Types of Synapse

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 entire galaxy!

Fig. 1 - Electron microscope photograph of a neurone (blue) with all the synapses ( yellow) connected to it. source: https://www.healththoroughfare.com/science/scientists-shed-more-light-on-the-brain-evolution-in-humans/14764

There are many types of synapses; they can be classified according to:

1. How they attach to the other cells.
2. Type of neurotransmitter released.
3. The effect they have on the postsynaptic membrane.

What is the function of a synapse?

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.

How are synapses named?

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.

What is the structure of a synapse?

The synapse consists of three parts:

• The pre-synapse - Axon terminal of the neurone that is sending information.
• The synaptic cleft - A tiny 20-30 nanometre wide gap between the two neurones filled with a fluid called the interstitium.
• The postsynaptic membrane of a second receiving cell is usually another neurone, but it might also be a gland, organ, or muscle. The postsynaptic membrane has protein channels called receptors, and they are more abundant here than in other parts of the cell.

Fig. 2 - Diagram of a synapse

What are the two main types of synapses?

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.

Fig. 3 - Diagram of an electrical and a chemical synapse, which both work in different ways

What is an electrical synapse?

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.

What is a chemical synapse?

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.

Fig. 4 - Electron microscope photograph of a synapse showing the synaptic cleft and vesicles. source: https://www.oist.jp/news-center/photos/high-magnification-image-synapse-obtained-electron-microscopy

Comparison between electrical and chemical synapses

Table 1. Differences between the electrical and chemical synapses.

How can synapses be classified?

Synapses can be grouped and classified in several ways.

Fig. 5 - Diagram of three different types of synaptic connections source: https://ib.bioninja.com.au/options/option-a-neurobiology-and/a1-neural-development/synaptic-formation.html

Cell attachment

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:

• Axodendritic: The axon of one neurone connects to the dendrites, by far the most common synapse in the human body.
• Axosomatic: The axon of one neurone connects to the cell membrane of the body or soma of another cell.
• Axo-axonic: The axon of one neurone connects to the axon of another neurone. Usually, these are inhibitory synapses.
• Dendro-dendritic: These are dendrite connections between two different neurones.
• Neuromuscular: The axon of one neurone connects to a muscle. These types of synapses are highly specialised. Usually, these are large synapses that convert the electrical impulses in the motor neuron into the electrical activity that causes muscle contractions. All neuromuscular junctions use acetylcholine as a neurotransmitter.

Neurones connect to all parts of the body. Various others include axons into the interstitial spaces or to a blood vessel, etc.

Type of neurotransmitter released.

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).

Effect on the postsynaptic membrane

• Excitatory ion channel synapses: Neuroreceptors contain sodium channels. The channels open and close on the postsynaptic membrane.
• Inhibitory ion channel synapses: Neuroreceptors contain chloride channels. The mechanism of the synapse causes the action potential to be less likely - they inhibit the impulse.
• Non-channel synapses: Neuroreceptors are membrane-bound enzymes. The enzymes catalyse a chemical messenger that affects the cell’s metabolism. These are involved in slow and long-lasting actions such as memory and learning.