1. Neurons, Synapses, and Signaling
Chapter 48
Neurons, Synapses, and Signaling

2. 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

3. Many animals have a complex nervous system which consists of:
Fig. 48-3
Sensory input
Integration
Sensor
Motor output
Figure 48.3 Summary of Information Processing
Nervous systems process information in three stages: sensory input, integration, and motor output
Sensors detect external stimuli and internal conditions and transmit information along sensory neurons
Sensory information is sent to the brain or ganglia, where interneurons integrate the information
Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity
Many animals have a complex nervous system which consists of:
A central nervous system (CNS) where integration takes place; this includes the brain and a nerve cord
A peripheral nervous system (PNS), which brings information into and out of the CNS
Effector
Peripheral nervous
system (PNS)
Central nervous
system (CNS)

4. Dendrites Stimulus Presynaptic Nucleus cell Axon hillock Cell body
Fig. 48-4
Dendrites
Stimulus
Nucleus
Presynaptic
cell
Axon
hillock
Cell
body
Axon
Synapse
Synaptic terminals
Figure 48.4 Neuron Structure and Organization
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
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
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
Structure fits function
many entry points for signal
one path out
transmits signal
Postsynaptic cell
Neurotransmitter

5. Dendrites Axon Cell body Portion of axon Cell bodies of
Fig. 48-5
Dendrites
Axon
Cell
body
Portion
of axon
Figure 48.5 Structural diversity of neurons
INFORMATION PROCESSING:
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 (muscles or glands)
80 µm
Cell bodies of
overlapping neurons
Sensory neuron
Interneurons
Motor neuron

6. 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

7. 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
CYTOSOL
EXTRACELLULAR FLUID
[Na+] 15 mM
[K+] 150 mM
[Cl–] 10 mM
[A–] 100 mM
[Na+] 150 mM
[K+] 5 mM
[Cl–] 120 mM – +
Plasma membrane
Figure 48.10

8. [Na+] 150 mM [Cl–] 120 mM [K+] 140 mM [A–] 100 mM Fig. 48-6
Key
Na+
Sodium-
potassium
pump
Potassium
channel
Sodium
channel K+
OUTSIDE
CELL
OUTSIDE
CELL
[K+]
5 mM
[Na+]
150 mM
[Cl–]
120 mM
INSIDE
CELL
[K+]
140 mM
[Na+]
15 mM
[Cl–]
10 mM
[A–]
100 mM
Figure 48.6 The basis of the membrane 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
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
In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady
INSIDE
CELL
(a)
(b)

9. 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

10. Action potentials are signals that carry information along axons
Fig. 48-9
Stimuli
Stimuli
Strong depolarizing stimulus
+50
+50
+50
Action
potential
Membrane potential (mV)
Membrane potential (mV)
Membrane potential (mV)
–50
Threshold
–50
Threshold
–50
Threshold
Resting
potential
Resting
potential
Resting
potential
Figure 48.9 Graded potentials and an action potential in a neuron
(A)
When gated K+ channels open, K+ diffuses out, making the inside of the cell more negative. This is hyperpolarization, an increase in magnitude of the membrane potential
(B)
Other stimuli trigger a depolarization, a reduction in the magnitude of the membrane potential
For example, depolarization occurs if gated Na+ channels open and Na+ diffuses into the cell
Graded potentials are changes in polarization where the magnitude of the change varies with the strength of the stimulus
(C)
Voltage-gated Na+ and K+ channels respond to a change in membrane potential
When a stimulus depolarizes the membrane, Na+ channels open, allowing Na+ to diffuse into the cell
The movement of Na+ into the cell increases the depolarization and causes even more Na+ channels to open
A strong stimulus results in a massive change in membrane voltage called an action potential
An action potential occurs if a stimulus causes the membrane voltage to cross a particular threshold
An action potential is a brief all-or-none depolarization of a neuron’s plasma membrane
Action potentials are signals that carry information along axons
Hyperpolarizations
Depolarizations
–100
–100
–100 1 2 3 4 5 1 2 3 4 5 1 2 3 4 5 6
Time (msec)
Time (msec)
Time (msec)
(a) Graded hyperpolarizations
(b) Graded depolarizations
(c) Action potential

11. 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

12. Fig
Key
Na+ K+ 3
Rising phase of the action potential 4
Falling phase of the action potential
+50
Action
potential 3
Membrane potential
(mV) 2 4
Threshold
–50 1 1 5
Resting potential 2
Depolarization
Figure The role of voltage-gated ion channels in the generation of an action potential
(1) RESTING STATE - The activation gates on the Na+ and K+ channels are closed, and the membrane’s resting potential is maintained.
(2) 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.
(3) RISING PHASE OF 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.
(4) FALLING PHASE OF 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.
(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.
During the refractory period after an action potential, a second action potential cannot be initiated. The refractory period is a result of a temporary inactivation of the Na+ channels.
–100
Time
Extracellular fluid
Sodium
channel
Potassium
channel
Plasma
membrane
Cytosol
Inactivation loop 5
Undershoot 1
Resting state

13. Figure 48.11 Conduction of an action potential
Axon
Plasma
membrane
Action
potential
Na+
Cytosol
Action
potential K+
Na+
Figure Conduction of an action potential
An action potential can travel long distances by regenerating itself along the axon - action potentials travel in only one direction: toward the synaptic terminals.
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. K+
Action
potential K+
Na+ K+

14. Figure 48.12 Schwann cells and the myelin sheath
Node of Ranvier
Layers of myelin
Axon
Schwann
cell
Schwann
cell
Nodes of
Ranvier
Nucleus of
Schwann cell
Axon
Myelin sheath
Figure Schwann cells and the myelin sheath
The speed of an action potential increases with the axon’s diameter
In vertebrates, axons are insulated by a myelin sheath, which causes an action potential’s speed to increase
Myelin sheaths are made by glia— oligodendrocytes in the CNS and Schwann cells in the PNS
Action potentials are formed only at nodes of Ranvier, gaps in the myelin sheath where voltage-gated Na+ channels are found
Action potentials in myelinated axons jump between the nodes of Ranvier in a process called saltatory conduction
A myelin sheath is a many-layered coating that wraps around the axon of a neuron and very efficiently insulates it.
At nodes of Ranvier, the axonal membrane is uninsulated and therefore capable of generating electrical activity.
0.1 µm

15. Schwann cell Depolarized region (node of Ranvier) Cell body Myelin
Fig
Schwann cell
Depolarized region
(node of Ranvier)
Cell body
Myelin
sheath
Axon
Figure Saltatory conduction
Structure fits function in the vertebrate neuron:
many entry points for signal (dendrites)
one path out – signal travels in one direction only (axon)
transmits signal quickly (myelin sheath) and effectively (nodes of Ranvier)

16. 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

17. Synaptic vesicles containing Presynaptic neurotransmitter membrane
Fig 5
Na+ K+
Synaptic vesicles
containing
neurotransmitter
Presynaptic
membrane
Voltage-gated
Ca2+ channel
Postsynaptic
membrane 1
Ca2+ 4 2 6
Figure A chemical synapse
When an action potential depolarizes the plasma membrane of the synaptic terminal, it
opens volted-gated Ca2+ channels in the membrane triggering an influx of Ca2+
The elevated Ca concentration in the terminal causes synaptic vesicles to fuse with the presynaptic membrane
The vesicles release neurotransmitter into the synaptic cleft
The neurotransmitter binds to the ion-channels in the postsynaptic membrane, opening the channels and allowing the flow of both Na and K
When the neurotransmitter releases from the receptors, the channels close and synaptic transmission ends.
Synaptic
cleft 3
Ligand-gated
ion channels

18. 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

19. There are five major classes of 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

20. Can be inhibitory or excitatory Biogenic amines
Table 48-1
Acetylcholine
Is one of the most common neurotransmitters in both vertebrates and invertebrates
Can be inhibitory or excitatory
Biogenic amines
Include epinephrine, norepinephrine, dopamine, and serotonin
Are active in the CNS and PNS
Various amino acids and peptides
Are active in the brain
Gases such as nitric oxide and carbon monoxide
Are local regulators in the PNS