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24 CPG - PSL302 Lecture 24 by Prof MacKay Friday Nov 12...

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Central Pattern Generators (CPG) - Reflexes can perform simple elements of mvts, but not complex patterns - E.g. Stretch reflexes 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 reflexes Properties of networks: (found in all CPGs) 1) Pacemaker neurons - Neurons that can spontaneously generate a.p. w/o specific signals - Just need background of diffuse excitation (no specific 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 reflexes - Reflexes 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 flexion (lift leg to max height) = F - First 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 - Final 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 flexor muscles, other extensor muscles 1) Flexor burst generator drives flexor 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 | E 3 | toe heel toe off strike off PSL302: Lecture 24, by Prof. MacKay Friday, Nov. 12, 2010 1 of 6
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Flexor burst generator - Flexion phase of swing has fixed 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 flexor motoneurons - When flexor network active, extensor burst network inhibited (reciprocal inhibition)
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