1. Nerve Cells and Electrical Signaling

2. Chapter Outline Overview of the Nervous System
Cells of the Nervous System
Establishment of the Resting Membrane Potential
Electrical Signaling Through Changes in Membrane Potential
Maintaining Neural Stability
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3. I. Overview of the Nervous System
Figure 7.1
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4. II. Cells of the Nervous System
Neurons
Excitable cells
Glial cells
Support cells
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5. Components of a Neuron Figure 7.2
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6. Structural Classes of Neurons
Figure 7.3
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7. Functional Classes of Neurons
Figure 7.4
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8. III. Establishment of the Resting Membrane Potential
Determining the equilibrium potentials for potassium and sodium ions
Resting membrane potential of neurons
Approximately -70 mV
Exists because more negative charges inside cell and more positive charges outside cell
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9. Electrical Potentials
Table 7.1
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10. Resting Membrane Potential of Neurons
Typical neuron
Permeable to potassium and sodium
25 times more permeable to potassium
Ion distribution
Outside cell
Sodium and chloride
Inside cell
Potassium and organic anions
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11. Resting Potential: Neuron
Chemical driving forces
K+ out
Na+ in
Figure 7.8a
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12. Resting Potential: Neuron
Membrane more permeable to K+
More K+ leaves cell than Na+ enters
Inside of cell becomes negative
Figure 7.8b
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13. Resting Potential: Neuron
Electrical forces develop
Na+ into cell
K+ into cell
Due to electrical forces
K+ outflow slows
Na+ inflow speeds
Figure 7.8c
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14. Resting Potential: Neuron
Steady state develops
Inflow of Na+ is balanced by outflow of K+
Resting membrane potential = -70mV
Figure 7.8d
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15. Resting Potential: Neuron
Sodium pump maintains the resting potential
Figure 7.8e
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16. Resting Membrane Potential
The resting membrane potential is closer to the potassium equilibrium potential
+60 mV
ENa
-94 mV EK
-70 mV
Resting Vm
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17. Forces Acting on Ions
If membrane potential is not at equilibrium for an ion, then the
Electrochemical force is not 0
Net force acts to move ion across membrane in the direction that favors its being at equilibrium
Strength of the net force increases the further away the membrane potential is from the equilibrium potential
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18. Resting Potential: Forces on K+
Resting potential = -70mV
EK = -94mV
Vm is 24mV less negative than EK
Electrical force is into cell (lower)
Chemical force is out of cell (higher)
Net force is weak: K+ out of cell, but membrane is highly permeable to K+
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19. Resting Potential: Forces on Na+
Resting potential = -70mV
ENa = +60mV
Vm is 130mV less negative than ENa
Electrical force is into cell
Chemical force is also into cell
Net force is strong: Na+ into cell, but membrane has low permeability to Na+
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20. A Neuron at Rest Small Na+ leak at rest (high force, low permeability)
Small K+ leak at rest (low force, high permeability)
Sodium pump returns Na+ and K+ to maintain gradients
Figure 7.8e
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21. IV. Electrical Signaling Through Changes in Membrane Potential
Describing changes in membrane potential
Graded potentials
Action potentials
Propagation of action potentials
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22. Membrane Potential Changes
Resting potential—reference point
Depolariation
Repolarization
Hyperpolarization
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23. Membrane Potential Changes
Figure 7.9
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24. Types of Electrical Signals
Graded potentials
Small
Communicate over short distances
Action potentials
Large
Communicate over long distances
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25. Graded Potentials Initiated by a stimulus
Small change in membrane potential
Magnitude varies (graded)
Figure 7.10a
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26. Graded Potentials Some are depolarizing Some are hyperpolarizing
Figure 7.10b
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27. Purpose of Graded Potentials
Graded potentials determine whether or not an action potential will occur
Threshold
Level of depolarization necessary to elicit action potential
Excitatory
Depolarization
Inhibitory
Hyperpolarization
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28. Graded Potentials Spread by electrotonic conduction Are decremental
Magnitude decays as it spreads
Figure 7.11
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29. Graded Potentials Can Sum
Temporal summation
Same stimulus
Repeated close together in time
Spatial summation
Different stimuli
Overlap in time
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30. Temporal Summation Figure 7.12a–b
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31. Spatial Summation Figure 7.12c
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32. Summation: Cancelling Effects
Figure 7.12d
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33. Action Potentials
Excitable membranes have ability to generate action potentials
Action potential
Rapid large depolarization used for communication
In neurons
Action potentials travel along axons from cell body to axon terminal (or if afferent neuron, from receptor to terminal)
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34. Graded Versus Action Potentials
Table 7.2
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35. Phases of an Action Potential
Depolarization
Repolarization
After-hyperpolarization
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36. Phases of an Action Potential
Figure 7.13a
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37. Phases of an Action Potential
Figure 7.13b
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38. Depolarization to Threshold
Graded potentials bring membrane to threshold
Threshold triggers
Rapid opening of sodium channels
Regenerative mechanism
Slow closing of sodium channels
Slow opening of potassium channels
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39. Voltage-Gated Sodium Channel
Two gates associated with channel
Activation gate
Voltage dependent
Opens at threshold and depolarization
Positive feedback
Inactivation gate
Voltage and time dependent
Close during depolarization
Open during depolarization
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40. Sodium Channel Gating Figure 7.14
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41. Sodium and Potassium Gating
Delayed effect
(1 msec)
Open potassium
channels
Membrane
sodium
permeability
Sodium flow
into cell
Potassium flow
out of cell
Net positive
charge in cell
(depolarization)
(repolarization)
Positive
feedback
Negative
Threshold stimulus
Depolarization of membrane
Open sodium channels
Sodium channel
inactivation
gates close
potassium
Figure 7.15
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42. Sodium and Potassium Gating Summary
Table 7.3
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43. Concept of Threshold Figure 7.16
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44. All-or-None Principle
Threshold
Minimum depolarization necessary to induce the regenerative mechanism for the opening of sodium channels
Threshold depolarization  action potential
Subthreshold depolarization  no action potential
Suprathreshold depolarization  action potential
Action potential from threshold and supra-threshold stimulus are same magnitude
100 mV
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45. Refractory Period Period of time following an action potential
Marked by decreased excitability
Types
Absolute
Relative
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46. Refractory Periods Absolute
Spans all of depolarization and most of the repolarization phase
Second action potential cannot be generated
Sodium gates are inactivated
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47. Refractory Periods Relative
Spans last part of repolarization phase and hyperpolarization
Second action potential can be generated—with a stronger stimulus
Some sodium gates closed, some inactivated
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48. Causes of Refractory Periods
Figure 7.17a
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49. Causes of Refractory Periods
Figure 7.17b
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50. Causes of Refractory Periods
Figure 7.17c
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51. Consequences of Refractory Periods
All-or-none principle
Frequency coding
Unidirectional propagation of action potentials
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52. Frequency Coding Figure 7.18a
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53. Frequency Coding Figure 7.18b
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54. Conduction: Unmyelinated
Unmyelinated axon +
Extracellular fluid
Resting
Plasma
membrane
Intracellular fluid –
Initiation
Site of
original
action
potential
Site A
Region of
depolarization
Site B
Axon hillock
Direction of action potential propagation
Propagation
Refractory
state
Site C
repolarization
(resting state)
Site D
continues
(a)
(b)
(c)
(d)
Figure 7.19
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55. Factors Affecting Propagation
Refractory period
Unidirectional
Axon diameter
Larger
Less resistance, faster
Smaller
More resistance, slower
Myelination
Saltatory conduction
Faster propagation
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56. Conduction: Myelinated Fibers+
Myelinated axon
Myelin
sheath
Node of
Ranvier
Extracellular fl
uid
Direction of action potential propagation
Intracellular –
Axon hillock
Figure 7.20
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57. Conduction Velocity Comparisons
Table 7.4
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58. V. Maintaining Neural Stability
Graded potentials and action potentials tend to dissipate Na+ and K+ concentration gradients
But only small percent of ions actually move
Na+ and K+ pump prevents dissipation
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