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Types of Synapse

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Biology

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!

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.

An odd one out of -ergic naming rule for synapses is the cholinergic synapse, which transmits acetylcholine.

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.

Pre- (in presynaptic) is before the gap (synaptic cleft), and post- (in postsynaptic) is after the gap.

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.

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.

Comparison between electrical and chemical synapses

Table 1. Differences between the electrical and chemical synapses.

Chemical synapsesElectrical 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 nmSmaller 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.

How can synapses be classified?

Synapses can be grouped and classified in several ways.

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.

Types of Synapse - Key takeaways

  • A synapse is the contact site where a neurone and another neurone or a neurone and another cell meet. The presynaptic neurone/cell is the transmitting cell; the postsynaptic neurone/cell is the receiving cell. There are two major types of synapse - electrical and chemical.
  • An electrical synapse is a protein channel called a gap junction, which directly connects two neurones and enables fast, bidirectional and transmission of electrical impulses and molecules.
  • A chemical synapse uses neurotransmitters diffused into the synaptic cleft to bind to receptors that open gates to allow ions to flow into the postsynaptic cell.
  • Synapses can have a variety of interfaces. The most common interfaces are axodendritic (presynaptic axon to postsynaptic dendrite, the most common), axosomatic (presynaptic axon to postsynaptic cell body), and axo-axonic (axon to axon).

Types of Synapse

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:  

  • according to how they attach to other cells (axo-axonic, axodendritic, axosomatic, etc.) 
  • according to what kind of neurotransmitter is released by them (dopaminergic for dopamine-releasing synapses)
  • what kind of effect they have on the postsynaptic membrane (excitatory ion channel, inhibitory ion channel or non-channel synapse)

Electrical synapses are much less common in higher invertebrates.

Final Types of Synapse Quiz

Question

What is a synapse?

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Answer

A synapse is the contact site where a neurone and another neurone or other cell meet.

Show question

Question

What is synaptic transmission of the chemical synapse?

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Answer

Synaptic transmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft.

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Question

What are the two main types of synapses?


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Answer

Chemical and electrical synapses.

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Question

Do electrical synapses use neurotransmitters?


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Answer

Electrical synapses don't use neurotransmitters, the transmission of electrical charge and molecules is direct.

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Question

Name two advantages of the electrical synapse.


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Answer

Impulse is fast and bi-directional.

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Question

Which type of synapse is the most common in humans?


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Answer

The chemical synapse.

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Question

The presynaptic membrane is where neurotransmitters bind to let positive or negative ions enter. True or False?


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Answer

False.

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Question

Synaptic transmission or neurotransmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft. True or False?


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Answer

True.

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Question

Fill in the blank: Some medication such as SSRIs which are used to treat depression by blocking the reuptake of neurotransmitters into the __________ cell.


Show answer

Answer

Presynaptic.

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Question

Fill in the blank: A synapse that transmits Acetylcholine is called ___________.


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Answer

Cholinergic.

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Question

Which of the following is not a way in which synapses interface? Choose one correct answer.


  1. Axodendritic
  2. Axosomatic
  3. Axoribosomic
  4. Axo-axonal

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Answer

Axoribosomic

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Question

Which type of connection of a synapse is the most common? Choose one correct answer.


  1. Axodendritic
  2. Axosomatic
  3. Axoribosomic
  4. Axo-axonal

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Answer

Axodendric

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Question

What is the neurotransmitter used at an adrenergic synapse?

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Answer

Adrenaline.

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Question

What’s the protein that gap junctions are made of? Choose one correct answer.


  1. Connexon
  2. Connexin
  3. Interstitium
  4. Receptors

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Answer

Connexin

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Question

You can look at synapses using any common light microscope. True or False?

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Answer

False.

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Question

What is synaptic transmission?

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Answer

Synaptic transmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft.

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Do electrical synapses use neurotransmitters for transmission across the synapse?

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Answer

Electrical synapses don't use neurotransmitters, transmission of electrical charge and molecules is direct.

Show question

Question

The presynaptic membrane is where neurotransmitters bind to let positive or negative ions enter. True or False?


Show answer

Answer

False.

Show question

Question

Synaptic transmission or neurotransmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft. True or False?


Show answer

Answer

True.

Show question

Question

When the opening of an ion channel leads to positive ions streaming into the cell, this is called exhibitory. True or False?


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Answer

False.

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Question

When action potential flows into the presynaptic axon terminal, if excitatory neurotransmitters are diffused into the synaptic cleft, this always leads to action potential in the postsynaptic cell. True or False?


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Answer

False.

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Question

After diffusing into the synaptic cleft, neurotransmitters enter the postsynaptic cell. True or False?


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Answer

False.

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Question

Fill in the blank. Some medication such as SSRIs which are used to treat depression by blocking the reuptake of neurotransmitters into the ________ cell.


Show answer

Answer

Presynaptic.

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Question

What is exocytosis?

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Answer

Exocytosis is when the membrane of the vesicles that hold the neurotransmitters fuse with the presynaptic membrane.

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Question

Fill in the blank. The liquid in the synaptic cleft is called _______.

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Answer

Interstitium.

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Question

What is special about transmission across neuromuscular junctions?


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Answer

In transmission across neuromuscular junctions, the postsynaptic cell is muscle and they are all cholinergic. 


The successful reaching of the excitatory threshold after synaptic transmission at the neuromuscular junction leads to contraction of the muscle.

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Question

What is the name given to drugs that mimic neurotransmitter?

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Answer

Antagonist/antagonistic.

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Question

Give an example of a drug that interacts with molecular components of the neurone cells.

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Answer

Marijuana.

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