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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
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 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)
• 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
TECHNIQUE
Microelectrode
–70 mV
Voltage
recorder
Figure 48.9
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Reference
electrode
The Resting Potential
• The resting potential
– Is the membrane potential of a neuron that is
not transmitting signals (-70 mV)
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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
–
+
Organic anions like charged
Amino acids.
Figure 48.10
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Plasma
membrane
• 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+
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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 (molecule that
binds to receptor site of another molecule)
– 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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• 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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• 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.
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
• 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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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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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 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
Forebrain  Telencephalon The Cerebrum
• The cerebrum - Is the cerebral cortex, where sensory
information is analyzed, motor commands are issued, and
language is generated
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• The cerebrum has right and left cerebral hemispheres
• A thick band of axons, the corpus callosum
– Provides communication between the right and left
cerebral cortices
Left cerebral
hemisphere
Right cerebral
hemisphere
Corpus
callosum
Neocortex
Figure 48.26
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Basal
nuclei
Forebrain The Diencephalon
• The embryonic diencephalon develops into
three adult brain regions
– The epithalamus, thalamus, and hypothalamus
Sensory, feeling,
etc..
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Midbrain  Mesencephalon  Midbrain
Hindbrain  Metencephalon  pons
H  Myelencephalon  medulla oblongata
• The brainstem consists of three parts
– The medulla oblongata, the pons, and the
midbrain
Controls viceral
function and some
sensory info.
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Hindbrain  Myelencephalon The Cerebellum
• The cerebellum
– Is important for coordination and error
checking during motor, perceptual, and
cognitive functions
–Is also
involved in
learning and
rememberin
g motor
skills
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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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• 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
• 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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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
• Many sensory and motor association areas of
the cerebral cortex
– Are involved in storing and retrieving words
and images
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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)
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
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
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
Neural Stem Cells
• The adult human brain
– Contains stem cells that can differentiate into
mature neurons
Figure 48.34
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings
• The induction of stem cell differentiation and
the transplantation of cultured stem cells
– Are potential methods for replacing neurons
lost to trauma or disease
Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings