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A synapse is the contact site where a neurone and another neurone or other cell meet. Specialised electron microscopes are used to visualise synapses. Through these, we know one average neuron has 1000 synapses. The cortex (the outermost layer of the brain) has around 125 trillion (125,000,000,000,000) synapses alone, which is more synapses in every brain than stars exist in our…
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Jetzt kostenlos anmeldenA synapse is the contact site where a neurone and another neurone or other cell meet. Specialised electron microscopes are used to visualise synapses. Through these, we know one average neuron has 1000 synapses. The cortex (the outermost layer of the brain) has around 125 trillion (125,000,000,000,000) synapses alone, which is more synapses in every brain than stars exist in our entire galaxy!
There are many types of synapses; they can be classified according to:
The function of a synapse is to transmit information from one neuron to another or from one neurone to another cell, depending on the type of synapse. Synapses are the interfaces between the specialised cells of the nervous system and each other/other cells.
Synapses are always named after the main neurotransmitter passed on at the synapse using -ergic as an affix. So if a synapse transmits dopamine, it’s called dopaminergic, a synapse transmitting adrenaline is called adrenergic, one transmitting GABA (primary inhibitory neurotransmitter) is called GABA-ergic, etc.
An odd one out of -ergic naming rule for synapses is the cholinergic synapse, which transmits acetylcholine.
The synapse consists of three parts:
Pre- (in presynaptic) is before the gap (synaptic cleft), and post- (in postsynaptic) is after the gap.
There are two major types of synapses: electrical synapses and chemical synapses. There are more chemical synapses in the human body than electrical, but both have important functions.
An electrical synapse features a channel made of connexin proteins. This protein channel is called a gap junction, connexon or a pore. The gap junction directly connects a neurone and another cell to bridge a gap filled with an interstitial liquid called the synaptic cleft.
Although electrical synapses are more frequent in animals such as squid and zebrafish, they are also in humans’ central nervous system, retina and olfactory bulbs, where it’s most important to have optimal synchronisation fast coordination of neurones.
Charged ions and messenger proteins can pass through gap junctions uninhibited. This direct connection makes the transmission of information in electrical synapses faster than in chemical synapses. In contrast to chemical synapses, the charge and the protein molecules can flow back and forth between the cells in some electrical synapses, making it bi-directional.
Chemical synapses are the most common synapses in the human body. The chemical synapse uses chemical messenger molecules to generate an electrical signal. These messengers that are generated in the postsynaptic cell are called neurotransmitters. They diffuse into the synaptic cleft to bind to receptors to open gates that allow ions to flow into the postsynaptic cell. Receptors are specialised protein channels that only allow positively or negatively charged ions into the cell. You can find out more about how this process works in our article on synaptic transmission.
Table 1. Differences between the electrical and chemical synapses.
Chemical synapses | Electrical synapses |
Found in higher vertebrates. | Found in both lower and higher vertebrates and invertebrates. |
Impulse is transmitted using a neurotransmitter. | Impulse is transmitted using ions. |
Unidirectional transmission. | Bi-directional transmission. |
Gaps between cells are around 20 nm | Smaller gaps - only 3 - 5 nm |
Transmission is relatively slow - several milliseconds. | Transmission is fast - almost instant. |
Either inhibitory or excitatory. | Excitatory. |
Signal remains strong. | Signal will disappear over time. |
Sensitive to pH and hypoxia. | Insensitive to pH and hypoxia. |
Vulnerability to fatigue. | Relatively less vulnerable to fatigue. |
Synapses can be grouped and classified in several ways.
We’ve looked at two different functional types of synapses, but synapses can also be classified according to how they connect to other neurones or cells.
Types of attachment between two cells include:
Neurones connect to all parts of the body. Various others include axons into the interstitial spaces or to a blood vessel, etc.
Synapses can be classified on the type of neurotransmitter released. Examples of neurotransmitters include dopamine, adrenaline, GABA, acetylcholine and others. These help name the synapses accordingly (except for acetylcholine).
There are more but the main ones we focus on are electrical synapses, neuromuscular junctions and inhibitory ion channel synapses.
The terms presynaptic and postsynaptic refer to either side of the gap or synaptic cleft, with the presynaptic side being axon terminal of the sending neurone and the postsynaptic side being the specialised membrane of the receiving cell (neurone, muscle or other cell).
Synapses can be classified in three ways:
Electrical synapses are much less common in higher invertebrates.
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