Transmission of nerve impulses

1. Transmission of nerve impulses

2. axon

3. During a nerve impulse Na+ axon

4. During a nerve impulse K+ Na+ axon

5. Resting state More positive – outside axon Less positive - inside

6. During an action potential Na+ axon Less positive - inside

7. During an action potential Na+ axon Inside becomes DEPOLARISED

8. During an action potential Na+ axon Inside becomes DEPOLARISED Action potential is generated

9. During an action potential K+ axon Inside becomes negative again

10. Resting state More positive – outside axon Less positive - inside

11. Transmission of nerve impulses • Neurones transmit impulses as electrical signals • These signals pass along the cell surface membrane of the axon as a nerve impulse

12. Transmission of nerve impulses • It is NOT the same as an electric current passing down a wire (which is much faster) • The mechanisms is the same throughout the animal kingdom

13. Transmission of nerve impulses • Experiments have been carried out using squid axons which are big enough to have electrodes inserted in them. • One electrode can be placed inside the axon and one on its surface.

14. Transmission of nerve impulses • When at rest, the inside of the membrane has a negative electrical potential compared to the outside. • This difference in potential is called the resting potential and is typically about between -70 mV to -80 mV • In this resting state the axon is said to be polarised.

15. Transmission of nerve impulses • This is maintained because the neurone has an internal composition which is different to the outside. • Sodium ions and potassium ions are transported across the membrane against their concentration gradients by active transport

16. Transmission of nerve impulses • Carrier proteins pick up Na+ ions and transport them to the outside. • At the same time K+ ions are transported into the axon. • This is known as the sodium-potassium pump and relies on ATP from respiration

17. Transmission of nerve impulses • Inside the axon there are large numbers of negatively charged organic ions which can not move out of the axon.

19. Transmission of nerve impulses • The Na+ ions are passed out faster than the K+ ions are bought in. • Approx. three Na+ ions leave for every two K+ ions that enter. • K+ ions can also diffuse back out quicker than Na+ ions can diffuse back in. • Net result is that the outside of the membrane is positive compared to the inside.

21. Axon membrane

22. Axon membrane Inside axon

23. The action potential • A nerve impulse can be initiated by mechanical, chemical, thermal or electrical stimulation • When the axon is stimulated the resting potential changes. • It changes from –70 mV inside the membrane to +40 mV • For a very brief period the inside of the axon becomes positive and the outside negative

24. The action potential • This change in potential is called the action potential and lasts about 3 milliseconds • When an action potential occurs, the axon is said to be depolarised. • When the resting potential is re-established the axon membrane is said to be repolarised

26. depolarisation

27. depolarisation reploarisation

28. depolarisation reploarisation ‘overshoot’

29. direction of impulse depolarisation reploarisation ‘overshoot’

30. Depolarisation • When the membrane depolarises changes occur in the membrane to the permeability of both Na+ ions and K+ ions

31. Depolarisation • When the axon is stimulated, channels open on its cell surface which allow Na+ ions to pass through. • Na+ ions flood in by diffusion • The Na+ ions create a positive charge of +40 mV inside the membrane, reversing the resting potential and causing the action potential

32. Repolarisation • Potassium channels open in the membrane and K+ ions diffuse out along their concentration gradient. • This starts of repolarisation • At the same time, sodium channels in the membrane close preventing any further influx of Na+ ions.

34. Repolarisation • The resting potential is re-established as the outside of the membrane becomes positive again compared to the inside. • So many K+ ions leave that the charge inside becomes more negative that it was originally. • This shows up as an ‘overshoot’.

35. resting potential (no net ion movement)

36. Na+ start to move in resting potential (no net ion movement)

37. Na+ ions diffuse in rapidly Na+ start to move in resting potential (no net ion movement)

38. K+ ions diffuse out rapidly Na+ ions diffuse in rapidly Na+ start to move in resting potential (no net ion movement)

39. K+ ions diffuse out rapidly Na+ ions diffuse in rapidly Sodium ions pumped out potassium ions pumped in Na+ start to move in resting potential (no net ion movement)

40. Repolarisation • The potassium channels close and the sodium-potassium pump starts again. • Normal concentrations of sodium and potassium ions is re-established. • The membrane is once again at its resting potential

42. direction of impulse + + + + + + + + - - - - - - - - - - - - - - - - + + + + + + + + a) In the resting axon, there is a high conc. of Na+ ions outside and a high conc. of K+ ions inside. But the net effect is that the outside is positive compared to the inside giving the resting potential

43. Leading edge of impulse + + + + + + + - - - - - - - - + - - - - - - - + + + + + + + + - b) The axon is stimulated producing an action potential, setting up local circuits on the axon membrane

44. direction of impulse Na+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - Na+ c) Sodium ions rush into the axon along a diffusion gradient depolarising the membrane causing an action potential

45. direction of impulse K+ + + + + + + - - - - - - - - + + - - - - - - + + + + + + + + - - K+ d) As the action potential passes along the axon potassium ions diffuse out along a concentration gradient, starting off the process of repolarisation

46. direction of impulse K+ Na+ K+ + + + + + - - + - - - - - + + - - - - - - + + - + + + + + - - + K+ Na+ K+ e) The sodium-potassium pump is re-established, fully repolarising the membrane