Download Synapses & Neurotransmitters

The Synapse
• A junction that mediates information transfer
from one neuron:
• To another neuron, or
• To an effector cell (muscle, secretory…)
• Presynaptic neuron—conducts impulses
toward the synapse (sending side)
• Postsynaptic neuron—transmits impulses
away from the synapse (receiving side)
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Types of Synapses
• Axodendritic—axon of one to dendrite of
another
• Axosomatic—axon of one to soma of another
• Less common types:
• Axoaxonic (axon to axon)
• Dendrodendritic (dendrite to dendrite)
• Dendrosomatic (dendrite to soma)
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Axodendritic
synapses
Dendrites
Axosomatic
synapses
Cell body
Axoaxonic synapses
(a)
Axon
Axon
Axosomatic
synapses
(b)
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Cell body (soma) of
postsynaptic neuron
Figure 11.16
Electrical Synapses
• Less common than chemical synapses
• Neurons are electrically coupled (joined by gap
junctions)
• Communication is very rapid, and may be
unidirectional or bidirectional
• Are important in:
• Embryonic nervous tissue
• Some brain regions
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Chemical Synapses
• Specialized for the release and reception of
neurotransmitters
• Typically composed of two parts
• Axon terminal of the presynaptic neuron, which
contains synaptic vesicles
• Receptor region on the postsynaptic neuron
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Synaptic Cleft
• Fluid-filled space between presynaptic and
postsynaptic neurons, found at chemical
synapses
• Transmission across the synaptic cleft
• Is a chemical event (not electrical)
• Involves release, diffusion, and binding of
neurotransmitters
• Provides unidirectional communication between
neurons
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Synaptic Transmission Summary
• Action potential arrives at presynaptic neuron’s axon
terminal and opens voltage-gated calcium channels
• Calcium enters neuron terminal and causes synaptic
vesicle fusion with cell membrane
• Neurotransmitter exocytosis occurs
• Neurotransmitter diffuses across cleft and binds to
receptors on postsynaptic neuron
• Ion channels in membrane of post-synaptic cell open,
causing excitation or inhibition (graded potential)
• Neurotransmitter diffuses away from receptors as it is
broken down in the cleft and/or taken back up by presynaptic neuron
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Chemical synapses
transmit signals from
one neuron to another
using neurotransmitters.
Presynaptic
neuron
Presynaptic
neuron
Postsynaptic
neuron
1 Action potential
arrives at axon terminal.
2 Voltage-gated Ca2+
channels open and Ca2+
enters the axon terminal.
Mitochondrion
Ca2+
Ca2+
3 Ca2+ entry causes
neurotransmittercontaining synaptic
vesicles to release their
contents by exocytosis.
4 Neurotransmitter
diffuses across the synaptic
cleft and binds to specific
receptors on the
postsynaptic membrane.
Axon
terminal
Ca2+
Ca2+
Synaptic
cleft
Synaptic
vesicles
Postsynaptic
neuron
Figure 11.17, step 4
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Ion movement
Graded potential
5 Binding of neurotransmitter
opens ion channels, resulting in
graded potentials.
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Figure 11.17, step 5
Enzymatic
degradation
Reuptake
Diffusion away
from synapse
6 Neurotransmitter effects are terminated
by reuptake through transport proteins,
enzymatic degradation, or diffusion away
from the synapse.
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Figure 11.17, step 6
Termination of Neurotransmitter Effects
• Neurotransmitter effect is terminated in a few
milliseconds by
• Degradation by enzymes
• Reuptake by astrocytes or axon terminal
• Diffusion away from synaptic cleft
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Synaptic Delay
• Neurotransmitter must be released, diffuse
across synapse, and bind to receptors
• Synaptic delay = time needed to do this (0.3-5.0
ms)
• Synaptic delay is the rate-limiting step of neural
transmission (in short neurons at least)
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Postsynaptic Potentials
• Graded potentials
• Strength determined by:
•
Amount of neurotransmitter released
•
Time the neurotransmitter is in the area
• Types of postsynaptic potentials
• EPSP—excitatory postsynaptic potentials
• IPSP—inhibitory postsynaptic potentials
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Table 11.2 (1 of 4)
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Table 11.2 (2 of 4)
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Table 11.2 (3 of 4)
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Table 11.2 (4 of 4)
Excitatory Synapses and EPSPs
• Neurotransmitter binds to and opens chemically
gated channels that allow simultaneous flow of
Na+ and K+ in opposite directions
• Na+ influx is greater than K+ efflux, causing a net
depolarization
• EPSP helps trigger AP at axon hillock if EPSP is
of threshold strength and opens the voltagegated channels
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Membrane potential (mV)
Threshold
An EPSP is a local
depolarization of the
postsynaptic membrane
that brings the neuron
closer to AP threshold.
Neurotransmitter binding
opens chemically gated
ion channels, allowing
the simultaneous passage of Na+ and K+.
Stimulus
Time (ms)
(a) Excitatory postsynaptic potential (EPSP)
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Figure 11.18a
Inhibitory Synapses and IPSPs
• Neurotransmitter binds to and opens channels
for K+ or Cl–
• Causes hyperpolarization (inside of cell
becomes more negative)
• Reduces the postsynaptic neuron’s ability to
produce an action potential
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Membrane potential (mV)
Threshold
An IPSP is a local
hyperpolarization of the
postsynaptic membrane
and drives the neuron
away from AP threshold.
Neurotransmitter binding
opens K+ or Cl– channels.
Stimulus
Time (ms)
(b) Inhibitory postsynaptic potential (IPSP)
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Figure 11.18b
Integration: Summation
• One EPSP cannot induce an action potential
• EPSPs can sum to reach threshold
• IPSPs and EPSPs can cancel each other out
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Integration: Summation
• Temporal summation
• One presynaptic neuron sends several or
many impulses in a short time to the
postsynaptic neuron
• Spatial summation
• Multiple presynaptic neurons stimulate the
post-synaptic neuron simultaneously
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E1
E1
Threshold of axon of
postsynaptic neuron
Resting potential
E1
E1
Time
(a) No summation:
2 stimuli separated in time
cause EPSPs that do not
add together.
E1 E1
Time
(b) Temporal summation:
2 excitatory stimuli close
in time cause EPSPs
that add together.
Excitatory synapse 1 (E1)
Excitatory synapse 2 (E2)
Inhibitory synapse (I1)
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Figure 11.19a, b
E1
E1
E2
I1
E1 + E2
Time
(c) Spatial summation:
2 simultaneous stimuli at
different locations cause
EPSPs that add together.
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I1
E1 + I1
Time
(d) Spatial summation of
EPSPs and IPSPs:
Changes in membane
potential can cancel each
other out.
Figure 11.19c, d
Neurotransmitters
• Most neurons make two or more
neurotransmitters, which are released at
different stimulation frequencies
• 50 or more neurotransmitters have been
identified
• Classified by chemical structure and by
function
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Chemical Classes of Neurotransmitters
• Acetylcholine (Ach)
• Released at neuromuscular junctions and
some autonomic neurons
• Synthesized in the pre-synaptic neuron
• Degraded by acetylcholinesterase (AChE)
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Chemical Classes of Neurotransmitters
• Biogenic amines include:
• Norepinephrine (NE)
• Epinephrine
• Serotonin
• Many others
• Broadly distributed in the brain
• Play roles in emotional behaviors and the
biological clock
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Chemical Classes of Neurotransmitters
• Amino acids include:
• GABA—Gamma ()-aminobutyric acid
• Glutamate
• Many others
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Functional Classification of
Neurotransmitters
• Excitatory (depolarizing) and/or inhibitory (hyperpol.)
• Determined by receptor type on postsynaptic neuron
• GABA usually inhibitory
• Glutamate, epinephrine usually excitatory
• Acetylcholine
• Excitatory at neuromuscular junctions in skeletal
muscle
• Inhibitory in cardiac muscle
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Neurotransmitter Actions
• Direct action
• Neurotransmitter binds to channel-linked
receptor and opens ion channels
• Promotes rapid responses
• Examples: ACh; glutamate and GABA at some
of their synapses
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Neurotransmitter Actions
• Indirect action
• Neurotransmitter binds to a G protein-linked
receptor and acts through an intracellular
second messenger
• Promotes long-lasting effects
• Examples: Norepinephrine, epinephrine,
serotonin; glutamate and GABA at some of
their synapses
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