Download Nerve Cells and Electrical Signaling

PRINCIPLES OF
HUMAN PHYSIOLOGY
7
THIRD EDITION
Cindy L. Stanfield | William J. Germann
Nerve Cells and
Electrical
Signaling
PowerPoint® Lecture Slides prepared by W.H. Preston, College of the Sequoias
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Chapter Outline
I. Overview of the Nervous System
II. Cells of the Nervous System
III. Establishment of the Resting Membrane
Potential
IV. Electrical Signaling Through Changes in
Membrane Potential
V. Maintaining Neural Stability
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I. Overview of the Nervous System
Figure 7.1
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II. Cells of the Nervous System
• Neurons
•
Excitable cells
• Glial cells
•
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Support cells
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Components of a Neuron
Figure 7.2
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Structural Classes of Neurons
Figure 7.3
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Functional Classes of Neurons
Figure 7.4
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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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Electrical Potentials
Table 7.1
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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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Resting Potential: Neuron
• Chemical driving
forces
•
K+ out
•
Na+ in
Figure 7.8a
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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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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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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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Resting Potential: Neuron
• Sodium pump
maintains the resting
potential
Figure 7.8e
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Resting Membrane Potential
The resting membrane potential is closer to the
potassium equilibrium potential
+60 mV
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ENa
-70 mV
Resting Vm
-94 mV
EK
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Forces Acting on Ions
• If membrane potential is not at equilibrium for an
ion, then the
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•
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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Resting Potential: Forces on K+
• Resting potential = -70mV
• EK = -94mV
• Vm is 24mV less negative than EK
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•
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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Resting Potential: Forces on Na+
• Resting potential = -70mV
• ENa = +60mV
• Vm is 130mV less negative than ENa
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•
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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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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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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Membrane Potential Changes
• Resting potential—reference point
• Depolariation
• Repolarization
• Hyperpolarization
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Membrane Potential Changes
Figure 7.9
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Types of Electrical Signals
• Graded potentials
•
Small
•
Communicate over short distances
• Action potentials
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•
Large
•
Communicate over long distances
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Graded Potentials
• Initiated by a stimulus
• Small change in membrane potential
• Magnitude varies (graded)
Figure 7.10a
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Graded Potentials
• Some are depolarizing
• Some are hyperpolarizing
Figure 7.10b
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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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Graded Potentials
• Spread by
electrotonic
conduction
• Are decremental
•
Magnitude decays
as it spreads
Figure 7.11
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Graded Potentials Can Sum
• Temporal summation
•
Same stimulus
•
Repeated close together in time
• Spatial summation
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•
Different stimuli
•
Overlap in time
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Temporal Summation
Figure 7.12a–b
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Spatial Summation
Figure 7.12c
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Summation: Cancelling Effects
Figure 7.12d
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Action Potentials
• Excitable membranes have ability to generate
action potentials
• Action potential
•
Rapid large depolarization used for communication
• In neurons
•
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Action potentials travel along axons from cell body to
axon terminal (or if afferent neuron, from receptor to
terminal)
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Graded Versus Action Potentials
Table 7.2
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Phases of an Action Potential
• Depolarization
• Repolarization
• After-hyperpolarization
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Phases of an Action Potential
Figure 7.13a
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Phases of an Action Potential
Figure 7.13b
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Depolarization to Threshold
• Graded potentials bring membrane to threshold
• Threshold triggers
•
Rapid opening of sodium channels
• Regenerative mechanism
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•
Slow closing of sodium channels
•
Slow opening of potassium channels
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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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Sodium Channel Gating
Figure 7.14
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Sodium and Potassium Gating
Threshold stimulus
Depolarization of membrane
Open sodium channels
Positive
feedback
Net positive
charge in cell
(depolarization)
Membrane
sodium
permeability
Sodium flow
into cell
Delayed effect
(1 msec)
Sodium channel
inactivation
gates close
Membrane
sodium
permeability
Delayed effect
(1 msec)
Open potassium
channels
Membrane
potassium
permeability
Potassium flow
out of cell
Sodium flow
into cell
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Negative
feedback
Net positive
charge in cell
(repolarization)
Figure 7.15
Sodium and Potassium Gating Summary
Table 7.3
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Concept of Threshold
Figure 7.16
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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 suprathreshold stimulus are same magnitude
•
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100 mV
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Refractory Period
• Period of time following an action potential
• Marked by decreased excitability
• Types
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•
Absolute
•
Relative
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Refractory Periods
• Absolute
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•
Spans all of depolarization and most of the
repolarization phase
•
Second action potential cannot be generated
•
Sodium gates are inactivated
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Refractory Periods
• Relative
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•
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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Causes of Refractory Periods
Figure 7.17a
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Causes of Refractory Periods
Figure 7.17b
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Causes of Refractory Periods
Figure 7.17c
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Consequences of Refractory Periods
• All-or-none principle
• Frequency coding
• Unidirectional propagation of action potentials
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Frequency Coding
Figure 7.18a
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Frequency Coding
Figure 7.18b
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Conduction: Unmyelinated
Extracellular fluid
Axon hillock
Unmyelinated axon
Plasma
membrane
(a) Resting
Site of
original
action
potential
+
Extracellular fluid
+ +
–
+ + + + + + + + + + + + + + + + +
– –
– – – – – – – – – – – – – – – – –
Intracellular fluid
– – – – – – – – – – – – – – – – –
– –
–
+ + + + + + + + + + + + + + + + +
+ +
Extracellular fluid
+
+
+ +
–
+ – – – –
– –
– + + + +
Site A
– + + + +
–
–
–
+ – – – –
+ +
+
+ + + +
– – – –
Site B
– – – –
+ + + +
+ + + + + + + +
– – – – – – – –
– – – – – – – –
+ + + + + + + +
Region of
depolarization
(b) Initiation
Direction of action potential propagation
+
+ +
–
+ + + + +
– –
– – – – –
Site A
– – – – –
– –
–
+ + + + +
+ +
+
(c) Propagation
– – – –
+ + + +
Site B
+ + + +
– – – –
+ + + +
– – – –
Site C
– – – –
+ + + +
+ + + +
– – – –
– – – –
+ + + +
RefractoryRegion of
state
depolarization
+
+ +
+ + + + +
– –
– – – – –
Site A
– – – – –
–
–
–
+ + + + +
+ +
+
–
(d) Propagation
continues
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+ + + +
– – – –
Site B
– – – –
+ + + +
– – – –
+ + + +
Site C
+ + + +
– – – –
+ + + +
– – – –
Site D
– – – –
+ + + +
Region of
Refractory Region of
depolarization
repolarization state
(resting state)
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Figure 7.19
Factors Affecting Propagation
• Refractory period
•
Unidirectional
• Axon diameter
•
Larger
• Less resistance, faster
•
Smaller
• More resistance, slower
• Myelination
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•
Saltatory conduction
•
Faster propagation
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Conduction: Myelinated Fibers
Extracellular
fl uid
Axon hillock
Myelinated axon
Myelin
sheath
+
Node of
Ranvier
+ + + + + + +
+ + + + + + +
– – –
+ + +
+
– + + + – – – – – – – – – – – – – – – – – –
Intracellular
– + + + – – – – – – – – – – – – – – – – – –
– – –
+ + +
+
+
+ + + + + + +
+ + + + + + +
Extracellular
+
+ +
– – –
fl uid
– – –
+ +
+
fl uid
Direction of action potential propagation
+
+ + + + + + +
+ + + + + + +
+
+ + +
– – –
+ + +
– – – – – – – – – – – ++ + – – – – – – – – – – –
– – – – – – – – – – – ++ + – – – – – – – – – – –
+ + +
+ + +
– – –
+
+ + + + + + +
+ + + + + + +
+
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Figure 7.20
Conduction Velocity Comparisons
Table 7.4
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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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