KEY CONCEPTS 48.1 Neuron structure and organization reflect function in information transfer 48.2 Ion pumps and ion channels establish the resting potential of a neuron 48.3 Action potentials are the signals conducted by axons 48.4 Neurons communicate with other cells at synapses

1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Overview: Command and Control Center
• The human brain contains about 100 billion nerve cells, or neurons
• Each neuron may communicate with thousands of other neurons
• Functional magnetic resonance imaging is a technology that can
reconstruct a three-dimensional map of brain activity
• Brain imaging and other methods reveal that groups of neurons
function in specialized circuits dedicated to different tasks

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Brainbow mouse section of brain
http://www.conncad.com/gallery/brainbow.html

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 48.1: Nervous systems consist of circuits
of neurons and supporting cells
• All animals except sponges have a nervous
system
• What distinguishes nervous systems of different
animal groups is how neurons are organized into
circuits

4. Nerve net
Hydra (cnidarian)
Radial
nerve
Nerve
ring
Sea star (echinoderm)
The simplest animals with
nervous systems, the cnidarians,
have neurons arranged in nerve
nets
Sea stars have a nerve net in
each arm connected by radial
nerves to a central nerve ring

5. Brain
Ganglia
Squid (mollusc)
Brain
Salamander (chordate)
Spinal
cord
(dorsal
nerve
cord)
Sensory
ganglion
•Nervous systems in molluscs correlate with lifestyles
•Sessile molluscs have simple systems, whereas more complex molluscs
have more sophisticated systems

6. LE 48-3
Sensor
Sensory input
Motor output
Integration
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
In vertebrates, the central nervous system consists of a brain and
dorsal spinal cord
The PNS connects to the CNS
Nervous systems process information in three stages: sensory input,
integration, and motor output

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• Sensory neurons transmit information from
sensors that detect external stimuli and internal
conditions
• Sensory information is sent to the CNS, where
interneurons integrate the information
• Motor output leaves the CNS via motor neurons,
which communicate with effector cells
• The three stages of information processing are
illustrated in the knee-jerk reflex

8. LE 48-4
Quadriceps
muscle
Cell body of
sensory neuron in
dorsal root
ganglion
Sensory neuron
Spinal cord
(cross section)
White
matter
Hamstring
muscle
Gray
matter
Motor neuron
Interneuron

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Neuron Structure
• Most of a neuron’s organelles are in the cell body
• Most neurons have dendrites, highly branched
extensions that receive signals from other neurons
• The axon is typically a much longer extension that
transmits signals to other cells at synapses
• Many axons are covered with a myelin sheath

10. LE 48-5
Dendrites
Cell body
Nucleus
Axon hillock Axon
Signal
direction
Presynaptic cell
Myelin sheath
Synaptic
terminals
Synapse
Postsynaptic cell

11. Dendrites
Cell
body
Axon
InterneuronsSensory neuron Motor neuron
Neurons have a wide variety of shapes that reflect
input and output interactions

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Supporting Cells (Glia)
• Glia are essential for structural integrity of the
nervous system and for functioning of neurons
• Types of glia: astrocytes, radial glia,
oligodendrocytes, and Schwann cells

13. In the CNS,
astrocytes provide
structural support
for neurons and
regulate
extracellular
concentrations of
ions and
neurotransmitters

14. LE 48-8
Axon Nodes of
Ranvier
Schwann
cell
Myelin sheath
Nucleus of
Schwann cell
Schwann
cell
Nodes of Ranvier
Layers of myelin
Axon
0.1 µm
Oligodendrocytes (in the CNS) and Schwann cells (in the
PNS) form the myelin sheaths around axons of many
vertebrate neurons

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Myelination in the central and peripheral nervous
systems

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Dorsal view of the human brain showing the progression of myelination
(“white matter”) over the cortical surface during adolescence

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Concept 48.2: Ion pumps and ion channels
maintain the resting potential of a neuron
• Across its plasma membrane, every cell has a
voltage called a membrane potential
• The cell’s inside is negative relative to the outside
• Membrane potential of a cell can be measured
• Oligodendrocytes (in the CNS) and Schwann cells
(in the PNS) form the myelin sheaths around
axons of many vertebrate neurons

18. Microelectrode
Reference
electrode
Voltage
recorder
–70 mV

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The Resting Potential
• Resting potential is the membrane potential of a
neuron that is not transmitting signals
• Resting potential depends on ionic gradients
across the plasma membrane

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• Concentration of Na+
is higher in the extracellular
fluid than in the cytosol
• The opposite is true for K+
• By modeling a neuron with an artificial membrane,
we can better understand resting potential

21. LE 48-10
CYTOSOL EXTRACELLULAR
FLUID
[Na+
]
15 mM
[K+
]
150 mM
[A–
]
100 mM
[Na+
]
150 mM
[K+
]
5 mM
[Cl–
]
120 mM
[Cl–
]
10 mM
Plasma
membrane

22. LE 48-11
150 mM
KCl
Inner
chamber
Outer
chamber
–92 mV
Potassium
channel
Membrane selectively permeable to K+
Membrane selectively permeable to Na+
5 mM
KCl
Artificial
membrane
K+
Cl–
150 mM
NaCl
Inner
chamber
Outer
chamber
+62 mV
Sodium
channel
15 mM
NaCl
Na+
Cl–

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• A neuron that is not transmitting signals contains
many open K+
channels and fewer open Na+
channels in its plasma membrane
• Diffusion of K+
and Na+
leads to a separation of
charges across the membrane, producing the
resting potential

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Gated Ion Channels
• Gated ion channels open or close in response to
one of three stimuli:
– Stretch-gated ion channels open when the
membrane is mechanically deformed
– Ligand-gated ion channels open or close when
a specific chemical binds to the channel
– Voltage-gated ion channels respond to a
change in membrane potential

25. Hyperpolarizations
Graded potential hyperpolarizations Graded potential depolarizations
5
Time (msec)
Resting
potential
43210
Threshold
–100
–50
0
Membranepotential(mV)
Stimuli
+50
Depolarizations
5
Time (msec)
Resting
potential
43210
Threshold
–100
–50
0
Membranepotential(mV)
Stimuli
+50
Action potential
5
Time (msec)
Resting
potential
43210
Threshold
–100
–50
0
Membranepotential(mV)
Stronger depolarizing stimulus
+50
Action
potential
6
If a cell has gated ion channels, its membrane potential may change
in response to stimuli that open or close those channels
Some stimuli trigger a hyperpolarization, an increase in magnitude
of the membrane potential
Other stimuli trigger a depolarization, a reduction in the magnitude of
the membrane potential

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• Hyperpolarization and depolarization are called
graded potentials
• The magnitude of the change in membrane
potential varies with the strength of the stimulus

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Production of Action Potentials
• Depolarizations are usually graded only up to a
certain membrane voltage, called the threshold
• A stimulus strong enough to produce
depolarization that reaches the threshold triggers
a response called an action potential
• An action potential is a brief all-or-none
depolarization of a neuron’s plasma membrane
• It carries information along axons

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• Voltage-gated Na+
and K+
channels are involved in
producing an action potential
• When a stimulus depolarizes the membrane, Na+
channels
open, allowing Na+
to diffuse into the cell
• As the action potential subsides, K+
channels open, and K+
flows out of the cell
• During the refractory period after an action potential, a
second action potential cannot be initiated
Action Potential

29. Resting potential
Threshold
Membranepotential
(mV)
Action
potential
Time
–100
–50
+50
0
Potassium
channel
Extracellular fluid
Plasma membrane
Na+
Resting state
Inactivation
gate
Activation
gates
Sodium
channel K+
Cytosol
Na+
Depolarization
K+
Na+
Na+
Rising phase of the action potential
K+
Na+
Na+
Falling phase of the action potential
K+
Na+
Na+
Undershoot
K+
Na+

30. An action potential is generated as Na+
flows inward
across the membrane at one location.
Na+
Action
potential
Axon
Na+
Action
potentialK+
The depolarization of the action potential spreads to the
neighboring region of the membrane, re-initiating the
action potential there. To the left of this region, the
membrane is repolarizing as K+
flows outward.
K+
Na+
Action
potentialK+
The depolarization-repolarization process is repeated in the
next region of the membrane. In this way, local currents of
ions across the plasma membrane cause the action
potential to be propagated along the length of the axon.
K+
Conduction of Action Potentials
•An action potential can travel
long distances by regenerating
itself along the axon
•At the site where the action
potential is generated, usually the
axon hillock, an electrical current
depolarizes the neighboring
region of the axon membrane

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Conduction Speed
• The speed of an action potential increases with
the axon’s diameter
• In vertebrates, axons are myelinated, also causing
an action potential’s speed to increase
• Action potentials in myelinated axons jump
between the nodes of Ranvier in a process called
saltatory conduction

32. LE 48-15
Cell body
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
Axon
Na+ and K+ channels are concentrated at the
nodes to initiate a series of action potentials

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Concept 48.4: Neurons communicate with other
cells at synapses
• In an electrical synapse, current flows directly from
one cell to another via a gap junction
• The vast majority of synapses are chemical
synapses
• In a chemical synapse, a presynaptic neuron
releases chemical neurotransmitters stored in the
synaptic terminal

34. LE 48-16
Postsynaptic
neuron
Synaptic
terminals
of pre-
synaptic
neurons
5µm

35. LE 48-17
Postsynaptic cellPresynaptic
cell
Synaptic vesicles
containing
neurotransmitter
Presynaptic
membrane
Voltage-gated
Ca2+
channel
Ca2+
Postsynaptic
membrane
Postsynaptic
membrane
Neuro-
transmitter
Ligand-
gated
ion channel
Na+
K+
Ligand-gated
ion channels
Synaptic cleft
When an action potential reaches a terminal, the final result
is release of neurotransmitters into the synaptic cleft
•Direct synaptic transmission involves binding of neurotransmitters to
ligand-gated ion channels

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• Postsynaptic potentials fall into two categories:
– Excitatory postsynaptic potentials (EPSPs)
– Inhibitory postsynaptic potentials (IPSPs)
• After release, the neurotransmitter diffuses out of
the synaptic cleft
• It may be taken up by surrounding cells and
degraded by enzymes

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Summation of Postsynaptic Potentials
• Unlike action potentials, postsynaptic potentials
are graded and do not regenerate
• Most neurons have many synapses on their
dendrites and cell body
• A single EPSP is usually too small to trigger an
action potential in a postsynaptic neuron

38. Postsynaptic
neuron
Terminal branch
of presynaptic
neuron
E1
E1
Axon
hillock
E1
E2
E1
I
Action
potential
E1E1 + E2
Spatial summation
of EPSP and IPSP
Spatial summation
I E1 + I
Action
potential
E1
Temporal summation
E1
Threshold of axon of
postsynaptic neuron
E1
Subthreshold, no
summation
E1
Resting
potential
Membranepotential(mV)
–70
0
•If two EPSPs are produced in rapid succession, an effect called
temporal summation occurs
•In spatial summation, EPSPs produced nearly simultaneously by
different synapses on the same postsynaptic neuron add together
•Through summation, an IPSP can counter the effect of an EPSP

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Indirect Synaptic Transmission
• In indirect synaptic transmission, a
neurotransmitter binds to a receptor that is not
part of an ion channel
• This binding activates a signal transduction
pathway involving a second messenger in the
postsynaptic cell
• Effects of indirect synaptic transmission have a
slower onset but last longer

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Neurotransmitters
• The same neurotransmitter can produce different effects in
different types of cells

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Acetylcholine
• Acetylcholine is a common neurotransmitter in
vertebrates and invertebrates
• It can be inhibitory or excitatory

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Biogenic Amines
• Biogenic amines include epinephrine,
norepinephrine, dopamine, and serotonin
• They are active in the CNS and PNS

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Amino Acids and Peptides
• Four amino acids are known to function as
neurotransmitters in the CNS
• Several neuropeptides, relatively short chains of
amino acids, also function as neurotransmitters

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Gases
• Gases such as nitric oxide and carbon monoxide
are local regulators in the PNS