Definition
A synapse is the contact point between two nerve cells. It is used to transmit a stimulus from one neuron to another. A synapse can also exist between neuron and muscle cell or sensory cell and gland.
There are two fundamentally different types of synapses, the electrical (gap junction) and the chemical. Each of these synapses uses a different way of transmitting excitation. The chemical synapses can also be subdivided according to the messenger substances (neurotransmitters).
These are used for transmission. In addition, the synapses can also be subdivided according to the type of excitation. There is an exciting and an inhibiting synapse.
Internal synapses (between two neurons) can also be subdivided according to localization, i.e. at which point of the neuron the synapse is located. In the brain alone, there are 100 trillion synapses. They can constantly rebuild and break down, this principle is called neural plasticity.
Structure, function and tasks
The electrical synapse (gap junction) works without delay over a very small gap, called synaptic gap. With the help of ion channels, this enables the transmission of stimuli directly from nerve cell to nerve cell. This type of synapse is found in smooth muscle cells, heart muscle cells and in the retina.
They are suitable for rapid transmission, such as for the eyelid reflex. Transmission is possible in both directions (bidirectional). The chemical synapse consists of a presynapse, a synaptic cleft and a post-synapse.
The presynapse is usually the end button of a neuron. The postsynapse is a site on the dendrite of the adjacent neuron or a designated section of the adjacent muscle cell or gland. Neurotransmitters transmit excitation through the synaptic cleft.
The previously electrical signal is converted into a chemical signal and then back into an electrical signal. This type of transmission is only possible in one direction (unidirectional). The electrical action potential is conducted via the axon of the neuron to the presynapse.
In the presynaptic membrane, voltage-controlled Ca channels are opened by the action potential. Small vesicles are located in the presynaptic membrane and are filled with the transmitters. The increased calcium concentration causes the vesicles to fuse with the presynaptic membrane and the neurotransmitters are released into the synaptic cleft.
This type of transport is called exocytosis. The higher the action potential frequency, the more vesicles release their stored neurotransmitters. The neurotransmitters then diffuse through the synaptic cleft, which is about 30 nm wide, and dock to neurotransmitter receptors.
These are located at the postsynaptic membrane. These are channels that are either ionotropic or metabotropic. If the postsynaptic membrane is a motor endplate, it is an ionotropic channel to which two molecules of the messenger substance (acetylcholine) dock and thus open it.
This allows cations (mainly sodium) to flow in. This polarizes the postsynapses and creates an excitatory postsynaptic potential (EPSP). It takes several EPSP ́s to turn it back into an action potential.
The EPSPs are summed up in time and space and at the so-called axon hill a postsynaptic action potential is generated. This action potential can then be passed on via the axon of this nerve cell and at the next synapse the whole process starts again. This is the effect of an excitatory synapse.
An inhibiting synapse, on the other hand, is hyperpolarized and inspiratory postsynaptic potentials (IPSP’s) are created. Inhibitory neurotransmitters such as glycine or GABA are used. The transmission of information via chemical synapses takes somewhat longer due to the release of the neurotransmitter and its diffusion.
By the way, the neurotransmitters are recycled. They return from the synaptic cleft to the presynapses and are repackaged in vesicles. The enzyme cholinesterase plays an important role in the transmitter substance acetylcholine.
It splits the neurotransmitter into choline and acetic acid (acetate). Thus the acetylcholine is inactive. There are also other ways to switch off the synaptic transmission. For example, the cation channels of the post-synapse can be inactivated.
