Слайд 1Chapter 48
Neurons, Synapses, and Signaling
Слайд 2Overview: 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).
Слайд 3The cone snail is a deadly predator. Why?
Слайд 4The 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.
Signals Travel along a Path
Слайд 5Neuron organization and structure reflect function in information transfer
The squid
possesses extremely large nerve cells and is a good model
for studying neuron function.
Nervous systems process information in three stages: sensory input, integration, and motor output.
Слайд 6Squid Nervous System
Nerves
with giant axons
Ganglia
Mantle
Eye
Brain
Arm
Nerve
Слайд 7Sensors detect external stimuli and internal conditions and transmit information
along sensory neurons.
Sensory information is sent to the brain or
ganglia, where interneurons integrate / process the information.
Motor output leaves the brain or ganglia via motor neurons, which trigger muscle or gland activity = response.
Слайд 8Many 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.
Слайд 9Information Processing
Sensor:
Detects stimulus
Sensory input
Integration
Processing
Effector:
Does response
Motor output
Peripheral nervous
system (PNS)
Central nervous
system
(CNS)
Слайд 10 Neuron - Structure / Function Signal Transmission
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 from its terminal branches to other cells at synapses.
An axon joins the cell body at the axon hillock.
Слайд 11Neurons
Dendrites
Stimulus
Nucleus
Cell
body
Axon
hillock
Presynaptic cell
Axon
Synaptic terminals
Synapse
Postsynaptic cell
Neurotransmitters
Слайд 12The 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.
A synapse is a junction between cells.
Слайд 13 Structural diversity of neurons
Dendrites
Axon
Cell
body
Sensory neuron
Interneurons
Portion
of axon
Cell bodies of
overlapping
neurons
80 µm
Motor neuron
Слайд 14Ion pumps and ion channels maintain the
resting potential of
a neuron
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.
Слайд 15 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.
Слайд 16The 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 / inside the neuron.
Слайд 17The Basis of the Membrane Potential
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
(a)
(b)
OUTSIDE
CELL
Na+
Key
K+
Sodium-
potassium
pump
Potassium
channel
Sodium
channel
INSIDE
CELL
Слайд 18OUTSIDE
CELL
Na+
Key
K+
Sodium-
potassium
pump
Potassium
channel
Sodium
channel
INSIDE
CELL
Слайд 19Modeling of the Resting Potential
Resting potential can be modeled by
an artificial membrane that separates two chambers.
At equilibrium, both the
electrical and chemical gradients are balanced.
In a resting neuron, the currents of K+ and Na+ are equal and opposite, and the resting potential across the membrane remains steady.
Слайд 20Action 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.
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 / increase in difference between sides / farther from threshold.
Слайд 21Graded potentials and an action potential in a neuron
Stimuli
+50
+50
Stimuli
0
0
Membrane potential
(mV)
Membrane potential (mV)
–50
–50
Threshold
Threshold
Resting
potential
Resting
potential
Hyperpolarizations
–100
–100
0
1
2
3
4
5
Time (msec)
(a) Graded Hyperpolarizations
Time (msec)
(b) Graded Depolarizations
Depolarizations
0
1
2
3
4
5
Strong depolarizing
stimulus
+50
0
Membrane potential (mV)
–50
Threshold
Resting
potential
–100
Time (msec)
0
1
2
3
4
5
6
(c) Action potential
Action
potential
Слайд 22Other 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.
Слайд 23Stimuli
+50
Membrane potential (mV)
–50
Threshold
Resting
potential
Depolarizations
–100
0
2
3
4
Time (msec)
(b) Graded depolarizations – magnitude of the
change varies
with the strength of the stimulus.
1
5
0
Слайд 24Production of Action Potentials
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 = signal.
Слайд 25Strong depolarizing stimulus
+50
Membrane potential (mV)
–50
Threshold
Resting
potential
–100
0
2
3
4
Time (msec)
(c) Action potential = change
in membrane voltage
1
5
0
Action
potential
6
Слайд 26An 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.
Слайд 27Generation 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.
Слайд 28The role of voltage-gated ion channels in the generation of
an action potential
Key
Na+
K+
+50
Action
potential
Threshold
0
1
4
5
1
–50
Resting potential
Membrane potential
(mV)
–100
Time
Extracellular fluid
Plasma
membrane
Cytosol
Inactivation loop
Resting state
Sodium
channel
Potassium
channel
Depolarization
Rising phase of
the action potential
Falling phase of the action potential
5
Undershoot
2
3
2
1
3
4
Слайд 29At resting potential
Most voltage-gated Na+ and K+ channels are closed,
but some K+ channels (not voltage-gated) are open.
Слайд 30Voltage-gated Na+ channels open first and Na+ flows into the
cell.
During the rising phase, the threshold is crossed, and the
membrane potential increases.
During the falling phase, voltage-gated Na+ channels become inactivated; voltage-gated K+ channels open, and K+ flows out of the cell.
Cell is now repolarized but is not normal until Na+ K+ pump restores original resting potential.
When an action potential is generated
Слайд 31During the refractory period after an action potential, a second
action potential cannot be initiated. This ensures that an impulse
moves along the axon in one direction only.
The refractory period is a result of a temporary inactivation of the Na+ channels.
The refractory period is a period of “normal” repolarization when the Na+ K+ pump restores the membrane to its original polarized condition.
Слайд 32Conduction of Action Potentials
An action potential can travel long distances
by regenerating itself along the axon.
At the site where the
action potential is generated, usually the axon hillock, an electrical current depolarizes the neighboring region of the axon membrane.
Inactivated Na+ channels behind the zone of depolarization prevent the action potential from traveling backwards. Action potentials travel in only one direction: toward the synaptic terminals.
Слайд 33Conduction of an
Action Potential
Signal
Transmission
Axon
Plasma
membrane
Cytosol
Action
potential
Na+
Action
potential
Na+
K+
K+
Action
potential
K+
K+
Na+
Слайд 34Conduction Speed
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.
Слайд 35Schwann cells and the myelin sheath
Axon
Myelin sheath
Schwann
cell
Nodes of
Ranvier
Schwann
cell
Nucleus of
Schwann cell
Node
of Ranvier
Layers of myelin
Axon
Слайд 36Action 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.
Слайд 37Saltatory conduction
Cell body
Schwann cell
Depolarized region
(node of Ranvier)
Myelin
sheath
Axon
Слайд 38Neurons 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 = synapse.
Most synapses are chemical synapses.
Слайд 39The 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.
Слайд 40Chemical synapse
Voltage-gated
Ca2+ channel
Ca2+
1
2
3
4
Synaptic
cleft
Ligand-gated
ion channels
Postsynaptic
membrane
Presynaptic
membrane
Synaptic vesicles
containing
neurotransmitter
5
6
K+
Na+
Слайд 41Generation 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.
Слайд 42Postsynaptic potentials fall into two categories:
Excitatory postsynaptic potentials (EPSPs) are
depolarizations that bring the membrane potential toward threshold.
Inhibitory postsynaptic potentials
(IPSPs) are hyperpolarizations that move the membrane potential farther from threshold.
Слайд 43After release, the neurotransmitter
May diffuse out of the synaptic cleft
May
be taken up by surrounding cells
May be degraded by
enzymes
Слайд 44Summation of Postsynaptic Potentials
Unlike action potentials, postsynaptic potentials are graded
and do not regenerate.
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.
If two EPSPs are produced in rapid succession, an effect called temporal summation occurs.
Слайд 45Summation of postsynaptic potentials
Terminal branch
of presynaptic
neuron
E1
E2
I
Postsynaptic
neuron
Threshold of axon of
postsynaptic neuron
Resting
potential
E1
E1
0
–70
Membrane
potential (mV)
(a) Subthreshold, no
summation
(b) Temporal summation
E1
E1
Action
potential
I
Axon
hillock
E1
E2
E2
E1
I
Action
potential
E1 +
E2
(c) Spatial summation
I
E1
E1 + I
(d) Spatial summation
of EPSP and IPSP
E2
E1
I
Слайд 46In spatial summation, EPSPs produced nearly simultaneously by different synapses
on the same postsynaptic neuron add together. The combination of
EPSPs through spatial and temporal summation can trigger an action potential.
Through summation, an IPSP can counter the effect of an EPSP. The summed effect of EPSPs and IPSPs determines whether an axon hillock will reach threshold and generate an action potential.
Слайд 47Modulated / 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.
Effects of indirect synaptic transmission have a slower onset but last longer.
Слайд 48Neurotransmitters
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.
Gases such as nitric oxide and carbon monoxide are local regulators in the PNS.
Слайд 50Acetylcholine
Acetylcholine is a common neurotransmitter in vertebrates and invertebrates.
In vertebrates
it is usually an excitatory transmitter.
Common at the neuro-muscular
junction.
Слайд 51Biogenic Amines & Amino Acids
Biogenic amines include epinephrine, norepinephrine, dopamine,
and serotonin. They are active in the CNS and PNS.
Two
amino acids are known to function as major neurotransmitters in the CNS: gamma-aminobutyric acid (GABA) and glutamate.
Слайд 52Neuropeptides
Several neuropeptides, relatively short chains of amino acids, also function
as neurotransmitters.
Neuropeptides include substance P and endorphins, which both affect
our perception of pain.
Opiates bind to the same receptors as endorphins and can be used as painkillers.
Слайд 53Review
Action potential
Falling
phase
Rising
phase
Threshold (–55)
Resting
potential
Undershoot
Time (msec)
Depolarization
–70
–100
–50
0
+50
Membrane potential (mV)
Слайд 54You should now be able to:
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; summation.
Explain the role of the sodium-potassium pump in maintaining the resting potential.
Слайд 55Describe the stages of an action potential; explain the role
of voltage-gated ion channels in this process.
Explain why the action
potential cannot travel back toward the cell body.
Describe saltatory conduction.
Describe the events that lead to the release of neurotransmitters into the synaptic cleft.
Слайд 56Explain the statement: “Unlike action potentials, which are all-or-none events,
postsynaptic potentials are graded.”
Name and describe five categories of neurotransmitters.
![The Basis of the Membrane PotentialOUTSIDECELL [K+]5 mM Na+150 mM [Cl–]120 mMINSIDECELL K+140 mM [Na+]15 mM](/img/thumbs/36c385354717262ba327c3da96798cb2-800x.jpg "Chapter 48 The Basis of the Membrane PotentialOUTSIDECELL [K+]5 mM Na+150 mM [Cl–]120")
