The Nerve Impulse

1. The Nerve Impulse An Electrochemical Event!

2. Today we know that... • Neurons use electrical signals to communicate with other neurons, muscles and glands • These signals, called nerve impulses, involve changes in the amount of electric charge across a cell’s plasma membrane

4. Neurons • Surrounded by plasma membrane (like all cells are ) • PM is impermeable to ions

5. Resting Membrane Potential • The potential difference (voltage) across the membrane in a resting neuron • The cytoplasmic side of the membrane (inside the cell) is negative, relative to the extracellular side (outside of the cell) • - 70 mV

6. Resting Membrane Potential • Provides energy for the generation of a nerve impulse in response to an appropriate stimulus How is this membrane potential maintained?

7. A closer look at a portion of a neuron membrane... Pg 354

8. Resting Membrane Potential • Large, negatively charged proteins are present in the intracellular fluid (but not outside the cell) – too large to pass through the cell membrane • PM contains ion specific channels that allow for the movement of Na and K ions across the membrane

9. Resting Membrane Potential (K+ channels are open more often – so more K+ can move out of the cell) 3. Sodium-potassium pump actively transports Na+ and K+ in ratios that leave the inside of the cell negatively charged compared to the outside

11. Na+ K+

12. Polarization • The process of generating a resting membrane potential of – 70 mV

13. Sodium-Potassium Pump • Uses ATP to transport Na+ OUT of the cells and K+ INTO the cells Pg 355

14. Sodium-Potassium Pump • 3 Na+ OUT to 2 K+ IN

15. The Resting Neuron- Animation http://bcs.whfreeman.com/thelifewire/content/chp44/4402001.html Is the membrane potential ALWAYS – 70 mV?

16. Membrane Potential Graph Pg 357

17. Depolarization • Occurs when the cell becomes less polarized (the inside of the cell becomes less negative relative to the outside of the cell) • Caused by action potentials

18. Action Potential • The movement of an electrical impulse along the plasma membrane of an axon • Results in a rapid change in polarity across the axon membrane

19. Action Potential All or None Phenomenon • a stimulus must cause the axon membrane to depolarize to a certain level (threshold potential) for an action potential to occur • The strength of the AP does not change based on the strength of the stimulus

20. Use your text (Pg 356) to describe the events occurring above. (Make sure to include terms like hyperpolarized, repolarized, refractory period)

21. Steps of an Action Potential Depolarization 1) An AP is triggered when the threshold potential is reached 2) Voltage-gated sodium channels open • Na+ rushes into neuron and the voltage across the membrane rapidly reverses (becomes positively charged inside)

22. Steps of an Action Potential: Hyperpolarization 3) Na+ gates close (because of change in membrane potential) Many voltage gated K+ gates open • membrane potential becomes more negative again (even more than resting potential)  hyperpolarized • K+ channels close

23. Steps of an Action Potential: Repolarization 4) sodium-potassium pump and the small amount of regular diffusion bring the membrane back to it’s normal resting potential of -70mV • for a short time (a few ms) after an AP, the membrane cannot be stimulated for another AP (refractory period)

24. The Action Potential: Animation • http://www.biology4all.com/resources_library/source/63.swf

25. Movement of the AP • An action potential occurs at one spot where the neuron is stimulated, and must move down the axon in order to stimulate another neuron.

26. STEPS: How the AP moves... • Depolarization occurs – Na+ ions rush into nerve cell causing a charge reversal • Positively charged ions that rush into nerve cell are attracted to the adjacent negative ions (pg 421)

28. STEPS: How the AP moves... • Flow of positively charged ions from AP toward adjacent resting membrane causes depolarization in adjoining area • This electrical disturbance causes Na+ channels to open  movement of the AP

29. STEPS: How the AP moves... • A wave of depolarization moves down the axon, followed by a wave of repolarization (Na+/K+ pump restoring resting potential)

30. Movement of an AP: Animation • Action potentials move differently down myelinated and unmyelinated neurons • http://www.blackwellpublishing.com/matthews/actionp.html

31. The “All or None” Response • Neurons either fire maximally or not at all, this is referred to as the “all or none” response • ↑ neuronal stimulation beyond a critical level will not result in an ↑ response

32. The “All or None” Response • Neurons respond to ↑ stimulation by ↑ the frequency of firing, not the intensity at which they fire. • The Threshold level is the minimum strength of stimulation required to produce an action potential

33. Communication Between Neurons • When an AP reaches the end of the axon the impulse must be transmitted to another adjacent neuron. • This communication occurs between neurons across spaces called synapses.

34. Communication Between Neurons • The message is not transmitted electrically as it was down the axon, but rather by the release of chemical messengers.

35. Synaptic Transmission

36. Neurotransmitters • Chemicals in the end plates of axons • Released from presynaptic membrane • Diffuse across synaptic cleft • Creates depolarization of dendrites on post synaptic neuron

37. The greater the number of synapses – the slower the speed of transmission

38. Neurotransmitters • Can be excitatory or inhibitory Acetylcholine • Excitatory • Makes post synaptic membrane more permeable to Na+ • Opens Na+ channels, causes depolarization and AP

39. Neurotransmitters Cholinesterase • Released from post synaptic membrane • Destroys acetylcholine • Allows Na+ channels to close so neuron can begin recovery and a new impulse can be transmitted

40. Neurotransmitters Inhibitory Neurotransmitters • Make the post synaptic membrane more permeable to K+ • K+ rushes out of cell • These neurons are hyperpolarized – because the resting membrane is even more negative • Prevents post synaptic neurons from becoming active

41. Summation Principle Pg 424

42. Summation Principle • The production of an AP in neuron D requires the SUM of two excitatory neurons

43. Homework Pg 426 # 3-13