Synapse = the place where a neuron with the help of a nerve ending (boutong, axon terminal) forms a functional information-relaying contact with another neuron.
In the synapse, electro-chemical information is conveyed unidirectionally (some exceptions exist, e.g. in the NMDA synapses) from a contacting neuron, the presynaptic neuron, to a receiving neuron, the postsynaptic neuron.
The synaptic information transfer occurs locally and is directed to the postsynaptic membrane (see below). The effect is short-lived and disappears after only a few milliseconds (1 millisecond = 1 thousandth of a second).
Depending on where a synapse is located, one speaks of
* axo-dendritic synapses, on the dendrites
* axo-somatic synapses, on the cell body
* axo-axonal synapses; on the initial axon segment,IS
* presynaptic synapses, synapse on synapse
* Dendro-dendritic synapses
Synapses occur not only between axon terminals and the receiving neuron, but are also found between dendrites (dendro-dendritic synapses) in certain places in the CNS, e.g. in the back horn of the spinal cord and in the olfactory nodules.
There are two main types of synapses:
1/ Propulsive/excitatory synapses
2/ Inhibitory/inhibitory synapses.
Two factors determine whether a synapse is excitatory or inhibitory.
1/ Most important are the characteristics of the post-synaptic membrane. Depending on the different types of special proteins (receptor proteins, channel proteins, pump proteins, G proteins, etc.) built into the membrane, synapse activity gives rise to either an excitatory or an inhibitory postsynaptic potential.
2. The second determining factor is the neurotransmitter (transmitter substance). There are a number of different types of neurotransmitters.
The main parts of the synapse. The synaptic complex.
1. The presynaptic region, where the synapse vesicles are located particularly tightly packed. The region is bordered by
2. The presynaptic membrane. This is the part of the cell membrane of the boutong that is closely related to the cell membrane of the post-synaptic neuron, but is separate from this by
3. synapse cleft, an approx. 20 nanometers (1 nanometer = 1 millionths of a mm.) wide gap that lies between the presynaptic membrane and
4. the postsynaptic membrane, bordering on
5. The postsynaptic region of the contacted neuron.
1. A presynaptic neuron has been activated and sends out a nerve impulse, an action potential, in its axon.
2. The action potential is spread out in all axon branches.
3. The action potential reaches a boutong.
4. The action potential alters the electrical voltage of the boutong, thus triggering a series of intricate chemical reactions.
5. These reactions result in the bouton releasing a certain amount of so-called transmitter substance/neurotransmitter (transmitter = transmitter). The discharge takes place through the presynaptic membrane into the synaptic cleft.
6. The transmitter attaches itself to the special receptor molecules of the postsynaptic membrane.
7. The postsynaptic membrane then reacts by briefly changing its electrical properties. A local "postsynaptic voltage/potential change" occurs and a local electric current begins to flow through the postsynaptic membrane.
8. The transmitter is rapidly lost by enzymatic degradation or otherwise disposed of (sucked up by surrounding astrocyte protrusions; pumped back into the end end).
9. The postsynaptic potential-for-change fades away from the synapse and spreads at high speed over the surface of the postsynaptic neuron while it becomes weaker and eventually disappears.
10. The postsynaptic membrane returns to its normal position. Note that the postsynaptic potential is not an action potential.
Synaptic transmission/transmission is lightning fast and locally limited. It takes less than 1 millisecond (millisecond = thousandth of a second) from when the nerve impulse reaches the boutong until the postsynaptic potential begins to fade away. Compare with volume transmission!
A neuron is constantly exposed to a varied influx of excitatory and inhibitory synaptic influences. Thus, at each moment a large amount of local excitatory and inhibitory postsynaptic potentials and circuits arise.
With decreasing strength, the postsynaptic potentials spread rapidly across the post-synaptic neuron and sweep forward towards the initial axon segment.
Excitatory and inhibitory potentials are mixed with each other and summed up. Now if this sum of synaptic activities in a given moment of time is large enough when it reaches the initial axon segment, then an action potential is triggered in IS and is carried forward with maintained strength throughout the axon.
Chang-Lu Tao, et al. 2018, Differentiation and Characterization of Excitatory and Inhibitory Synapses by Cryo-electron Tomography and Correlative Microscopy. The Journal of Neuroscience, 38:1493-1510; doi:10.1523/JNEUROSCI.1548-17.2017