2. Transmission of Nerve Impulses WALT Neurones transmit impulses as a series of electrical signals A neurone has a resting potential of – 70 mV Depolarisation causes an action potential to be transmitted along the axon

3. Resting Potential Experiments have been carried out using Giant Squid axons These are large enough to have microelectodes inserted into then to measure changes in electrical charge. One electrode is inserted into the axon and one is placed on the outside of the cell membrane

4. The difference between the two potential charges is called the resting potential The membrane of a neuron is negatively charged internally with respect to outside This generates a potential difference of around - 50 - 90 mV (resting potential) Resting Potential

6. Maintaining the Resting Potential Cation pumps (Na pumps) maintain active transport of K + ions in and Na + out of the neurone 3 Na + ions are pumped out at the same time 2 K+ ions are pumped in This is done by the Sodium Potassium ATPase pump

7. Sodium Potassium Pump

8. Diffusion back Also within the membrane are channel proteins that allow both Na + and K + ions to diffuse back down their concentration gradient However there are many more K + channels so K + ions diffuse back much faster than the Na + ions The net result is that the outside of the axon is positively charged compared to inside

10. An Action Potential Action Potential An action potential is produced when membrane of neuron stimulated, the charge is reversed: The inside of the axon was -70 mV and this changes to +40 mV and membrane is said to be depolarized

11. An Action Potential A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation Experiment show that when a small electrical current is applied to the axon the resting potential changes from – 70 mV to + 40 mV This change in potential is called the action potential

13. An Action Potential An Action Potential is produced due to a sudden increase in the permeability of the membrane to Na + : Na + ions rush into neuron through the Na+ channels to depolarize the membrane, and then further increases its permeability to Na + This leads to greater influx & further depolarization --- positive feedback

14. The Action Potential The Na+ ions move into the axon causing the charge to change to +40mV This reversal of charge causes the action potential

15. The Action Potential When inside becomes sufficiently positively charged, permeability to Na + ions start to decrease. At the same time as Na + begins to move inward, K + begins to move in the opposite direction along a diffusion gradient slowly until the membrane is repolarized.

16. An Action Potential Within about 2 milliseconds, the same portion of the membrane returns to resting potential of -70 mV inside this is called repolarisation Provided the stimulus exceeds a certain value (the threshold value), an action potential results.

17. All or none response Above the threshold value, the size of the Action Potential ( A P ) remains constant, regardless of the size of the stimulus The size of the A P does not decrease as it is transmitted along the neuron but always remains the same

18. Progression of The impulse When a nerve impulse reaches any point on the axon an action potential is generated. Small local circuits exist at the leading edge of the action potential. Sodium ions move towards the negatively charged regions. This excites the next part of the axon and so the action potential progresses

19. The Refractory Period Absolute refractory period: This lasts for about 1 msec during which no impulses can be propagated however intense the stimulusThis lasts for about 1 msec during which no impulses can be propagated however intense the stimulus Relative refractory period: This lasts for about 5 msec during which new impulses can only be generated if the stimulus is more intense than the normal threshold

21. The refractory Period The refractory period ensures that: Impulses can flow in only one direction as the region behind the impulse cannot be depolarised It limits the frequency at which successive impulses can pass along an axon.