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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)

1

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