1. Nervous coordination 2
The nerve impulse

2. NERVE IMPULSES
The nerve impulse involves the movement of ions through the axon membrane
When a neurone is not conducting an impulse it is in the Resting State and the inside has a slight negative charge
When positively charged Na ions enter, the inside is briefly positive; Depolarised, generating an Action Potential.

3. The ions causing the resting state include sodium, potassium, chloride and large negatively charged organic ions
The concs of these vary across the axon membrane due to its relative permeability to the ions
During the resting state K+ ions can move out through protein channels (gates); but negative organic ions can’t move out so, negative inside keeps K+ inside
Na+ ions are in high conc outside but membrane has low permeability with few sodium channels, those that do enter are expelled by ion pumps.
This all results in a negative (-70mV) charge inside the axon during its resting state

4. Action Potential
When nerve impulse generated, more sodium channels are opened so Na+ ions move in faster than can be expelled by pumps
The charge inside briefly becomes positive (+40mV)and an action potential caused

5. The permeability to sodium ions is quickly decreased again; simultaneously potassium channels open allowing K+ ions to flood out
This restores the negative charge inside the axon; Repolarised
The potassium channel opening causes a slight “overshoot” causing a slightly too negative charge inside the axon; Hyperpolaristaion, but soon charge stabilises at resting state

6. Transmission speeds vary from 0.5msec-1 to over 100 msec –1
So the nerve impulse is caused by a wave of depolarisation travelling along the axon from each region of membrane to the next
The ion channels are “voltage dependent” opening in response to depolarisation
Impulses jump from one Ranvier node to the next in myelinated neurones, greatly increasing the speed of transmission; Saltatory Conduction
Transmission speeds vary from 0.5msec-1 to over 100 msec –1
Controlled by:
Diameter of the axon (greater=faster)
Myelin sheath (myelinated=faster)

7. Resting potential
The inside of a resting nerve fibre is about 60 mV negative compared to the surface of the cell
This electrical potential difference across the fibre membrane (axolemma) is called the resting potential
The resting potential is maintained by differences in the concentration of ions and other charged particles between the cytoplasm and the tissue fluid around the fibre

8. Resting potential
Under normal conditions there is a greater concentration of sodium ions in the extracellular fluid than in the axon, and a greater concentration of potassium ions in the axon than outside.
As a result there is a very slow inward diffusion of sodium ions, and a slightly faster outward diffusion of potassium ions.
At rest this is balanced by the action of the sodium-potassium pump.
As a result there is a very slow inward diffusion of sodium ions, and a slightly faster outward diffusion of potassium ions
... but a greater concentration of negatively charged protein molecules in the axoplasm keeps the inside negative with respect to the outside.
There is also a very slow inward diffusion of chloride ions ...
The membrane of the resting axon contains channels (gates) specific for sodium, potassium and chloride ions. At rest, more K+ channels are open than Na+ channels. Like most animal cell membranes, it also contains the sodium-potassium pump (Na+/K+-activated ATPase
The result of all of these differences in concentration and permeability is the resting potential of -60 mV.

9. The action potential

10. The action potential
The action potential is followed by a refractory period, when the sodium gates are inactivated. During the refractory period no stimulus, however strong, can trigger another action potential.
The action potential is an all-or-nothing event: if the initial depolarisation reaches threshold value, a full action potential is triggered; if it does not, there is no action potential.
When the initial depolarisation reaches threshold, Na+ gates open: Na+ ions flood in, causing rapid depolarisation
1 ms later Na+ gates are inactivated, and K+ gates open: K+ ions flood out
The axon is hyper-polarised and refractory
Oscilloscope trace
Resting potential
Resting potential restored 1 2 3 4
Time / ms
Sodium gates inactivated
K+ gates open
Sodium gates closed
Some K+ gates open
Sodium gates reactivated and closed
Sodium gates closed
Events in the axon
-60
+45
-70
-60
Some K+ gates open
Most K+ gates open

11. The action potential: how much have you understood?
What causes this sharp rise in potential?
What causes this sharp fall in potential?
What is this potential called?
Oscilloscope trace
What is this potential called?
What is the state of the axon here? 1 2 3 4
Time / ms
Events in the axon

12. Propagation of the action potential
Repolarisation
Threshold
Local currents occur between the depolarised and resting regions of the membrane
Initial depolar-isation
At the trailing edge of the impulse local currents cannot open sodium gates because they are inactivated for the refractory period.
At the leading edge of the impulse local currents cause initial depolarisation, which opens sodium gates when it reaches threshold.
Hyperpolarised and refractory
Resting
Direction of impulse
Sodium in
Potassium out: refractory

13. Propagation of the action potential
Na+ gates behind a.p. inactivated (refractory period), preventing backward propagation.
Threshold depolarisation opens Na+ gates ahead of a.p., moving it forward.
Action potential initiated by opening sodium gates
Resting axon: sodium gates closed, some potassium gates open
Local currents propagated, causing initial depolarisation

14. Propagation of the action potential
The sodium inflow and potassium outflow at each action potential are restored by the sodium-potassium pump: the ion gradients maintained by the pump represent the energy store for action potential transmission
The sodium and potassium ions exchanged at each action potential represent about one ten-millionth of those available: even if the sodium-potassium pump is stopped by metabolic inhibitors, the axon can still transmit thousands of action potentials

15. Properties of the action potential
Action potentials are all-or-nothing: there either is one, or there isn’t; this is therefore a digital signalling system
Action potentials are self-propagating: once initiated, an action potential propagates itself along the axon using the ion gradients maintained by the sodium-potassium pump
Action potentials do not decay: unlike analogue signalling systems, there is no loss of signal strength with distance travelled

16. Properties of the action potential
Frequency of action potentials conveys information about different intensities of stimulation
Sensory neurones commonly have a resting transmission frequency: raising or lowering this frequency can therefore convey opposite kinds of information. E.g. hair cells in the inner ear:
Hair bent to right
Resting frequency
Hair bent to left

17. Properties of the action potential
The absolute refractory period following an action potential lasts about 1 ms, giving a theoretical maximum frequency of transmission approaching 1000 per second. In practice maximum frequencies observed are about 200 per second.
Unmyelinated human nerve fibres conduct action potentials at about 1-2 ms-1; squid giant axons conduct at about 20 ms-1; the largest myelinated fibres in mammals conduct at over 100 ms-1. Myelination speeds up by transmission by allowing saltatory conduction (see next slide)

18. Saltatory conduction
… so the action potential ‘jumps’ from one node to the next, greatly speeding up conduction.
Insulation by myelin allows local currents to flow only between nodes …

19. Comprehension check
Suppose that two action potentials have been generated at opposite ends of a fibre, and are travelling toward each other. What will happen when they meet?
The membrane on both sides is refractory and incapable of being depolarised: both action potentials will therefore fade out.
When the actions potentials merge, what is the state of the membrane on both sides?
Make your prediction now …