Transmission Across a Synapse
What is a synapse?
A synapse is the junction between two neurons. An action potential cannot cross the gap between two neighbouring neurons and instead the nerve impulse is 'carried' by neurotransmitters, a type of chemical messenger.
How does synaptic transmission occur?
The pre-synaptic neuron ends with an enlarged tip, called the synaptic knob, which houses vesicles containing neurotransmitters (often acetylcholine). The synpatic knob fits into a depression on the postsynaptic neuron, creating a gap called the synaptic cleft; an important thing to note is that there is no physical contact between axons and the dendrites - there is a gap.
When the nerve impulse reaches the end of the axon, it causes calcium channels to open, causing calcium ions to flood into the synpatic knob. This causes the neurotransmitter-containing vesicles to fuse with the cell membrane of the pre-synaptic knob. The neurotransmitter is then released into the synaptic cleft via exocytosis (active transport). The neurotransmitters then bind to specific receptor proteins which are found on the dendrites of the post-synaptic neuron. This triggers the depolarisation in the dendrites of the following neuron.