Transcript Slide 1

TOPIC 6:
The Sensorimotor System
How You Do What You Do
Motor Systems
 Functions
Movement
Posture
& balance
Communication
 Guided
by sensory systems
Internal
representation of world & self
Detect changes in environment
 external
& internal ~
3 Classes of Movement
1. Voluntary
Complex
actions
reading, writing, playing piano
Purposeful, goal-oriented
Learned
 improve with practice ~
Continue… 3 Classes of Movement
2. Reflexes
Involuntary,
rapid, stereotyped
eye-blink, coughing, knee jerk
Graded control by eliciting stimulus
3. Rhythmic motor patterns
Combines
voluntary & reflexive acts
chewing, walking, running
Initiation & termination voluntary
once initiated, repetitive & reflexive ~
Control of movement by the
brain
 What
are the principles of
sensorimotor Function?
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.
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.
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.
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
A. Posterior Parietal Association
Cortex
 Before
we respond to sensory input,
we integrates information about
Body
part location (Where are they?)
External objects
 Receives
visual, auditory, and
somatosensory information
 Most outputs go to secondary motor
cortex.
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
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.

Secondary Motor Cortex (SMC)
Input mainly from association cortex
 Output mainly to primary motor cortex
 At least 7 different areas of SMC in each
hemisphere

2
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.
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
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)

Motor
homunculus
The Motor Homunculus
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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)
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.
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.

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)

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)?
4 Descending Motor Pathways:

2 dorsolateral areas of spinal cord
 Corticospinal
tract
 Corticorubrospinal tract

2 ventromedial (each side of Spinal cord)
 Corticospinal
tract
 Cortico-brainstem-spinal tract tract

Both corticospinal tracts are direct
Dorsolateral Tracts

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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
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
Dorsolateral Vs Ventromedial
Motor Pathways
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Dorsolateral
One direct tract, one
that synapses in the
brain stem
Terminate in one
contralateral spinal
segment
Distal muscles
Limb movements

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
Ventromedial
One direct tract, one
that synapses in the
brain stem
More diffuse
Bilateral innervation/
stimulation
Proximal muscles
Posture and whole
body movement
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

Muscles

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
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:
muscle fibers – fatigue quickly
 Slow muscle fibers – capable of sustained contraction
due to vascularization
 Fast

Muscles are a mix of slow and fast
Muscles
Movement occurs at joints
Flexors – bend or flex a joint
Extensors – straighten or extend limb
Contraction & relaxation of opposing
muscles
Movement & Muscles
Synergistic muscles – any 2 muscles
whose contraction produces the same
movement
Agonists: prime movers
 Antagonistic muscles – any 2 muscles
that act in opposition
Antagonists

 counterbalance
agonists
 decelerate movement ~
Functions of muscles:
 Movement
control more than
contraction & relaxation
Accurately
time control of many muscles
Make postural adjustment during
movement
Adjust for mechanical properties of joints
& muscles
 inertia,
changing positions ~
Receptor Organs of Tendons and
Muscles
 Golgi
tendon organs
Embedded
in tendons
Tendons connect muscle to bone
Detect muscle tension
 Muscle
spindles
Embedded
in muscle tissue
Detect changes in muscle length
Knee-jerk
reflex
Types of Reflexes:
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.
 Stretch
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.

Motor equivalence
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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).
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 chunking
 Shifting control to lower levels
The Development of Central
Sensorimotor Programs
 Response
chunking
Practice
combines the central programs
controlling individual response
 Shifting
Frees
control to lower levels
up higher levels to do more
complex tasks
Permits greater speed
Hierarchical Control of Movement
3
levels of control
 Spinal
cord (SC)
 Brainstem
 Cortex
 Division
of responsibility
Higher
levels: general commands
Spinal cord: complex & specific
 Each
receives sensory input
Relevant
to levels function ~
Hierarchical Control: Spinal Cord
 Automatic
& stereotyped responses
reflexes
rhythmic
motor patterns
 Can
function without brain
 Spinal interneurons
same
circuits as voluntary movement
 Pathways
converge on a motor
neurons
final
common path ~
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?