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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
Overview: Lines of Communication
• 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)
• The transmission of information depends on the
path of neurons along which a signal travels
• Processing of information takes place in simple
clusters of neurons called ganglia or a more
complex organization of neurons called a brain
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-3
Sensory input
Integration
Sensor
Motor output
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)
Fig. 48-4
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic
cell
Axon
Synapse
Synaptic terminals
Postsynaptic cell
Neurotransmitter
Fig. 48-5
Dendrites
Axon
Cell
body
Portion
of axon
Sensory neuron
Interneurons
Cell bodies of
overlapping neurons
80 µm
Motor neuron
Concept 48.2: Membrane & Resting Potential
• Every cell has a voltage (difference in electrical
charge) across its plasma membrane called a
membrane potential
– Messages are transmitted as changes in
membrane potential
• The resting potential is the membrane
potential of a neuron not sending signals
– Ion pumps and ion channels maintain the
resting potential of a neuron
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
The Resting Potential
•
The resting potential is the membrane potential of a neuron that is not
transmitting signals
•
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
–
+
10 mM
–
[Cl–]
+ 120 mM
[A–]
100 mM
–
+
[Cl–]
Plasma
membrane
Figure 48.10
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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
Concept 48.3: Action Potentials
• Action potentials are the signals conducted
by axons
• Neurons contain gated ion channels that
open or close in response to stimuli
– Membrane potential changes in response to opening
or closing of these channels
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-9
http://bcs.whfreeman.com/thelifewire/content/chp44/4402s.swf
Stimuli
Stimuli
Strong depolarizing stimulus
+50
+50
+50
0
–50
Threshold
Membrane potential (mV)
Membrane potential (mV)
Membrane potential (mV)
Action
potential
0
–50
Resting
potential
Threshold
0
–50
Resting
potential
Resting
potential
Depolarizations
Hyperpolarizations
–100
–100
0
1
2 3 4 5
Time (msec)
(a) Graded hyperpolarizations
Threshold
–100
0
1 2 3 4
Time (msec)
(b) Graded depolarizations
5
0
(c) Action potential
1
2 3 4 5
Time (msec)
6
Generation of Action Potentials: A Closer Look
• A neuron can produce hundreds of action
potentials per second
• The frequency of action potentials can reflect
the strength of a stimulus
• An action potential can be broken down into a
series of stages
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-10-5
Key
Na+
K+
3
4
Rising phase of the action potential
Falling phase of the action potential
Membrane potential
(mV)
+50
Action
potential
–50
2
2
4
Threshold
1
1
5
Resting potential
Depolarization
Extracellular fluid
3
0
–100
Sodium
channel
Time
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop
5
1
Resting state
Undershoot
Fig. 48-11-3 - http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter14/animation__the_nerve_impulse.html
Axon
Plasma
membrane
Action
potential
Cytosol
Na+
K+
Action
potential
Na+
K+
K+
Action
potential
Na+
K+
Fig. 48-12
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Axon
Nodes of
Myelin sheath Ranvier
Schwann
cell
Nucleus of
Schwann cell
0.1 µm
Fig. 48-13
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
Synapses – Communication Between Neurons
• Neurons communicate with other cells at
synapses
• The vast majority of synapses
– Are chemical synapses
• In a chemical synapse, a presynaptic neuron
releases chemical neurotransmitters, which are
stored in the synaptic terminal
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Fig. 48-15
http://bcs.whfreeman.com/thelifewire/content/chp44/4402003.html
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+
Synapses – Communication Between Neurons
• After release, the neurotransmitter
– May diffuse out of the synaptic cleft
– May be taken up by surrounding cells
– May be degraded by enzymes
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Neurotransmitters
• The same neurotransmitter can produce different
effects in different types of cells
• There are five major classes of
neurotransmitters:
– acetylcholine,
– biogenic amines,
– amino acids,
– neuropeptides,
– and gases
Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
Table 48-1