1. THE NERVE IMPULSE
© 2016 Paul Billiet ODWS

2. Cells and membrane potentials
All animal cells generate a small voltage across their membranes
There are a large amount of small organic molecules in the cytoplasm (e.g. amino acids)
To balance this, animal cells pump Na+ out of the cells
This regulates osmosis but it leaves a large number of organic molecules
These organic molecules are overall negatively charged (anions) in the cytoplasm
Thus the cell has a potential difference (voltage) across its membrane.
© 2016 Paul Billiet ODWS

3. Experiments on the neuron of a giant squid
Ion
Concentration /mmol kg-1 water
Axoplasm
(the cytoplasm in an axon)
Blood plasma
Sea water K+
400 20 10
Na+ 50
440
460
Cl-
120
560
540
Organic anions
(-ve ions)
360 -
© 2016 Paul Billiet ODWS

4. The neuron © 2016 Paul Billiet ODWS
© 2016 Paul Billiet ODWS

5. The neuron Dendrites Myelin sheath Schwann cell
Nucleus of Schwann cell
Axon
Nodes of Ranvier
Terminal dendrites
Cell body
© 2016 Paul Billiet ODWS

6. Neurons
Neurons, like other cells, are more negatively charged inside than outside
This results in a membrane potential of about – 70 milliVolts
This is called the resting potential of the neuron.
© 2016 Paul Billiet ODWS

7. Potassium & Sodium Ions
The two important ions: K+ and Na+
Both are positively charged ions
Na+ ions move more slowly across the membrane than K+ or Cl- ions
The Na+ ion is smaller than the K+ ion (Na+ has a larger coating of water molecules giving it a bigger diameter)
This makes the plasma membrane 25 times more permeable to K+ than Na+.
© 2016 Paul Billiet ODWS

8. Potassium & Sodium Ions
K+ ions leak out a little from K+ ion pores cell is negative inside pulling K+ in but there is a very high concentration of K+ inside pulling K+ out
K+ has to be actively pumped inwards a bit
The resting potential of the neuron is almost at the equilibrium for K+ ions
K+ leak out a bit and need pumping in
Na+ ions, however, are actively pumped out and kept out.
© 2016 Paul Billiet ODWS

9. A coupled Na+-K+ pump plasma membrane K+ K+ coupled ion pump Na+ Na+
Cytoplasm
ECF K+ K+
coupled ion pump
Na+
Na+
© 2016 Paul Billiet ODWS

10. Getting excited!
The neuron’s membrane at rest is more negative inside than outside
The neuron is said to be polarised
Neurons are excitable cells
Neurons are excited when their membranes become depolarised.
© 2016 Paul Billiet ODWS

11. Depolarisation Depolarising membranes may be achieved by:
a stimulus arriving at a receptor cell (e.g. vibration of a hair cell in the ear)
a chemical fitting into a receptor site (e.g. a neurotransmitter)
a nerve impulse travelling down a neuron.
© 2016 Paul Billiet ODWS

12. Nerve impulses
Nerve impulses are self-propagating like a trail of gunpowder
Localised currents in the ions occur just ahead of the impulse causing localised depolarisation
Nerve impulses are not like electrical signals travelling down a wire.
© 2016 Paul Billiet ODWS

13. The action potential
The action potential is the state of the neuron membrane when a nerve impulse passes by.
© 2016 Paul Billiet ODWS

14. The action potential
Localised currents cause Na+ channels to flip open
Voltage-gated Na+ channels
As Na+ moves into the cell, more and more Na+ channels open
A small change in the membrane permeability to Na+ results in a big change in membrane potential
The volume of the axon is minute compared to the volume of the extracellular fluid.
© 2016 Paul Billiet ODWS

15. Time mV Resting potential Action potential
+35
More Na+ channels open Na+ floods into neuron mV
Na+ voltage-gated channels open
-55
Threshold
-70
Resting potential
Action potential
© 2016 Paul Billiet ODWS

16. All-or-nothing Na+s move in, the cell it will become more positive
Ion pumps resist the change in the membrane potential
If it rises by 15mV and the pumps cannot restore the equilibrium
Na+ floods in and neuron is depolarised
Nerve impulses all look the same, there are not big ones and little ones
This is the all-or-nothing law.
© 2016 Paul Billiet ODWS

17. The threshold –55mV represents the threshold potential
Beyond this we get a full action potential
The membrane potential rises to +35mV this is the peak of the action potential
The cells are almost at the equilibrium for Na+ ions.
© 2016 Paul Billiet ODWS

18. mV Time Resting potential Action potential Resting potential
-70
-55
+35
Threshold mV
Time
Resting potential
Action potential
Na+ channels close and K+ channels open, K+ floods out of neuron
Resting potential
© 2016 Paul Billiet ODWS

19. Potassium takes over
Na+ moves in passively until it reaches equilibrium
At the same time K+ permeability increases as voltage-gated K+ channels open –
K+ channels are a bit slower to respond to the depolarisation than the Na+ channels
K+ ions move out
The cell becomes negative inside with respect to outside again
The membrane potential falls
The cell become repolarised.
© 2016 Paul Billiet ODWS

20. Potassium ion channel
OPEN
CLOSED

21. Hyperpolarisation
The membrane potential falls below the resting potential of –70mV
It is said to be hyperpolarised
The axon is negative inside but the ion concentration is not the same
Gradually active pumping of the ions (K+ in and Na+ out) restores the resting potential
During this period no impulses can pass along that part of the membrane
This is called the refractory period.
© 2016 Paul Billiet ODWS

22. Time mV Resting potential Action potential
-70
-55
+35
Threshold
Time mV
Resting potential
Action potential
Hyperpolarisation of the membrane
Active pumping of Na+ out and K+ in during the refractory period
© 2016 Paul Billiet ODWS

23. The neuron Dendrites Myelin sheath Schwann cell
Nucleus of Schwann cell
Axon
Nodes of Ranvier
Terminal dendrites
Cell body
© 2016 Paul Billiet ODWS

24. Myelinated neurones
Non-myelinated
neuron
Myelinated neuron
In myelinated neurons the cell membrane of the Schwann cell wraps around the axon many times (myelin sheet).
© 2016 Paul Billiet ODWS

25. Saltation No depolarisation occurs under the myelin
Depolarisation only happens at the nodes (0.5μm)
All the Na+ channels are concentrated at the nodes.
© 2016 Paul Billiet ODWS

26. Saltation
An impulse is triggered by local currents that depolarise the next bit of the membrane
In myelinated nerves the triggering jumps from one node to the next
Much quicker than depolarising all the membrane along the whole axon.
© 2016 Paul Billiet ODWS

27. Grey matter and White matter
White matter = myelinated for long distance transmission
Grey matter = non-myelinated for short distance transmission
© 2016 Paul Billiet ODWS