How is a nervous impulse transmitted across a synapse?

A nervous impulse is transmitted across the synapse from a pre-synaptic neurone to a post-synaptic neurone through the use of neurotransmitter diffusion. To explain this in more detail let’s take the example of a cholinergic synapse; a synapse that uses the neurotransmitter Acetylcholine. The transmission across a cholinergic synapse can be summarised in 10 steps: 1. Firstly, an action potential (change in electrical potential) arrives at the pre-synaptic neurone. 2. This changes the voltage in the neurone causing the voltage-gated calcium channels on the pre-synaptic neurone to open. 3. Calcium ions then diffuse into the pre-synaptic neurone. 4. The increased concentration of Calcium in the neurone then causes synaptic vesicles, containing the neurotransmitter acetylcholine, to move towards the membrane on the pre-synaptic neurone. 5. The vesicles fuse to the membrane and the neurotransmitter is released into the gap between the two neurones (known as the synaptic cleft). 6. The acetylcholine neurotransmitter then diffuses across the synaptic cleft towards the post-synaptic neurones membrane. 7. Here, the acetylcholine neurotransmitter then binds to the complimentary receptors on the post-synaptic neurone’s membrane. 8. The increase in concentration of the neurotransmitter causes ligand (chemical) gated sodium channels in the post-synaptic neurone membrane to open, allowing sodium to diffuse into the post-synaptic neurone. 9. The increased concentration of sodium ions now in the post-synaptic neurone depolarise the neurone’s membrane causing EPSPs (excitatory post-synaptic potentials). 10. If these EPSPs reach a certain threshold, then an action potential is initiated in the post-synaptic neurone and the impulse has been successfully transmitted from one neurone to the next! If you can remember these 10 steps then you can thoroughly explain transmission of a nervous impulse across a synapse. To help you remember these steps try making a poster showing the process visually, or perhaps try creating a mnemonic.