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.

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