MEDI 258 Info Processing Part 2 PDF

MEDI 258 - Pre-programmed reactions Info processing in the Motor System 1. What are the basic components of a purely “feed-forward” mode of IP? Open loop control 2. How is feedback incorporated into information processing? Closed loop control 3. What types of feedback contribute to CL and how? A/Pro...

MEDI 258 - Pre-programmed reactions Info processing in the Motor System 1. What are the basic components of a purely “feed-forward” mode of IP? Open loop control 2. How is feedback incorporated into information processing? Closed loop control 3. What types of feedback contribute to CL and how? A/Prof Paul Stapley, MEDI 330 Sensorimotor Control, Week 3 1 Open-loop control Movements planned using a predetermined ‘program’ and run-off without feedback, Executive and effector No feedback is used Input delivered to executive/control centre ⇒ Input processed ⇒ decision made about appropriate action ⇒ Instructions transmitted to effector Without feedback, system is “unaware” if output was successful in achieving goal. A/Prof Paul Stapley, MEDI 330 Sensorimotor Control, Week 3 Open-loop control (“feed-forward”) Effective IF circumstances surrounding action = unchanged Inflexible when unexpected changes occur Summary (open loop mode): 1) Advance instructions to specify operations (sequencing and timing), 2) Once initiated, program executes instructions (no corrections) 3) No capability for detecting errors and updating A/Prof Paul Stapley, MEDI 330 Sensorimotor Control, Week 3  Closed loop control Feedback from actual movement is compared to expected sensory feedback of the desired state. Difference between actual and expected sensory feedback = error used to update movement. Every time error signal goes to executive for correction must pass through each of the stages. This controlled processing requires attention and time. Reflexes also contribute to movement control, within a CL mode of control. Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  What does a closed loop system look like? Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  M1 response First brief burst in activity at about 30-50 ms Is induced by unexpected stretching of muscle spindles (length) Information about stretch is relayed to spinal cord and straight back to muscles: Involves only 1 synapse “monosynaptic” M1 plays a role in correcting small muscle stretches (e.g., postural sway) Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  M2 response 2nd burst of activity 50-80 ms after experimenter adds load. It generates a higher burst of activity than the M1 reflex Longer duration. Provides greater adjustment than M1 response Signals also arises from muscle spindles but travel higher than the spinal cord (motor cortex and/or cerebellum) Thus, longer distance travelled and additional synapses account for the added time lag of the response M2 is responsible for well-known patellar-tap reflex ** (workshop) Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  Triggered reaction Longer latency still than M2, but too fast to be a voluntary reaction (80-120 ms) Can affect muscles far from stimulation (loaded) site. Can be learned and become more or less automatic Example: wineglass effect - when someone lifts a wineglass and it begins to slip. Vibrations of glass slipping generate compensation in forearm muscles Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  M3 response This is the voluntary action Is powerful and sustained. In this example, it brings the limb back to the desired position Latency = 120-180 ms, can affect all body muscles, not just stretched ones However, long delay in response means it is sensitive to number of S-R alternatives Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 *Reflex responses in the closed loop model* M1 loop carries FB about muscle length (stretch) to SC. Loop = fast and inflexible. Lowest level of FB-based corrections, minimal contact with higher brain centres M2 feedback loop = long-loop or functional stretch reflex. Carries info about muscle force/length and joint/body position. M3 Loop = longer & slower than M1. Involves higher control (brain) centres Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  How does the CNS know what muscles are doing? Combination of 3 input sources: 1) Peripheral sensing of joint angles and skin deformation* 2) Sensing of muscle lengths (spindles) 3) Sensing of muscle forces generated by GTOs *don’t really touch on this… Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  Feedback from the muscles Muscle has 2 important mechanoreceptors: Muscle spindles - changes in muscle length GTO’s - changes in muscle force (tension) How do these 2 receptors contribute to a closed loop mode of control (as sources of feedback)? Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 12  Structure of muscle spindles Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 13  Muscle spindle architecture 2 types of intrafusal fibres: 1) Primary (1a) endings - large, fast conducting, and 2) Secondary (II) - smaller, slower conducting Ia spirals around central region of all intrafusal fibres. II’s adjacent to central region of static bag and chain fibres Gamma efferent neurons can be static or dynamic 2 phases of stretch: Dynamic (when muscle length changes) and Static (muscle stable at new length) Primary endings = small, unexpected changes in length Secondary endings = steady state length Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 14  Sensory transduction in spindles When intrafusal fibre is stretched => sensory endings undergo mechanical deformation that results in receptor (action) potential Impulse-initiating region of nerve membrane produces action potentials conducted along sensory nerve fibres to spinal cord (afferent transmission) Regions near ends (polar zones) are stiffer than sensory zones (centre of fibres). When spindle stretched most of length changes occur in sensory zone Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 15  Golgi Tendon Organs What are they? Mechanoreceptors that respond to force Bundles of collagen strands and sensory nerve endings enclosed in connective tissue Located at both superficial muscle-tendon junctions and deep intramuscular muscle- tendon junctions Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 16  Golgi Tendon Organs - Architecture Collagen strands originate from compact bundles that leave tendon. Bundles split into compartments. Compartments break into fine strands woven with nerve endings, 10-25 muscle fibres form with single GTO. Fibres attached to GTO originate from several motor units, Slow and fast twitch motor units may connect to same GTO, Some muscle fibres insert around tendinous end of GTO in parallel. These signal unload of tendon organ Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 17  Golgi Tendon Organs - Sensory Transduction Similar to other mechanoreceptors. Force transmitted from tendon-muscle ends via collagen bundles and capsule cells. Mechanical deformation => receptor potential => nerve impulses Sensory nerve fibres = Ib afferents. GTO afferents discharge at onset of contraction of in-series muscle fibres. Linear regression between discharge rate and force Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 18  Feedback control: Closed loop How is sensory information used to control movements? Goal = maintain constant movement Uses a comparator, an executive and an effector Comparator = senses difference between desired and actual state. If error => signal to executive (command of system) When executive receives signal issues command to effector responsible for the action (e.g., online correction of arm movement) Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3  Summary - Pre-programmed reactions Q1: What are the basic components of a FF mode of open loop control? (executive and effector) Q2: How is feedback incorporated into information processing (Closed Loop mode of control). Q3: What types of feedback are used in a CL mode of control? Muscle spindles = length (dynamic and static) GTO’s = force. Dr Paul Stapley, MEDI 320 Motor Control & Dysfunction, Week 3 20