1. Transmission of nerve impulses

2. axon

3. During a nerve impulse
Na+
axon

4. During a nerve impulse K+
Na+
axon

5. More positive – outside
Resting state
More positive – outside
axon
Less positive - inside

6. During an action potential
axon
Less positive - inside

7. Inside becomes DEPOLARISED
During an action potential
Na+
axon
Inside becomes DEPOLARISED

8. Inside becomes DEPOLARISED
During an action potential
Na+
axon
Inside becomes DEPOLARISED
Action potential is generated

9. Inside becomes negative again
During an action potential K+
axon
Inside becomes negative again

10. More positive – outside
Resting state
More positive – outside
axon
Less positive - inside

11. Transmission of nerve impulses
Neurones transmit impulses as electrical signals
These signals pass along the cell surface membrane of the axon as a nerve impulse

12. Transmission of nerve impulses
It is NOT the same as an electric current passing down a wire (which is much faster)
The mechanisms is the same throughout the animal kingdom

13. Transmission of nerve impulses
Experiments have been carried out using squid axons which are big enough to have electrodes inserted in them.
One electrode can be placed inside the axon and one on its surface.

14. Transmission of nerve impulses
When at rest, the inside of the membrane has a negative electrical potential compared to the outside.
This difference in potential is called the resting potential and is typically about between -70 mV to -80 mV
In this resting state the axon is said to be polarised.

15. Transmission of nerve impulses
This is maintained because the neurone has an internal composition which is different to the outside.
Sodium ions and potassium ions are transported across the membrane against their concentration gradients by active transport

16. Transmission of nerve impulses
Carrier proteins pick up Na+ ions and transport them to the outside.
At the same time K+ ions are transported into the axon.
This is known as the sodium-potassium pump and relies on ATP from respiration

17. Transmission of nerve impulses
Inside the axon there are large numbers of negatively charged organic ions which can not move out of the axon.

19. Transmission of nerve impulses
The Na+ ions are passed out faster than the K+ ions are bought in.
Approx. three Na+ ions leave for every two K+ ions that enter.
K+ ions can also diffuse back out quicker than Na+ ions can diffuse back in.
Net result is that the outside of the membrane is positive compared to the inside.

21. Axon membrane

22. Axon membrane
Inside axon

23. The action potential
A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation
When the axon is stimulated the resting potential changes.
It changes from –70 mV inside the membrane to +40 mV
For a very brief period the inside of the axon becomes positive and the outside negative

24. The action potential
This change in potential is called the action potential and lasts about 3 milliseconds
When an action potential occurs, the axon is said to be depolarised.
When the resting potential is re-established the axon membrane is said to be repolarised

26. depolarisation

27. depolarisation
reploarisation

28. depolarisation
reploarisation
‘overshoot’

29. direction of impulse
depolarisation
reploarisation
‘overshoot’

30. Depolarisation
When the membrane depolarises changes occur in the membrane to the permeability of both Na+ ions and K+ ions

31. Depolarisation
When the axon is stimulated, channels open on its cell surface which allow Na+ ions to pass through.
Na+ ions flood in by diffusion
The Na+ ions create a positive charge of +40 mV inside the membrane, reversing the resting potential and causing the action potential

32. Repolarisation
Potassium channels open in the membrane and K+ ions diffuse out along their concentration gradient.
This starts of repolarisation
At the same time, sodium channels in the membrane close preventing any further influx of Na+ ions.

34. Repolarisation
The resting potential is re-established as the outside of the membrane becomes positive again compared to the inside.
So many K+ ions leave that the charge inside becomes more negative that it was originally.
This shows up as an ‘overshoot’.

35. resting potential
(no net ion movement)

36. Na+ start to move in
resting potential
(no net ion movement)

37. Na+ ions diffuse in rapidly
Na+ start to move in
resting potential
(no net ion movement)

38. K+ ions diffuse out rapidly
Na+ ions diffuse in rapidly
Na+ start to move in
resting potential
(no net ion movement)

39. K+ ions diffuse out rapidly
Na+ ions diffuse in rapidly
Sodium ions pumped out potassium ions pumped in
Na+ start to move in
resting potential
(no net ion movement)

40. Repolarisation
The potassium channels close and the sodium-potassium pump starts again.
Normal concentrations of sodium and potassium ions is re-established.
The membrane is once again at its resting potential

42. direction of impulse
a) In the resting axon, there is a high conc. of Na+ ions outside and a high conc. of K+ ions inside. But the net effect is that the outside is positive compared to the inside giving the resting potential

43. + + + + + + + - - - - - - - - + - - - - - - - + + + + + + + + -
Leading edge of impulse
b) The axon is stimulated producing an action potential, setting up local circuits on the axon membrane

44. Na+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - -
direction of impulse
Na+
Na+
c) Sodium ions rush into the axon along a diffusion gradient depolarising the membrane causing an action potential

45. K+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - K+
direction of impulse K+ K+
d) As the action potential passes along the axon potassium ions diffuse out along a concentration gradient, starting off the process of repolarisation

46. K+ Na+ K+ + + + + + - - + - - - - - + + - - - - - - + + -
direction of impulse K+
Na+ K+ K+
Na+ K+
e) The sodium-potassium pump is re-established, fully repolarising the membrane