2. Speed of the nerve impulse
Aim: To recap work done last lesson!

3. Nerve impulses – key facts
A nerve impulse is the movement of an action potential
Nerve impulses are always a fixed size and speed in a given neurone
Speed 0.5ms-1 – 120ms-1

4. Factors affecting speed of action potential
Presence of myelin sheath
Diameter:
Temperature:

5. Presence of myelin sheath
Impulse faster with in a myelinated axon than a unmyelinated axon
What term is used to describe conduction in a myelinated axon?
saltatory conduction!

6. Diameter Larger diameter, faster impulse Why?
Larger diameter relatively less leakage of ions therefore membrane potentials easier to maintain.
Narrower = greater resistance
In thinner axons the localised circuits form in a much smaller area therefore conduction is slower.

7. Temperature
The higher the temperature the faster the impulse … Up to a point!
Why?
Higher temperature gives a faster rate of diffusion
Energy for active transport from respiration, which is dependant on enzymes therefore higher temp faster rate of reaction.
Too high a temp … enzymes and plasma membrane proteins denature … No more impulses!

8. Refractory period
After impulse sodium voltage gated channels remain closed for a period of time.
Therefore its impossible to generate another action potential

9. Purpose of refractory period
Only propagated in one direction. The action potential cannot be propagated in a region that is refractory so it can only move forwards
Ensures action potentials separated. This limits the number of action potentials in a given time

11. Extra info Actually there are 2 refractory periods!
Absolute refractory period – Lasts 1ms and no impulses can pass
Relative refractory period – Lasts 5ms. New impulse only if the stimulus is more intense than the normal threshold

12. All or nothing principle
Impulse only triggered at a certain level of stimulus, this is called the threshold value
The impulse is always the same size and speed

13. How do organisms determine the size of the stimulus?
Larger stimulus: More impulses in a given time
Different Neurones with different threshold values