CH 8: The Sensorimotor System Flashcards
WRT the Sensorimotor System, explain what’s meant by ‘HIERARCHICALLY ORGANIZED” & “FUNCTIONAL SEGREGATION”
- Sensorimotor sys directed by commands cascading down levels of hierarchy from the association cortex = hierarchically organized
- Hierarchy characterized by FUNCTIONAL SEGREGATION = each level composed of diff units that perform diff functions
Describe the ASSOCIATION CORTEX
- Top of the sensorimotor hierarchy
- Specifies general goals rather than specific plans of advantage
Name an advantage of higher levels of hierarchy?
Higher levels of hierarchy are free to perform & complex functions
What’s the primary difference b/w the flow of info in SENSORY systems vs. SENSORIMOTOR systems?
- The primary direct of info flow
- Sensory sys: info mainly flows UP through hierarchy
- Sensorimotor sys: info flows DOWN
Describe SENSORY FEEDBACK and its important role of sensory input for motor output.
- Sensory signals that are prod by a response & are used to guide the continuation of the response
- Eyes, organs of balance, skin receptors, muscles & joints monitor body’s responses
- -> Feed info back into sensorimotor circuits
- Motor output adjustments that occur IRT sensory feedback = unconsciously controlled via lower levels of sensorimotor hierarchy
Describe how learning changes the nature & locus of sensorimotor control.
- Initial motor learning stages = each individual response performed under CONSCIOUS control
- -> practice
- -> individual responses organized into cont. integrated sequences of action
- -> smoothly flows & adjusted via sensory feedback
- -> transfer control to lower levels of CNS
- ie) typing, knitting, dancing, etc.
Explain the role of the POSTERIOR PARIETAL ASSOCIATION CORTEX in sensorimotor function.
Its input & output?
Posterior Parietal Association Cortex:
= Area of association cortex
- Receives input from visual, auditory & somatosensory systems
- **Involved in perception of spatial location & guidance of voluntary behaviour
- INPUT from sensory sys that play role in localization of body & external objects in space
- OUTPUT from posterior parietal cortex goes to MOTOR CORTEX (located in frontal cortex
List the deficits that occur as a result of damage to the Posterior Parietal Association Cortex (4).
DAMAGE –> deficits in:
- perception & memory of spacial relations
- accurate reaching & grasping
- eye movement control
- attention
List the 2 largest consequences of damage to the Posterior Parietal Association Cortex.
- Apraxia
2. Contralateral Neglect
Describe APRAXIA
- Disorder of voluntary movement that’s not attributable to simple motor deficit (ie. not to paralysis or weakness)
- Difficulty making specific movements when requested
- -> Can do same movements naturally when not thinking
Describe CONTRALATERAL NEGLECT
- Disturbance of ability to respond to stimuli on side of body OPPOSITE to side of brain lesion in absence of simple sensory or motor deficits
- -> Behave as if (L) side of their world doesn’t exist
- ie) Does makeup on only (R) side of face
- ie) Doesn’t notice food on (L) side of plate
- ie) Only turns right
Explain the role of the DORSOLATERAL PREFRONTAL ASSOCIATION CORTEX in sensorimotor function.
Where does it receive its inputs from?
Dorsolateral Prefrontal Association Cortex:
= Area of prefrontal association cortex
- **Plays role in evaluation of external stimuli & initiation of voluntary motor responses
- Receives input from POSTERIOR PARIETAL ASS. CORTEX
- -> Sends input to areas of secondary motor cortex, primary motor cortex, & Frontal eye field
Describe the response properties of neurons in the Dorsolateral Prefrontal Association Cortex.
- Activity of dorsolateral prefrontal neurons are related to RESPONSE rather than object
- -> Begins to fire before response & continues to fire until response complete
- Decisions to initiate voluntary movements depends on critical interactions w/ posterior parietal cortex & other areas of frontal cortex
Explain the general role of areas of SECONDARY MOTOR CORTEX (aka what are its neurons responsible for (2)?
Secondary Motor Cortex:
= supplementary motor area + premotor cortex
- Neurons in this area:
1. Become ^active just prior to initiating voluntary movement, & continue being active throughout the movement
2. Are involved in programming specific patterns of movement after taking general instructions from Dorsolateral Prefrontal Cortex.
Input & output of the secondary motor cortex?
- Receives input from association cortex & sends output to primary cortex
Association cortex = _________ + ________.
- Posterior Parietal Cortex + Dorsolateral Prefrontal Cortex
When do MIRROR NEURONS fire?
- Fires when an individual performs a particular goal-directed hand movement, or when they observe the same goal directed performed by another
Explain why Mirror Neurons have received so much attention from neuroscientists.
- **Provides possible mech for SOCIAL COGNITION = knowledge of the perceptions, ideas & intentions of others
- Mapping actions of others onto own action repertoire = social understanding, cooperation & imitation
- Mirror neurons responds to the understanding of the purpose of an action
Describe the PRIMARY MOTOR CORTEX.
- Cortex of the pre-central gyrus (in frontal lobe)
- Major point of departure for motor signals descending from cerebral cortex into lower levels of the sensorimotor system
Describe the CONVENTIONAL view of primary motor cortex function & the evidence up which it was based.
- Motor cortex organized via SOMATOTOPIC LAYOUT = organized, like the primary somatosensory system, to a map of the surface of the body
- Each site in primary motor cortex receives sensory feedback from receptors in the muscles that the site influences
- Each primary motor cortex neuron thought to encode the direction of movement
- Learned by stimulating parts of body & seeing which parts of brain are active
- -> via brief pulses of current just above threshold to produce a reaction
**Many primary motor cortex neurons are tuned to movement in a particular direction
Describe the CURRENT view of primary motor cortex function & the evidence up which it was based.
- Map primary motor cortex via longer bursts of currents
- Rather than eliciting individual muscle contractions, currents elicit complex natural-looking response sequences
- **Signals from every site in primary motor cortex greatly diverge
- -> So each particular site has ability to get a body part to a target location regardless of starting position
**Primary motor cortex neurons play MAJOR role in initiating body movements
Describe the structure of the CEREBELLUM (2).
Cerebellum:
- Organized systematically in lobes, columns & layers
- Contains >1/2 of brain’s neurons
Describe the 3 types of info the cerebellum receives & explain the current view of cerebellar function.
- Info from primary & secondary motor cortex
- Info about descending motor signals from brain stem motor nuclei
- Feedback from motor responses via somatosensory & vestibular systems
- -> Compares these 3 sources of input & corrects ongoing movement that deviate from intended course
- -> Thus plays major role in MOTOR LEARNING
Describe the consequence of cerebellar damage.
- Disrupts motor function via balance, gait, speech & eye movement control
Describe the anatomy of the BASAL GANGLIA.
- Not as many neurons as cerebellum, but is more complex
- Organized via complex heterogeneous collection of interconnected nuclei
Explain the current view of the function of the Basal Ganglia (4).
- Part of neural loops that receive cortical input from various cortical areas
- -> Transmits it back to cortex via thalamus - Plays role in modulation of motor output (like cerebellum)
- Involved in variety of cognitive functions
- Play role in habit learning = motor learning
Descending Motor Pathways
- 2 pathways descend in the dorsolateral region of spinal cord = Dorsolateral
Motor Pathways - 2 pathways descend in the ventromedial region of spinal cord = Ventromedial Motor Pathways
- -> Signals conducted over these pathways act together
- -> Controls voluntary movement
List the 2 descending Dorsolateral motor pathways of the spinal cord.
- Dorsolateral Corticospinal Tract
- Dorsolateral Corticorubrospinal Tract
**See Figure 8.7*
- Describe the DORSOLATERAL CORTICISPINAL TRACT.
Dorsolateral Corticospinal Tract:
= Direct tract/divison
= Motor tract that leaves the primary motor cortex
–> Descends to the medullary pyramids
–> Decussates
–> Descends in the contralateral dorsolateral spinal white matter
What happens to the axons of the Dorsolateral Corticospinal Tract?
- Most axons of this tract synapse on small interneurons of the spinal grey matter
- -> synapse on motor neurons of distal muscles of the wrist, fingers, & toes
Describe the DORSOLATERAL CORTICORUBROSPINAL TRACT.
Dorsolateral Corticorubrospinal Tract:
= Indirect tract/division
= The descending motor tract that synapses in the red nucleus of the midbrain
–> Decussate
–> Descends in the dorsolateral spinal white matter
What happens to the axons of the Dorsolateral Corticorubrospinal Tract?
- The axons of this tract synapse on interneurons that in turn synapse on motor neurons that project to the distal muscles of the arms & legs
List the 2 descending Ventromedial Motor Pathways of the spinal cord.
- Ventromedial Corticospinal Tract
- Ventromedial Cortico-brainstem-spinal Tract
See Figure 8.8
Describe the VENTROMEDIAL CORTICOSPINAL TRACT.
Ventromedial Corticospinal Tract
= Direct tract
- Descends ipsilaterally from the primary cortex directly into the ventromedial areas of the spinal white matter
What happens to the axons of the Ventromedial Corticospinal Tract?
- As each axon descends, it branches diffusely
- -> Innervates the interneuron circuits in several diff signal segments on both sides of the spinal grey matter
Describe the VENTROMEDIAL CORTICO-BRAINSTEM-SPINAL TRACT
Ventromedial Cortico-brainstem-spinal Tract
= Indirect tract
- Descends bilaterally from the primary motor cortex to reveal interconnected brain stem motor structures
–> Descends the ventromedial portions of the spinal cord
The Ventromedial Cortico-brainstem-spinal Tract contains motor neurons that feed into a complex network of which brain structures (4)?
- TECTUM = receives auditory & visual info about spatial
- VESTIBULAR NUCLEUS = receives info about balance from receptors in the semicircular canals of the inner ear
- RETICULAR FORMATION = contains motor programs that regulate complex species-typical movements (ie. walking, jumping, etc)
- MOTOR NUCLEI of CRANIAL NERVES = control face muscles
Describe the SIMILARITIES b/w the 2 dorsolateral motor pathways & the 2 ventromedial motor pathways.
Each composed of 2 major tracts
- One whose axons descend directly to the spinal cord
- Another whose axons synapse in the brain stem on neurons that in turn descend to the spinal cord
Describe the DIFFERENCES b/w the 2 dorsolateral motor pathways & the 2 ventromedial motor pathways (3).
- Ventromedial tracts = ^^diffuse
- Their axons innervate interneurons on both sides of the spinal grey matter in several diff segments
- vs. axons of the Dorsolateral tracts terminate in the contralateral half of 1 spinal cord segments - Motor neurons activated by the 2 ventromedial tracts project to PROXIMAL muscles of the trunk & limp (ie. shoulder muscles)
- vs. motor neurons activated by the 2 Dorsolateral tracts project to DISTAL muscles (ie. finger muscles)
- ** 3. All originate in cerebral cortex, BUT diff in their routes & destinations
- Ventromedial tracts = control of posture & whole body movements (ie. walking, climbing)
- -> Exert control over limb movements involved in such activities
- Dorsolateral tracts = control movements of the limbs
Describe SPINAL CIRCUITS.
Describe its office workers analogy.
Spinal Circuits:
= Lowest level of sensorimotor hierarchy
- Capable of independent functioning
- ie) spinal circuits = office workers able to function effectively when all executives & branch managers are at convention in Hawaii
Describe the components of a MOTOR UNIT.
Motor Units:
= Smallest units of motor activity
- Comprised of a single motor neuron & all individual skeletal muscle fibres it innervates
- When motor neuron fires, all the muscle fibres of its unit contract together
Describe SKELETAL MUSCLE.
- Comprised of 100-1000s of threadlike muscle fibres bound together in tough membrane attached to bone via tendon
- Each muscle has both fast & slow muscle fibres
- -> Either FLEXORS (=bend/flex at joint) vs. EXTENSORS (=straight/extend joint)
List the 2 types of Skeletal Muscle.
- Fast muscle fibres
2. Slow muscle fibres
Describe FAST MUSCLE FIBRES
Fast Muscle Fibres:
- Contract & relax quickly
- Capable of generating great force but fatigue quickly bc poorly vascularized
- Participates in quick movements (ie. jumping)
Describe SLOW MUSCLE FIBRES
Slow Muscle Fibres:
- Slower & weaker
- Capable of ^sustained contraction bc ^vascularized
- Participate in gradual movements (ie. walking)
List the 2 receptor organs of tendons & muscles.
- Golgi Tendon Organs
2. Muscle Spindles
Describe GOLGI TENDON ORGANS & their functions.
Golgi Tendon Organs:
= Receptors embedded in tendons
- Are sensitive to the amount of tension in skeletal muscles to which their tendons are attached
- Insensitive to changes in muscle length
- Monitors the activity of the skeletal muscles they’re attached to via TENDONS (=attaches skeletal muscle to bone)
- Responds to ^muscle tension (ie. pull of muscle on tendon)
- Provides CNS w/ info about muscle tension & serves a protective function
- -> when ^^extreme contraction of muscle that risk damage
- -> golgi tendon organ excite inhibitory interneurons in spinal cord
- -> cause muscle to relax
Describe MUSCLE SPINDLES & their functions.
Muscle Spindles:
= Receptors embedded in skeletal muscle tissue
- Sensitive to changes in muscle length
- Monitors the activity of skeletal muscles they’re attached to via tendons
- Does NOT respond to changes in muscle tension
- Has its own muscle & motor neuron:
- -> Intrafusal Muscle = threadlike muscle that adjusts the tension on a muscle spindle
- -> Intrafusal Motor Neuron = motor neuron that innervates an intramural muscle
See Figure 8.12
Golgi Tendon Organs & Muscle Spindles have diff locations, thus_____.
- Thus they respond to diff aspects of muscle contraction
Describe the STRETCH REFLEX.
Stretch Reflex:
= Reflex elicited by a sudden external stretching force on a muscle
Explain the mechanism behind the Stretch Reflex.
ie) doctor striking tendon on knee while sitting on table
- -> Extensor muscle running along thigh is stretched
- -> Initiates chain of events in FIGURE 8.13
- -> Sudden thigh muscle stretch then stretches its muscle-spindle stretch receptors
- -> Initiate volley of action potentials carried from stretch receptors into spinal cord via SPINDLE AFFERENT NEURONS
- -> Excites motor neurons in the ventral horn of spinal cord
- -> Sends action potentials back to the muscle whose stretch originally excited them
- -> Impulses arrive back at starting point
- -> Compensatory muscle contraction & sudden leg extension
- *see Figure 8.13**
- *see pg 212-213**
Describe the WITHDRAWAL REFLEX.
Withdrawal Reflex:
= Reflexive withdrawal of a limb when it comes in contact w/ a painful stimulus
Explain the mechanism behind the Withdrawal Reflex.
ie) pulling hand back from hot pot
- Painful stimulus applied to hand
- -> 1st responses recorded in motor neurons of the arm flexor muscles ~1.6ms later = time it takes a neural fire to cross 2 synapses
- -> Shortest route in withdrawal-reflex circuit involves 1 interneuron
Explain what’s meant by ‘RECIPROCAL INNERVATION’.
Reciprocal Innervation:
= The principle of spinal cord circuit that causes a muscle to automatically relax when a muscle that’s antagonistic to it contracts
- When 1 muscle contracts, the other relaxes
- see pg 214
Explain ‘RECURRENT COLLATERAL INHIBITION’
Recurrent Collateral Inhibition:
= Inhibition of a neuron that’s prod by its own activity via a collateral branch of its axon & inhibitory interneuron
- Allows muscle fibres & the muscle neurons that innate them to break
- Each time a motor neuron fires, it momentarily inhibits itself & shifts the responsibility for the contract of a particular muscle to other member’s of the muscle’s motor pool
- see Figure 8.16
Describe the phenomenon of WALKING.
Walking:
- ^complex than withdrawal & stretch reflexes
= Program of reflexes that integrate:
- Visual info from eyes
- Somatosensory info from feet, knees, hips, arms, etc.
- Info RE: balance from semicircular canals of inner ears
= ^plastic program of reflexes
–> Able to adjust output imm. to changes in ie_ terrain, slope, brain instructions, sudden external forces etc.
Describe the degree to which walking is controlled by spinal circuits.
- Can control walking via spinal cord w/o contribution from brain!
- -> Tested on cats & others sp who walked when placed on a sling above moving treadmill
- -> Had spinal cords separated from brains
Describe the company analogies of various structures of the sensorimotor system.
- Dorsolateral prefrontal cortex
- Secondary motor cortexes
- Posterior Parietal Cortex
- Primary motor cortex
- Dorsolateral & Ventromedial spinal motor pathways
- Spinal sensorimotor circuits
- Muscles
***Sensorimotor sys = hierarchy of large company
- Executives = Dorsolateral Prefrontal Cortex & Secondary Motor Cortexes
- Issues commands based o info suppled to them by Posterior Parietal Cortex - Director of ops = Primary Motor Cortex
- Takes commands from executives & distributes them over 4 main channels of communication - Main channels of communication = 2 Dorsolateral & 2 Ventromedial Spinal Motor Pathways
- Takes commands to office managers of the sensorimotor hierarchy - Office mangers = Spinal Sensorimotor Circuits
- Directs the activities of the workers - Workers = muscles
Explain what’s meant by a hierarchy of CENTRAL SENSORIMOTOR PROGRAMS, & explain the importance of this arrangement of sensorimotor functioning.
Central Sensorimotor Program:
= Patterns of activity that are programmed into the sensorimotor sys
- Sensorimotor sys comprised of a hierarchy of central sensorimotor programs
- All but highest levels of sensorimotor sys have certain patterns of activity programmed into them
- Complex movements are prod by activating the appropriate combo of these programs
- -> Once activated, each level of the sensorimotor sys is capable of operating on basis of current sensory feedback w/o direct control of higher levels
- -> Thus even though highest levels of sensorimotor sys retain option of directly controlling your activities, most of the individual responses that you make are performed w/ direct cortical involvement
- -> Often barely aware of them
- see pg 216 for example
Describe the 1st characteristic of Central Sensorimotor Programs.
- Central sensorimotor programs are capable of motor equivalence
- MOTOR EQUIVALENCE = ability of sensorimotor sys to carry out basic movement in diff ways that involve diff muscles
- ie) can sign name w/ hand vs toe & still share same characteristics
- Suggests that ie) specific sensorimotor programs for signing name are NOT stored in neural circuits that directly control hand
- -> But general programs are stored higher in sensorimotor hierarchy
- -> Adapted to situation as required
Describe the 2nd characteristic of Central Sensorimotor Programs.
- Sensory info that controls central sensorimotor programs is NOT necessarily conscious
- see pg 216-217
Describe the 3rd characteristic of Central Sensorimotor Programs.
- Central sensorimotor programs can develop WITHOUT practice
- ie) mice w/o normal tongue-forepaw contact would still mimic grooming
Describe the 4th characteristic of Central Sensorimotor Programs.
- Central sensorimotor programs can develop WITH practice
- via (a) Response Chunking Hypothesis
- via (b) Shifting Control to Lower Levels
Describe the RESPONSE CHUNKING HYPOTHESIS.
Response Chunking Hypothesis:
= Idea that practice combines the central sensorimotor programs that control individual responses into programs that control sequences (chunks) of behaviour
- Chunks can be combined into higher-order chunks
- ie) responses needed to type the individual letters of one’s address may be chunked into longer sequences necessary to produce individual words, etc.
Describe the concept of SHIFTING CONTROLS TO LOWER LEVELS.
- Shifting control to lower levels of sensorimotor hierarchy
- Frees up higher levels of sys to deal w/ ^complex aspects of performance
- ^speed bc lower levels work simultaneously w/ higher levels
- ie) skilled pianist can concentrate on interpreting piece of music bc not consciously focussing on pressing right keys
Explain how the classic Jenkins et al. PET study of simple motor learning summarizes the main points of this chapter.
- Recorded PET activity from humans performing 2 diff sequences of key presses
- (1) Involvement of cortical sensorimotor areas introduced in this chapter
- (2) Involvement of association areas & the cerebellum diminished when sequence was well practiced
- Identifies where changes occur in brain while volunteers learn sensorimotor tasks
- -> But haven’ yet provided any indication of the nature of these changes
