15 Impulses & action potentials

  • Created by: lee8444
  • Created on: 27-02-20 08:17

Resting potential

  • Phospholipid bilayer of the axon plasma membrane prevents sodium and potassium ions from diffusing across
  • Channel proteins that span the bilayer can transport the ions across the membrane
  • Some of these channels have gates which can be open or closed
  • Some carrier proteins actively transport potassium ions ito the axon and sodium ions out of the axon - sodium-potassium pump
  • Inside of the axon is negatively charged at -65mV
  • This is called being polarised
  • Active transport of sodium out of the cell is greater than the amount of potassium coming into the cell. 3 sodium per 2 potassium
  • Slow diffusion too but this is cancelled out
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Action potential

  • A stimulus can cause a reversal of the charge by opening channel proteins to allow diffusion
  • If it reaches +40mV an action potential is created and the membrane is depolarised
  • There are voltage-gated channels that open/close depending on the charge of its environment
  • At resting potential sodium voltage-gated channels are closed
  • Energy of the stimulus causes some of the sodium voltage-gated channels to open so sodium diffuses in along their electrochemical gradient
  • As sodium diffuses in, more sodium channels open causeing a greater influx of sodium
  • Once it is at +40mV the sodium voltage-gated channels close and potassium channels open
  • The potassium ions are now able to diffuse out more easily causing more potassium ion channels to open repolarising the axon
  • This causes an overshoot where the axon becomes hyperpolarised. Here the potassium channels close and the sodium-potassium pump restarts
  • The resting potential of -65mV is re-established and the axon is repolarised
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Passage of an action potential on an unmyelinated

  • At resting potential, there is a high concentration of sodium ions outside the axon compared to inside whereas there is a high concentration of potassium on the inside compared to the outside. The axon is polarised
  • A stimulus causes a sudden influx of sodium ions - depolarisation
  • Localised electrical currents open sodium voltage-gated channels further along the axon
  • This causes depolarisation further along the axon
  • behind this new region, the older region now shuts its sodium channels stopping the influx and potassium ions diffuse out decrease its charge
  • This causes depolaristion to move along the membrane
  • The section behind the depolarised region is being repolarised and becomes hyperpolarised to prevent the backwards flow of an impulse
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Passage of an action potential on a myelinated axo

  • Fatty sheath of myelin acts as an electrical insulator
  • every 1-3 mm there is a break called the nodes of Ranvier
  • Action potentials can occur at these points
  • Localised currents happen between adjacent nodes of Ranvier
  • The action potential can jump from gap to gap
  • This is called saltatory conduction
  • Action potential can happen faster when myelinated as depolarisation doesn't have to happen the whole way along the neurone, only at parts
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