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Chapter 48
Nervous Systems
PowerPoint Lectures for
Biology, Seventh Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• Overview: Command and Control Center
• The human brain
– Contains an estimated 100 billion nerve cells,
or neurons
• Each neuron
– May communicate with thousands of other
neurons
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• Functional magnetic resonance imaging
– Is a technology that can reconstruct a threedimensional map of brain activity
Figure 48.1
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• The results of brain imaging and other research
methods
– Reveal that groups of neurons function in
specialized circuits dedicated to different tasks
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• Concept 48.1: Nervous systems consist of
circuits of neurons and supporting cells
• All animals except sponges
– Have some type of nervous system
• What distinguishes the nervous systems of
different animal groups
– Is how the neurons are organized into circuits
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Organization of Nervous Systems
• The simplest animals with nervous systems,
the cnidarians
– Have neurons arranged in nerve nets
Nerve net
Figure 48.2a
(a) Hydra (cnidarian)
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• Sea stars have a nerve net in each arm
– Connected by radial nerves to a central nerve
ring
Radial
nerve
Nerve
ring
Figure 48.2b
(b) Sea star (echinoderm)
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• In relatively simple cephalized animals, such as
flatworms
– A central nervous system (CNS) is evident
Eyespot
Brain
Nerve
cord
Transverse
nerve
Figure 48.2c
(c) Planarian (flatworm)
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• Annelids and arthropods
– Have segmentally arranged clusters of
neurons called ganglia
• These ganglia connect to the CNS
– And make up a peripheral nervous system
(PNS)
Brain
Brain
Ventral
nerve
cord
Segmental
ganglia
Segmental
ganglion
Figure 48.2d, e
Ventral
nerve cord
(d) Leech (annelid)
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(e) Insect (arthropod)
• Nervous systems in molluscs
– Correlate with the animals’ lifestyles
• Sessile molluscs have simple systems
– While more complex molluscs have more
sophisticated systems
Anterior
nerve ring
Ganglia
Brain
Longitudinal
nerve cords
Figure 48.2f, g
(f) Chiton (mollusc)
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Ganglia
(g) Squid (mollusc)
• In vertebrates
– The central nervous system consists of a brain
and dorsal spinal cord
– The PNS connects to the CNS
Brain
Spinal
cord
(dorsal
nerve
cord)
Figure 48.2h
Sensory
ganglion
(h) Salamander (chordate)
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Information Processing
• Nervous systems process information in three
stages
– Sensory input, integration, and motor output
Sensory input
Integration
Sensor
Motor output
Effector
Figure 48.3
Peripheral nervous
system (PNS)
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Central nervous
system (CNS)
• 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
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• The three stages of information processing
– Are illustrated in the knee-jerk reflex
2 Sensors detect
a sudden stretch in
the quadriceps.
3 Sensory neurons
convey the information
to the spinal cord.
Cell body of
sensory neuron
in dorsal
root ganglion
4 The sensory neurons communicate with
motor neurons that supply the quadriceps. The
motor neurons convey signals to the quadriceps,
causing it to contract and jerking the lower leg forward.
Gray matter
5 Sensory neurons
from the quadriceps
also communicate
with interneurons
in the spinal cord.
Quadriceps
muscle
White
matter
Hamstring
muscle
Spinal cord
(cross section)
Sensory neuron
Motor neuron
Figure 48.4
1 The reflex is
initiated by tapping
the tendon connected
to the quadriceps
(extensor) muscle.
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Interneuron
6 The interneurons
inhibit motor neurons
that supply the
hamstring (flexor)
muscle. This inhibition
prevents the hamstring
from contracting,
which would resist
the action of
the quadriceps.
Neuron Structure
• Most of a neuron’s organelles
– Are located in the cell body
Dendrites
Cell body
Nucleus
Synapse
Signal
Axon direction
Axon hillock
Presynaptic cell
Postsynaptic cell
Myelin sheath
Figure 48.5
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Synaptic
terminals
• 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
– That may be covered with a myelin sheath
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• Neurons have a wide variety of shapes
– That reflect their input and output interactions
Dendrites
Axon
Cell
body
Figure 48.6a–c (a) Sensory neuron
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(b) Interneurons
(c) Motor neuron
Supporting Cells (Glia)
• Glia are supporting cells
– That are essential for the structural integrity of
the nervous system and for the normal
functioning of neurons
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• In the CNS, astrocytes
Figure 48.7
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50 µm
– Provide structural support for neurons and
regulate the extracellular concentrations of
ions and neurotransmitters
• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS)
– Are glia that form the myelin sheaths around
the axons of many vertebrate neurons
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Myelin sheath
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
Figure 48.8
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0.1 µm
• 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 inside of a cell is negative
– Relative to the outside
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• The membrane potential of a cell can be
measured
APPLICATION Electrophysiologists use intracellular recording to measure the membrane potential of
neurons and other cells.
TECHNIQUE
A microelectrode is made from a glass capillary tube filled with an electrically conductive
salt solution. One end of the tube tapers to an extremely fine tip (diameter < 1 µm). While looking through a
microscope, the experimenter uses a micropositioner to insert the tip of the microelectrode into a cell. A
voltage recorder (usually an oscilloscope or a computer-based system) measures the voltage between the
microelectrode tip inside the cell and a reference electrode placed in the solution outside the cell.
Microelectrode
–70 mV
Voltage
recorder
Figure 48.9
Reference
electrode
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The Resting Potential
• The resting potential
– Is the membrane potential of a neuron that is
not transmitting signals
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• In all neurons, the resting potential
– Depends on the ionic gradients that exist
across the plasma membrane
EXTRACELLULAR
FLUID
CYTOSOL
[Na+]
15 mM
–
+
[Na+]
150 mM
[K+]
150 mM
–
+
[K+]
5 mM
–
+
[Cl–]
10 mM
–
[Cl–]
+ 120 mM
[A–]
100 mM
–
+
Plasma
membrane
Figure 48.10
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• The concentration of Na+ is higher in the
extracellular fluid than in the cytosol
– While the opposite is true for K+
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• By modeling a mammalian neuron with an
artificial membrane
– We can gain a better understanding of the
resting potential of a neuron
Outer
chamber
–92 mV
+62 mV
+
–
150 mM
KCL
5 mM
KCL
+
Cl–
Artificial
membrane
–
+
–
Figure 48.11a, b (a) Membrane selectively permeable to K+
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Outer
chamber
–
150 mM
NaCl
15 mM
NaCl
Cl–
K+
Potassium
channel
Inner
chamber
+
Inner
chamber
+
–
Sodium +
channel
–
Na+
(b) Membrane selectively permeable to Na+
• A neuron that is not transmitting signals
– Contains many open K+ channels and fewer
open Na+ channels in its plasma membrane
• The diffusion of K+ and Na+ through these
channels
– 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 membrane stretch or the
binding of a specific ligand
– In response to a change in the membrane
potential
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• Concept 48.3: Action potentials are the signals
conducted by axons
• If a cell has gated ion channels
– Its membrane potential may change in
response to stimuli that open or close those
channels
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• Some stimuli trigger a hyperpolarization
– An increase in the magnitude of the membrane
Stimuli
potential
Membrane potential (mV)
+50
0
–50
Threshold
Resting
potential Hyperpolarizations
–100
0 1 2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
produced by two stimuli that
increase membrane permeability
to K+. The larger stimulus produces
Figure 48.12a a larger hyperpolarization.
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• Other stimuli trigger a depolarization
– A reduction in the magnitude of the membrane
Stimuli
potential
Membrane potential (mV)
+50
0
–50
Threshold
Resting Depolarizations
potential
–100
0 1 2 3 4 5
Time (msec)
(b) Graded depolarizations produced
by two stimuli that increase
membrane permeability to Na+.
The larger stimulus produces a
Figure 48.12b larger depolarization.
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• Hyperpolarization and depolarization
– Are both called graded potentials because the
magnitude of the change in membrane
potential varies with the strength of the
stimulus
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Production of Action Potentials
• In most neurons, depolarizations
– Are graded only up to a certain membrane
voltage, called the threshold
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• A stimulus strong enough to produce a
depolarization that reaches the threshold
– Triggers a different type of response, called an
Stronger depolarizing stimulus
action potential
Membrane potential (mV)
+50
Action
potential
0
–50
Threshold
Resting
potential
–100
Figure 48.12c
0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
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• An action potential
– Is a brief all-or-none depolarization of a
neuron’s plasma membrane
– Is the type of signal that carries information
along axons
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• Both voltage-gated Na+ channels and voltagegated K+ channels
– Are involved in the production of an action
potential
• When a stimulus depolarizes the membrane
– Na+ channels open, allowing Na+ to diffuse into
the cell
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• As the action potential subsides
– K+ channels open, and K+ flows out of the cell
• A refractory period follows the action potential
– During which a second action potential cannot
be initiated
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• The generation of an action potential
Na+
Na+
– –
– –
– –
– –
+ +
+ +
+ +
+ +
K+
Rising phase of the action potential
Depolarization opens the activation
gates on most Na+ channels, while the
K+ channels’ activation gates remain
closed. Na+ influx makes the inside of
the membrane positive with respect
to the outside.
Na+
+ +
+ +
– –
– –
+50
+ +
– –
K+
– –
–50
Na+
+ + + + + + + +
+ +
– –
– –
– –
– –
3
2
4
Threshold
5
1
1
Resting potential
Na+
Potassium
channel
+ +
Activation
gates
+ +
+ +
– –
– –
+ +
+ +
– –
– –
+ +
K+
– – – – – – – –
Cytosol
– –
Sodium
channel
1
Na+
+ +
Plasma membrane
Figure 48.13
Falling phase of the action potential
The inactivation gates on
most Na+ channels close,
blocking Na+ influx. The
activation gates on most
K+ channels open,
permitting K+ efflux
which again makes
the inside of the cell
negative.
Time
Depolarization A stimulus opens the
activation gates on some Na+ channels. Na+
influx through those channels depolarizes the
membrane. If the depolarization reaches the
threshold, it triggers an action potential.
Extracellular fluid
+ +
Action
potential
0
–100
2
+ +
4
Na+
+ +
+ +
K+
Membrane potential
(mV)
3
Na+
Na+
– –
K+
– –
Inactivation
gate
Resting state
The activation gates on the Na+ and K+ channels
are closed, and the membrane’s resting potential is maintained.
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5
Undershoot
Both gates of the Na+ channels
are closed, but the activation gates on some K+
channels are still open. As these gates close on
most K+ channels, and the inactivation gates
open on Na+ channels, the membrane returns to
its resting state.
Conduction of Action Potentials
• An action potential can travel long distances
– By regenerating itself along the axon
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• At the site where the action potential is
generated, usually the axon hillock
– An electrical current depolarizes the
neighboring region of the axon membrane
Axon
Action
potential
– –
+ ++
Na
+ +
– –
K+
+ +
– –
– –
+ +
K+
Figure 48.14
+
–
–
+
+
–
–
+
+
+
+
+
+
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
–
–
+
+
–
–
+
+
–
–
+
Action
potential
–
+
Na
–
+
+
+ +
–
–
K+
+ +
– –
– –
+ +
K+
+
–
–
+
Action
potential
– –
+ ++
Na
+ +
– –
–
+
+
–
1
An action potential is generated
as Na+ flows inward across the
membrane at one location.
2
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.
3
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.
+
–
–
+
–
+
+
–
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Conduction Speed
• The speed of an action potential
– Increases with the diameter of an axon
• In vertebrates, axons are myelinated
– Also causing the speed of an action potential
to increase
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• Action potentials in myelinated axons
– Jump between the nodes of Ranvier in a
process called saltatory conduction
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
––
–
Cell body
+
++
+
++
–––
––
–
+
+
Axon
+
++
––
–
Figure 48.15
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• Concept 48.4: Neurons communicate with
other cells at synapses
• In an electrical synapse
– Electrical current flows directly from one cell to
another via a gap junction
• The vast majority of synapses
– Are chemical synapses
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• In a chemical synapse, a presynaptic neuron
– Releases chemical neurotransmitters, which
are stored in the synaptic terminal
Postsynaptic
neuron
5 µm
Synaptic
terminal
of presynaptic
neurons
Figure 48.16
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• When an action potential reaches a terminal
– The final result is the release of
neurotransmitters into the synaptic cleft
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
neurotransmitter
5
Presynaptic
membrane
Na+
K+
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
3
Synaptic cleft
Figure 48.17
Ligand-gated
ion channels
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Postsynaptic
membrane
6
Direct Synaptic Transmission
• The process of direct synaptic transmission
– Involves the binding of neurotransmitters to
ligand-gated ion channels
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• Neurotransmitter binding
– Causes the ion channels to open, generating a
postsynaptic potential
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• Postsynaptic potentials fall into two categories
– Excitatory postsynaptic potentials (EPSPs)
– Inhibitory postsynaptic potentials (IPSPs)
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• After its release, the neurotransmitter
– Diffuses out of the synaptic cleft
– 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 themselves
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• Since 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
Terminal branch of
presynaptic neuron
Membrane potential (mV)
Postsynaptic
neuron
Figure 48.18a
E1
Threshold of axon of
postsynaptic neuron
0
Resting
potential
–70
E1
E1
(a) Subthreshold, no
summation
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• If two EPSPs are produced in rapid succession
– An effect called temporal summation occurs
E1
Axon
hillock
Action
potential
E1
E1
(b) Temporal summation
Figure 48.18b
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• In spatial summation
– EPSPs produced nearly simultaneously by
different synapses on the same postsynaptic
neuron add together
E
E2
1
Action
potential
E1 + E2
(c) Spatial summation
Figure 48.18c
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• Through summation
– An IPSP can counter the effect of an EPSP
E1
I
E1
Figure 48.18d
I
E1 + I
(d) Spatial summation
of EPSP and IPSP
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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, producing a slowly
developing but long-lasting effect
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Neurotransmitters
• The same neurotransmitter
– Can produce different effects in different types
of cells
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• Major neurotransmitters
Table 48.1
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Acetylcholine
• Acetylcholine
– Is one of the most common neurotransmitters
in both vertebrates and invertebrates
– Can be inhibitory or excitatory
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Biogenic Amines
• Biogenic amines
– Include epinephrine, norepinephrine,
dopamine, and serotonin
– Are active in the CNS and PNS
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Amino Acids and Peptides
• Various amino acids and peptides
– Are active in the brain
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Gases
• Gases such as nitric oxide and carbon
monoxide
– Are local regulators in the PNS
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• Concept 48.5: The vertebrate nervous system
is regionally specialized
• In all vertebrates, the nervous system
– Shows a high degree of cephalization and
distinct CNS and PNS components
Central nervous
system (CNS)
Brain
Spinal cord
Peripheral nervous
system (PNS)
Cranial
nerves
Ganglia
outside
CNS
Spinal
nerves
Figure 48.19
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• The brain provides the integrative power
– That underlies the complex behavior of
vertebrates
• The spinal cord integrates simple responses to
certain kinds of stimuli
– And conveys information to and from the brain
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• The central canal of the spinal cord and the
four ventricles of the brain
– Are hollow, since they are derived from the
dorsal embryonic nerve cord
Gray matter
White
matter
Ventricles
Figure 48.20
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The Peripheral Nervous System
• The PNS transmits information to and from the
CNS
– And plays a large role in regulating a
vertebrate’s movement and internal
environment
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• The cranial nerves originate in the brain
– And terminate mostly in organs of the head
and upper body
• The spinal nerves originate in the spinal cord
– And extend to parts of the body below the
head
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• The PNS can be divided into two functional
components
– The somatic nervous system and the
autonomic nervous system
Peripheral
nervous system
Somatic
nervous
system
Autonomic
nervous
system
Sympathetic
division
Figure 48.21
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Parasympathetic
division
Enteric
division
• The somatic nervous system
– Carries signals to skeletal muscles
• The autonomic nervous system
– Regulates the internal environment, in an
involuntary manner
– Is divided into the sympathetic,
parasympathetic, and enteric divisions
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• The sympathetic and parasympathetic divisions
– Have antagonistic effects on target organs
Parasympathetic division
Sympathetic division
Action on target organs:
Location of
preganglionic neurons:
brainstem and sacral
segments of spinal cord
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Action on target organs:
Dilates pupil
of eye
Constricts pupil
of eye
Inhibits salivary
gland secretion
Stimulates salivary
gland secretion
Constricts
bronchi in lungs
Sympathetic
ganglia
Cervical
Accelerates heart
Slows heart
Location of
postganglionic neurons:
in ganglia close to or
within target organs
Stimulates activity
of stomach and
intestines
Stimulates
gallbladder
Thoracic
Inhibits activity
of pancreas
Stimulates glucose
release from liver;
inhibits gallbladder
Promotes emptying
of bladder
Figure 48.22
Location of
postganglionic neurons:
some in ganglia close to
target organs; others in
a chain of ganglia near
spinal cord
Lumbar
Stimulates
adrenal medulla
Promotes erection
of genitalia
Neurotransmitter
released by
preganglionic neurons:
acetylcholine
Inhibits activity of
stomach and intestines
Stimulates activity
of pancreas
Neurotransmitter
released by
postganglionic neurons:
acetylcholine
Relaxes bronchi
in lungs
Location of
preganglionic neurons:
thoracic and lumbar
segments of spinal cord
Inhibits emptying
of bladder
Synapse
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Sacral
Promotes ejaculation and
vaginal contractions
Neurotransmitter
released by
postganglionic neurons:
norepinephrine
• The sympathetic division
– Correlates with the “fight-or-flight” response
• The parasympathetic division
– Promotes a return to self-maintenance
functions
• The enteric division
– Controls the activity of the digestive tract,
pancreas, and gallbladder
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Embryonic Development of the Brain
• In all vertebrates
– The brain develops from three embryonic
regions: the forebrain, the midbrain, and the
hindbrain
Embryonic brain regions
Forebrain
Midbrain
Hindbrain
Midbrain
Hindbrain
Forebrain
Figure 48.23a
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(a) Embryo at one month
• By the fifth week of human embryonic
development
– Five brain regions have formed from the three
embryonic regions
Embryonic brain regions
Telencephalon
Diencephalon
Mesencephalon
Metencephalon
Myelencephalon
Mesencephalon
Metencephalon
Diencephalon
Myelencephalon
Spinal cord
Telencephalon
Figure 48.23b
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(b) Embryo at five weeks
• As a human brain develops further
– The most profound change occurs in the
forebrain, which gives rise to the cerebrum
Brain structures present in adult
Cerebrum (cerebral hemispheres; includes cerebral
cortex, white matter, basal nuclei)
Diencephalon (thalamus, hypothalamus, epithalamus)
Midbrain (part of brainstem)
Pons (part of brainstem),
cerebellum
Medulla oblongata (part of brainstem)
Cerebral hemisphere
Diencephalon:
Hypothalamus
Thalamus
Pineal gland
(part of epithalamus)
Brainstem:
Midbrain
Pons
Pituitary
gland
Spinal cord
Cerebellum
Central canal
Figure 48.23c
(c) Adult
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Medulla
oblongata
The Brainstem
• The brainstem consists of three parts
– The medulla oblongata, the pons, and the
midbrain
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• The medulla oblongata
– Contains centers that control several visceral
functions
• The pons
– Also participates in visceral functions
• The midbrain
– Contains centers for the receipt and integration
of several types of sensory information
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Arousal and Sleep
• A diffuse network of neurons called the reticular
formation
– Is present in the core of the brainstem
Eye
Reticular formation
Figure 48.24
Input from touch,
pain, and temperature
receptors
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Input from ears
• A part of the reticular formation, the reticular
activating system (RAS)
– Regulates sleep and arousal
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The Cerebellum
• The cerebellum
– Is important for coordination and error
checking during motor, perceptual, and
cognitive functions
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• The cerebellum
– Is also involved in learning and remembering
motor skills
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The Diencephalon
• The embryonic diencephalon develops into
three adult brain regions
– The epithalamus, thalamus, and hypothalamus
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• The epithalamus
– Includes the pineal gland and the choroid
plexus
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• The thalamus
– Is the main input center for sensory information
going to the cerebrum and the main output
center for motor information leaving the
cerebrum
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• The hypothalamus regulates
– Homeostasis
– Basic survival behaviors such as feeding,
fighting, fleeing, and reproducing
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Circadian Rhythms
• The hypothalamus also regulates circadian
rhythms
– Such as the sleep/wake cycle
• Animals usually have a biological clock
– Which is a pair of suprachiasmatic nuclei
(SCN) found in the hypothalamus
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• Biological clocks usually require external cues
– To remain synchronized with environmental cycles
EXPERIMENT
In the northern flying squirrel (Glaucomys sabrinus), activity normally begins with the onset of darkness and ends
at dawn, which suggests that light is an important external cue for the squirrel. To test this idea, researchers monitored the activity of captive
squirrels for 23 days under two sets of conditions: (a) a regular cycle of 12 hours of light and 12 hours of darkness and (b) constant darkness.
The squirrels were given free access to an exercise wheel and a rest cage. A recorder automatically noted when the wheel was rotating and
when it was still.
(a) 12 hr light-12 hr dark cycle
Light
Dark
Light
Dark
1
Days of experiment
RESULTS
When the squirrels
were exposed to a regular light/dark
cycle, their wheel-turning activity
(indicated by the dark bars) occurred
at roughly the same time every day.
However, when they were kept in
constant darkness, their activity phase
began about 21 minutes later each day.
(b) Constant darkness
5
10
15
20
Figure 48.25
12
16
20
24
4
Time of day (hr)
8
12
12
16
20
24
Time of day (hr)
CONCLUSION
The northern flying squirrel’s internal clock can run in constant darkness, but it does so on
its own cycle, which lasts about 24 hours and 21 minutes. External (light) cues keep the clock running on a 24-hour cycle.
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4
8
12
The Cerebrum
• The cerebrum
– Develops from the embryonic telencephalon
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• The cerebrum has right and left cerebral
hemispheres
– That each consist of cerebral cortex overlying
white matter and basal nuclei
Left cerebral
hemisphere
Right cerebral
hemisphere
Corpus
callosum
Neocortex
Figure 48.26
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Basal
nuclei
• The basal nuclei
– Are important centers for planning and learning
movement sequences
• In mammals
– The cerebral cortex has a convoluted surface
called the neocortex
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• In humans, the largest and most complex part
of the brain
– Is the cerebral cortex, where sensory
information is analyzed, motor commands are
issued, and language is generated
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• A thick band of axons, the corpus callosum
– Provides communication between the right and
left cerebral cortices
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• Concept 48.6: The cerebral cortex controls
voluntary movement and cognitive functions
• Each side of the cerebral cortex has four lobes
– Frontal, parietal, temporal, and occipital
Frontal lobe
Parietal lobe
Speech
Frontal
association
area
Taste
Speech
Smell
Somatosensory
association
area
Reading
Hearing
Auditory
association
area
Visual
association
area
Vision
Figure 48.27
Temporal lobe
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Occipital lobe
• Each of the lobes
– Contains primary sensory areas and
association areas
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Information Processing in the Cerebral Cortex
• Specific types of sensory input
– Enter the primary sensory areas
• Adjacent association areas
– Process particular features in the sensory input
and integrate information from different
sensory areas
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• In the somatosensory cortex and motor cortex
– Neurons are distributed according to the part
of the body that generates sensory input or
receives motor input
Frontal lobe
Parietal lobe
Toes
Lips
Jaw
Tongue
Genitalia
Tongue
Pharynx
Primary
motor cortex
Figure 48.28
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Abdominal
organs
Primary
somatosensory
cortex
Lateralization of Cortical Function
• During brain development, in a process called
lateralization
– Competing functions segregate and displace
each other in the cortex of the left and right
cerebral hemispheres
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• The left hemisphere
– Becomes more adept at language, math,
logical operations, and the processing of serial
sequences
• The right hemisphere
– Is stronger at pattern recognition, nonverbal
thinking, and emotional processing
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Language and Speech
• Studies of brain activity
– Have mapped specific areas of the brain
responsible for language and speech
Max
Hearing
words
Seeing
words
Min
Figure 48.29
Speaking
words
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Generating
words
• Portions of the frontal lobe, Broca’s area and
Wernicke’s area
– Are essential for the generation and
understanding of language
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Emotions
• The limbic system
– Is a ring of structures around the brainstem
Thalamus
Hypothalamus
Prefrontal cortex
Olfactory
bulb
Amygdala
Figure 48.30
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Hippocampus
• This limbic system includes three parts of the
cerebral cortex
– The amygdala, hippocampus, and olfactory
bulb
• These structures interact with the neocortex to
mediate primary emotions
– And attach emotional “feelings” to survivalrelated functions
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• Structures of the limbic system form in early
development
– And provide a foundation for emotional
memory, associating emotions with particular
events or experiences
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Memory and Learning
• The frontal lobes
– Are a site of short-term memory
– Interact with the hippocampus and amygdala
to consolidate long-term memory
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• Many sensory and motor association areas of
the cerebral cortex
– Are involved in storing and retrieving words
and images
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Cellular Mechanisms of Learning
• Experiments on invertebrates
– Have revealed the cellular basis of some types
of learning
(a) Touching the siphon triggers a reflex that
causes the gill to withdraw. If the tail is
shocked just before the siphon is touched,
the withdrawal reflex is stronger. This
strengthening of the reflex is a simple form
of learning called sensitization.
Siphon
Mantle
Gill
Tail
Head
Figure 48.31a, b
(b) Sensitization involves interneurons that
make synapses on the synaptic terminals of
the siphon sensory neurons. When the tail
is shocked, the interneurons release
serotonin, which activates a signal
transduction pathway that closes K+
channels in the synaptic terminals of
the siphon sensory neurons. As a result,
action potentials in the siphon sensory
neurons produce a prolonged
depolarization of the terminals. That allows
more Ca2+ to diffuse into the terminals,
which causes the terminals to release more
of their excitatory neurotransmitter onto the gill
motor neurons. In response, the motor neurons
generate action potentials at a higher frequency,
producing a more forceful gill withdrawal.
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Gill withdrawal pathway
Touching
the siphon
Siphon sensory
neuron
Gill motor
neuron
Sensitization pathway
Shocking
the tail
Interneuron
Tail sensory
neuron
Gill
• In the vertebrate brain, a form of learning called
long-term potentiation (LTP)
– Involves an increase in the strength of synaptic
transmission
1 The presynaptic
neuron releases glutamate.
2 Glutamate binds to AMPA
receptors, opening the AMPAreceptor channel and depolarizing
the postsynaptic membrane.
PRESYNAPTIC NEURON
7 NO diffuses into the
presynaptic neuron, causing
it to release more glutamate.
NO
6 Ca2+ stimulates the
postsynaptic neuron to
produce nitric oxide (NO).
Glutamate
AMPA receptor
NO
Figure 48.32
5 Ca2+ initiates the phosphorylation of AMPA receptors,
making them more responsive.
Ca2+ also causes more AMPA
receptors to appear in the
postsynaptic membrane.
NMDA
receptor
3 Glutamate also binds to NMDA
receptors. If the postsynaptic
membrane is simultaneously
depolarized, the NMDA-receptor
channel opens.
P
Ca2+
Signal transduction pathways
POSTSYNAPTIC NEURON
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4 Ca2+ diffuses into the
postsynaptic neuron.
Consciousness
• Modern brain-imaging techniques
– Suggest that consciousness may be an
emergent property of the brain that is based on
activity in many areas of the cortex
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• Concept 48.7: CNS injuries and diseases are
the focus of much research
• Unlike the PNS, the mammalian CNS
– Cannot repair itself when damaged or
assaulted by disease
• Current research on nerve cell development
and stem cells
– May one day make it possible for physicians to
repair or replace damaged neurons
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Nerve Cell Development
• Signal molecules direct an axon’s growth
– By binding to receptors on the plasma
membrane of the growth cone
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• This receptor binding triggers a signal
transduction pathway
– Which may cause an axon to grow toward or
away from the source of the signal
Midline of
spinal cord
Developing axon
of interneuron
Developing axon
of motor neuron
Growth
cone
Netrin-1
receptor
Netrin-1
receptor
Slit
receptor
Netrin-1
Floor
plate
1 Growth toward the floor plate.
2
Cells in the floor plate of the
spinal cord release Netrin-1, which
diffuses away from the floor plate
and binds to receptors on the
growth cone of a developing
interneuron axon. Binding stimulates
axon growth toward the floor plate.
Figure 48.33a, b
Slit
Netrin-1
Cell
adhesion
molecules
Growth across the mid-line.
3
Once the axon reaches the
floor plate, cell adhesion molecules
on the axon bind to complementary
molecules on floor plate cells,
directing the growth of the axon
across the midline.
Slit
No turning back.
Now the axon synthesizes
receptors that bind to Slit,
a repulsion protein released by floor plate cells.
This prevents the axon
from growing back across
the midline.
(a) Growth of an interneuron axon toward and across the midline of the spinal cord
(diagrammed here in cross section)
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Slit
receptor
Netrin-1 and Slit, produced by cells
of the floor plate, bind to receptors
on the axons of motor neurons. In
this case, both proteins act to repel
the axon, directing the motor neuron
to grow away from the spinal cord.
(b) Growth of a motor neuron axon away
from the midline of the spinal cord
• The genes and basic events involved in axon
guidance
– Are similar in invertebrates and vertebrates
• Knowledge of these events may be applied one
day
– To stimulate axonal regrowth following CNS
damage
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Neural Stem Cells
• The adult human brain
– Contains stem cells that can differentiate into
mature neurons
Figure 48.34
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• The induction of stem cell differentiation and
the transplantation of cultured stem cells
– Are potential methods for replacing neurons
lost to trauma or disease
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Diseases and Disorders of the Nervous System
• Mental illnesses and neurological disorders
– Take an enormous toll on society, in both the
patient’s loss of a productive life and the high
cost of long-term health care
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Schizophrenia
• About 1% of the world’s population
– Suffers from schizophrenia
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• Schizophrenia is characterized by
– Hallucinations, delusions, blunted emotions,
and many other symptoms
• Available treatments have focused on
– Brain pathways that use dopamine as a
neurotransmitter
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Depression
• Two broad forms of depressive illness are
known
– Bipolar disorder and major depression
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• Bipolar disorder is characterized by
– Manic (high-mood) and depressive (low-mood)
phases
• In major depression
– Patients have a persistent low mood
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• Treatments for these types of depression
include
– A variety of drugs such as Prozac and lithium
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Alzheimer’s Disease
• Alzheimer’s disease (AD)
– Is a mental deterioration characterized by
confusion, memory loss, and other symptoms
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• AD is caused by the formation of
– Neurofibrillary tangles and senile plaques in
the brain
20 m
Senile plaque
Neurofibrillary tangle
Figure 48.35
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• A successful treatment for AD in humans
– May hinge on early detection of senile plaques
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Parkinson’s Disease
• Parkinson’s disease is a motor disorder
– Caused by the death of dopamine-secreting
neurons in the substantia nigra
– Characterized by difficulty in initiating
movements, slowness of movement, and
rigidity
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• There is no cure for Parkinson’s disease
– Although various approaches are used to
manage the symptoms
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