24 CPG - PSL302: Lecture 24, by Prof. MacKay! Friday, Nov....

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Central Pattern Generators (CPG) - Refexes can perForm simple elements oF mvts, but not complex patterns - E.g. Stretch refexes cannot restore postural stability aFter a perturbation - A centrally coordinated response is required - Posture and mvt are programmed by CPGs (Fxnal networks oF neurons in CNS) - Coordinate seqce oF muscle contractions to perform fxnal mvts - Manipulate reFexes Properties oF networks: (Found in all CPGs) 1) Pacemaker neurons - Neurons that can spontaneously generate a.p. w/o speci±c signals - Just need background oF diffuse excitation (no speci±c inFo about timing) -> elevated membrane potential 2) Reciprocal inhibition - Activity in one muscle => inhibition oF antagonistic muscle pair - Important For seqcing muscle contractions 3) Phase-dependent reFexes - Refexes manipulated: turned on when useFul + oFF when not useFul Locomotion: Leg-Step Cycle - The step cycle oF each leg during walking is programmed by a network oF neurons w/i the intermediate zone of the lumbar cord - Swing phase : leg is in air - Toe off: initial fexion (liFt leg to max height) = ± - ²irst extension phase (bring leg down using gravity ) = E1 - Heel strike! - Stance phase: leg is on ground, supporting body weight - Second extension phase ( active muscle contraction) = E2 - Mid-stance: center oF gravity over sole oF Foot = entire body weight supported while other leg is mid-swing - ²inal extension phase (heel comes oFF ground, push off substrate) = E3 - Power stroke = moving center oF gravity Fwd -> toe off ‘HalF-center’ structure - CPG Fxnally organized into 2 half-centers: 1 driving fexor muscles, other extensor muscles 1) ±lexor burst generator drives fexor motor nuclei 2) Extensor burst generator drives extensors - Mutually inhibit one another = reciprocal inhibition - Presence oF pacemaker neuron + background oF diFFuse excitation: both spontaneously active when not inhibited - The way the nervous system preFers to operate: generalized excitation but inhibitions are very precise -> all motor patterns based on inhibition patterns | SWING | STANCE | |F | E 1 |E 2 3 | toe heel toe off strike off PSL302: Lecture 24, by ProF. MacKay ²riday, Nov. 12, 2010 1 oF 6
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Flexor burst generator - Flexion phase of swing has ±xed duration (same cycle period), regardless of speed of locomotion (whether you’re walking or running) - Flexor network has build-up of inhibition in sp.c. that eventually stops burst of a.p.’s - End of activation of ±exor motoneurons - When ±exor network active, extensor burst network inhibited (reciprocal inhibition)
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This note was uploaded on 03/27/2012 for the course PSL PSL300 taught by Professor Mackayfrench during the Fall '11 term at University of Toronto- Toronto.

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24 CPG - PSL302: Lecture 24, by Prof. MacKay! Friday, Nov....

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