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Lecture 15

Lecture 15 - Reflexes

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University of Toronto St. George
Michelle French

PSY300H1F L15; Oct. 17, 2011 • during movement, reflex suppressed– when actually • Need efferents to stop out hair cells from amplifying, want to get out off posture and move, inhibit the stretch otherwise getting ringing in ears reflex Reflexes • Basic element of motor control in motor system Golgi tendon reflex • ‘bending back’ of a sensory stimulus within the CNS to •Stimulus: active tension/contraction in muscle produce a motor response – sensory-motor linkage at • Response: relaxation or reduction of tension basic level • GTO afferents (Ib) synapse on interneurons in • perform a specific task; often a feedback loop intermediate zone of spinal cord – slightly slower than 1a regulating force, position, etc.; called up often, otherwise but still fast, less direct access suppressed •Ib-interneurons inhibit α-motoneurons (perform contraction vs gamma when sensitize and don’t do Reflex Loop physical work) of same muscle – so opposite effect on muscle – so inhibit contraction • Signal sent by afferents to dorsal horn in spinal cord or sensory nuc in brainstem (involving cranial nerves) • Then directed to motor nuc to specifc muscles to get motor response in periphery of body • Over-exerting bicep muscle, over-contract trying to maintain stability; negative-feedback/inhibitory most Stretch reflex of the time • Stimulus: passive stretch of a muscle by applied load • Motorneurons inhibited to lower muscle activity to or contraction of antagonist muscle; basic level, functions in opposite direction to muscle • most important reflex, often unaware of; ex. Bang on spindle (stretch relex sees as not enough, golgi knee quadriceps by doctor tendon reflex sees as too much) • Response: active contraction of muscle; e.g. patellar tendon reflex Golgi tendon reflex properties • very sensitive(can detect 1-2 µ) and fast due to muscle • Prevents movement spindle afferents (so fastest in NS), and monosynaptic • acts in concert with stretch reflex to stabilize posture, connection to motoneurons (only sensory afferent that just operate in dif directions goes straight to motorneurons) – Grp 1a, 2 • suppressed when net motion is desired • in certain conditions reflex output is reversed (positive feedback instead of negative); e.g. stance phase of walking • Over contracting biceps, object brought up too high • 1b afferents activated in over-contracted muscle • Triceps stretched – slight upward movement  stretch on opposite side, activate • Get motor signal to stabilize load on arm muscle spindles, volley in 1a afferents  stretch reflex causes small contraction in Stretch reflex function stretched muscle • Essential for stabilizing posture; reflex strongest in • 1b afferents in postural muscles; constantly detecting stretches and making adjustments to keep posture upright; ex. If doze overcontracted muscle cause off makes you stand up inhibition so loosen up to stabilize position • latencies (delays) of about 25 ms for forearm muscles, • So same overall function 37 ms for ankle extensors (longer conduction distance, Flexion withdrawal reflex still short delay) • Actually conscious of, but happens very rarely • parallel multisynaptic paths through spinal cord, and a transcortical path – sustains reflex for longer, branch • Stimulus: noxious injury of limb sent up dorsal column medial meniscus system to cortex • Response: flexion of joints proximal to stimulus (not purely extension since distal limbs extend to stimulus) – pull limb away from site of injury • Aδ, C nociceptor afferents synapse on interneurons in • If injurious stimulus to foot  superficial dorsal horn – so slowly conducting since less flexion withdrawal reflex lifts important and flexes leg to move away • multi-synaptic path to motoneurons from dorsal horn from site of injury to ventral horn – slow due to integrations in the path • On opposite side, commissural • Holding object in hand, axon crosses midline  move arm over candle activates extensor (anti-gravity •  flex at elbow at muscles) muscles  increased shoulder to move limb postural support in opposite leg of of way; but also (must double support in extend fingers to remaining leg if one not on remove object in hands ground) (say if object heavy • Otherwise collapse on top of the enough support that it injurious stimulus would slow your • Also happens in arm but not as movement) strong (in humans, would be as • Pattern designed to strong in cats and dogs since move site of injury from use all four limbs for support stimulus Extensor thrust reflex • also not conscious of, only evident in certain conditions Reciprocal inhibition/inervation • Stimulus: pressure on sole of foot (cutaneous, • default mode of spinal cord innocuous mechanical pressure like weight of body on • Basic property of intermediate zone • activation of one motor nucleus is coupled to inhibition foot so not painful) of antagonistic motor nucleus  so don’t have muscle • Response: activates large myelinated Aβ mechanocreceptors in skin (ex. Merkel disks)  goes from each side of joint fighting, don’t want both active at into spinal cord intermediate zone interneurons  same time; default pattern activate leg extensor muscles to increase activity and • e.g. activation of flexor motoneurons elicits inhibition support of antagonist extensors • reflex operates only during stance when support • when co-contraction of antagonists desired (for joint stiffness) this circuit is suppressed; ex. When catching weight on foot, otherwise not needed • Supporting weight on right leg ball stiffen up shoulder and elbow – use another set of activates mechanoreceptors on interneurons sole of foot • Using as part of flexion • Increase postural support in withdrawal reflex that leg to keep upright • Activate
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