Today: the first activity system. Neural Control of Movement LOCOMOTION GAIT DESCRIPTIONS. Review: that amazing spinal cord! What we do: gait patterns

Transcription

1 Neural Control of Movement LOCOMOTION 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 AP photo 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 GAIT DESCRIPTIONS Sites of reflex regulation 1. alpha motoneuron 2. interneuron 3. afferent axon It was discovered from photographs of horses that gait patterns change with speed of locomotion. 1

2 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? Source: energy used or power required (watts) RUNNING WALKING L V L = radius of center of mass (leg length) V =velocity of forward movement 1 3 WALKING speed of locomotion (m/sec) acceleration of center of mass = V 2 / L (from physics) g < V 2 / L since V 2 / L cannot exceed g unless force exerted downwards V < g L g = 9.8 m/sec 2 and L = 0.9 m V = 3.0 m/sec cannot comfortably walk faster than this THEREFORE, CHANGE GAIT TO BYPASS PROBLEM The exception! support phase (stance) = 2 to 3 & 3 to 4 transfer phase (swing) = 1 to 2 & 4 to 1 1. FLEXED 4. LEG EXTENDS BACK, WEIGHT MOVED FORWARD 2. LEG EXTENDED IN FRONT OF TORSO 3. BODY WEIGHT OVER FOOT GAIT NOMENCLATURE 2

3 ANALYSIS OF GAIT PATTERNS find regularities which provide insight into underlying mechanisms Two examples 1) relationship of stance to swing duration stance swing swing more constant than stance across species with 4 legs because this phase determined 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? Cycle time Another way to show joint angle patterns: Timing of muscles to produce these angle changes also consistent, but increase in amplitude as speed increases Source:Winter, J Mot Beh,15:302-30, rad = pi rads = Below-knee amputee Source: Enoka et al.am.j.phys.med. 61:66-84, angle-angle (phase-phase) diagrams used to analyze gaits in amputees and accident victims An interesting perceptual phenomenon that arises from these regularities is known as biological motion 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 Able to recognize animate from inanimate objects only seeing motion at joints Nervous system tuned to recognizing movement in living animals Source: Johansson, Sci Am.,232:76-88,1975 3

4 critical finding was that there are central pattern generators or rhythm generators in the ventral (towards the front) columns of the spinal cord Dorsal (sensory) roots cut 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 Source: Grillner & Wallen, Ann Rev Neurosci, 8:233-61, 1985 Question: how are central pattern generators organized?? cockroaches used to develop models of locomotion because of relatively simple circuitry, on assumption that mechanism is similar throughout animal kingdom 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 CNS + clock-like flexor burst generator + = excitatory synapses - = inhibitory synapses + + flexor (swing) motor neuron + interneuron _ extensor (stance) motor neuron exit signal EXTENSION OF THE MODEL TO TWO LEGS left leg right leg 1 clock-like flexor burst generator clock-like flexor burst generator 1: inhibition between adjacent legs by flexor burst generators 2 : inhibition of leg s burst generator by sensory receptor in leg during stance phase 2 right leg left leg swing (flexor) stance (extensor) 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? 4

5 Sensory feedback important for tuning movements and reflexes swing flexors Can see reflex reversal depending on the phase of the step cycle that an obstacle is encountered What s the functional relevance of this? stance extensors flexors extensors Source: Forssberg J Neurophys 42:936-53, 1979 Role of higher brain structures Subcortical and brain stem areas can affect gait patterns In cats and rats: In cats, can also stimulate circuits directly with neurotransmitter to restore locomotion Areas related to speed, direction, posture (dopamine levels) caudate N.accumbens Subs. nigra STIM HERE affects turning, gait pattern (dopamine cells) CUT HERE, still get locomotion (noradrenergic agonist) Lastly, if lesion corticospinal tract (from cortex to spinal cord), affects locomotion requiring visual guidance So brain serves more of a supervisory role over central pattern generator (cpg) Sensory feedback acts to tune the cpg Next class how robotics have helped in studying the control of locomotion development, navigation Neural circuitry Control system Gait patterns 5

6 Neural Control of Movement KINE 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!) ergonomics PSYCHOLOGY behaviour cognition neuropsychology Motor Control Research ENGINEERING physical systems PHYSIOLOGY cells muscles biomechanics biomedical engineering 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 df problem Ghengis, 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. 6

7 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 reflex disappears 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 Three-month olds can learn to flex knee about 85 degree point to move mobile Source: Angulo-Kinzler R, Horn CL, Infant Beh.&Devel(2001) 24:239 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 hip knee ankle 7

8 Reduction in leg synergies by 10 months What principles underlie these developmental changes? Myelination of axons correlation Source: Thelen, Dev.Psychobiol.18:1-22,1985. Age (months) Knee-Ankle Hip-Knee Hip-Ankle 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 layers central nerves myelinated by oligodendrocytes begins during later fetal development and during first postnatal year. Amount of myelin continues to increases from birth to maturity. 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 Source: Tortora, Princ. Anat.Physiol. Harper&Row,1984 Navigation: locomotion through an environment Combination of perception, memory, and motor control Perception (visual): Optic flow affects 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? 8

9 Experiment with kittens: One walks and sees visual environment, the other just sees Result: Carted kitten showed no placing reaction no integration of visual and motor experience trouble recognizing approaching surface Source: Held, Sci.Am.,213:84-94,1965 Similar study with humans shows adaptation to altered visual environment impaired if moved passively prism goggle adaptation experiment Subject who actively moved with goggles on showed adaptation. Passively moved subjects did not. Source: Held, Sci.Am.,213:84-94,1965 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 9