The Nerve Impulse Part 2

6,221 views
5,811 views

Published on

Published in: Business, Technology
0 Comments
3 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
6,221
On SlideShare
0
From Embeds
0
Number of Embeds
30
Actions
Shares
0
Downloads
115
Comments
0
Likes
3
Embeds 0
No embeds

No notes for slide

The Nerve Impulse Part 2

  1. 1. The nerve impulse Part 2
  2. 2. Progress of an impulse <ul><li>When an impulse reaches any point on the axon an action potential (AP) is generated </li></ul><ul><li>Small local currents occur at the leading edge of the AP </li></ul><ul><li>Sodium ions move across the membrane towards negatively charged regions. </li></ul><ul><li>This excites the next part of the axon so the AP progresses along its length </li></ul><ul><li>The local currents change the potential of the membrane, creating a new action potential ahead of the impulse. </li></ul>
  3. 3. stimulus The passage of an impulse
  4. 4. stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - +
  5. 5. stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - + Na + Na+
  6. 6. stimulus The passage of an impulse + + + + + - + + + + + - - - - - - + - - - - - + Na + Na+ local electrical circuit
  7. 7. The all or nothing law <ul><li>An AP can only be generated if the stimulus reaches a certain threshold intensity </li></ul><ul><li>Below this threshold, no AP can be created </li></ul><ul><li>Once the threshold level is reached, the size of an impulse is independent of the stimulus </li></ul><ul><li>So, a greater stimulus does not give a greater action potential. </li></ul>
  8. 8. successive stimuli
  9. 9. successive stimuli increasing intensity of stimulation
  10. 10. successive stimuli increasing intensity of stimulation threshold intensity
  11. 11. successive stimuli increasing intensity of stimulation threshold intensity below threshold intensity: no action potentials
  12. 12. successive stimuli increasing intensity of stimulation below threshold intensity: no action potentials threshold intensity
  13. 13. successive stimuli increasing intensity of stimulation below threshold intensity: no action potentials threshold intensity action potentials generated
  14. 14. The all or nothing law <ul><li>The difference between a weak and a strong stimuli is due to the frequency of the APs </li></ul><ul><li>A weak stimulus gives few APs </li></ul><ul><li>A strong stimulus gives more APs </li></ul><ul><li>… .(and is also likely to result in APs in more neurones) </li></ul>
  15. 15. The refractory period <ul><li>Following the passage of an AP, there is a time delay before the next one can pass </li></ul><ul><li>This is called the refractory period </li></ul><ul><li>During this time sodium channels in the membrane are closed, preventing the inward movement of Na + ions </li></ul><ul><li>This is known as the absolute refractory period (about 1 ms) </li></ul>
  16. 16. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms
  17. 17. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability
  18. 18. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  19. 19. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  20. 20. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus
  21. 21. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period
  22. 22. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period
  23. 23. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period normal resting excitability
  24. 24. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period relative refractory period normal resting excitability
  25. 25. neurone excitability 0 1 2 3 4 5 6 7 8 time / ms resting excitability stimulus absolute refractory period relative refractory period normal resting excitability refractory period
  26. 26. The refractory period <ul><li>The membrane starts to recover and the potassium channels open </li></ul><ul><li>Even before it is completely repolarised an AP can occur if the stimulus is more intense than the normal threshold level </li></ul><ul><li>This period is known as the relative refractory period and lasts about 5 ms. </li></ul>
  27. 27. The refractory period <ul><li>The refractory period means that impulses can only travel one way down the axon as the region behind the impulse can not be depolarised. </li></ul>
  28. 28. The refractory period <ul><li>It also limits the frequency at which successive impulses can pass along the axon </li></ul>
  29. 29. Speed of transmission <ul><li>In myelinated neurones speed of transmission is up to 100 metres per millisecond. </li></ul><ul><li>In unmyelinated neurones it is much slower at about </li></ul><ul><li>2 m ms -1. </li></ul>
  30. 30. Speed of transmission <ul><li>Myelin speeds up the speed of the impulse by insulating the axon. </li></ul><ul><li>Myelin is fatty and does not allow Na + or K + to pass through it. </li></ul><ul><li>So depolarisation (and APs) can only occur at the nodes of Ranvier. </li></ul><ul><li>So the AP ‘jumps’ from one node to the next. </li></ul><ul><li>This is known as salatory conduction . </li></ul>
  31. 31. Salatory conduction <ul><li>Advantages </li></ul><ul><li>Increase speed of transmission 100 fold. </li></ul><ul><li>Conserve energy as sodium-potassium pump only has to operate at the nodes and fewer ions have to be transported </li></ul>Nerve fibres growing through cylindrical Schwann cell formation .
  32. 32. axon myelin sheath
  33. 33. axon myelin sheath direction of impulse
  34. 34. axon myelin sheath direction of impulse + - + - + + - -
  35. 35. axon myelin sheath direction of impulse + - + - + + - - polarised depolarised
  36. 36. axon myelin sheath direction of impulse + - + - + + - - polarised depolarised local circuit
  37. 37. <ul><li>Any thing that affects the rate of respiration, such as temperature, will affect the transmission rate in a nerve. </li></ul><ul><li>This is because the restoration of the resting potential is an energy-requiring process relying upon ATP </li></ul>
  38. 38. Axon diameter <ul><li>The thicker the axon, the faster the rate of transmission. </li></ul><ul><li>Probably due to the greater surface area of the membrane over which ion exchange can occur </li></ul>
  39. 39. Axon diameter <ul><li>Giant axons found in some invertebrates (earthworms, marine annelids) are thought to be associated with rapid escape responses </li></ul>

×