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Section 4 Motor Control & Learning General intro Motor = movement • A very diverse area An area of many questions • • • • • • How do we move? Why do we move the way we do? How do movement skills improve? How does memory help learning? Are experts “born” or “made”? Should we tell learners what they ought to do? General intro Inference • We can’t just look into the CNS • Stroke, injury, observation…a slow accumulation of knowledge • The most diverse of our sub-disciplines. See fig. 1.1… General intro General intro Sub-areas • Neurophysiology – the wiring (ch. 14) • Cognitive science – the behavior • Together, they describe the development of movement control in the growing human (ch. 16), the means by which movements are eventually controlled (ch. 14 & 15), and give insight into the complex ideas underlying how movement skills are acquired in the child and adult (ch. 17) • First, a quick look at the basic structure of the problem… Ch. 14 Neurophysiological Approaches to Motor Control Intro What is the problem of control? • • • • • 792 muscles, 100 joints. degrees of freedom problem (Bernstein, 1967) Motor equivalence We never quite repeat ourselves (Bartlett, 1932) We are not aware of what we do well (James, 1890) Components of the nervous system Basic components (neurophysiologically speaking): • Sensory receptors • Motor units • Neurons (nerves) & synapses How complex? • 1,000,000,000,000-100,000,000,000,000 neurons • UK: 1 billion to 100 billion • US: 1 trillion to 100 trillion • Each neuron has up to 10,000 connections • “More connections than there are particles in the known universe” Components of the nervous system Basic function • CNS, PNS • Afference, efference Efferent information PNS CNS Afferent information Neurons and Synapses Structure and function of neurons Neurons and Synapses Structure and function of neurons Neurons and Synapses Structure and function of neurons • Vary by # of dendrites, length of axon • Structure determines role – many types (bipolar, multipolar, golgi I, golgi II…etc.) Purkinje cell (in cerebellum) Basic reflex arc, with interneuron Neurons and Synapses Structure and function of neurons • Neural communication: • Dendrites pick up electrical signal • If total signal strength exceeds a threshold, neuron fires…pulse sent along axon • Pulse is carried more efficiently if axon is myelinated (myelin is fatty stuff that insulates) • So signal transmission is complex, determined by total strength of arriving signal and subsequent strength of descending signal Neurons and Synapses Structure & function of synapses • Joints of the nervous system • Electrical or chemical communication across synaptic gap • Chemical most common, via neurotransmitter • Can be either inhibitory or excitatory synapse Neurons and Synapses Structure & function of synapses • So, typical signal transmission is: • Electrical activity • Chemical activity • Electrical activity • Excitation, inhibition necessary for complex patterns of communication synapse Sensory Receptor Systems for Movement Primarily from vision and proprioception • Purpose is to communicate information • Achieved by converting signals through several energy types Sensory Receptor Systems for Movement The visual system • Light retina rods, cones optic nerve LGN (70%) visual cortex…focal vision • Light retina rods, cones optic nerve SC…ambient vision • Focal vision pathway = “what” pathway • Ambient vision pathway = “vision for action” pathway Sensory Receptor Systems for Movement The visual system • Rods, cones • All over retina but more concentrated in fovea • Cones sense color, require high light • Rods only sense B&W, only require low light • Hence seeing at night in B &W Sensory Receptor Systems for Movement The kinesthetic system • Sensory receptors are distributed through muscle, tendon, joints and skin Sensory Receptor Systems for Movement Muscle receptors • Muscle spindles • In all muscles – esp. small ones, used for fine control (think about rate control…see open & closed loop control later) • Made up of intrafusal muscle fibers and sensory receptors • Transmits info about amount and rate of stretch in muscle Sensory Receptor Systems for Movement Muscle receptors • Muscle spindles • Basically, they lie alongside the main muscle fibers, and are stretched by them(at endpoints) Sensory Receptor Systems for Movement Muscle receptors • Muscle spindles • Intrafusal fibers: • Innervated by gamma motor neuron activity • Contract at end points only • Loaded with sensory connections returning signals to the spinal cord Fires when entire muscle stretches Type Ia afferent: connected at the non-contractile center. Fires on mismatch between contraction due to alpha motor neuron and contraction due to gamma motor neuron activation Sensory Receptor Systems for Movement Tendon receptors • Golgi tendon organs • Impulse returns via type Ib afferent fiber • Respond to tension in the tendon • Fire when entire muscle shortens (contrast w/spindle) • Function: • protection – like a fuse in a circuit • Feedback to spinal cord • Stiffness? Tension? – fine tuning Exact role of spindles & GTO’s still debated Sensory Receptor Systems for Movement Skin (cutaneous) receptors Respond to light vibration Responds to high frequency vibration, & compression Sensory Receptor Systems for Movement Skin (cutaneous) receptors • Density of cutaneous receptors varies around the body • Gives rise to “just noticeable differences” varying in different parts of the skin • Loss or damage to these receptors ruins fine control of movements • Problems w/robots lacking these senses Sensory Receptor Systems for Movement Joint receptors • Modified Ruffini corpuscles, Modified Pacinian corpuscles • In joint capsule • Golgi organs • In ligaments Thought to signal problems w/extreme ranges of motion Sensory Receptor Systems for Movement The vestibular system • Signals body orientation in space Highly integrated with vision…mismatch leads to motion sickness • Semicircular canals, otolith organs (utricle, saccule) Canals allow for both linear and angular acceleration sense…like internal accelerometers Sensory Receptor Systems for Movement Intersensory integration and sensory dominance • Overall sense of what is going on dependent on information flowing from many receptors simultaneously • Occasionally they contradict each other • Vision is dominant…can lead to some amusing experiments (and experiences) • The swinging room (see next 4 slides) • The maze at navy pier • Alcohol, helicopters, and keeping the lights on • See, for example, http://icbmp.uaeu.ac.ae/Proceedings/PDFPAPERS/59_ICBMP.p df if you’re curious. Sensory Receptor Systems for Movement The swinging room (Lee & Aronson, ‘74) The swinging “room” was (is?) a box hanging from the ceiling of a large hall Experimenter (Lee) stands behind fake room, holding a rod connecting the back walls, so that he can swing the room back and forth an inch or 2 Participant stands in center of fake room, looking at front wall (field of view filled with fake room) Sensory Receptor Systems for Movement The swinging room (Lee & Aronson, ‘74) Direction of motion of swinging room Sensory Receptor Systems for Movement So which way would you sway (relative to the room)? Direction of motion of swinging room ? Sensory Receptor Systems for Movement Other applications of (tau - time to contact - used to guide movement)… • Echolocation in bats • Diving gannets • Driving, feeding (hand to mouth), kicking, running, steering, catching, etc., etc., etc. Effector Systems for Movement The motor unit • Covered in Chapter 2. Motor units containing smaller muscle fibers are recruited first – size principle Motor Control Functions of the Spinal Cord Structure of the spinal cord • 2 basic functions: Surprisingly functional • Two-way communications route • Maintains movements in progress (& prevents bad things getting worse) • Mesencephalic cat • 31 pairs of nerves attach – see figure • It’s protected by bone along it’s route, and is a massive information superhighway Motor Control Functions of the Spinal Cord Spinal reflexes • 4 components: • • • • Sensory receptor Afferent neuron Efferent neuron Effector • The stretch reflex • The simplest example of the above 4 components • Knee-jerk reflex is an example • Muscle is stretched, reflex makes muscle contract in response to the stretch Motor Control Functions of the Spinal Cord Spinal reflexes • The flexion reflex • Typical response to pain or threat • Limbs flex as a consequence of reflexive contraction of flexor muscles & inhibition of contraction of extensor muscles Motor Control Functions of the Spinal Cord Spinal reflexes • The crossed extensor reflex • Works with flexion reflex to get away from the pain/threat source • Opposite pattern of contraction Motor Control Functions of the Spinal Cord Spinal reflexes • The extensor thrust reflex • Responds to pressure on sole of foot by contracting extensor muscles in leg • Aids balance, walking • Spinal reflexes for gait control • Overall, these types of reflex show patterns of reciprocal inhibition that set limbs to work in cycles and initiate rhythm…leads to spine as… • Central pattern generator (mes. cat again) Motor Control Functions of the Spinal Cord The role of reflexes in voluntary movement control • All paths lead to α–motor neuron activation • α-γ coactivation • The γ activation of the intrafusal fibers serves as a reflexive check on the α activated extrafusal fibers • If there’s a match, all is well • If there’s a mismatch, the α–motor neuron fires some more • Basic idea: reflexes are incorporated in voluntary movement control Motor Control Functions of the Brain Basal ganglia (deep) brainstem Motor Control Functions of the Brain The motor cortex • Proportional representation • Mostly opposite to side of body • As you move to the left, planned movements are more gross, and involve more muscles • Takes in signals from a # of sources • Sends out final plan to spine Motor Control Functions of the Brain The motor cortex • Pyramidal tract: • primary path to muscles (via α–motor neurons and interneurons) • Damage results in paralysis • Extrapyramidal tract: • Secondary pathway, via basal ganglia, cerebellum, thalamus, brainstem • Primarily inhibitory function (required for coordination, of course) • Damage results in spasticity Motor Control Functions of the Brain The cerebellum • “the seat of coordination” • Receives input from all over…cortex, brainstem, vestibular apparatus, sensory receptors from spine • Output to thalamus and brainstem Motor Control Functions of the Brain The basal ganglia • Input from motor cortex & brainstem • Output to thalamus & brainstem • No direct link to α-motor neurons, but still important in regulation of movements • Parkinson’s disease • Huntington’s disease Motor Control Functions of the Brain The brainstem • Pons, medulla, reticular formation • Integration of both afferent & efferent information • Regulates many long loop reflexes (righting reflex, tonic reflexes) • Also tunes lower reflexes, regulates arousal Integrative Brain Mechanisms for Movement Understanding how all systems work together in movement regulation is incredibly difficult…final understanding is a long way off So there are a number of theories that deal instead with a different level of analysis • Cognitive science – see next chapter