Neurotransmission at chemical synapses & Excitory and inhibitory potentials

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• A series of events occur at chemical synapses in order to communicate with the adjacent cell.
• The action potential arrives at the presynaptic membrane.
• The depolarization phase of the action potential opens voltage gated Ca+ channels.

• increased inflow of Ca+' into the cytosol triggers exocytosis of vesicles carrying neurotransmitter chemicals.
• Released into the synaptic cleft, neurotransmitters diffuse across the cleft and bind to receptors (often, ligand gated ion channels).
• gated channels open allowing ions to flow according to their concentration gradient.
• Sodium flows into the cell making its interior slightly more positive.
• Potassium flows out of the cell making its interior slightly less positive.
• The ionic flow through the channels will cause either a graded depolarization or hyperpolarization of the postsynaptic cell membrane.
• Large graded depolarizations tend to generate action potentials.

Excitory and inhibitory potentials - EPSP

• The ionic flow made possible because of the opening of the ligand gated channels in the postsynaptic membrane determines whether a graded depolarization or hyperpolarization occurs.
• If Na+ gates open, the depolarization of the membrane charge will move closer to threshold and the ability to generate an action potential.
• These depolarizations are called excitatory postsynaptic membrane potentials (EPSP).

Excitory and inhibitory potentials - IPSP
• If Cl- or K+ gates open, this creates hyperpolarizations which will inhibit the generation of an action potential.
• These hyperpolarizations are called inhibitory postsynaptic membrane potentials (IPSP).

Excitory and inhibitory potentials - role
• The sum of all IPSPs and EPSPs, from all synapses, determines whether an action potential will be generated at a neuron's trigger zone.