Explain how nerve transmission occurs at the synapse.

As an action potential arrives at the presynaptic knob, the membrane is depolarised. This causes calcium channels in the membrane to open and calcium ions to flood in. This influx of calcium causes vesicles that contain neurotransmitters (e.g. acetylcholine) to fuse with the presynaptic membrane, emptying their contents into the synaptic cleft. The neurotransmitters then bind with complementary receptors on the post-synaptic membrane causing sodium ion channels to open and sodium ions move into the post-synaptic membrane. If the generator potential created by this influx of sodium ions reaches the threshold, an action potential begins along the next neuron.