical Control of Movement 3 levels of control
Spinal cord (SC) Brainstem Cortex
Division of responsibilityHigher levels: general commandsSpinal cord: complex & specific
Each receives sensory inputRelevant to levels function ~
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Hierarchical Control: Spinal Cord Automatic & stereotyped responses
reflexesrhythmic motor patterns
Can function without brain Spinal interneurons
same circuits as voluntary movement Pathways converge on motor neurons
final common path ~
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SCL:
How does our brain control muscles? What are the neural pathways?
What are the concepts related to Central Sensorimotor program and its important functions?