Transcript How do neurotransmitters generate electrochemical signals in

Psychology 304: Brain and Behaviour
Lecture 14
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Transmission of Electrochemical Neural Signals and
Neuropharmacology
1. How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
2. What mechanisms terminate synaptic transmission?
3. What neurotransmitters have been identified?
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How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
• Neurotransmitters produce signals in postsynaptic
neurons by binding to receptors in the postsynaptic
membrane.
• A neurotransmitter that binds to a receptor is referred to
as a ligand.
• Most neurotransmitters can bind to a number of receptor
subtypes. Among these subtypes are:
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Ionotropic receptors: When activated by a
neurotransmitter, ionotropic receptors cause ligandactivated ion (e.g., Na+, K+, Cl-) channels to open or
close, producing an immediate change in potential (EPSP
or IPSP) on the postsynaptic membrane.
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Ionotropic Receptor Activity
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Metabotropic receptors: When activated by a
neurotransmitter, metabotropic receptors cause a
subunit of an associated G protein to break away. The
subunit either:
1. binds to a ligand-activated ion (e.g., Na+) channel,
causing the channel to open or close, producing an
immediate change in potential (EPSP or IPSP) on
the postsynaptic membrane (Figure A).
2. triggers the synthesis of a second messenger
which: (a) binds to a ligand-activated ion channel,
causing the channel to open or close, or (b)
diffuses through the cytoplasm where it influences
the activities of the neuron (Figure B).
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Figure A
Figure B
Metabotropic Receptor Activity
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• If a sufficient number of EPSPs are generated on the
postsynaptic membrane by the binding of
neurotransmitters to receptors, an action potential will be
generated in the postsynaptic neuron.
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What mechanisms terminate synaptic transmission?
• Two mechanisms terminate synaptic transmission:
1. Reuptake.
Neurotransmitters are repackaged into vesicles in
the cystoplasm.
2. Enzymatic degradation.
Example: Acetylcholinestrase.
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Mechanisms of Neurotransmitter Deactivation
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What neurotransmitters have been identified?
• Four classes of small-molecule neurotransmitters have
been identified:
1. Acetylcholine (Ach)
 Found at neuromuscular synapses, synapses in the
autonomic nervous system, and synapses in parts of
the central nervous system.
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 Cholinergic neurons: associated with movement,
autonomic function, learning, and memory.
2. Monoamine neurotransmitters
 Found in neurons whose cell bodies are largely
located in the brain stem. These neurons tend to
have long and highly branched axons with many
varicosities from which neurotransmitters are diffusely
released.
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A Branched Axon with Varicosities
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 Subdivided into two groups:
(a) Catecholamines:
Include dopamine, epinephrine, and
norepinephrine.
Dopaminergic neurons: associated with
movement, attention, reinforcement/reward, verbal
learning, and planning.
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Production of the Catecholamines
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Adrenergic neurons: associated with attention and
arousal.
Noradrenergic neurons: associated with attention,
arousal, mood, feeding, and sexual behaviour.
(b) Indoleamines:
Include serotonin (5-HT or 5-hydroxytryptamine)
and melatonin.
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Serotonergic neurons: associated with mood,
sleep, feeding, sexual behaviour, and pain.
Melatonergic neurons: associated with sleep.
3. Amino acid neurotransmitters
 Found in fast-acting neurons in the central nervous
system.
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 Include glutamate, gamma-aminobutyric acid (GABA),
glycine, and aspartate. Of these, the first two are
most important.
 Glutamatergic neurons: the principle excitatory
neurotransmitter in the brain and spinal cord;
associated with learning and memory.
 GABA-secreting neurons: the principle inhibitory
neurotransmitter in the brain and spinal cord;
associated with mood and the “seizure threshold.”
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4. Soluble gas neurotransmitters
 Include nitric oxide (NO) and carbon monoxide (CO).
 Unconventional neurotransmitters. After production in
the cytoplasm, they immediately diffuse through the
presynaptic cell membrane. Thereafter, the diffuse
through the postsynaptic cell membrane and stimulate
the production of a second messenger.
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 Function as retrograde transmitters: Diffuse back to
the presynaptic neuron and regulate its activity.
 Little is known about the functions of CO. NO is
associated with autonomic activity, learning, and
memory.
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Transmission of Electrochemical Neural Signals and
Neuropharmacology
1. How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
2. What mechanisms terminate synaptic transmission?
3. What neurotransmitters have been identified?
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