Synaptic Neurotransmission

What is synaptic neurotransmission?

Synaptic neurotransmission refers to the steps that occur in the axon terminal of the pre-synaptic neuron to convert action potentials (electrical signals) into the release of neurotransmitter into the synaptic cleft (chemical signal). This chemical signal then carries information across the synapse to the post-synaptic neurons.

What do small synaptic vesicles in pre-synaptic neurons typically contain?

Small synaptic vesicles of pre-synaptic neurons typically contain only excitatory or inhibitory neurotransmitter. Neurons containing these vesicles are usually called excitatory or inhibitory neurons.

What neurotransmitter do excitatory neurons store and release in this course?

Excitatory neurons store and release glutamate.

What neurotransmitter do inhibitory neurons store and release in this course?

Inhibitory neurons store and release GABA.

Briefly describe the steps in excitatory neurotransmission after an action potential reaches the axon terminal.

The action potential depolarizes the axon terminal.

Depolarization opens voltage-gated Ca2+ channels, and Ca2+ ions rush in.

Ca2+ ions bind to synaptotagmin on vesicles containing glutamate.

V-SNAREs on vesicles bind to T-SNAREs on the plasma membrane, docking the vesicle.

Vesicles fuse with the membrane, releasing glutamate into the synaptic cleft via exocytosis.

Glutamate diffuses across the cleft and binds to glutamate receptors on the post-synaptic neuron.

Activated glutamate receptors produce excitatory post-synaptic potentials (EPSPs), which can lead to an action potential if the threshold is reached.

What are the two main types of glutamate receptors?

The two main types of glutamate receptors are ionotropic receptors and metabotropic receptors.

Describe ionotropic glutamate receptors.

Ionotropic glutamate receptors are glutamate-gated ion channels for Na+ and Ca2+ that directly cause depolarizations called EPSPs. There are three types: NMDA, AMPA, and Kainate receptors.

Describe metabotropic glutamate receptors.

Metabotropic glutamate receptors (mGluR) are G-protein coupled receptors activated by glutamate. They produce second messengers that open Na+ or Ca2+ channels and produce depolarizations called EPSPs.

Briefly describe the steps in inhibitory neurotransmission after an action potential reaches the axon terminal.

The action potential depolarizes the axon terminal.

Depolarization opens voltage-gated Ca2+ channels, and Ca2+ ions rush in.

Ca2+ ions bind to synaptotagmin on vesicles containing GABA.

V-SNAREs on vesicles bind to T-SNAREs on the plasma membrane, docking the vesicle.

Vesicles fuse with the membrane, releasing GABA into the synaptic cleft via exocytosis.

GABA diffuses across the cleft and binds to GABA receptors on the post-synaptic neuron.

Activated GABA receptors produce Inhibitory Post-Synaptic Potentials (IPSPs), which move the membrane potential away from threshold.

What are the two main types of GABA receptors?

The two main types of GABA receptors are ionotropic and metabotropic receptors.

Describe ionotropic GABA receptors.

Ionotropic GABA receptors are GABA-gated ion channels for Cl- and K+ that directly cause hyperpolarizations called IPSPs. There are two types: GABAA and GABAC receptors.

Describe metabotropic GABA receptors.

Metabotropic GABA receptors (GABAB) are G-protein coupled receptors activated by GABA. They produce second messengers that open K+ channels and produce hyperpolarizations called IPSPs

What is essentially the same for excitatory and inhibitory neurotransmission regarding neurotransmitter release?

The steps that cause the release of neurotransmitter (steps 1-6 of both processes) are essentially the same for both excitatory and inhibitory neurotransmission. Specifically, how the neurotransmitter is released is the same.

What are the key differences between excitatory and inhibitory neurotransmission?

The key differences lie in what receptor the neurotransmitter binds to and how it affects the post-synaptic neuron. Excitatory neurotransmitters (glutamate) typically cause depolarization (EPSPs), while inhibitory neurotransmitters (GABA) typically cause hyperpolarization (IPSPs).

What are the two main methods for terminating the signal after neurotransmitter release?

The two main methods for removing previously released neurotransmitter are neurotransmitter transporters and degradation enzymes.

Describe neurotransmitter transporters and their role in signal termination.

Neurotransmitter transporters are Na+-Neurotransmitter cotransporters located in the pre-synaptic neurons and surrounding glial cells. They clear neurotransmitters from the synapse, acting like a vacuum cleaner. Neurotransmitter taken up by the pre-synaptic neuron can be recycled.

Describe degradation enzymes and their role in signal termination.

Degradation enzymes are located facing or in the synaptic cleft and function to break down neurotransmitters, thus clearing them from the synapse.