NERVE IMPULSE

1.   Membrane potential:
      -     difference in electrical charge
      -     across the plasma membrane.











2.   Resting potential:
     -     neurone in its resting state
     -     usually has a membrane potential of -70mV
     -     maintained by the sodium-potassium pumps
     -     which actively transport sodium ions ß out of  the cell
     -     potassium ions à into the cell.
     -     require ATP from respiration.

3.   In resting neurone:
     -     membrane is more permeable to K⁺ than to Na⁺.
     -     potassium ions move out
     -      sodium ions move in
     -     thru passive ion channels.

4.   When the axon is stimulated:
     -      Na⁺ ions diffuse in
     -     polarity of the membrane in reversed
     -     from -70mV inside value
     -     to +40mV  inside.
     -     axon is depolarised.

5.   Polarity change = action potential = nerve impulse.

6.   Stimulus:
     -     opens up Na⁺ channels,
     -     which allow Na⁺ to flood in by diffusion
     -     thus depolarising the axon
     -     sodium-potassium pumps stop.

7.   In response to depolarisation:
     -     K⁺ ions diffuse out along a concentration gradient
     -     starting off repolarisation.
     -     inside membrane becomes more -ve than resting potential
     -     thus creating a refractory period.
     -     sodium-potassium pumps then start again.
     -     re-establishes the resting potential

8.      Axon can also be excited by direct application of:

     -     mechanical stimuli
     -     osmotic stimuli
     -     chemical stimuli
     -     thermal stimuli
     -     electrical stimuli.

9.      Action potential:

     -     can be generated if stimulus reaches certain threshold level.
     -     more intense stimulus:
         à will not give greater action potential.
     -     once threshold level is reached:
         à size of an impulse
         à is independent of the intensity of the stimulus
         = all-or-nothing law.

10.  During refractory period:

      -     neurone is insensitive to depolarisation.

11.  In absolute refractory period à cannot transmit another action potential à no matter how great a stimulus is applied.

12.  In relative refractory period à axon can transmit impulses à but stimulus must be stronger than usual.

13.  2 factors determine the speed of impulse transmission:

      -     axon diameter
      -     myelin sheath.

14.  An action potential:

      -    self-propagation (regenerating repeatedly along the axon)
      -     when wave of depolarization travels down the axon.

15.  In non-myelinated neurones:

      à speed of impulse transmission

      à proportional to diameter of axon.

16.  In myelinated neurones:

      à action potential jumps from node of Ranvier to the next

      à speed up impulse transmission = saltatory conduction.