How to transmit an action potential across a synapse

Channel proteins in the pre-synaptic membrane open, allowing Ca2+ ions from the synaptic cleft to enter the synaptic knob.

Vesicles in the synaptic knob merge with the pre-synaptic membrane, and transmitter molecules such as acetylcholine are secreted into the cleft.

The transmitter molecules diffuse across the cleft and bind with specific sites on the receptor proteins in the post-synaptic membrane.

Each receptor protein then opens a protein channel, allowing ions such as Na+ to pass through the membrane and enter the cell. This depolarises the post-synaptic membrane.

If enough Na+ ions enter then depolarisation reaches a threshold level; an action potential is generated in the cell. If not enough Na+ ions enter the cell, an action potential won't be generated.

Enzymes in the cleft remove the transmitter molecules from binding sites, eg. acetylcholine esterase hydrolyses acetylcholine to choline and ethanoic acid, which are reabsorbed into the synaptic knob.