The Organization of
Movement
From Ch. 33
“Principles of Neural Science”, 4th Ed.
Kandel et al
The sensory and motor systems
• The sensory system integrates sensory information to construct
internal representations
– Physical energy is transformed into neural signals
• The motor system uses internal motor representation to generate
movement
- Neural signals are transformed into physical energy
• Purposeful action is only possible because the motor system has
access to an on-going stream of sensory information
The motor system
• The motor system plans, coordinates and execute movements
• The motor system is hierarchically organized
– Spinal cord
– Brain stem
– Motor cortex
• Successively higher levels of the motor hierarchy specify
increasingly complex aspects of a motor task
– Reflexive movements (withdrawal reflexes)
– Rhythmic movements (chewing, swallowing, scratching)
– Voluntary movements (goal directed)
Reflexes
•
Reflexes are involuntary coordinated patterns of
muscle contractions and relaxations elicited by
peripheral stimuli
–
–
•
Different reflexes have different spatial and temporal patterns
of muscle contractions
Reflexes varies with stimulus site and receptor type activated
Standardized response
–
–
Same response time after time
Modulation my context of behaviour by descending input
•
Reflexes circuits are used to coordinate muscles in
complex voluntary movements
•
Reflexes and rhythmic motor patterns lie in the
spinal cord and the brain stem
Voluntary movements
• Initiated to accomplish a specific goal
• Improve with practice
– Learning to anticipate and correct for obstacles that perturb the body
• Take external perturbations into account
– Feedback control: sensory feedback directly from the moving limb
– Feed-forward control: use of multimodal sensory information to detect imminent
perturbations to initiate proactive strategies (based on experience)
Motor control circuits
•
Signals from sensors are compared
with a desired state (reference
signal)
•
The difference (error signal) is used
to adjust the output
•
Feedback is especially important to
maintain limb position or forces
applied to objects
•
Without feedback the limb begins to
drift as muscle fiber fatigue goes
undetected
Motor control circuits
Example: Catching a ball
•
Feed-forward systems act in advance
of certain perturbations
•
Experience is important to generate
relevant anticipatory commands
•
Accuracy requires prior knowledge of
trajectories
•
Without feedback the limb begins to
drift as muscle fiber fatigue goes
undetected
Feed-forward: catching a ball
3 key principles for feed-forward control:
Feed-forward is essential for rapid action
Feed-forward depends on predicting the
consequences of sensory events
Feed-forward mechanisms can modify feedback
mechanisms in the spinal cord
Arrows: anticipatory response onset
Modification of stretch reflex: flexor (F) and extensor (E)
co-activated to stiffen elbow.
Only spinal reflexes can mediate such rapid feedback
Voluntary movements
• 3 laws govern voluntary movements (modifiable by learning)
1. The brain represents the outcome of actions independently of the specific muscle
used or the specific way the action is achieved
2. The time taken to respond to a stimulus depends on the amount of information
needed to be processed
3. There is a trade-off between the speed of movement and the accuracy of the
movement
Voluntary movements
• 3 laws govern voluntary movements (modifiable by learning)
1. The brain represents the outcome of actions independently of the specific muscle
used or the specific way the action is achieved
2. The time taken to respond to a stimulus depends on the amount of information
needed to be processed
3. There is a trade-off between the speed of movement and the accuracy of the
movement
T5
T6
Invariant features
•
•
•
All paths have similar shape
All speed profiles have similar shapes
Speed profiles scale linearly with the
distance covered
Variant features
•
Profiles of shoulder and elbow angles
changes
Conclusions
•
•
Planning of movement is done with
reference to the hand
Represented by in abstract form
rather than relative to joints or
muscles
Is the movement planned entirely before
execution or is it continuously assesses?
Is the movement planned?
Speed and acceleration are scaled
proportionately to the distance of
the target
The extent of movement is
planned before the movement is
initiated
The representation of this plan is
called a motor program
Movement kinematics: spatial
features and angles
Distances: 2.5, 5, 10, 20,
and 30 cm
Movement dynamics: forces and
torques
Lifting a slippery object
(Absence of
feedback)
(Constant
force)
Both feed-forward and feedback controls are used.
(B) Load and grip force increase proportionally to the weight of the objects (scaling of preprogrammed force profile)
(C) Training: 400 g object presented several times (dashed line). 800 g object then given. Force is increased in absence of
sensory feedback via spinal circuit
Voluntary movements
• 3 laws govern voluntary movements (modifiable by learning)
1. The brain represents the outcome of actions independently of the specific muscle
used or the specific way the action is achieved
2. The time taken to respond to a stimulus depends on the amount of information
needed to be processed
3. There is a trade-off between the speed of movement and the accuracy of the
movement
Reaction time
RT increases with choice and decreases with learning
Reaction time indicates the amount of neural processing taking place
(Also depends on sensory modality and conduction distance)
Parallel processing of movement
To overcome the slowness of serial processing, multiple
stimuli and responses can be processed in parallel pathways
>200 ms: trajectories are stereotyped (pre-programmed),
match the target force and direction
0-100 ms: before extent or direction of movement is
processed: no scaling, random direction
100-200 ms: Scaling but no direction = they are specified
independently in parellel.
Paradigm:
Fixed arm, produce a pulse of elbow force (isometric)
Match the peak of the force
6 possible target levels: three up (flexion) and three
down (extension)
Voluntary movements
• 3 laws govern voluntary movements (modifiable by learning)
1. The brain represents the outcome of actions independently of the specific muscle
used or the specific way the action is achieved
2. The time taken to respond to a stimulus depends on the amount of information
needed to be processed
3. There is a trade-off between the speed of movement and the accuracy of the
movement
Accuracy and speed
Fast movements are less accurate than slow
ones due to less time for feedback corrections.
A rapid increase in force requires recruitment of
additional motor neurons
A constant incremental increase in force is
produced my a progressively smaller number of
motor neurons
As forces increases, fluctuations in the smaller
number of motor neurons will be
proportionately greater
Learning will eliminate uncertainties and
increase accuracy and speed
Accuracy and learning
Movement path
becomes straighter
and less variable
Hierarchical organization
•
•
The motor system can perform many
different motor tasks with speed and
accuracy
–
Sensory inputs to motor neurons and muscles is
distributed in hierarchically interconnected areas of
the spinal cord, brain stem and forebrain
–
Movement-related sensory information is processed
in a different system that operates in parallel to the
motor system
–
Reflex circuits in spinal cord and brain stem
simplifies the instructions the cortex sends to lower
levels
3 levels of organization
–
The spinal cord
–
The brain stem (medial/ lateral systems)
–
The cerebral cortex (direct/indirect projections)
Hierarchical organization
•
•
The cerebellum and basal ganglia
–
Feedback circuits for modulation
–
Forms two loop
–
Relay through different parts of thalamus
–
No direct output to the spinal cord
–
Are necessary for smooth movement and posture
Diseases of the cerebellum and basal
ganglia
–
Parkinson and Huntington diseases produce
involuntary movements and abnormal posture
–
Cerebellar ataxias: abnormal timing and
coordination of movements in progress
The spinal cord
Primary afferents from peripheral tissue
•
Connects to local interneurons
•
Connects to propriospinal interneurons
•
Connects to projection neurons to the brain
•
Connects to motor neurons
The proximal-distal rule
•
Motor neurons innervating the most proximal
muscles lie most medially
•
Motor neurons innervating distal muscles are
located more laterally
Propriospinal interneurons
•
•
Short range projects dorsolaterally (distal limbs)
Long range projects ventromedially (axial limbs)
Hierarchical organization
• The brain stem
•
•
–
Contains motor neurons controlling facial muscles
–
Projection neurons to the spinal cord
–
Receives descending input from the cerebral cortex
–
Other brain stem systems control movements of eyes and head
Projection systems
–
The medial system: control of posture; integrates visual, vestibular and somatosensory input
–
The lateral systems: controls distal limb muscles; goal-directed movements of hand and arm
Medial pathways
–
Vestibulospinal
–
Reticulospinal
–
Tectospinal tracts
Ventromedial
Dorsolateral