Chapter 12b

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Transcript of Chapter 12b

  • 1. Chapter 12b Neurophysiology

2.

  • Processing of sensory information and communication
    • Messages are conveyed as action potentials
    • Communication depends on membrane potentials, graded potentials and action potentials

3.

  • Resting potential :
    • transmembrane potential(TMP) of resting cell
    • Results from uneven distribution of ions across membrane
    • Usually -70mV for average neuron

5 Neural Membrane Processes 4.

  • Graded potential :
    • temporary, localized change in TMP
    • caused by stimulus
    • Generated in soma or dendrite
  • Action potential :
    • electrical impulse
    • produced by graded potential
    • moves along surface of axon to synapse

5.

  • Synaptic activity :
    • releases neurotransmitters at presynaptic membrane
    • produces graded potentials in postsynaptic membrane
  • Information processing :
    • response (integration of stimuli) of postsynaptic cell

6. Figure 127 (Navigator) 7. How is resting potentialcreated and maintained? 8.

  • Concentration gradient of ions (Na + , K + )
    • ECF has high concentration of Na+ & Cl-
    • Cytosol has high concentration of K+
  • Selectively permeable through channels
  • Maintains charge difference across membrane ( - 70 mV)

9. Figure 128 (Navigator) Product of both passive and active forces 10. PassiveForcesAcross the Membrane

  • Chemical gradients :
    • concentration gradients of ions (Na + , K + )
  • Electrical gradients :
    • potential difference across membrane
    • Slightly negative on inner surface
    • Slightly positive charge on outer surface
  • Electrochemical gradient:
    • Sum of chemical and electrical forces

11. Electrical Currentsand Resistance

  • Electrical current :
    • movement of charges to eliminate potential difference
  • Resistance :
    • the amount of current a membrane resists
    • May be altered by opening/closing channels creating a current

12. Electrochemical Gradients Figure 129a, b 13. Electrochemical Gradients Figure 129c, d 14. ActiveForcesAcross the Membrane

  • Sodiumpotassium ATPase(exchange pump):
    • are powered by ATP
    • carries 3 Na +out and 2 K +in
    • balances passive forces of diffusion
    • maintainsresting potential(70 mV)

15. Changes inTransmembrane Potential

  • Transmembrane potential rises or falls:
    • in response to temporary changes in membrane permeability
    • resulting from opening or closing specific membrane channels

16. Sodium and Potassium Channels

  • Membrane permeability to Na +and K +determines transmembrane potential
  • Sodium and potassium channels are eitherpassiveoractive

17. Passive Channels

  • Also calledleak channels
  • Are always open
  • Permeability changes with conditions

18. Active Channels

  • Also calledgated channels
  • Open and close in response to stimuli
  • At resting potential, most gated channels are closed

19. Gated Channels Figure 1210 20. 3Classesof Gated Channels

  • Chemically regulated channels :
    • open in presence of specific chemicals at a binding site
    • found on neuron cell body and dendrites

21.

  • Voltage-regulated channels :
    • respond to changes in transmembrane potential
    • characteristic ofexcitable membrane
    • found in neural axons, skeletal muscle sarcolemma, cardiac muscle

22.

  • Mechanically regulated channels :
    • respond to membrane distortion
    • found in sensory receptors (touch, pressure, vibration)

23. Graded Potentials

  • Any stimulus that opens a gated channel:
    • produces a graded potential
    • Also calledlocal potentials
  • Changes in transmembrane potential:
    • cant spread far from site of stimulation

24.

  • Opening sodium channel produces graded potential

Figure 1211 (Navigator) 25. Figure 1211 (Step 1) Graded Potentials: Step 1 26. Figure 1211 (Step 2) Graded Potentials: Step 2 27.

  • Repolarization
    • stimulus is removed, transmembrane potential returns to normal
  • Hyperpolarization
    • Increasing the negativity of the resting potential
    • Result of opening a potassium channel

28. Figure 1212 29. Effects of Graded Potentials

  • At cell dendrites or cell bodies:
    • trigger specific cell functions
  • At motor end plate:
    • releases ACh into synaptic cleft

30. What events are involvedin the generationand propagation ofan action potential? 31. Action Potentials

  • Propagated changes in transmembrane potential
  • Affect an entire excitable membrane
  • Link graded potentials at cell body with motor end plate actions

32. Initiating Action Potential

  • Initial stimulus:
    • a graded depolarization to change resting potential tothresholdlevel (60 to 55 mV)
  • All or none principle
    • stimulus exceeds threshold amount and action potential is triggered or it wont

33. Generating the Action Potential Figure 1213 (Navigator) 34. Steps of A P Generation

  • Depolarization to threshold
  • Activation of Na +channels and rapid depolarization
  • Inactivation of Na +channels, activation of K +channels
  • Return to normal permeability

35. The Refractory Period

  • time period:
    • from beginning of action potential
    • to return to resting state
    • during which membrane will not respond normally to additional stimuli
    • Absolutevs.Relative

36. Propagation of Action Potentials

  • moves along entire length of axon
    • series of repeated actions, not passive flow
  • Continuous propagation :
    • unmyelinated axons
  • Saltatory propagation :
    • myelinated axons

37. Saltatory Propagation

  • Faster and uses less energy than continuous propagation
  • Myelin insulates axon, prevents continuous propagation
  • Local current jumps from node to node
  • Depolarization occurs only at nodes

38. Comparison of graded andaction potentials 39. Graded Potential

  • Depolarizes or hyperpolarizes
  • No threshold value
  • Dependent of intensity of stimuli
  • Effect decreases with distance
  • No refractory period
  • Occurs in most cell types

40. Action Potential

  • Depolarizes only
  • Distinct threshold value
  • All or none phenomenon
  • No decrease in strength along axon
  • Refractory period occurs
  • Occurs only in excitable cells

41. What factors affectthe propagation speedof action potentials? 42. Axon Diameterand Propagation Speed

  • Ion movement is related to cytoplasm concentration
  • Axon diameter affects action potential speed
  • The larger diameter, the lower the resistance

43. 3 Groups of Axons

  • Classified by:
    • diameter
    • myelination
    • speed of action potentials
  • Type A ,Type B , andType C fibers

44.

  • Information travels within the nervous system as propagated electrical signals (action potentials)
  • The most important information (vision, balance, motor co