Transcript How are neurotransmitters released from presynaptic neurons?

Psychology 304:
Brain and Behaviour
Lecture 14
1
Transmission of Electrochemical Neural Signals
1. How are neurotransmitters released from presynaptic
neurons?
2. How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
3. What mechanisms terminate synaptic transmission?
2
By the end of today’s class, you should be able to:
1. explain the process by which neurotransmitters are
released from neurons.
2. distinguish between ionotropic and metabotropic
receptors.
3. describe the processes of reuptake and enzymatic
degradation.
4. describe the neural basis of “myasthenia gravis.”
3
How are neurotransmitters released from presynaptic
neurons?
• When stimulated by an action potential, voltage-activated
calcium channels open and Ca2+ ions enter the terminal
button.
This causes vesicles in the button to fuse with the
presynaptic membrane and release their contents into
the synaptic cleft.
4
Release of Neurotransmitters into the Synaptic Cleft
5
Synaptic cleft
Neurotransmitter
Presynaptic
membrane
Release of Neurotransmitters into the Synaptic Cleft
6
How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
• Neurotransmitters produce signals in postsynaptic
neurons by binding to receptors.
• A neurotransmitter that binds to a receptor is referred to
as a ligand.
• Neurotransmitters may bind to distinct receptor subtypes.
Two broad subtypes are:
7
Ionotropic receptors (IRs): When activated by a
neurotransmitter, IRs cause ligand-activated ion (e.g., Na+,
K+, Cl-) channels to open or close, producing an immediate
change in potential (EPSP or IPSP) on the postsynaptic
membrane.
8
Ionotropic Receptor Activity
9
Metabotropic receptors (MRs): When activated by a
neurotransmitter, MRs 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).
10
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).
11
Figure A
Figure B
Metabotropic Receptor Activity
12
What mechanisms terminate synaptic transmission?
• Two mechanisms terminate synaptic transmission:
1. Reuptake.
Neurotransmitters are reabsorbed and repackaged into
vesicles in the cystoplasm.
2. Enzymatic degradation.
Example: Acetylcholinestrase.
13
Mechanisms of Neurotransmitter Deactivation
14
Transmission of Electrochemical Neural Signals
1. How are neurotransmitters released from presynaptic
neurons?
2. How do neurotransmitters generate electrochemical
signals in postsynaptic neurons?
3. What mechanisms terminate synaptic transmission?
15