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Chapter 48
Neurons, Synapses, and
Signaling
PowerPoint® Lecture Presentations for
Biology
Eighth Edition
Neil Campbell and Jane Reece
Lectures by Chris Romero, updated by Erin Barley with contributions from Joan Sharp
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
What you need to know…
• The components of a reflex arc and how they
work.
• The organization and function of the major
parts of the nervous system.
• One function of each major brain region.
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Overview: Lines of Communication
• The cone snail kills prey with venom that
disables neurons
• Neurons are nerve cells that transfer
information within the body
• Neurons use two types of signals to
communicate: electrical signals (long-distance)
and chemical signals (short-distance)
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Fig. 48-1
48.1: Nervous Systems consist of Neuron Circuits
• All animals except sponges may 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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Quick Evolution of Nervous System Overview
• Nerve Net – cnidarians
• Cephalization – trend toward clustering sensory
neurons and interneurons at anterior end
– Flatworms – small brain and longitudinal nerve
cord; simplest clearly defined nervous system
– Annelids (earthworms, arthropods) – have a
ventral nerve cord
– Vertebrates – have a hollow dorsal nerve cord
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Evolution of Nervous Systems
Most Simplest CNS
Nerve Nets
Ventral Nerve
Cord
Vertebrates – hollow
dorsal nerve cord
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Information Processing
• Nervous Systems process information in three
stages
– Sensory input, integration, and motor output
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Information Processing
• Sensory neurons
– Transmit information from sensors that detect
external stimuli and internal conditions
• Sensory information
– 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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Information Processing Example
• Reflex – simple autonomic nerve circuit in
response to a stimulus
– Ex: Jerking finger off a flame – stimulus is
detected by a receptor in the skin, conveyed
via a sensory neuron to an interneuron in the
spinal cord, which synapses with a motor
neuron, which will causes the effector, a
muscle cell, to contract
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Information Processing Example
• The 3 stages of information processing are
illustrated in the knee-jerk reflex
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Definitions to know!
• Cerebrospinal fluid – circulates through central
canal in spinal cord and ventricles of brain –
bathes cells with nutrients, carries away wastes
• Grey Matter – consists of mainly neuron cell
bodies and unmyelinated axons
• White matter – white because of the myelin
sheaths around the axons
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neuron Structure and Function
• 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
• An axon joins the cell body at the axon hillock
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
• A synapse is a junction between an axon and
another cell
• The synaptic terminal of one axon passes
information across the synapse in the form of
chemical messengers called
neurotransmitters
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• Information is transmitted from a presynaptic
cell (a neuron) to a postsynaptic cell (a
neuron, muscle, or gland cell)
• Most neurons are nourished or insulated by
cells called glia
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-5
Dendrites
Axon
Cell
body
Portion
of axon
Sensory neuron
Interneurons
Cell bodies of
overlapping neurons
80 µm
Motor neuron
CNS vs PNS
• Central Nervous System (CNS)
– Brain and spinal cord
• Peripheral Nervous System (PNS)
– Consists of paired cranial and spinal nerves
associated with ganglia
– Divided into
• Motor (somatic) nervous system
• Autonomic nervous system
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Motor (somatic) and Autonomic Nervous System
• Motor (somatic) Nervous System
– Carries signals to skeletal muscles
– Voluntary
• Autonomic Nervous System
– Regulates the primarily automatic, visceral
functions of smooth and cardiac muscles
– Involuntary
– 2 Divisions: Sympathetic Division and
Parasympathetic Division
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Sympathetic and Parasympathetic Divisions
• Autonomic Nervous System – transmits
signals that regulate the internal environment
by controlling smooth and cardiac muscle
– Sympathetic – when activated causes heart to
beat faster and adrenaline to be secreted (with all
its effects)
– Parasympathetic – has the opposite effect when
activated – slowing heartbeat and digestions
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Nervous System Flow Chart
Nervous System
Central Nervous System (CNS)
Brain
Spinal Cord: Nerve
bundle that
communicates with
body
Peripheral Nervous System (PNS)
Somatic Nervous
System: Voluntary
control over
muscles
Sympathetic Division:
Fight or flight
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Automatic
Nervous System:
Involuntary
Control over
Organs
Parasympathetic
Division: Rest and
Digest
• Oligodendrocytes (in the CNS) and Schwann
cells (in the PNS)
– Are glia that form from the myelin sheaths
around the axons of many vertebrate neurons
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Concept 48.2: Vertebrate Brain is Regionally
Specialized
• Brain
– Provides the integrative power that
underlies the complex behavior of
vertebrates
• Spinal Cord
– Integrates simple responses to certain kinds
of stimuli and conveys information to and
from the brain
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The Brain Parts
• Brainstem –
– Made up of the medulla oblongata, pons, and
midbrain
– Controls homeostatic functions such as
breathing rate
– Conducts sensory and motor signals between
the spinal cord and higher brain centers
– Regulates arousal and sleep
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The Brain Parts (cont)
• Cerebellum
– Helps coordinate motor, perceptual, and cognitive
functions
• Thalamus
– Main center through which sensory and motor
information passes to and from the cerebellum
• Hypothalamus
– Regulates homeostasis – basic survival features such
as feeding, fighting, fleeing, and reproducing;
thermostat, appestat, thirst center, and circadian
rhythms
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The Brain Parts (cont)
• Cerebrum
– 2 hemispheres – each with a covering of grey matter
over white matter
– Information processing is centered here, and this
region is extensive in mammals
• Cerebral cortex –
– Controls voluntary movement and cognitive functions
• Corpus callosum
– Thick band of axons that enables communication
between right and left cortices
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The Brain Parts (cont)
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Membrane Potential: Formation of the Resting
Potential
• In a mammalian neuron at resting potential, the
concentration of K+ is greater inside the cell,
while the concentration of Na+ is greater outside
the cell
• Sodium-potassium pumps use the energy of
ATP to maintain these K+ and Na+ gradients
across the plasma membrane
• These concentration gradients represent
chemical potential energy
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• The opening of ion channels in the plasma
membrane converts chemical potential to
electrical potential
• A neuron at resting potential contains many
open K+ channels and fewer open Na+
channels; K+ diffuses out of the cell
• Anions trapped inside the cell contribute to the
negative charge within the neuron
Animation: Resting Potential
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Fig. 48-6
Key
Na+
K+
OUTSIDE
CELL
OUTSIDE [K+]
CELL
5 mM
INSIDE [K+]
CELL 140 mM
[Na+]
[Cl–]
150 mM 120 mM
[Na+]
15 mM
[Cl–]
10 mM
[A–]
100 mM
INSIDE
CELL
(a)
(b)
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
Fig. 48-6a
OUTSIDE [K+]
CELL
5 mM
INSIDE [K+]
CELL 140 mM
(a)
[Na+]
[Cl–]
150 mM 120 mM
[Na+]
15 mM
[Cl–]
10 mM
[A–]
100 mM
Fig. 48-6b
Key
Na+
K+
OUTSIDE
CELL
INSIDE
CELL
(b)
Sodiumpotassium
pump
Potassium
channel
Sodium
channel
Modeling of the Resting Potential
• Resting potential can be modeled by an
artificial membrane that separates two
chambers
– The concentration of KCl is higher in the inner
chamber and lower in the outer chamber
– K+ diffuses down its gradient to the outer
chamber
– Negative charge builds up in the inner
chamber
• At equilibrium, both the electrical and chemical
gradients are balanced
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Fig. 48-7
–90 mV
Inner
chamber
+62 mV
Outer
chamber
140 mM
KCI
150 mM
15 mM
NaCI
5 mM
KCI
NaCI
Cl–
K+
Cl–
Potassium
channel
(a) Membrane selectively permeable to K+
(
EK = 62 mV log
5 mM
140 mM
)
= –90 mV
Na+
Sodium
channel
(b) Membrane selectively permeable to Na+
(
ENa = 62 mV log
150 mM
15 mM
)
= +62 mV
Fig. 48-7a
Inner
chamber
–90 mV
Outer
chamber
140 mM
KCI
5 mM
KCI
K+
Cl–
Potassium
channel
(a) Membrane selectively permeable to K+
(
5 mM
EK = 62 mV log
140 mM
) = –90 mV
• In a resting neuron, the currents of K+ and Na+
are equal and opposite, and the resting
potential across the membrane remains steady
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Fig. 48-7b
+62 mV
150 mM
NaCI
15 mM
NaCI
Cl–
Na+
Sodium
channel
(b) Membrane selectively permeable to Na+
(
ENa = 62 mV log
) = +62 mV
150 mM
15 mM
Neurons communicate with other cells at synapses
• At electrical synapses, the electrical current
flows from one neuron to another
• At chemical synapses, a chemical
neurotransmitter carries information across the
gap junction
• Most synapses are chemical synapses
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Fig. 48-14
Synaptic
terminals
of presynaptic
neurons
5 µm
Postsynaptic
neuron
• The presynaptic neuron synthesizes and
packages the neurotransmitter in synaptic
vesicles located in the synaptic terminal
• The action potential causes the release of the
neurotransmitter
• The neurotransmitter diffuses across the
synaptic cleft and is received by the
postsynaptic cell
Animation: Synapse
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Fig. 48-15
5
Synaptic vesicles
containing
neurotransmitter
Voltage-gated
Ca2+ channel
Postsynaptic
membrane
1 Ca2+
4
2
Synaptic
cleft
Presynaptic
membrane
3
Ligand-gated
ion channels
6
K+
Na+
Generation of Postsynaptic Potentials
• Direct synaptic transmission involves binding of
neurotransmitters to ligand-gated ion channels
in the postsynaptic cell
• Neurotransmitter binding causes ion channels
to open, generating a postsynaptic potential
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Fig. 48-16
Terminal branch
of presynaptic
neuron
E2
E1
E2
Membrane potential (mV)
Postsynaptic
neuron
E1
E1
E1
E2
E2
I
I
Axon
hillock
I
I
0
Action
potential
Threshold of axon of
postsynaptic neuron
Action
potential
Resting
potential
–70
E1
E1
(a) Subthreshold, no
summation
E1
E1
(b) Temporal summation
E1 + E2
(c) Spatial summation
E1
I
E1 + I
(d) Spatial summation
of EPSP and IPSP
Acetylcholine
• Acetylcholine is a common neurotransmitter
in vertebrates and invertebrates
• In vertebrates it is usually an excitatory
transmitter
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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
• Two amino acids are known to function as
major neurotransmitters in the CNS: gammaaminobutyric acid (GABA) and glutamate
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
You should now be able to:
1. Distinguish among the following sets of terms:
sensory neurons, interneurons, and motor
neurons; membrane potential and resting
potential; ungated and gated ion channels;
electrical synapse and chemical synapse;
EPSP and IPSP; temporal and spatial
summation
2. Explain the role of the sodium-potassium
pump in maintaining the resting potential
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3. Describe the stages of an action potential;
explain the role of voltage-gated ion channels
in this process
4. Explain why the action potential cannot travel
back toward the cell body
5. Describe saltatory conduction
6. Describe the events that lead to the release of
neurotransmitters into the synaptic cleft
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7. Explain the statement: “Unlike action
potentials, which are all-or-none events,
postsynaptic potentials are graded”
8. Name and describe five categories of
neurotransmitters
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings