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Chapter 7
Nerve Cells and Electrical Signaling
1.
Overview of the nervous system 神經系統概論
2.
Cells of the nervous system 神經系統的細胞
3.
Establishment of the resting membrane potential
靜止膜電位的建立
4.
Electrical signaling through changes in membrane potential
經由膜電位的改變產生電訊息
5.
Maintaining neural stability 維持神經的穩定性
I. Overview of the Nervous System
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 7.1 Organization of the nervous system. The nervous system has two
main parts: the central nervous and the peripheral nervous system. The
peripheral nervous system is functionally divided into afferent and efferent
divisions. Arrows indicate the direction of information flow.
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II. Cells of the Nervous System
Neurons
 Soma 細胞本體  contains nucleus and most organelles
 Dendrites 樹突  reception of incoming information
 Axon 軸突  transmits electrical impulses called action
potentials
 Axon hillock 軸突丘  where axon originates and action
potentials initiated
 Axon terminal 軸突末梢  releases neurotransmitter
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Structure of Typical Neurons
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Inc., publishing as Benjamin Cummings.
 Site of communication
between two neurons
or between a neuron
and an effector organ
 synapse 突觸；聯會
Figure 7.2 Structure of a typical neurons. Two neurons are shown; the upper
neuron communicates with the lower neuron, as indicated by the arrows representing
information flow. The main parts of a neuron include the cell body (soma); dendrites,
which receive communication from other neurons; and an axon, which is specialized
for transmitting electrical impulses. The axon terminals of one neuron release a
chemical messenger (neurotransmitter) that communicates with another neuron.
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Localization of Ion Channels in Neurons
 Leak channels
 always open
 which is found in the plasma membrane throughout a neuron
 are responsible for the resting membrane potential
 Ligand-gated channels
 open or close in response to the binding of a chemical messenger to a
specific receptor in the plasma membrane
 are most densely located in the dendrites and cell body that receive
communication
 Voltage-gated channels
 open or close in response to change in membrane potential
 sodium & potassium channel are more densely in the axon that are
necessary for initiation and propagation of action potentials
 calcium channels are more densely in the axon terminals that are necessary
for the release of neurotransmitter
P168-169
Structural Classes of Neurons

Multipolar neurons, the
most common neurons,
have multiple projections
from the cell body, one
projection is an axon & all
the others are dendrites
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 7.3 Structural classes of neurons. (a) A bipolar neuron. Afferent neurons
associated with vision and olfaction are bipolar neuron, which are generally sensory
neurons with two projections, an axon and a dendrite, coming off the cell body. (b) A
pesudo-unipolar neuron. The vast majority of afferent neurons are pseudo-unipolar,
because the axon and dendrite projections appear as a single process that extends in
two directions from the cell body. (c) A multipolar neuron. Most neurons are multipolar
neurons.
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Functional Classes of Neurons

There are three functional classes of neurons: efferent neurons, afferent neurons,
and interneurons

Efferent neurons originate in the central nervous system (CNS), where the cell body
and dendrites receive synaptic communication from other neurons

Efferent axons are part of the peripheral nervous system (PNS) and terminate at a
synapse with an effector organ

Efferent neurons  transmit information from the CNS to effector organs

Afferent neurons originate in the periphery with sensory or visceral receptors 
transmit information from the receptors to CNS for further processing

The peripheral axons of afferent neurons are part of the PNS, but the axon terminals
are located in the CNS, where they communicate with other neurons

The third functional class of neurons is interneurons which account for 99% of all
neurons in the body

Interneurons lie entirely in the CNS and can communicate with afferent neurons,
efferent neurons, or other interneurons
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Functional Classes of Neurons
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
Figure 7.4 Functional classes of neurons. Afferent neurons originate in the periphery with
sensory or visceral receptors. The peripheral axons of afferent neurons are part of the peripheral
nervous system, but the axon terminals are located in the central nervous system, where they
communicate with other neurons. Efferent neurons originate in the central nervous system, where
the cell body and dendrites receive synaptic communication from other neurons. Efferent axons,
however, are part of the peripheral nervous system and terminate at a synapse with an effector
organ. Interneurons lie entirely in the central nervous system and can communicate with afferent
neurons, efferent neurons, or other interneurons.
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Structural organization of neurons
 In the CNS 中樞神經系統, cell bodies of neurons are often
grouped into nuclei (nucleus) 神經核, and the axons travel
together in the bundles called pathways 路徑, tracts, or
commissures
 In the PNS 末梢神經系統, cell bodies of neurons are clustered
together in ganglia (ganglion) 神經節, and the axons travel
together in bundles called nerves 神經纖維
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Glial Cells
 Glial cells, the second class of cell found in the nervous system,
account for 90% of all cells in the nervous system
 They provide structural integrity to the nervous system and are
necessary for neurons to carry out their functions
 There are five types of glial cells: astrocytes, ependymal cells,
microglia, oligodendrocytes, and Schwann cells
 Of these glial cells, only Schwann cells are located in the PNS, the
rest are in the CNS
 The primary function of oligodendrocytes and Schwann cells is to
form an insulating wrap of myelin around the axons of neurons
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Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
Figure 7.5 Formation and origins of myelin sheaths. (a) Formation of a myelin sheath by a
Schwann cell. Myelin, which consists of concentric layers of plasma membrane provided by either a
Schwann cell or an oligodendrocyte, forms a layer of insulation around an axon. (b) Arrangement of
myelin sheaths form by oligodendrocytes in the central nervous system. A single oligodendrocyte
sends out cytoplasmic processes that form myelin sheaths around several axons. Note the nodes
of Ranvier, gaps in the myelin sheaths. (c) Arrangement of myelin sheaths formed by Schwann cells
in the peripheral nervous system. A given Schwann cell sheaths only a single axon.
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III. Establishment of the Resting
Membrane Potential
 Resting membrane potential 靜止膜電位--a cell at rest has
a potential difference across its membrane such that the
inside of the cell is negatively charged relative to the outside
 The resting membrane potential of neurons is approximately
-70 mV
 Neurons communicate by generating electrical signals in the
form of changes in membrane potential
P172
Table 7.1 Types of Electrical Potentials in Biological systems
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
P172
Determining the Equilibrium Potentials
for Potassium and Sodium Ions
 The resting membrane potential depends on two critical factors:
 the concentration gradients of ion (particularly sodium ions and
potassium ions) across the plasma membrane
 the presence of ion channels in the plasma membrane
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 20% of resting membrane potential directly due to Na+/K+-ATPase
 electrogenic: 3 Na+ out, 2 K+ in  net +1 out
 80% of resting membrane potential indirectly due to Na+/K+-ATPase
 produces concentration gradients  Na+: high outside, low inside &
K+: low outside, high inside
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Membrane Potential of a Cell Permeable Only to Potassium
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Inc., publishing as Benjamin Cummings.
Figure 7.6 Membrane potential of a cell permeable to potassium only. Potassium (K+)
and organic anions (A-) are located in greater concentration inside the cell. Sodium (Na+) and
chloride (Cl-) ions are located in greater concentration outside the cell. The width of an arrow
is relative to the strength of ion movement in the direction of the arrow. (a) Potassium ions
move out of the cell because of a chemical force. (b) As some potassium ions leave the cell,
taking with them a positive charge, the inside of the cell becomes negative relative to the
outside. This change in charge distribution creates an electrical force to move potassium ions
into the cell, opposing the chemical force. (c) Eventually, enough potassium leaves the cell
that the electrical force becomes strong enough to oppose further movement of potassium
ions out of the cell because of the chemical force, resulting in no net movement of potassium
ions. At this membrane potential, potassium is at equilibrium. This potential is thus the
potassium equilibrium potential and is approximately -94 mV in neurons.
P175
Membrane Potential of a Cell Permeable Only to Sodium
Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
Figure 7.7 Membrane potential of a cell freely permeable to sodium only. Potassium
(K+) and organic anions (A-) are located in greater concentration inside the cell. Sodium (Na+)
and chloride (Cl-) ions are located in greater concentration outside the cell. The width of an
arrow is relative to the strength of ion movement in the direction of the arrow. (a) Sodium ions
move into of the cell because of a chemical force. (b) As some sodium ions enter the cell,
taking with them a positive charge, the inside of the cell becomes positive relative to the
outside. The change in charge distribution creates an electrical force to move sodium ions out
of the cell, opposing the chemical force. (c) Eventually, enough sodium enter the cell that the
electrical force becomes strong enough to oppose further movement of sodium ions into the
cell because of the chemical force, resulting in no net movement of sodium ions. At this
membrane potential, sodium is at equilibrium. This potential is thus the sodium equilibrium
potential and is approximately +60 mV in neurons.
P176
Resting Membrane Potential of Neurons
Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
Figure 7.8 Establish a steady state resting membrane potential. Potassium (K+) and
organic anions (A-) are located in greater concentration inside the cell. Sodium (Na+) and
chloride (Cl-) ions are located in greater concentration outside the cell. The width of an arrow
is relative to the strength of ion movement in the direction of the arrow. The cell is permeable
to both sodium and potassium ions, but more permeable to potassium. (a) Chemical force act
on potassium ions to leave the cell and sodium ions to enter the cell. (b) More potassium
leaves the cell than sodium enters because of the greater permeability for potassium. With
more positive charge leaving the cell, a negative membrane develops. (c) Electrical forces
now act on the ions, drawing both sodium and potassium ions into the cell, creating a
stronger electrochemical force for sodium to enter the cell and a weaker electrochemical
force for potassium to leave the cell.
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Resting Membrane Potential of Neurons
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Figure 7.8 Establish a steady state resting membrane potential. (d) Eventually, a
steady state is established, whereby the movement of sodium into the cell is balanced
by the movement of potassium out of the cell, and no net charge -70 mV in neurons.
(e) To prevent the sodium and potassium concentration gradients from dissipating, the
Na+/K+ pump moves Na+ out of the cell and K+ into the cell, establishing a steady state
at -70 mV.
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Neurons at Rest
 The membrane potential depends
on the relative permeabilities of the
membrane to the different ions
+60 mV
ENa
 The strength of the electrochemical
force acting on a specific ion is
proportional to the difference
between the membrane potential
and the equilibrium potential for
that ion
-70 mV
 The sodium and potassium
channels that are responsible for
the resting membrane potential are
leak channels, which are always
open  in addition to these leak
channels, neurons also have gated
ion channels.
-94 mV
Resting Vm
EK
Resting Membrane Potential Closer
to Potassium Equilibrium Potential
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Inc., publishing as Benjamin Cummings.
P178-179
IV. Electrical Signaling Through Changes
in Membrane Potential
Describing Changes in Membrane Potential
Figure 7.9 Changes in membrane potential.
The membrane potential can change with
opening or closing of ion channels. A change in
membrane potential to a less negative value is
called depolarization. The return of a
depolarized membrane to the resting membrane
potential is called repolarization. A change in
membrane potential to a more negative value is
called hyperpolarization. If gated K+ channels
open, then K+ move out the cell, bringing the
membrane potential toward potassium
equilibrium potential (EK), or hyperpolarizing the
cell. If gated Na+ channels open, then Na+ move
into the cell, bringing the membrane potential
toward the sodium equilibrium potential potential
(ENa), or depolarzing the cell.
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publishing as Benjamin Cummings.
P179
Describing Changes in Membrane Potential
 Neurons communicate via two different types of
electrical signals that result from the opening or closing
of gated ion channels: graded potentials & action
potentials
 Graded potentials, which are small electrical signals
that act over short ranges only because they diminish in
size with distance
 Action potentials, which are large signals capable of
traveling long distances without decreasing in size
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Graded Potentials
 Graded potentials are small changes in
membrane potential that occur when ion
channels open or close in response to a
stimulus acting on the cell
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publishing as Benjamin Cummings.
 A weak stimulus (eg, 500 molecules)
produces a small change in membrane
potential (eg, 5 mV), whereas a stronger
stimulus (eg, 1000 molecules) produces a
greater change in membrane potential (eg,
10 mV)
 Depending on the stimulus and neuron,
graded potential can cause either
hyperpolarization (eg, K+ channel) or
depolarization (eg, Na+)
Figure 7.10 Properties of
graded potential.
 The primary significance of graded potentials is that they determine whether
or not a cell will generate an action potential  if they depolarize a neuron to
a certain level of membrane potential called threshold, a critical valve of
membrane potential that must be met or exceeded if an action potential is to
be generated
P180-181
Graded Potentials are Decremental
 A graded potentials dissipates
as it moves to adjacent areas
of the plasma membrane
(solid arrows) and across the
plasma membrane (dashed
arrows)  because a
membrane has leaks to ions
Copyright © 2008 Pearson Education, Inc., publishing as Benjamin Cummings.
 Therefore, the potential
change recorded at the site of
stimulus is greater than that
recorded distant from the site
of stimulus
Figure 7.11 Decremental property of graded
potential.
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Graded Potentials Can Sum
 A single graded potential is generally not of sufficient strength
to elicit an action potential  if graded overlap in time, then
they can sum, both temporally and spatially
 Temporal Summation
–
Same stimulus
–
Repeated close together in time
 Spatial Summation
–
Different stimuli
–
Overlap in time
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Temporal Summation
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Education, Inc., publishing as
Benjamin Cummings.
Figure 7.12 Temporal and spatial summation.
 Stimulus W and X depolarize the cell; Y hyperpolarizes the cell (a)
 Temporal summation of stimulus W (same stimulus) resulting in
depolarization to above threshold and generation of an action
potential (b)
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Spatial Summation of Depolarization
 Stimulus W and X depolarize the
cell; Y hyperpolarizes the cell
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publishing as Benjamin Cummings.
 Spatial summation of stimulus W
and X (different stimuli) resulting
in depolarization to above
threshold and generation of an
action potential (c)
 Spatial summation of stimulus W
and Y (different stimuli) resulting
in no change in membrane
potential and therefore no action
potential (d)
Figure 7.12 Temporal
and spatial summation. P182
Action Potentials
 Excitable membranes have ability to
generate action potentials
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Education, Inc., publishing as
Benjamin Cummings.
 Action potential = rapid large
depolarization used for communication
 In neurons--action potentials travel along
axons from cell body to axon terminal
(long distance) without any decrease in
strength
 An action potential in a neuron consists
of three phase:
 depolarization (phase 1)
 repolarization (phase 2)
 after-hyperpolarization (phase 3)
Figure 7.13 The phases
and ionic basis of an
action potential.
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P183
Ionic Basis of an Action Potential
 The rapid depolarization of phase 1
is caused by a rapid Na+ permeability
increases that enables Na+ to move
into the cell  -70 mV to +30 mV
Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
 The repolarization of phase 2 is
caused by a slower increase in K+
permeability, enabling greater
movement of K+ out of the cell
compared to resting conditions
 +30 mV to -70 mV
 The after-hyperpolarization of phase 3
is caused by the continuing
movement of K+ out of the cell
 -70 mV to -94 mV, then to -70 mV
Figure 7.13 The phases and ionic
basis of an action potential.
P182-183
The Role of Voltage-Gated Ion Channels in Action
Potentials
Sodium Channels  two gates
 Activation gates  voltage-dependent; open with
depolarization; positive feedback
 Inactivation gates  voltage-dependent; time-dependent;
close with depolarization; open with repolarization
Potassium Channels  one gate
 Voltage- and time-dependent; negative feedback
P184-185
Sodium Channel Gating

At rest, the
inactivating gate is
open and the activation gate is
closed but can open in response to
a depolarizing stimulus

Following a depolarization stimulus
to threshold, both the activation and
inactivation gates are open, and
Na+ can move through the channel

Approximately 1 msec after a
depolarizing stimulus, the
inactivation gate closes and
remains closed until the cell has
repolarized to the resting state

Before repolarization the channel
cannot open in response to a new
depolarizing stimulus
Na+
Copyright © 2008 Pearson Education,
Inc., publishing as Benjamin Cummings.
Figure 7.14 A model for the operation
of voltage-gated sodium channels.
P184
Figure 7.15 Gating of sodium and
potassium channels during an action
potential. Sodium channel opening is
part of a positive feedback loop that
allows for the rapid depolarization of the
cell. When the cell is depolarized to
threshold, sodium channels open.
Opening allows sodium to move into the
cell, causing further depolarization and
opening more sodium channels. The
feedback loop continues until the
sodium inactivation gates close,
approximately 1 msec after the
depolarization to threshold. Potassium
channel opening and closing is part of a
negative feedback loop. Depolarization
stimulate the slow open of potassium
channels. This allow potassium to move
out of the cell, repolarizing it. Since
repolarization opposes the depolarizing
stimulus for opening potassium
channels, the potassium channels close.
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Inc., publishing as Benjamin Cummings.
P186
P188
Concept of Threshold
 A stimulus must reach a critical
level of depolarization--threshold-before an action potential is
generated
Copyright © 2008 Pearson Education, Inc.,
publishing as Benjamin Cummings.
 A stimulus less than threshold (a
sub-threshold stimulus) cannot
generate an action potential
 Any stimulus greater than
threshold (a supra-threshold
stimulus) generates an action
potential of the same magnitude
and duration as a threshold
stimulus
Figure 7.16 The concept of a
threshold stimulus.
P187
All-or-None Principle
 Initiation of action potentials follows the all-or-none principle: whether a
membrane is depolarized to threshold or above, the amplitude of the
resulting action potential is the same; if the membrane is not
depolarized to threshold, no action potential occurs
 Threshold = minimum depolarization necessary to induce the
regenerative mechanism for the opening of sodium channels
 Threshold depolarization  action potential
 Sub-threshold depolarization  no action potential
 Supra-threshold depolarization  action potential
 Action potential from threshold and supra-threshold stimulus are same
magnitude – 100 mV
P185
Refractory Periods
 During and immediately after an action potential, the membrane is
less excitable than it is at rest  this period of reduced excitability is called
the refractory period (RP)
 The RP can be divided into two phases, the absolute RP and the relatively RP
 Absolute refractory period
–
Immediately follows action potential
–
No action potential possible
–
Na+ channels open & inactivated
 Relative refractory period
–
Follows absolute refractory period
–
Action potential possible with
stronger stimulus
–
Some Na+ channels still inactivated
& High PK
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Inc., publishing as Benjamin Cummings.
Figure 7.17 Refractory periods
associated with an action potential.
P185,187
Causes of Refractory Periods
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Figure 7.17 Refractory periods associated with an action potential.
 There are two reasons for the absolute RP:
— during the rapid depolarization phase, the regenerative opening of Na+ channels
that has been set into motion will not be affected by a second stimulus
— during the beginning of the repolarization phase, most of the Na+ inactivation
gates are closed and cannot be opened by a second stimulus
 A second action potential cannot be generated until the majority of the Na+
channels have returned to their resting state, a situation that occurs near the
end of the repolarization phase
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Consequences of Refractory Periods
The RP establish several properties of action potential:
 All-or-none principle
— action potential cannot sum, because the absolute RP prevents an overlap of
action potentials
 Frequency coding of information
— information pertaining to stimulus intensity is encoded by changes in the
frequency of action potentials--that is, changes in the number of action
potentials that occur in a given period of time
 Unidirectional propagation of action potentials
— the RP prevents action potentials from traveling backward, ensuring
unidirectional propagation of action potentials
P187-188
Long Duration Stimulus Can Produce More
Than One Action Potential
 The entire refractory period for this
neuron is 15 msec
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Benjamin Cummings.
 The sub-threshold stimulus does not
generate an action potential
 A 10-msec threshold stimulus
generates a single action potential,
whereas a 20-msec threshold stimulus
(longer than the refractory period)
generates a second action potential
 A threshold stimulus generates a single
action potential, whereas suprathreshold stimuli generate a burst of
action potentials
 The stronger of the supra-threshold
stimuli generates a higher frequency of
action potentials
Figure 7.18 Frequency coding: how action potentials convey intensity of stimuli.
P189
Propagation of Action Potentials
 Once an action potential is initiated in an axon, it is propagated down the
length of the axon from the trigger zone to the axon terminal without
decrement
 An action potential does not actually travel down the axon; instead, an
action potential sets up electrochemical gradients in the extracellular and
intracellular fluids
 The first action potential produced at the trigger zone produces current
that causes a second action potential in the adjacent membrane  third
 and so on until an action potential is produced at the axon terminal
 Current flows to adjacent areas of the axon’s plasma membrane by
electrotonic conduction
 The propagation mechanisms differ, however, depending on where the
axon is unmyelinated or myelinated
P189
Conduction in Unmyelinated Axons: Initiation of
Action Potential
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Benjamin Cummings.
Figure 7.19 Action potential conduction in unmyelinated axons.
 State of the membrane at the resting membrane potential (a)
 When an action potential occurs at site A on the membrane, there is
separation of charge in the intracellular fluid and extracellular fluid  the
charge separation is a force for current to move
 The local currents produced by positive ions moving toward the negative
regions of the intracellular fluid are show by arrow
P189-190
Conduction in Unmyelinated Axons: Propagation of
Action Potential
 The current depolarizes the
adjacent region of membrane
(site B) to threshold, eliciting an
action potential there (b)
 Depolarization of adjacent
regions continues until the action
potential has been propagated
all the way to the axon terminal
 Refractory periods prevent
action potentials from traveling in
the reverse direction
Copyright © 2008 Pearson Education, Inc., publishing as
Benjamin Cummings.
Figure 7.19 Action potential conduction in unmyelinated axons.
P189-190
Factors Affecting Propagation
 Refractory periods
 The refractory period prevents action potentials from traveling
backward, ensuring unidirectional propagation of action
potentials
 Fiber diameter
 The larger the diameter, the less resistance to longitudinal
current flow
 Action potentials are propagated more quickly from axon
hillock to axon terminal in large-diameter axons
P190
Saltatory Conduction in Myelinated Axons
 An action potential in a myelinated axon
produces electrical gradients in the
intracellular and extracellular fluids that
are similar to those observed in
unmyelinated axons
 Because very little current flows across
the membrane where myelin insulates it,
the current must flow all the way to the
next node of Ranvier, where it
depolarizes this area of the membrane to
threshold and initiates an action potential
 The jumping of action potentials from
node to node is the basis of the term
saltatory conduction
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Benjamin Cummings.
Figure 7.20 Saltatory conduction in myelinated axons.
P190-191
Conduction Velocities in Axons of Various Nerve
Fiber Types
 Because action potentials can move in large jumps in myelinated axons,
conduction velocities in myelinated axons are greater than those in
unmyelinated axons
 The fastest conducting axons are both large-diameter and myelinated
 are generally found in pathways requiring quick action
 control skeletal muscle contractions
P191
V. Maintaining Neural Stability
 Graded potentials and action potentials tend to
dissipate Na+ and K+ concentration gradients
 Na+/K+ pump in the context of its importance in
developing concentration gradients and
sustaining the gradients when the cell is at rest
prevents dissipation
P191-192