Synaptic transmission Domina Petric, MD

Types of synapses Electrical synapses function by means of proteins that form channels (gap junctions). Chemical synapses work via the release the packages of neurotransmitter (synaptic vesicles) into the synaptic cleft.

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Electrical synapses Allow very rapid communication of electrical signals from one cell to another. Proteins that form gap junctions are called connexons. They aggregate to provide an aqueous channel that allows for a current carrying molecule to pass directly from the cytoplasm of one neuron to another neuron. This is the fastes way of communication of neurons.

Electrical synapses are usefull when: It is important to synchronise a local population of cells. Synchronisation is important for example in hypothalamus: synchronised release of the hormone in enough quantity.

Molecules that pass through the gap junctions: calcium ions (calcium messanger system) ATP other ions

Chemical synapses The principal type of synapses that are found in adult nervous system. They can be dendrite-axon, axon terminal-cell body or axon terminal-another axon terminal. Chemical synapses are four to five time slower than electrical synapses. Chemical synapses are mostly unidirectional while electrical signals are bidirectional.

Chemical synapses Transmitter is synthesized and stored in vesicles. Action potential invades the presynaptic terminal. Depolarisation of presynaptic terminal causes the opening of voltage-gated calcium channels. Influx of calcium ions through the channels. Calcium ions cause the vesicles to fuse with presynaptic membrane.

Chemical synapses Transmitter is released into the synaptic cleft via exocytosis. Transmitter binds to receptor molecules in postsynaptic membrane. Opening or closing of postsynaptic channels. Postsynaptic current causes excitatory or inhibitory postsynaptic potential that changes the excitability of the postsynaptic cell. Removal of neurotransmitter by glial uptake or enzymatic degradation.

Calcium ions Sudden rise of calcium ions in presynaptic terminal is critical for the release of neurotransmitter and the generation of an action potential in the postsynaptic cell.

It is a molecule in the vesicular membrane that binds to calcium. Synaptotagmin It is a molecule in the vesicular membrane that binds to calcium.

SNARE complex of proteins SNARE complex of proteins is involved in snaring the vesicle membrane to the cytoplasmic face of the presynaptic terminal.

Clathrin Clathrin coats the vesicle membrane. Interacts with other proteins (for example dynamin protein) and there is retreival of these membranes. Clathrin forms triskelion. Triskelion can associate with other molecules and form a ring around the vesicular membrane.

Literature https://www.coursera.org/learn/medical-neuroscience/lecture: Leonard E. White, PhD, Duke University Quora.com