Synaptic Transmission

• .An action potential arrives at the presynaptic membrane, causing depolarisation through the opening of voltage-dependent calcium ion channels, and the consequent influx of calcium ions.
• The increased concentration of calcium ions within the membrane causes the vesicles, containing neurotransmitter, to fuse with the presynaptic membrane and release their contents into the synaptic cleft through exocytosis.
• The neurotransmitter diffuses across the synaptic cleft, down a concentration gradient, and binds to complementary receptors on the post-synaptic membrane.
• The resultant action potential will then be transmitted along the axon of the following neuron, resulting in a ‘cascade’ of neurotransmission
• Neurotransmitters can either have an inhibitory or excitatory effect. Inhibitory neurotransmitters (e.g. serotonin) reduce the difference across the postsynaptic membrane through the closing of the sodium ion channels, reducing the likelihood that an action potential will be generated.
• Excitatory neurotransmitters (e.g. dopamine) increase the potential difference across the postsynaptic membrane through triggering the opening of more voltage-dependent sodium ion channels, increasing the likelihood that an action potential will be generated.