nervous transmission

• resting potential = -70mV
• resting potential is the result of:

  • the action of the active transport system called the Sodium potassium pump
  • three sodium ions out
  • two potassium ions in
• Resting potential:

  • membrane is very impermeable to sodium ions so these cannot diffuse back in
  • membrane is slightly more permeable to potassium ions so these diffuse out slowly (leaky channel down concentration gradient)
  • establishes electrochemical diffusion gradient
  • the outside is more positive than inside
  • neurone is said to be polarised
• 1. Resting potential - The membrane is at rest and polarised at -70 mV.
  2. Stimulus - Voltage-gated sodium channels open, so sodium ions diffuse into neuron by sodium gated channel, inside becomes positive and become depolarised. threshold = -45mV
  3. Repolarisation - At +40 mV, sodium channels close and potassium channels open, so potassium flows out through potassium gated channel making the inside more negative - this is repolarisation
• 1. Hyperpolarisation - An excess of K+ leaves the axon, dropping the potential below the -70 mV resting level.
  2. Refractory period - ion pumps and channels work together to restore the membrane back to the resting potential. sodium and potassium are redistributed by sodium potassium pump
• potassium gated channels are open all the time as they are leaky
• action potentials have a:

  • threshold - there is a minimum level of stimulus necessary to cause depolarisation
  • are all or nothing
  • all action potentials are the same size, irrespective of the intensity of the stimulus
• refractory period

  • the ion gated channels remain closed and cannot be made to reopen so no further depolarisation is possible
  • separates impulses from one another
  • ensures impulses can only pass in one direction along the axon
• speed of conduction of impulses

  • increase diameter of axon
  • myelin sheath
  • temperature
• myelin sheath

  • acts as an insulator
  • rich in lipid (myelin) which makes the axon impermeable to ions so they are unable to diffuse between the tissue fluid and neuron
  • action potential cannot be generated by the myelinated regions
  • action potential can only be generated at the nodes of ranvier
  • action potential seen as jumping from node to node - SALTATORY CONDUCTION
• temperature

  • affects the rate of diffusion and the rate of energy release by respiration for active transport
• axon diameter

  • the bigger the axon diameter, the faster the impulse is transmitted. This is because there is less resistance to the flow of the ions in the cytoplasm
• threshold value = -45mV
• depolarised value = +40mV
• all or nothing - stimulus has to exceed the threshold (-45 mV) to generate an action potential