The transmembrane potential
• Electrochemical gradient
• Sum of all chemical and electrical forces acting
across the cell membrane
• Sodium-potassium exchange pump stabilizes
resting potential at ~70 mV
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Figure 12.11 An Introduction to the Resting
Potential
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Figure 12.11
Figure 12.12 Electrochemical Gradients
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Figure 12.12
Changes in the transmembrane potential
• Membrane contains
• Passive (leak) channels that are always open
• Active (gated) channels that open and close in
response to stimuli
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Figure 12.13 Gated Channels
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Figure 12.13
Three types of active channels
• Chemically regulated channels
• Voltage-regulated channels
• Mechanically regulated channels
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Graded potential
• A change in potential that decreases with
distance
• Localized depolarization or hyperpolarization
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Figure 12.14 Graded Potentials
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Figure 12.14.1
Figure 12.14 Graded Potentials
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Figure 12.14.2
Figure 12.15 Depolarization and
Hyperpolarization
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Figure 12.15
Action Potential
• Appears when region of excitable membrane
depolarizes to threshold
• Steps involved
• Membrane depolarization and sodium
channel activation
• Sodium channel inactivation
• Potassium channel activation
• Return to normal permeability
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Figure 12.16 The Generation of an Action
Potential
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Figure 12.16.1
Figure 12.16 The Generation of an Action
Potential
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Figure 12.16.2
Characteristics of action potentials
• Generation of action potential follows allor-none principle
• Refractory period lasts from time action
potential begins until normal resting
potential returns
• Continuous propagation
• spread of action potential across entire
membrane in series of small steps
• salutatory propagation
• action potential spreads from node to
node, skipping internodal membrane
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Figure 12.17 Propagation of an Action Potential
along an Unmyelinated Axon
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Figure 12.17
Figure 12.18 Saltatory Propagation along a
Myelinated Axon
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Figure 12.18.1
Figure 12.18 Saltatory Propagation along a
Myelinated Axon
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Figure 12.18.2
Axon classification
• Type A fibers
• Type B fibers
• Type C fibers
• Based on diameter, myelination and
propagation speed
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Muscle action potential versus neural action
potential
• Muscle tissue has higher resting potential
• Muscle tissue action potentials are longer lasting
• Muscle tissue has slower propagation of action
potentials
PLAY
Animation: The action potential
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