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Chapter 2
Synapses
© Cengage Learning 2016
© Cengage Learning 2016
Action Potentials
• We have been talking about action
potentials and how they allow an
electrical impulse to travel from the
dendrites to the end plates of a neuron.
• These action potentials do not just move
down a single neuron and then stop.
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Action Potentials
• Your brain is a network of millions of
neurons that are, in essence, talking to
one another.
• Action potentials are the message and
the synapse is how the message is
transferred from one neuron to another.
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2.1 The Concept of the Synapse
• Neurons communicate by transmitting
chemicals at junctions, called “synapses”
– The term was coined by Charles Scott
Sherrington in 1906 to describe the
specialized gap that existed between neurons
– Sherrington’s discovery was a major feat of
scientific reasoning
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The Properties of Synapses
• Sherrington
– Investigated how neurons communicate with
each other by studying reflexes (automatic
muscular responses to stimuli) in a process
known as a reflex arc
• Example
– Leg flexion reflex: a sensory neuron excites a
second neuron, which excites a motor neuron,
which excites a muscle
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Synaptic Transmission
• The terminal branches of a single
neuron allow it to join with many
different neurons allowing the
message from one to be multiplied
very quickly by sending it to many
other neurons.
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Synaptic Transmission
• Small vesicles in the end plates of neurons
contain chemical messengers called
neurotransmitters.
• As an impulse moves along a neuron, it
causes the release of these
neurotransmitters from the end plates.
• Neurotransmitters are released from the
presynaptic neuron into the synaptic cleft.
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• Presynaptic neuron: neuron that delivers
the synaptic transmission
• Postsynaptic neuron: neuron that receives
the message
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Synaptic Transmission
• Once neurotransmitters are in the
synapse, they diffuse across it until
they attach to receptors on the
dendrites, axon, or cell body of the
postsynaptic neuron.
• This binding of neurotransmitters
creates a depolarization of the
postsynaptic neuron stimulating an
action potential and allowing the
message to move on.
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Synaptic Transmission Stages:
1. Action potential moves toward end plates
stimulating calcium channels to open stimulating
movement of vesicles.
2. Vesicles with neurotransmitter move towards
endplate of presynaptic neuron.
3. Neurotransmitters are released into synapse
through exocytosis.
4. Neurotransmitters diffuse across synaptic cleft.
5. Neurotransmitters bind to receptors on
postsynaptic neuron.
6. Bound neurotransmitter stimulates response.
7. Neurotransmitter fragments released after use.
8. Fragments move back to presynaptic neuron
and re-enter cell through endocytosis for
recycling.
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Synaptic Transmission
• A synaptic cleft is extremely
small (about 20nm wide).
• Even with this small space,
diffusion is a slow process.
• So when neurotransmitters
are diffusing across the
synapse, the transmission of
the message slows down a
bit.
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Neurotransmitters
• Neurotransmitters are used by
neurons to change the membrane
potential of postsynaptic neurons.
• They can either stimulate an action
potential or inhibit one.
• Neurotransmitters that cause action
potentials to occur are said to be
excitatory while those that stop them
from happening are called inhibitory.
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Neurotransmitters
• Acetylcholine (a-see-tyl-kol-een) is an
excitatory neurotransmitter found in the
end plates of most neurons.
• When it attaches to the receptors on a
postsynaptic neuron it causes sodium
channels to open.
• Once this occurs, sodium ions rush in
causing depolarization to occur in this
neuron.
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Neurotransmitters
• This stimulation of an action
potential means that the message is
being passed on, which is a good
thing.
• But, remembering action potentials,
we know that the cell needs to reach
a repolarization stage which means
that sodium channels need to close.
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Neurotransmitters
• If acetylcholine remains
attached, the postsynaptic
neuron is stuck in the
depolarization stage.
• An enzyme called
cholinesterase (colonesteraze) is released by the
presynaptic neuron.
• This enzyme destroys
acetylcholine allowing the
postsynaptic neuron to begin
the recovery stages of action
potential.
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Neurotransmitters
• In humans, low levels of
acetylcholine has been related to
deterioration of memory and
mental capacity giving evidence to
this depletion being a cause of
Alzheimer’s disease.
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Neurotransmitters
• Although acetylcholine is
considered an excitatory
neurotransmitter, there are
some cases where it can also
be inhibitory.
• Inhibitory neurotransmitters
cause the membrane of the
postsynaptic neuron to
become more permeable to
potassium ions.
• This leads to a
hyperpolarization of the
membrane which means that
an action potential cannot
occur.
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• An example of an inhibitory
neurotransmitter is serotonin.
Action potentials are blocked
to allow your brain to enter a
state of rest and allows you to
sleep.
• People with low levels of
serotonin generally have a
hard time falling asleep or
staying asleep.
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• Another inhibitory neurotransmitter is
gamma aminobutyric acid (GABA) GABA
is the most abundant neurotransmitter in
the brain and is used to calm action
potentials in the brain.
• Having GABA in the brain allows you to
prioritize information and to focus on
many different things at once.
• People with low levels of GABA
neurotransmitters can suffer from certain
anxiety disorders, panic disorders, and
Parkinson’s disease.
• Certain drugs, like caffeine, inhibits the
release of GABA causing your brain to
become ‘more alert.’ AKA removing the
inhibiting effect on action potentials.
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Summation
• It needs to be understood
that in many cases, the
neurotransmitters released
from a single neuron are not
enough to reach the
threshold level in the
postsynaptic neuron which
means an action potential will
NOT occur.
• The effect produced by the
accumulation of
neurotransmitters released
from two or more neurons is
called summation.
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Spatial Summation, Part 1
• Sherrington also noticed that several small
stimuli in a similar location produced a
reflex when a single stimuli did not
• Thus, idea of spatial summation
– Synaptic input from several locations can
have a cumulative effect and trigger a nerve
impulse
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Recordings From a Postsynaptic Neuron
During Synaptic Activation
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Spatial Summation, Part 2
• Spatial summation is critical to brain
functioning
• Each neuron receives many incoming
axons that frequently produce
synchronized responses
• Temporal summation and spatial
summation ordinarily occur together
• The order of a series of axons influences
the results
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Temporal Summation
• Sherrington observed that repeated stimuli
over a short period of time produced a
stronger response
• Thus, the idea of temporal summation
– Repeated stimuli can have a cumulative effect
and can produce a nerve impulse when a
single stimuli is too weak
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Three Important Points About Reflexes
• Sherrington’s observations
– Reflexes are slower than conduction along an
axon
– Several weak stimuli present at slightly
different times or slightly different locations
produce a stronger reflex than a single
stimulus
– As one set of muscles becomes excited,
another set relaxes
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Spatial Summation, Part 1
• Sherrington also noticed that several small
stimuli in a similar location produced a
reflex when a single stimuli did not
• Thus, idea of spatial summation
– Synaptic input from several locations can
have a cumulative effect and trigger a nerve
impulse
© Cengage Learning 2016
Recordings From a Postsynaptic Neuron
During Synaptic Activation
© Cengage Learning 2016
Spatial Summation, Part 2
• Spatial summation is critical to brain
functioning
• Each neuron receives many incoming
axons that frequently produce
synchronized responses
• Temporal summation and spatial
summation ordinarily occur together
• The order of a series of axons influences
the results
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Temporal and Spatial Summation
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Spontaneous Firing Rate
• The periodic production of action
potentials despite synaptic input
– EPSPs increase the number of action
potentials above the spontaneous firing rate
– IPSPs decrease the number of action
potentials below the spontaneous firing rate
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Activating Receptors of the Postsynaptic
Cell
• The effect of a neurotransmitter depends
on its receptor on the postsynaptic cell
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https://www.youtube.com/watch
?v=Tqwo9dmIXAQ
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Increasing and decreasing the effect of NTMs
Agonist:
A drug (or poison) increases activity of a NTM. How?
• Mimics shape
• Prevents reuptake by pre-synaptic neuron
• Blocks enzymes that break down NTM in synapse
Antagonist:
A drug (or poison) that reduces NTM activity
• Blocks release of NTM from its terminal button
• Blocks receptor on post synaptic dendrite
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