1 Psychology 304: Brain and Behaviour Lecture 14

2 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?

3 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.”

4 How are neurotransmitters released from presynaptic neurons? When stimulated by an action potential, voltage-activated calcium channels open and Ca 2+ 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.

5 Release of Neurotransmitters into the Synaptic Cleft

6 Presynaptic membrane Release of Neurotransmitters into the Synaptic Cleft Synaptic cleft Neurotrans- mitter

7 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:

8 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.

9 Ionotropic Receptor Activity

10 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).

11 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).

12 Metabotropic Receptor Activity Figure AFigure B

13 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.

14 Mechanisms of Neurotransmitter Deactivation

15 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?