1. Transmission of Nerve Impulses
GHB 2004

2. Information is carried along a neurone as an electrical impulse

3. The signal in a neurone is not an electrical current
The signal is a fleeting change in the potential difference across the cell surface membrane of the neurone.
This change in potential difference sweeps along the neurone from one end to the other.

4. Generating The Resting Potential
The resting potential of a cell is generally about -70mV. This means that the inside of the cell contains more negative ions and/or fewer positive ions than the outside.
When a neurone is not transmitting a signal it is said to be resting
‘Resting’ neurones are NOT resting!
Cell surface membranes contain sodium-potassium pumps……these use ATP to move sodium ions out of the cell and potassium ions in.
Three sodium ions are moved for every two potassium ions – (inside is -ve)

5. 2K+ 3Na+ K+ Na+ Outside Axon Na+ K+ Na – K Pump ATP ADP + Pi
Inside Axon

6. Potassium ions are able to diffuse back out of the cell, down their concentration gradient, however…
The membrane is less permeable to sodium ions so…
…they diffuse in more slowly.
This means that the tissue fluid outside the cell contains more positive ions than the cytoplasm, so…
…the inside is negatively charged compared to the outside

7. Potential Difference (70mv)
Na+ K+
Outside Axon K+ - +
Potential Difference (70mv)
Na – K Pump
Inside Axon

8. Depolarisation Action Potential
If something happens to reduce the difference in charge across the membrane, the neurone is said to be depolarised
Action Potential
Cell surface membrane contains sodium and potassium channels which are sensitive to potential difference…
…they are voltage gated channels
When the neurone is resting, and the potential difference across the membrane is –70mV, the gated channels are closed.

9. Threshold for depolarisation =
The Sodium Channels
When the membrane is depolarised such that the potential difference across the membrane is less than –40mV, the sodium channels open
Sodium ions diffuse in down the electro-chemical gradient.
Positive charge of sodium ions further depolarise the membrane, opening more channels.
Within less than a millisecond, the potential difference across the membrane changes from –40mV inside to +40mV inside
The sodium channels then begin to close. The sodium ions stop moving
Resting potential =
-70 Mv
Threshold for depolarisation =
-40 Mv
Depolarised =
+ 40 Mv

10. The Potassium Channels
As the sodium channels close, the potassium channels open
Potassium ions flow rapidly outwards down their electro-chemical gradient.
As the potassium ions take positive charge back out of the neurone, the potential difference across the membrane switches back to a value of about –75mV inside…
… overshooting the resting value
The Potassium ion channels close, and the resting potential is restored once more.
THE WHOLE PROCESS LASTS ABOUT 4 ms!

11. -70mV +40mV -75mV - + + - + - Outside Axon Na+ K+ Na+ K+
Sodium channel
Potassium channel
Inside Axon

12. The Refractory Period
When an action potential has just taken place at a particular point in a neurone, that point is unable to produce a second action potential immediately…
…the sodium channels have closed and cannot immediately reopen
The membrane must wait until a good proportion of sodium channels are ready to reopen.
This waiting time is called the refractory period

13. Define resting potential and action potential.
Resting potential: The electrical potential difference (measured in mV) between the inside and outside of a neurone when not generating an impulse
Action potential: The localised reversal and then restoration of the electrical potential across the membrane of a cell as the impulse passes along it.
Resting potential =
-70 Mv
Threshold for depolarisation =
-40 Mv
Depolarised =
+ 40 Mv
Repolarised =
- 75 Mv

14. How the Impulse Travels
When positively charged sodium ions have rushed into a depolarised region of the neurone, they are attracted sideways…
… as the regions on either side of the depolarised region have more negative charge
A localised current or circuit is set up.
The sodium ions depolarise the adjacent regions…
…and so the action potential travels along the neurone.

15. + - Direction of Impulse
In the resting axon, there is a high concentration of sodium ions ouside the axon…
Na+ + -
The net effect is that the outside is positively charged compared to the inside, giving the…
…and a high concentration of potassium ions inside K+
AXON
…resting potential

16. Leading edge of impulse
Direction of Impulse
Leading edge of impulse
2. The action potential sets up local circuits in the axon membrane + - + -
… action potential
AXON + -
1. Axon is stimulated producing an…

17. + - - + Direction of Impulse Na+ 3. Sodium ions rush in…
…and causing an action potential + -
…depolarising the membrane… + -
AXON + -

18. + - - + Direction of Impulse + -
4. As the action potential passes along the axon… K+
… potassium ions diffuse out along a concentration gradient… + - + -
AXON + -
… starting the process of repolarisation.

19. + - - + Na+ K+ 5. The sodium-potassium pump is re-established…
Direction of Impulse K+ + - + - + - - +
… fully repolarising the membrane
AXON + -

20. Leading edge of impulse
Direction of Impulse K+
Na+ - + + -
Na+ K+ + -
Na+ K+
Na+ + - K+ - +
Na+ K+ - +
Na+ + -
Leading edge of impulse + - + -
AXON + -

21. Details of the electrical activity of neurones were discovered by studying the giant axons of squid…
… these axons have a diameter of 1mm!
giant axon
bathing saline solution
• The giant axon is placed in a bathing solution (saline)
• The potential difference across the axon’s membrane is detected by two electrodes:
microelectrode
- a microelectrode inserted into the axon.
earth electrode
- the earth electrode in the bathing solution, and;

22. oscilloscope
• The earth electrode and the microelectrode are connected to a dual beam oscilloscope
• When the tip of the microelectrode is inserted into the axon, the beams of the oscilloscope separate…
… the distance between the beams indicates the potential difference between the two electrodes.
microelectrode
• This value is called the resting potential
earth electrode
giant axon
bathing saline solution

23. oscilloscope
stimulator
• A stimulator produces a current…
… which generates an action potential in the axon
… and can be displayed on the oscilloscope.
• This action potential is detected by the microelectrode…
microelectrode
earth electrode
giant axon
bathing saline solution

24. Membrane Potential (mV)
+50
+30
+10
Membrane Potential (mV)
-10
-30
-50
-70
time (ms) 1 2 3 4 5

25. Membrane Potential (mV)
+50
depolarisation
+30
+10
repolarisation
Membrane Potential (mV)
-10
-30
resting potential restored
-50
‘overshoot’
-70
resting potential
time (ms) 1 2 3 4 5

26. How do neurones transmit information?
Action potentials in a neurone are always the same size…
…a depolarisation is either large enough to produce a full-sized action potential or not.
Action potentials are BINARY – they are either ON or OFF!
Information about the strength of stimulus is contained in the frequency of action potentials in a neurone.
The brain recognises the type of stimulus (e.g. light or sound) by the position of the neurone.

27. The best diagrams will receive a merit
To Do
Label the handout to show:
Resting potential, threshold of depolarisation, depolarised, re-polarised – include the relative voltages and what is happening to the Na-K pump/Na, K voltage gated channels
2. Alongside, draw a clear labelled diagrams to explain the propagation of an action potential:
Showing how the resting potential is set up and maintained
Showing how depolarisation will lead to an action potential
Showing how depolarisation leads to repolarisation
The best diagrams will receive a merit

28. Show me when completed and you can move onto the next task
To Do
Label the handout to show:
Resting potential, threshold of depolarisation, depolarised, re-polarised –
include the relative voltages and what is happening to the Na-K pump/Na, K voltage gated channels
2. Alongside, draw a clear labelled diagram of the cell surface membrane and its proteins channels/pumps to explain the propagation of an action potential:
Showing how the resting potential is set up/maintained
Showing how depolarisation happens
Showing how depolarisation leads to repolarisation
(L7) Showing how depolarisation leads to threshold potential being reached in adjacent neuron
Show me when completed and you can move onto the next task

29. Synapses Tasks
Info on pg 8-9 of booklet Pg 8 = BioNinja upto Level 5 Pg 9 = More in depth Level 6-7 Complete exam questions at back of booklet, see me when finished for mark scheme

30. Synaptic Transmission
Once the action potential has reached the end of the axon, it will go into a SYNAPSE: a junction with another neurone or an effector
These synapses can be chemical or electrical. You need to know about chemical synapses.