Post on 29-Apr-2018
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Neural Control of Movement LOCOMOTION
AP photo
Today: the first “activity system”
• Review: that amazing spinal cord!
• What we do: gait patterns
• How we do it: neural circuitry and the role of sensory feedback and higher centres
Most spinal reflexes are polysynaptic• Allows the reflex to
be modified• Muscle action
around a joint is coordinated by inhibitory interneurons• Muscle group
around a joint linked by a reflex pathway called a myotatic unit
Key concept: ‘descending’ motor signals and multisensory inputs can change the balance of inputs to interneurons
• Can alter transmission in reflex pathways
• may lead to ‘reflex reversal’
• Regulates strength of spinal reflex
• Combine to regulate movements
Sites of reflex regulation1. alpha motoneuron2. interneuron3. afferent axon • It was discovered from photographs of horses
that gait patterns change with speed of locomotion.
GAIT DESCRIPTIONS
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Source: http://web.inter.nl.net/users/anima/chronoph/muybridge/
• Physical factors relating to the efficiency of control -- remember the solutions to the d.f. problem?
• An example is human locomotion -- both feet are on the ground when walking, but only one foot when running -- WHY?
energy used or powerrequired (watts)
speed of locomotion(m/sec)
1 3
WALKING
RUNNING
WALKING
L
V
L = radius of center of mass (leg length)
V =velocity of forward movement
acceleration of center of mass = V2 / L (from physics)
g < V2 / L since V2 / L cannot exceed g unless force exerted downwards
V < g L g = 9.8 m/sec2 and L = 0.9 m
V = 3.0 m/sec cannot comfortably walk faster than this
THEREFORE, CHANGE GAIT TO BYPASS PROBLEM
The exception!
3. BODY WEIGHT OVER FOOT
support phase (stance) = 2 to 3 & 3 to 4transfer phase (swing)= 1 to 2 & 4 to 1
1. FLEXED
2. LEG EXTENDED IN FRONT OF TORSO
GAIT NOMENCLATURE
4. LEG EXTENDS BACK, WEIGHT MOVED FORWARD
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ANALYSIS OF GAIT PATTERNS• find regularities which provide insight into underlying mechanisms
Two examples
1) relationship of stance to swing
duration
Cycle time
stance
swing
swing more constant than stance across species with4 legs because this phasedetermined by gravity
2) relationship between joint angles• Joint angle
patterns over the stride period are quite invariant, and do not change with cadence• What degree of
freedom solution does this represent?
Source:Winter, J Mot Beh,15:302-30,1983.
• Timing of muscles to produce these angle changes also consistent, but increase in amplitude as speed increases
1 rad = 57.3 0 2 pi rads = 360 0
-- angle-angle (phase-phase) diagrams used to analyze gaits in amputees and accident victims
Another way to show joint angle patterns:
Below-knee amputee
Source: Enoka et al.Am.J.Phys.Med. 61:66-84,1982
An interesting perceptual
phenomenon that arises from these
regularities is known as
biological motion
• Able to recognize animate from inanimate objects only seeing motion at joints• Nervous system tuned to recognizing
movement in living animals
Source: Johansson, SciAm
.,232:76-88,1975
AT WHAT LEVEL IN THE CNS IS GAIT CONTROLLED?
• TO ADDRESS THIS QUESTION, ELECTRICAL ACTIVITY IN LEG MUSCLES IS ANALYZED WITH THE ELECTROMYOGRAM (EMG)
• FLEXORS -- RETRACT LEG AND PULL IT FORWARD (SWING PHASE)
• EXTENSORS -- PUSH LEG DOWN AND PUSH IT FORWARDS (STANCE PHASE)
• ACTIVITY IN CAT DURING TREADMILL WALKING
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• critical finding was that there are central pattern generators or rhythm generators in the ventral (towards the front) columns of the spinal cord
• this was discovered in preparations that functioned without feedback or input from the CNS • “fictive locomotion”: still see signals
from ventral roots if everything else blocked, even muscle activity
Dorsal (sensory) roots cut
Source: Grillner & Wallen, Ann Rev Neurosci, 8:233-61, 1985
A MODEL OF LOCOMOTION FOR 1 LEG SHOWING THAT THE D.F. PROBLEM CAN BE OVERCOME BY CONTROLLING THE SYSTEM WITH A SINGLE PARAMETER --
FLEXOR BURST RATE
cockroaches used to develop models of locomotion because of
relatively simple circuitry, on assumption that mechanism is
similar throughout animal kingdom
Question: how are central pattern generators organized??
+
+
+
_+
CNS
interneuron
flexor (swing)motor neuron
extensor (stance)motor neuron
+ = excitatory synapses- = inhibitory synapses
exit signal
clock-likeflexor burstgenerator
right leg
left leg
swing (flexor)
stance (extensor)
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1: inhibition between adjacent legs by flexor burst generators
2 : inhibition of leg’s burst generator by sensory receptor in leg during stance phase
right leg
left leg
EXTENSION OF THE MODEL TO TWO LEGS
2clock-likeflexor burstgenerator
clock-likeflexor burstgenerator
• So, seen that inhibition of each leg’s flexor burst generator by mechanical receptors in that leg • thus sensory feedback important to locomotion -
- it tunes the system without complicated timing arrangements
• the d.f. problem is solved without taking up computational capacity
• BUT, some studies have shown that have central pattern generated without sensory feedback• So what is it’s role?
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Sensory feedback important for tuning movements and reflexes
• Can see reflex reversal depending on the phase of the step cycle that an obstacle is encountered
• What’s the functional relevance of this?
flexors
flexors
extensors
swing
stance
Source: Forssberg J Neurophys 42:936-53, 1979
extensors
Role of higher brain structures• Subcortical and brain stem areas can affect gait
patterns
caudate
N.accumbens
Subs. nigra
In cats and rats:
CUT HERE, still get locomotion
STIM HERE affects turning, gait pattern(dopamine cells)
Areas relatedto speed,direction, posture(dopaminelevels)
• In cats, can also stimulate circuits directly with neurotransmitter to restore locomotion
• Lastly, if lesion corticospinal tract (from cortex to spinal cord), affects locomotion requiring visual guidance
(noradrenergic agonist)
• So brain serves more of a supervisory role over central pattern generator (cpg)
• Sensory feedback acts to tune the cpg
Neural circuitry Control system Gait patterns
Next class• how robotics have helped in studying the
control of locomotion • development, navigation
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Neural Control of MovementKINE 4500
Locomotion, continued
Locomotion and robotics: Walking machines
• Useful for testing ideas about biological motor control, or to come up with solutions that can then be tested in biological systems
(and interesting and useful in their own right!)
Motor ControlResearch
PSYCHOLOGY
behaviour
cognition PHYSIOLOGY
cells
muscles
ENGINEERINGphysical systems
neuropsychology
ergonomics biomec
hanic
s
biomed
ical
engin
eerin
g
Essential conditions any walking system must meet:• Regulate its sequence of footfalls• Not tip over• distribute load and lateral forces among all its
legs• ensure legs do not move beyond their travel
limit or bump into each other• ensure that chosen footholds provide
adequate support
An early attempt:• Built by General
electric in 1968• Tele-operated:
mimicked control of human on top, so not autonomous control
• proved difficult for driver to control
Source: Raibert&Sutherland,Sci.Am.248:44-53,1983.
More recent work by Rodney Brooks at MIT
• Based on insect morphology• six legs =
more stability• received
feedback from sensors
• uses absorption at legs for stability• one way to
reduce dfproblem “Ghengis”, http://www.ai.mit.edu/
And the ultimate use for all this robotics technology?
• 18 degrees of freedom
• ccd camera (eyes)• microphone and
speakers (hearing, barks)
• memory and learning algorithms
• approx. $4500
AIBO!
Sony Corp.
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Locomotion: Development• Some reflexes are present
at birth that would seem to facilitate walking• righting reflex:
hold head up• crawling reflex
alternation of flexions and extensions of arms and legs
• step reflexdisappears and reappears
between 4 weeks and 8 months:
WHY?
Stepping reflex Dissappearance and reappearance of stepping in infants
• One theory: psychological
• walking requires cognitive sequencing abilities. Only develop these later in first year.
• Evidence: can train babies to step more
Source: Angulo-Kinzler R, Horn CL, Infant Beh.&Devel(2001) 24:239
Three-month olds can learn to flex knee about 85 degree point to move mobile
Source: Zelazo,JMotBeh,15:99-137,1983.
• More accepted theory: physical reasons• can step if held up in water
(supported)• kicking patterns similar to stepping
throughout first year• stepping stops due to leg weakness
• Do see changes in coordination over first year, however ankle
hip
knee
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Source: Thelen, Dev.Psychobiol.18:1-22,1985.
Knee-Ankle
Hip-AnkleHip-Knee
Reduction in leg synergies by 10 monthsco
rrel
a tio
n
Age (months)
What principles underlie these developmental changes?
• Myelination of axons
• myelin is a fatty substance that surrounds
neurons, allowing a faster conduction rate of
nerve impulses• Schwann cell surrounds axon of peripheral
nerves, wraps around in layerscentral nerves myelinated by
“oligodendrocytes”• begins during later fetal development and during
first postnatal year. Amount of myelin continues to increases from birth to maturity.
Source: Tortora, Princ. Anat.Physiol. Harper&Row,1984
• This neural maturation proceeds in a “head-to-tail” direction (cephalo-caudal) and a proximal-to-distal direction• head and mouth movements refined earlier
than fine finger movements
• Cortical areas take over certain functions from, or inhibit, subcortical areas• why some infant reflexes reappear after
brain damage
Navigation: locomotion through an environment
• Combination of perception, memory, and motor control
• Perception (visual): Optic flowaffects perception of body position• “visual kinesthesis”• common example: IMAX • illustrates visual system’s
dominance over other sensory systems
• used for ‘time to contact’estimates
• Alzheimer’s patients may be impaired in this. Perhaps why ‘wander off’.
Development of visual guidance
• Toddlers more sensitive to visual cues• ‘toddler with the beach blanket’ trick!
• Question: How is the coordination of vision and motion developed?
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Experiment with kittens:• One walks and sees visual environment, the
other just sees
Source: Held, Sci.Am.,213:84-94,1965
• Result: Carted kitten showed no “placing” reactionno integration of visual and motor experience trouble recognizing approaching surface
• Similar study with humans shows adaptation to altered visual environment impaired if moved passively
• Subject who actively moved with goggles on showed adaptation. Passively moved subjects did not.
Source: Held, Sci.Am.,213:84-94,1965
prism goggle adaptationexperiment
Summary - locomotion
• Gait patterns change with speed• physics!• Joint coordination same within a
pattern• In the spinal cord, have central pattern
generators• sensory feedback can tune these
patterns• higher centres can select patterns and
alter responsiveness (gain)
• Developmentally, see innate stepping patterns that reorganize towards end of 1st year• physiological reasons, perhaps
psychological too• Vision can affect body sense used
in exploring the environment• experience with both concurrently
necessary for visual-motor integration