The synaptic cleft is the space between two neurons. It connects the presynaptic nerve cell with the postsynaptic nerve cell and ensures that the transmission of impulses can take place between the two.
Structure of the synaptic cleft
Figure 1 shows the structure of a synapse. But the synaptic cleft is only part of it. If you look at an entire nerve cell, the synaptic cleft is at the end of the terminal buds.
Of the synaptic cleft is the section between the presynaptic and the postsynaptic membrane of two cells. It is therefore located between two nerve cells or a nerve cell and a muscle cell.
It is about 10–50 nm wide and enables the transmission of an action potential between two cells with the help of neurotransmitters. It also contains enzymes that break down used neurotransmitters.
The following process occurs:
- The synaptic vesicles, which contain the neurotransmitters, fuse with the presynaptic membrane.
- The messenger substances are released into the synaptic cleft.
- Due to the small distance, the messenger substances can diffuse through the synaptic cleft to the postsynaptic membrane.
- The transmitters bind to specific receptor molecules, which change their spatial structure (conformation change) and open ion channels as a result of the binding.
- Sodium ions can flow in through the ion channels and depolarize the postsynaptic membrane. The postsynaptic cell is excited by the signal and transmits it.
- Once the signal has been passed on, the messenger substances on the receptors are split by enzymes and transported back into the presynaptic cell.
- Ion pumps ensure that the potential in both cells is reset to the initial state.
Significance of the synaptic cleft
The synaptic cleft thus ensures that an action potential can be transmitted between two cells. For example, information is passed on to the next nerve cell or a muscle cell is stimulated to contract.
The flow of information is directed, ie it can only run in one direction (from the presynaptic cell to the postsynaptic cell).
Importance of the enzymes of the synaptic cleft
The enzymes break down some of the neurotransmitters that activate the receptors back into their individual parts. If they did not do this, the ion channels would remain open and thereby permanently excite the postsynaptic cell. For example, such a muscle would be permanently tense.
To prevent this from happening, the transmitters are removed by the enzymes, transported out of the synaptic cleft and packed back into vesicles in the presynaptic cell.
Synaptic cleft: Effects of different substances
Many different substances that affect the nervous system unfold at the synaptic cleft. These can be drugs, poisons or medication. But what exactly do these substances do? As you have just read, neurotransmitters are important players in the synaptic cleft. Now there are different types of transmitters that have different effects on the nervous system accordingly.
For example, the messenger substance acetylcholine ensures, among other things, that the ion channels in the synaptic gap between a nerve cell and a muscle cell are opened, i.e. the muscle can be contracted.
Effect of drugs on the synaptic cleft
Certain types of antidepressants work by blocking the reuptake of neurotransmitters. These include the selective serotonin reuptake inhibitorsselective serotonin reuptake inhibitorsSSRIs for short).
These active ingredients ensure that the reuptake of serotonin is inhibited by blocking the transport of the transmitter back into the presynaptic cell. As a result, the transmitter serotonin stays longer in the synaptic cleft and can also stimulate the postsynapse several times.
Effect of drugs on the synaptic cleft
In addition to acetylcholine, dopamine also serves as a neurotransmitter. Dopamine plays a major role in the human reward system.
The human reward system is called the mesocorticolimbic reward system. With positive impressions, it releases high amounts of dopamine and thus provides conditioning to repeat this positive stimulus – motivation arises.
This is exactly what many intoxicating substances take advantage of. Drugs such as nicotine or cocaine stimulate an increased release of dopamine and thus provide a certain feeling of happiness. As the drug wears off, dopamine levels drop and the feeling of happiness disappears. What remains is the desire to repeat this «kick».
Effect of alcohol on the synaptic cleft
If alcohol is consumed, the formation of gamma-aminobutyric acid, commonly abbreviated as GABA, is stimulated. This is an inhibitory neurotransmitter that reduces the excitability of nerve cells. As a result, information flows are inhibited and the alcohol unfolds its calming effect.
For more detailed information you can read the Originals Neurotransmitter, Neuromodulation and Neurotoxin.
Synaptic cleft versus gap junctions
A synaptic cleft is only found at a chemical synapse. In electrical synapses, the connection between two cells is instead called gap junctions or nexus.
In contrast to the synaptic gap, a gap junction is only about 2-3.5 nm wide. It transmits electrical signals in the form of ion currents, but unlike the chemical synapse, it does not need transmitters that open ion channels.
While chemical cells experience a delay of about 0.5 milliseconds in information transmission across the synaptic gap, the transmission at a gap junction is instantaneous. Also, at gap junctions, the signal can be propagated from either direction.
Synaptic cleft – the most important things at a glance
- The synaptic cleft is the connection between neurons.
- In it, transmitters diffuse to receptors so that the electrical signal can be passed on.
- Enzymes in the synaptic cleft cleave transmitters, thus the postsynaptic cell is not permanently excited.
- The process in the synaptic cleft can be disturbed by drugs, toxins or medicines.
- The synaptic gap connects chemical synapses, its counterpart for electrical synapses are gap junctions.