Transmission Of Nerve Impulses

1. Transmission of nerve impulses

2. axon

3. axon
During a nerve impulse
Na
+

4. axon
During a nerve impulse
Na
+
K+

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

6. axon
During an action potential
Less positive - inside
Na
+

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

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

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

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

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. depolarisation
reploarisation
‘overshoot’
direction of impulse

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

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

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

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

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

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

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

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