17 Turn over 6 The speed of a nerve impulse along a neurone depends on the diameter and the presence or absence of myelin. (a) The graph shows an action potential in a neurone. +50 +30 +10 –10 –30 –50 –70 –90 0.5 1.0 1.5 2.0 2.5 Membrane potential / mV Time / ms When are voltage-gated sodium channels open? (1) A 0.25 ms B 0.75 ms C 1.00 ms D 1.25 ms 18 (b) The graph shows the relationship between the diameter of the neurone and the speed of nerve impulses along myelinated and non-myelinated motor neurones. 10.0 8.0 6.0 4.0 2.0 0.0 –2.0 speed of nerve impulse / m s–1 motor neurone diameter / μm 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 myelinated motor neurone non-myelinated motor neurone 19 Turn over (i) Compare and contrast the effect of diameter on the speed of an impulse in myelinated and non-myelinated motor neurones. 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(ii) Determine the speed of a nerve impulse along a myelinated motor neurone with a diameter of 5 μm. The equation for a linear relationship is y = mx + c (3) Answer .............................................................. 20 (iii) Multiple sclerosis (MS) is a disease that results in the loss of myelin around motor neurones. Explain why the speed of nerve impulses along axons is slower in people with MS. 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(Total for Question 6 = 11 marks) 21 Turn over BLANK PAGE