A synapse is the contact between two neurons or a neuron and a muscle cell where an electrical or chemical signal is passed. A chemical synapse is composed of three parts:
Presynapse or presynaptic site, where the excitation is provoked
Postsynapse or postsynaptic site, where the excitation is received
- Synaptic vesicle with neurotransmitters
- Synaptic cleft with released neurotransmitters
- Postsynaptic receptors activated by neurotransmitters (inducting a postsynaptic potential
- Calcium channel
- Exocytosis of avesicle
- Neurotransmitter re-uptake
For transmitting the excitation in a chemical synapse, the presynaptic site is releasing neurotransmitters from the vesicles into the synaptic cleft (exocytosis). These transmitters diffuse through the synaptic cleft, to the postsynaptic site, where they bind to special receptors. This is only working in one direction.
There are two types of synapses:
At excitatory synapses, the binding of neurotransmitters trigger an excitatory postsynaptic potential (EPSP) that can trigger depolarization and lead to an action potential.
At an inhibitory synapse, the binding of neurotransmitters triggers a inhibitory postsynaptic potential (IPSP) that suppresses depolarization.
The released neurotransmitters have to be removed from the synaptic cleft after they have performed their function of transmitting a neural impulse. Reuptake is the reabsorption of not bound neurotransmitters in the synaptic cleft; they are packed back into vesicles for reuse. Because neurotransmitters are too large and hydrophilic to diffuse through the membrane, specific transport proteins take over the job.