Nerve Impulse
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Nerve Impulse

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A2 biology edexcel

A2 biology edexcel

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Nerve Impulse Nerve Impulse Presentation Transcript

  • Nerve impulse
    • Starter activity
    • Explain the role of the ‘nodes of ranvier’
  • Nerve impulse
    • Learning objectives
    • Understand the nature of the nerve impulse
    • Describe the propagation of action potentials of nerve impulses
    • Describe the permeability of membranes to sodium ions during action potential
  • Nerve impulse
    • http://www.phschool.com/atschool/science_activity_library/path_nerve_impulse.html
    • Each nerve impulse involves the movement of ions through the axon membrane
    • Na+ move into the axon
    • K+ move out of the axon
    • When a neurone is resting (not conducting an impulse) it has a negative electrical charge (resting potential), but when Na+ enter it briefly becomes positive (depolarised) which generates the action potential. When the K+ leave the inside becomes negative again therefore restoring the resting potential. The resting potential is the effect of differences in the ion concentration across the axon membrane (Na+, K+, Cl-, and organic ions)
    View slide
  • Nerve impulse (resting state) Exclusive to inside Organic ions Greater Cl- Greater K+ Greater Na+ Outside the axon Inside the axon Ions region of axon View slide
  • Nerve impulse (resting state)
    • The difference in the ion concentrations is all dependent on the permeability of the axon membrane
    • During the resting state the permeability of the membrane to K+ is high, due to the presence of the protein channels (gates)
    • Due to there being no (gates) for the organic ions, they remain on the inside of the membrane
    • Due to the ions, this is the reason why there is a negative charge on the inside of the axon membrane
    • Very few K+ ions will diffuse out of the membrane, due to the fact that there is an overall negative charge on the inside of the axon membrane
    • Cl- concentration gradient is towards the inside of the axon membrane but due to overall negative charge, they are repelled back out.
    • Na+ would be expected to move into the axon membrane, due to the overall negative charge. But there is a low permeability on the axon membrane to sodium ions, so in turn they are moved inside slowly and are captured by the ion pumps and expelled to the outside
  • Nerve impulse (resting state)
    • http://www.biology4all.com/resources_library/source/63.swf
  • Nerve impulse (generating)
    • When a nerve impulse is generated the following occur
    • Na+ permeability slightly increases, so more enter the axon membrane than are being expelled
    • Due to this movement the potential difference reaches a positive value (action potential)
    • So as the permeability to Na+ decrease in turn the permeability to K+ increases due to more (K) channels being open.
    • So as the K+ flow out, the potential difference inside the axon decrease back to the negative resting value. There is slight decrease in the membrane potential which is slightly lower than resting potential, due to more K+ released than needed
    • This effect is called the Hyperpolarisation. But this is temporarily until the Na+ and K+ return to their resting concentrations
  • Action potential
  • Action potential
    • A nerve impulse is the propagated (spread) action potential or a wave of depolarisation which travels along the axon membrane.
    • The ion channels in the axon membrane are voltage dependent, which only open when the membrane is depolarised
    • This sets up a current which spreads from the action potential region of the membrane to the region ahead of it.
    • This stimulates the Na+ channels to open. This occurs in a small segment of the axon membrane
    • Due to the propagated action potential, this moves at a constant velocity along the axon membrane.
    • Saltatory conduction is a type of conduction which only occurs when the action potential jumps from one ‘node of ranvier’ to another. This is because of the axon being myelinated. The result is an increase in the conduction velocity
    • During the passage of the nerve impulse the axon will gain Na+ and lose K+, but these ions are re-exchanged by the Na+/ K+ pumps which will actively pump out Na+ and pump K+ into the axon
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