- 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.
- 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.