Neural Signaling

1. Neural Signaling Chapter 40

2. Learning Objective 1 • Describe the processes involved in neural signaling: reception, transmission, integration, and action by effectors

3. Neural Signaling 1 (1) Reception of information • by a sensory receptor (2) Transmission by an afferent neuron • to the central nervous system (CNS) (3) Integration by interneurons • in the central nervous system (CNS)

4. Neural Signaling 2 (4) Transmission by an efferent neuron • to other neurons or effector (5) Action by effectors • the muscles and glands

5. Peripheral Nervous System (PNS) • Made up of • sensory receptors • neurons outside the CNS

6. Response to Stimulus

7. External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by external sense organs Detection by internal sense organs TRANSMISSION Sensory (afferent) neurons transmit information Fig. 40-1a, p. 846

8. Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses ACTION BY EFFECTORS (muscles and glands) e.g., espiration rate increases; blood pressure rises e.g., animal runs away Fig. 40-1b, p. 846

9. External stimulus (e.g., vibration, movement, light, odor) Internal stimulus (e.g., change in blood pH or blood pressure) RECEPTION Detection by internal sense organs Detection by external sense organs TRANSMISSION Sensory (afferent) neurons transmit information Central Nervous System (brain and spinal cord) INTEGRATION Interneurons sort and interpret information TRANSMISSION Motor (efferent) neurons transmit impulses ACTION BY EFFECTORS (muscles and glands) e.g., espiration rate increases; blood pressure rises e.g., animal runs away Stepped Art Fig. 40-1, p. 846

10. KEY CONCEPTS • Neural signaling involves reception, transmission, integration, and action by effectors

11. Learning Objective 2 • What is the structure of a typical neuron? • Give the function of each of its parts

12. Neurons • Specialized to • receive stimuli • transmit electrical and chemical signals • Cell body • contains nucleus and organelles

13. Dendrites • Many branched dendrites • extend from cell body of neuron • specialized to receive stimuli and send signals to the cell body

14. Axons 1 • A single long axon • extends from neuron cell body • forms branches (axon collaterals) • Transmits signals into terminal branches • which end in synaptic terminals

15. Axons 2 • Myelin sheath • surrounds many axons • insulates • Schwann cells • form the myelin sheath in the PNS

16. Axons 3 • In the CNS • sheath is formed by other glial cells • Nodes of Ranvier • gaps in sheath between successive Schwann cells

17. Neuron Structure

18. Dendrites covered with dendritic spines Cytoplasm of Schwann cell Synaptic terminals Axon Axon collateral Cell body Nucleus Myelin sheath Nucleus Axon Nodes of Ranvier Schwann cell Terminal branches Fig. 40-2, p. 847

19. Nerves and Ganglia • Nerve • several hundred axons • wrapped in connective tissue • Ganglion • mass of neuron cell bodies in the PNS

20. Nerve Structure

21. Ganglion Cell bodies Myelin sheath Vein Axon Artery (a) Fig. 40-3a, p. 848

22. 100 µm (b) Fig. 40-3b, p. 848

23. Learn more about the structure of neurons and nerves by clicking on the figures in ThomsonNOW.

24. Learning Objective 3 • Name the main types of glial cells • Describe the functions of each

25. Glial Cells • Support and nourish neurons • Are important in neural communication

26. Glial Cell Types 1 • Astrocytes • physically support neurons • regulate extracellular fluid in CNS (by taking up excess potassium ions) • communicate with one another (and with neurons) • induce and stabilize synapses

27. Glial Cell Types 2 • Oligodendrocytes • form myelin sheaths around axons in CNS • Schwann cells • form sheaths around axons in PNS

28. Glial Cell Types 3 • Microglia • Phagocytic cells • Ependymal Cells • line cavities in the CNS • contribute to formation of cerebrospinal fluid • serve as neural stem cells

29. KEY CONCEPTS • Neurons are specialized to receive stimuli and transmit signals; glial cells are supporting cells that protect and nourish neurons and that can modify neural signals

30. Learning Objective 4 • How does the neuron develop and maintain a resting potential?

31. Neural Signals • Electrical signals transmit information • along axons • Plasma membrane of resting neuron (not transmitting an impulse) is polarized • Inner surface of plasma membrane is negatively charged • relative to extracellular fluid

32. Resting Potential • Potential difference of about -70 mV • across the membrane • Magnitude of resting potential (1) differences in ion concentrations (Na+, K+) inside cell relative to extracellular fluid (2) selective permeability of plasma membrane to these ions

33. Axon 40 20 0 –20 –40 –60 –70 mV –80 Time Amplifier Plasma membrane Electrode placed inside the cell Electrode placed outside the cell + + + – + – + + – – – – – – – – + + – – + + + + (a) Measuring the resting potential of a neuron. Fig. 40-4a, p. 850

34. Ions • Pass through specific passive ion channels • K+ leak out faster than Na+ leak in • Cl- accumulate at inner surface of plasma membrane • Large anions (proteins) • cannot cross plasma membrane • contribute negative charges

35. Sodium–Potassium Pumps • Maintain gradients that determine resting potential • transport 3 Na+ out for each 2 K+ in • Require ATP

36. Extracellular fluid 3 Na+ CI– Na+ CI– Na+ Na+ Diffusion out K+ Na+ K+ K+ Na+ Plasma membrane + + + + + + – – – – – – Na /K pump K+ K+ Na+ K+ K+ K+ Diffusion in CI– K+ 2 K+ Na+ A_ A_ A_ CI– CI– CI– Cytoplasm (b) Permeability of the neuron membrane. Fig. 40-4b, p. 850

37. KEY CONCEPTS • The resting potential of a neuron is maintained by differences in concentrations of specific ions inside the cell relative to the extracellular fluid and by selective permeability of the plasma membrane to these ions

38. Learning Objective 5 • Compare a graded potential with an action potential • Describe the production and transmission of each

39. Membrane Potential • Membrane is depolarized • if stimulus causes membrane potential to become less negative • Membrane is hyperpolarized • if membrane potential becomes more negative than resting potential

40. Graded Potential • A local response • Varies in magnitude • depending on strength of applied stimulus • Fades out • within a few millimeters of point of origin

41. Action Potential 1 • Action potential is a wave of depolarization • that moves down the axon • Generated when • voltage across the membrane declines to a critical point (threshold level) • voltage-activated ion channels open • Na+ ions flow into the neuron

42. Voltage-Activated Ion Channels

43. Extracellular fluid Activation gate Cytoplasm Inactivation gate (a) Sodium channels. (b) Potassium channels. Fig. 40-6, p. 852

44. Voltage-Activated Ion ChannelsDuring an Action Potential

45. Spike Depolarization Repolarization Membrane potential (mV) Resting state Threshold level Time (milliseconds) (a) Action potential. Fig. 40-7a, p. 853

46. Axon Extracellular fluid Potassium channel Sodium channel Cytoplasm Return to resting state. Resting state. Depolarization. Repolarization. 1 2 3 4 (b) The action of the ion channels in the plasma membrane determines the state of the neuron. Fig. 40-7b, p. 853

47. Action Potential 2 • An all-or-none response • no variation in strength of a single impulse • either membrane potential exceeds threshold level or it does not • Once begun, an action potential is self-propagating

48. Repolarization • As an action potential moves down an axon, repolarization occurs behind it

49. Transmission of an Action Potential

50. Stimulus Axon Area of depolarization Potassium channel Sodium channel Action potential (1) Action potential is transmitted as wave of depolarization that travels down axon. At region of depolarization, Na+ diffuse into cell. Fig. 40-8a, p. 854