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NESC 2470 (13)

Lecture 2, Nov 14th 2011.docx

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Dalhousie University
NESC 2470

Lecture 2, Nov 14th 2011 November-22-11 6:14 AM Spinal Control of the nervous system Muscle Spindle  Also called a stretch receptor  Tough fibrous exterior  Inside composed of muscle fibres o Type 1a sensory axon wrapped around each fibre  Sensitive to changes in muscle length  Are proprioceptors  Information conveyed by 1a afferent axon into spinal cord  Could potentially contact every alpha motor neuron in the motor pool supplying the muscle in which the spindle resides Sherrington  Stretch reflex (myotatic) o Involves sensory feedback from the muscle  Shown by cutting dorsal roots  Alpha motor neurons were still intact, but reflex disappeared o Faster firing rate as muscle lengthens o Type 1a afferent axons important  Discharge is closely related to the length of the muscle  Stretching muscle increases discharge rate Knee-jerk reflex  Hitting tendon on knee causes quad contraction and knee straightens o Because it causes tendon to stretch, opening mechanosensitive ion channels Intrafusal muscle fibres  Inside spindle  Receive motor innervation from Gamma motor neurons o Another type of lower motor neuron o Innervate the intrafusal muscle fiber at the two ends of the muscle spindle o Gamma activation alone increases Ia activity  Not strong enough to contract muscle Extrafusal muscle fibres  Outside of spindle  Can contract muscle Gamma muscle fibres  When not receiving signal (When muscle is slack)  Muscle tightens to begin receiving signal again Gamma loop  Gamma motor neuron -> intrafusal muscle fiber -> Ia afferent axon -> alpha motor neuron -> extrafusal muscle fiber Golgi tendon organ  Acts like a strain gauge o Monitors muscle tension or force of contraction  Located at the junction of the muscle and the tendon o Innervated by group 1b sensory axons Type 1b afferent axons  Fast conducting  Slightly smaller than 1a axons  Wrapped around collagen fibrils in tendon  Very sensitive to tension  Enter spinal cord, branch repeatedly  Synapse on interneurons in the ventral horn  Some form inhibitory connections with the alpha motor neurons innervating the same muscle Biceps reflex  Find bicep tendon, push on to make tight  Other hand taps tendon  Bicep 1a axons send signal to contract  Tricep alpha motor neuron sends inhibitory signal Reciprocal inhibition The contraction of one set of muscles accompanied by the relaxation of the antagonist muscles  Crossed extensor reflex  After stepping on something sharp  Used to compensate for the extra load imposed by the limb withdrawal on
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