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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
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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• 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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• 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)
• The three stages of information processing
– Are illustrated in the knee-jerk reflex
(monosynaptic)
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
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
• The resting potential
– Is the membrane potential of a neuron that is
not transmitting signals
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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 concentration of Na+ is higher in the
extracellular fluid than in the cytosol
– While the opposite is true for K+
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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• Some stimuli trigger a hyperpolarization
– An increase in the magnitude of the membrane
Stimuli
potential
– Harder to fire impulse
– Further from threshold
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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• 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
– all-or-none depolarization
Membrane potential (mV)
+50
Action
potential
0
–50
Threshold
Resting
potential
–100
Figure 48.12c
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0 1 2 3 4 5 6
Time (msec)
(c) Action potential triggered by a
depolarization that reaches the
threshold.
• 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 (undershoot) follows the
action potential (brief hyperpolarization)
– 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 however
– 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
– Ca++ enters promoting vesicle fusion and NT
release
– The final result is the release of neurotransmitters
into the synaptic cleft (gap)
Postsynaptic cell
Presynaptic
cell
Synaptic vesicles
containing
Presynaptic
neurotransmitter membrane
Voltage-gated
Ca2+ channel
1 Ca2+
4
2
Synaptic cleft
Figure 48.17
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3
Ligand-gated
ion channels
Postsynaptic
membrane
5
Na+
K+
Neurotransmitter
Postsynaptic
membrane
Ligandgated
ion channel
6
Direct Synaptic Transmission
• The process of direct synaptic transmission
– Involves the binding of neurotransmitters to
ligand-gated ion channels
• Neurotransmitter binding
– Causes the ion channels to open, generating a
postsynaptic potential
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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
• Unlike action potentials
– Postsynaptic potentials are graded and do not
regenerate themselves
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• Postsynaptic potentials fall into two categories
– Excitatory postsynaptic potentials (EPSPs)
– Inhibitory postsynaptic potentials (IPSPs)
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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 can
occur
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
(d) Spatial summation
of EPSP and IPSP
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E1 + I
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 (skeletal
muscle)
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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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• 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 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 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
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
The Cerebellum
• The cerebellum
– Is important for coordination and error
checking during motor, perceptual, and
cognitive functions
• The cerebellum
– Is also involved in
learning and remembering
motor skills
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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
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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 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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• 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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• 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
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
•Broca’s area and Wernicke’s area
– Are essential for the generation and understanding of
language
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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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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