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The Nervous System > How Neurons Communicate
How Neurons Communicate
• Nerve Impulse Transmission within a Neuron: Resting Potential
• Nerve Impulse Transmission within a Neuron: Action Potential
• Synaptic Transmission
• Signal Summation
• Synaptic Plasticity
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The Nervous System > How Neurons Communicate
Nerve Impulse Transmission within a Neuron: Resting Potential
• When the neuronal membrane is at rest, the resting potential is negative due to
the accumulation of more sodium ions outside the cell than potassium ions inside
the cell.
• Potassium ions diffuse out of the cell at a much faster rate than sodium ions
diffuse into the cell because neurons have many more potassium leakage
channels than sodium leakage channels.
• Sodium-potassium pumps move two potassium ions inside the cell as three
sodium ions are pumped out to maintain the negatively-charged membrane inside
the cell; this helps maintain the resting potential.
Ion channel configurations
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The Nervous System > How Neurons Communicate
Nerve Impulse Transmission within a Neuron: Action Potential
• Action potentials are formed when a stimulus causes the cell membrane to
depolarize past the threshold of excitation, causing all sodium ion channels to
open.
• When the potassium ion channels are opened and sodium ion channels are
closed, the cell membrane becomes hyperpolarized as potassium ions leave the
cell; the cell cannot fire during this refractory period.
• The action potential travels down the axon as the membrane of the axon
depolarizes and repolarizes.
• Myelin insulates the axon to prevent leakage of the current as it travels down the
Formation of an action potential
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axon.
• Nodes of Ranvier are gaps in the myelin along the axons; they contain sodium
and potassium ion channels, allowing the action potential to travel quickly down
the axon by jumping from one node to the next.
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The Nervous System > How Neurons Communicate
Synaptic Transmission
• Both presynaptic and post-synaptic cell membranes are depolarized by sodium
ions before and after transmission of the signal across a chemical synapse.
• Calcium ions play a role in signal transmission across chemical synapses by
signaling a cascade that eventually releases the neurotransmitter that binds to
receptors and opens ion channels on the postsynaptic membrane.
• Neurotransmitters facilitate the signal transmission across chemical synapses.
• In an electrical synapse, gap junctions, which are formed by the channel proteins
connecting the presynaptic and postsynaptic membranes of two neurons, allow
the current to pass directly from one neuron to the next.
Synaptic vesicles inside a Nneuron
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The Nervous System > How Neurons Communicate
Signal Summation
• Simultaneous impulses may add together from different places on the neuron to
reach the threshold of excitation during spatial summation.
• When individual impulses cannot reach the threshold of excitation on their own,
they can can add up at the same location on the neuron over a short time; this is
known as temporal summation.
• The action potential of a neuron is fired only when the net change of excitatory
and inhibitory impulses is non-zero.
Signal summation at the axon hillock
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The Nervous System > How Neurons Communicate
Synaptic Plasticity
• Short-term synaptic enhancement occurs when the amount of available
neurotransmitter is increased, while short-term synaptic depression occurs when
the amount of vesicles with neurotransmitters is decreased.
• Synapses are strengthened in long-term potentiation (LTP) when AMPA receptors
(which bind to negatively-charged glutamate) are increased, allowing more
calcium ions to enter the cell, causing a higher excitatory response.
• Long-term depression (LTD) occurs when the AMPA receptors are decreased,
which decreases the amount of calcium ions entering the cell, weakening the
synaptic response to the release of neurotransmitters.
• The strengthening and weakening of synapses over time controls learning and
Long-term potentiation and depression
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memory in the brain.
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Appendix
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The Nervous System
Key terms
• action potential a short term change in the electrical potential that travels along a cell
• axon hillock the specialized part of the soma of a neuron that is connected to the axon and where impulses are added together
• depolarization a decrease in the difference in voltage between the inside and outside of the neuron
• hyperpolarize to increase the polarity of something, especially the polarity across a biological membrane
• ion channel a protein complex or single protein that penetrates a cell membrane and catalyzes the passage of specific ions
through that membrane
• long-term depression a long-term weakening of a synaptic connection
• long-term potentiation a long-lasting (hours in vitro, weeks to months in vivo) increase, typically in amplitude, of the response of
a postsynaptic neuron to a particular pattern of stimuli from a presynaptic neuron
• membrane potential the difference in electrical potential across the enclosing membrane of a cell
• neurotransmitter any substance, such as acetylcholine or dopamine, responsible for sending nerve signals across a synapse
between two neurons
• node of Ranvier a small constriction in the myelin sheath of axons
• plasticity the property of neuron that allows it to be strengthened or weakened
• postsynaptic in a synapse, of or pertaining to the neuron that bears receptors for the neurotransmitter to receive the signal
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The Nervous System
• presynaptic in a synapse, of or pertaining to the neuron that sends the signal by releasing the neurotransmitter into the synaptic
cleft
• resting potential the nearly latent membrane potential of inactive cells
• saltatory conduction the process of regenerating the action potential at each node of Ranvier
• spatial summation the effect when simultaneous impulses received at different places on the neuron add up to fire the neuron
• synapse the junction between the terminal of a neuron and either another neuron or a muscle or gland cell, over which nerve
impulses pass
• temporal summation the effect when impulses received at the same place on the neuron add up
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The Nervous System
Ion channel configurations
Voltage-gated ion channels are closed at the resting potential and open in response to changes in membrane voltage.After activation, they become
inactivated for a brief period and will no longer open in response to a signal.
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The Nervous System
Membrane potential
The (a) resting membrane potential is a result of different concentrations of Na+ and K+ ions inside and outside the cell.A nerve impulse causes Na+ to
enter the cell, resulting in (b) depolarization.At the peak action potential, K+ channels open and the cell becomes (c) hyperpolarized.
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The Nervous System
Formation of an action potential
The formation of an action potential can be divided into five steps.(1) A stimulus from a sensory cell or another neuron causes the target cell to
depolarize toward the threshold potential.(2) If the threshold of excitation is reached, all Na+ channels open and the membrane depolarizes.(3) At the
peak action potential, K+ channels open and K+ begins to leave the cell.At the same time, Na+ channels close.(4) The membrane becomes
hyperpolarized as K+ ions continue to leave the cell.The hyperpolarized membrane is in a refractory period and cannot fire.(5) The K+ channels close
and the Na+/K+ transporter restores the resting potential.
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The Nervous System
Nodes of Ranvier
Nodes of Ranvier are gaps in myelin coverage along axons.Nodes contain voltage-gated K+ and Na+ channels.Action potentials travel down the axon by
jumping from one node to the next.
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The Nervous System
Action potential travel along a neuronal axon
The action potential is conducted down the axon as the axon membrane depolarizes, then repolarizes.
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The Nervous System
Synaptic vesicles inside a Nneuron
This pseudocolored image taken with a scanning electron microscope shows an axon terminal that was broken open to reveal synaptic vesicles (blue
and orange) inside the neuron.
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The Nervous System
Communication at a chemical synapse
Communication at chemical synapses requires release of neurotransmitters.When the presynaptic membrane is depolarized, voltage-gated Ca2+
channels open and allow Ca2+ to enter the cell.The calcium entry causes synaptic vesicles to fuse with the membrane and release neurotransmitter
molecules into the synaptic cleft.The neurotransmitter diffuses across the synaptic cleft and binds to ligand-gated ion channels in the postsynaptic
membrane, resulting in a localized depolarization or hyperpolarization of the postsynaptic neuron.
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The Nervous System
Signal summation at the axon hillock
A single neuron can receive both excitatory and inhibitory inputs from multiple neurons.All these inputs are added together at the axon hillock.If the
EPSPs are strong enough to overcome the IPSPs and reach the threshold of excitation, the neuron will fire.
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The Nervous System
Long-term potentiation and depression
Calcium entry through postsynaptic NMDA receptors can initiate two different forms of synaptic plasticity: long-term potentiation (LTP) and long-term
depression (LTD).LTP arises when a single synapse is repeatedly stimulated.This stimulation causes a calcium- and CaMKII-dependent cellular
cascade, which results in the insertion of more AMPA receptors into the postsynaptic membrane.The next time glutamate is released from the
presynaptic cell, it will bind to both NMDA and the newly-inserted AMPA receptors, thus depolarizing the membrane more efficiently.LTD occurs when
few glutamate molecules bind to NMDA receptors at a synapse (due to a low firing rate of the presynaptic neuron).The calcium that does flow through
NMDA receptors initiates a different calcineurin and protein phosphatase 1-dependent cascade, which results in the endocytosis of AMPA receptors.This
makes the postsynaptic neuron less responsive to glutamate released from the presynaptic neuron.
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The Nervous System
The resting potential may be achieved by:
A) the sodium-potassium pump maintaining a higher level of K+ ions
inside the cell than outside.
B) ion channels acting as batteries to allow ion pumps to transmit a
signal.
C) ion channels allowing any ion to flow into the cell.
D) the flow of negative ions into the cell.
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The Nervous System
The resting potential may be achieved by:
A) the sodium-potassium pump maintaining a higher level of K+ ions
inside the cell than outside.
B) ion channels acting as batteries to allow ion pumps to transmit a
signal.
C) ion channels allowing any ion to flow into the cell.
D) the flow of negative ions into the cell.
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The Nervous System
For a neuron to fire an action potential, its membrane must reach:
A) inhibitory postsynaptic potential.
B) the threshold of excitation.
C) hyperpolarization.
D) the refractory period.
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The Nervous System
For a neuron to fire an action potential, its membrane must reach:
A) inhibitory postsynaptic potential.
B) the threshold of excitation.
C) hyperpolarization.
D) the refractory period.
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The Nervous System
During transmission across a chemical synapse:
A) All of these.
B) the neurotransmitter is released, causing the postsynaptic ion
channels to open.
C) gap junctions block current from traveling from one cell to the next.
D) sodium ions hyperpolarize the post-synaptic cell membranes.
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The Nervous System
During transmission across a chemical synapse:
A) All of these.
B) the neurotransmitter is released, causing the postsynaptic ion
channels to open.
C) gap junctions block current from traveling from one cell to the next.
D) sodium ions hyperpolarize the post-synaptic cell membranes.
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The Nervous System
Signal summation occurs when:
A) excitatory and inhibitory impulses are received in equal amounts.
B) All of these.
C) the impulses received by different places on the neuron add up to fire
the neuron.
D) the threshold for excitation is not reached by multiple impulses.
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The Nervous System
Signal summation occurs when:
A) excitatory and inhibitory impulses are received in equal amounts.
B) All of these.
C) the impulses received by different places on the neuron add up to fire
the neuron.
D) the threshold for excitation is not reached by multiple impulses.
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The Nervous System
A decrease in AMPA receptors causes fewer calcium ions to enter
the neuron, weakening the synaptic response in the process of:
A) long-term depression.
B) long-term potentiation and depression.
C) None of these.
D) long-term potentiation.
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The Nervous System
A decrease in AMPA receptors causes fewer calcium ions to enter
the neuron, weakening the synaptic response in the process of:
A) long-term depression.
B) long-term potentiation and depression.
C) None of these.
D) long-term potentiation.
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The Nervous System
Attribution
• Connexions. "How Neurons Communicate." CC BY 3.0 http://cnx.org/content/m44748/latest/?collection=col11448/latest
• Wikipedia. "Resting membrane potential." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Resting_membrane_potential
• Wiktionary. "resting potential." CC BY-SA 3.0 http://en.wiktionary.org/wiki/resting+potential
• Wiktionary. "ion channel." CC BY-SA 3.0 http://en.wiktionary.org/wiki/ion+channel
• Wiktionary. "membrane potential." CC BY-SA 3.0 http://en.wiktionary.org/wiki/membrane+potential
• Connexions. "How Neurons Communicate." CC BY 3.0 http://cnx.org/content/m44748/latest/?collection=col11448/latest
• Wikipedia. "Action potential." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Action_potential
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/saltatory-conduction
• Wiktionary. "node of Ranvier." CC BY-SA 3.0 http://en.wiktionary.org/wiki/node+of+Ranvier
• Wiktionary. "hyperpolarize." CC BY-SA 3.0 http://en.wiktionary.org/wiki/hyperpolarize
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/depolarization
• Wiktionary. "action potential." CC BY-SA 3.0 http://en.wiktionary.org/wiki/action+potential
• Connexions. "How Neurons Communicate." CC BY 3.0 http://cnx.org/content/m44748/latest/?collection=col11448/latest
• Wikipedia. "Synaptic transmission." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Synaptic_transmission
• Wiktionary. "postsynaptic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/postsynaptic
• Wiktionary. "presynaptic." CC BY-SA 3.0 http://en.wiktionary.org/wiki/presynaptic
• Wiktionary. "neurotransmitter." CC BY-SA 3.0 http://en.wiktionary.org/wiki/neurotransmitter
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The Nervous System
• Wiktionary. "synapse." CC BY-SA 3.0 http://en.wiktionary.org/wiki/synapse
• Connexions. "How Neurons Communicate." CC BY 3.0 http://cnx.org/content/m44748/latest/?collection=col11448/latest
• Wikipedia. "Synaptic transmission." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Synaptic_transmission
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/axon-hillock
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/spatial-summation
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/temporal-summation
• Connexions. "How Neurons Communicate." CC BY 3.0 http://cnx.org/content/m44748/latest/?collection=col11448/latest
• Wikipedia. "Synaptic plasticity." CC BY-SA 3.0 http://en.wikipedia.org/wiki/Synaptic_plasticity
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/plasticity
• Boundless Learning. "Boundless." CC BY-SA 3.0 http://www.boundless.com//biology/definition/long-term-depression
• Wiktionary. "long-term potentiation." CC BY-SA 3.0 http://en.wiktionary.org/wiki/long-term+potentiation
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