The nerve impulse

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The nerve impulse

  1. 1. The nerve impulseLearning objectives:• How a resting potential is achieved• How changes in permeability create an action potential (and the all-or-nothing idea)• How passage of action potentials along a neurone leads to a nerve impulse being conducted• The nature and importance of the refractory period in producing discrete nerve impulses
  2. 2. A resting neurone• In a resting neurone there are more positive ions on the outside of the axon and fewer positive ions inside the axon• Resting potential – the potential difference (p.d.) across a membrane when a neurone is not conducting an impulse• The p.d. across the membrane is –70mV and is polarised
  3. 3. Q: How is a resting potential achieved? A: The sodium-potassium pump!
  4. 4. Sodium – potassium pumpa) Uses energy from ATP to pump 3 x Na ions OUT in exchange for 2 x K ionsb) Allows both Na and K ions to diffuse but is MORE permeable to K + ions than to Na2+ ions
  5. 5. How does and impulse start? Depolarisation!
  6. 6. Gated channels• The channels are closed in a resting membrane• Only open when the p.d. across the membrane reaches a specific value
  7. 7. Gated channels• Na gated channels open at the point of a stimulus• NA2+ ions move in• If enough move in, more channels open, causing MORE to come in.• If not enough, nothing happens (all-or-nothing nature)• Reduction in p.d. = depolarisation of the membrane
  8. 8. Gated channelsSodium ions diffuse along:• Concentration gradient• Electrical gradientSo many have moved in that inside becomes more positive than outside. The p.d. is now +40mV
  9. 9. Repolarisation• Sodium gated channels now close and potassium gated channels open• K+ ions now flow along a concentration and electrical gradient• Membrane is repolarised to its original resting potential of -70mV
  10. 10. Recovery• There is a brief time of hyperpolarisation when the p.d. is more negative due to potassium channels staying open a millisecond longer• The is also a brief refractory period – when the sodium channels have closed before the neurone can respond to the next stimulus• The original balance of ions is restored by the sodium- The whole process is called potassium pump an action potential
  11. 11. Changes in permeability of the neurone during anaction potential
  12. 12. How does an impulse travel along a neurone? Passage of action potentials!
  13. 13. Different sized neurones• Nerve impulses move at constant speed• Wide neurones - conduct impulses more quickly than narrow• Myelinated neurones - conduct impulses more quickly unmyelinated

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