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Spinal reflexes

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Physiology 3120
Tom Stavraky

Human Physiology Friday, November 13, 2009 “Neuro X” Spinal Reflexes • Knee-jerk reflex  Tapping the quad muscle, you produce a reflexive quadriceps contraction  Ia afferents enter the spinal cord and stimulate its own efferent alpha motor-neuron  Also inhibit the antagonist muscle (i.e. the hamstring)  uses a branch (collateral) of the same Ia afferent fibre  Collaterals also feed up the dosal-column medial lemniscal system (not shown in diagram)  Reciprocal inhibition  Property of all spinal reflexes  Agonist muscle excited, but the antagonist inhibited  Functions as the first defence in overcoming unexpected muscle stretches (i.e. think of someone pushing you over; you want to contract the same muscle that was stretched by the perturbation); second defence is the vestibular postural reflex  Extraordinarily fast (contraction 30ms after the stretch); voluntary contractions caused by a bone tap take 150ms • Electromyogram (EMG)  Record electrical activity of entire muscle because all the individual muscle fibres in the motor unit simultaneously send action potentials  Extracellular recording  Much easier than recording from the nerve • Different speeds of stretch  Can’t manually generate a stretch reflex in the biceps because they can’t go fast enough to summate & reach threshold  Medium = as fast as we can go manually  Note: IAafferent responds to velocity of stretch  Frequency of discharge proportional to velocity of stretch  Get greater summation of EPSPs with faster stretch  With tendon
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