Synapses have different functions:
When it comes to the transmission of electrical impulses in neurons, one differentiates essentially two different types of synapses. Those which provide a transmission of an impulse, and those which are responsible for arrest.
Excitatory postsynaptic potential (EPSP)
The excitatory postsynaptic potential describes the electrically positive change in the membrane potential responsible for the triggering of the action potential. Excitatory means "exciting".
After the neurotransmitters bind to the receptors of the postsynaptic membrane, they open the sodium ion channels and allow Na + to enter the cell. As a result, depolarization of the membrane of the ensuing dendrites occurs. The dendrite forwards the arousal through the soma to the axon hillock. There, the incoming EPSP add up. The excitement in the form of a further action potential is only passed on if the threshold value (about -50 mV) is exceeded.
The probability that an action potential will be triggered is the higher if:
1. Several consecutive EPSP arrive at the axon hill (summation) and
2. The depolarization lasts longer. The more neurotransmitters are released and bind to the receptors, the longer the Na + channels are opened.
Inhibitory postsynaptic potential (IPSP)
But there are also synapses that provide an inhibition of the excitement. As with the EPSP, the transmitters dock to the receptors, but provide an opening for the potassium and chloride channels. Potassium channels are only passable from the inside to the outside for potassium ions, so that K+ diffused outwards. Episode: The cell interior becomes more negative. In addition, the chloride channels are opened and from the outside are charged negatively charged Cl- Ions into the cell. Both factors provide hyperpolarization of the postsynaptic membrane. The voltage is then even below the actual rest potential and stops in this way the excitement.
Whether a synapse propagates or amplifies (EPSP) or inhibits (IPSP) is not due to the transmitter molecules, but to the synapses themselves. There are only reinforcing or inhibitory synapses, but never a synapse that could do both.