How are action potentials transmitted across synapses?

When an action potential is transmitted from one neurone to another it has to be sent across a small gap - called the synapse. Firstly, the action potential arrives at the presynaptic neurone (‘pre’ means before, i.e. before the synapse). This causes voltage-gated calcium ion channels to open in the membrane of the presynaptic neurone. The channels open because they are voltage gated and action potentials are a form of electrical activity. The calcium ions diffuse into the presynaptic neurone which causes vesicles (small sacks formed of phospholipids) containing neurotransmitter molecules to move to and fuse with the presynaptic membrane. They then secrete the neurotransmitter molecules into the synaptic cleft by a process known as exocytosis. The neurotransmitter diffuses across the synapse and binds to specific complementary protein receptors on the membrane of the postsynaptic neurone. This causes sodium ion channels in the membrane of the postsynaptic neurone to open so sodium ions flood in down their concentration gradient. This increases the membrane potential of the neurone (as sodium ions are positively charged) and this is known as depolarisation. If the threshold is reached (approximately -50mV) an action potential is triggered and the action potential continues along the postsynaptic neurone as a wave of depolarisation. Let’s recap the key points: 1. An action potential arrives at the presynaptic neurone, 2. This causes calcium ion channels to open, 3. Calcium ions diffuse into the neurone, 4. This causes vesicles to move to and fuse with the presynaptic membrane, 5. Neurotransmitter molecules are released from the vesicles by exocytosis, 6. Neurotransmitter molecules diffuse across the synaptic cleft and bind to protein receptors on the surface of the postsynaptic membrane, 7. This causes sodium ion channels to open, 8. Sodium ions diffuse into the postsynaptic neurone triggering an action potential (depolarisation)