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Process of Synaptic Transmission

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Process of Synaptic Transmission

Imagine having to share the Nobel prize for your life’s work with your intellectual arch-nemesis. This is exactly what happened with early neuroscientists Golgi and Cajal in 1906. They had radically different ideas about how the nervous system worked - Golgi believed that neurones are part of one connected network similar to the bloodstream, called the reticular theory. Cajal thought that the nervous system was made of separate units, known as the neuron theory.

What is a synapse?

What separates the two theories is the mechanism of how neurones connect. The development of microscopy has confirmed Cajal’s theory - neurones are separate cells that don’t connect directly; there are gaps between them. It’s via these gaps or synapses that information is exchanged. This exchange of information is known as synaptic transmission.

A synapse is the contact site where a neurone and another neurone or other cell meet. Being tiny structures on the cell, specialised electron microscopes visualise synapses. Through these, we know that the average neurone has 1000 synapses, but estimates go from one up to 200,000 synapses per cell. The cortex (the outermost layer of the brain) has around 125 trillion (125,000,000,000,000) synapses alone, which means there are many more synapses in every brain than stars exist in our entire galaxy.

There’s usually a small gap between neurones called the synaptic cleft. In the most common type of synapse, the chemical synapse neurones don’t touch except via specific protein structures that act like scaffolding. It’s here at the synapse that neurones and cells communicate via chemical molecules called neurotransmitters. The synapse converts electrical signals into chemical information through its unique mechanism, which is again converted into electrical signals. Another type of synapse, the electrical synapse, works a bit differently. Let’s have a closer look at the two types of synapses.

What are the two different types of synapses?

There are two types of synapses; electrical synapses and chemical synapses. There are more chemical synapses in the human body than electrical, but both have essential functions.

What is an electrical synapse?

An electrical synapse features a channel made of proteins called a gap junction, connexons or pore. The gap junction directly connects a neurone and another cell, bridging the synaptic cleft. Although electrical synapses are more frequent in animals such as squid and zebrafish, they can also be found in humans’ central nervous system, retina and olfactory bulbs, where it’s most important to have optimal synchronisation and 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 bidirectional.

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, and it includes:

  • The axon terminal of the presynaptic neurone, meaning the neurone that is sending information.

  • The synaptic cleft is a tiny 20-30 nanometre wide gap between the two neurones filled with the interstitium fluid.

  • The postsynaptic membrane of a second receiving cell would usually be 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. We’ll have a closer look at receptors later.

Now we know what a chemical synapse is, but let’s look at it in action- during synaptic transmission.

Study tip: The chemical synapse is what most people refer to when speaking about synapses.

What is the synaptic transmission of a chemical synapse?

Synaptic transmission or neurotransmission is when a neurone communicates with another neurone or cell by releasing neurotransmitters into the synaptic cleft. When action potential (electrical charge firing along the axon) arrives in the axon terminal, neurotransmitters are released into the synaptic cleft. These then bind to receptors, allowing only negatively charged or only positively charged ions to enter into the next cell and depolarise it.

What do neurotransmitters do in a synapse?

Each synapse usually specialises in one type of neurotransmitter. These are specific messenger molecules produced in the cell body and transported along the cytoskeleton (a network of protein strings and tubes that act like scaffolding for the cell) to the end of the axon. Once they arrive in the axon terminal, they are wrapped in membrane sacs called vesicles and gather at the presynaptic end of the axon, ready to be released.


What are receptors?

A receptor is a protein molecule in the cell membrane that reacts to a specific neurotransmitter, hormone or other molecules.

You can think of it as a gate or door that opens when unlocked by one specific molecule. When a neurotransmitter binds to a receptor, the gate opens to let other specific molecules in, often either ions with a positive charge or ions with a negative charge.

What is an excitatory and inhibitory synaptic transmission?

Synaptic transmission can either be excitatory or inhibitory, depending on the neurotransmitter released. The impulse received on the postsynaptic membrane is either called excitatory postsynaptic potential (EPSP) or inhibitory postsynaptic potential (IPSP), depending on whether the neurotransmitter is excitatory or inhibitory.

Excitatory: This means that the gates opened by the neurotransmitters let positive ions such as Na+ or K+ flow into the cell, resulting in depolarisation of the cell membrane (the inside of the cell becomes positively charged). This makes it more likely for an action potential to be produced. Examples of excitatory neurotransmitters include glutamate and dopamine.

Inhibitory: This means that the gates opened by these neurotransmitters let negative ions such as Cl- into the cell, resulting in hyperpolarisation of the cell membrane (the inside of the cell becomes more negatively charged than usual). This makes it less likely for an action potential to be produced. Examples of inhibitory neurotransmitters include GABA and glycine.

A third possibility is that the neurotransmitter released doesn’t open an ion channel but rather that it sets off a protein chain reaction that has more long-term consequences. These are called g-protein cascades or second-messenger cascades.

When does synaptic transmission lead to action potential?

Action potentials or the electrical impulses that travel along the axon can only be initiated if a certain voltage threshold is reached (usually -55mV). Action potentials follow the all-or-nothing principle and only travel in one direction. One incoming impulse is usually not enough to initiate the action potential, which starts transmission to the next cell via the axon. The addition of a few incoming signals is needed. This process is called summation. Two types of summation can lead to depolarisation/action potential:

  • Spatial summation: When enough excitatory impulses arrive on one cell from different locations.

  • Temporal summation: When enough excitatory impulses arrive on one cell from one other cell in quick succession.


What is the process of synaptic transmission?

In synaptic transmission, electrical charge is converted to chemicals that bridge a gap between the two cells. These chemicals react with the cell membrane to create an electrical charge in the receiving cell.

Let’s go step by step to see how the process of synaptic transmission works:

Action potential (electrical current) arrives in the axon terminal from the cell body. The electrical charge opens Ca++ channels in the axon terminal. These Ca++ channels are voltage-gated, meaning they open up in response to electrical current. Ca++ is more abundant outside the cell and is attracted to the negative charge in the cell, so as soon as the gates open, Ca++ rushes into the cell. Ca++ enters the axon terminal, enabling exocytosis. Exocytosis means that the vesicles’ membrane containing the neurotransmitters fuse with the presynaptic membrane. Vesicles open up, and neurotransmitters diffuse into the synaptic cleft. Neurotransmitters diffuse across the synaptic cleft and bind with receptors on the postsynaptic membrane. The ion channels open up, and either negative ions or positive ions flow into the postsynaptic cell. The remaining neurotransmitters are recycled by the presynaptic cell.

What do synapses connect to?

There are a variety of types of synaptic connections:

  • Axodendritic: The axon of one neurone connects to the dendrites of another - by far the most common synapse in the human body.

  • Axo-somatic: The axon of one neurone connects to the cell membrane of the body or soma of another cell.

  • Axo-axonal: The axon of one neurone connects to the axon of another. Usually, these are inhibitory synapses.

  • Neuromuscular junction: The axon of one neurone connects to a muscle. Usually, these are large synapses and lead to muscle contraction.

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

Process of Synaptic Transmission - Key takeaways

  • A synapse is the contact site where a neurone and another neurone or a neurone and another cell meet.
  • Synaptic transmission is the communication of one neurone with another neurone or cell.
  • The presynaptic neurone/cell is the transmitting cell; the postsynaptic neurone/cell is the receiving cell.
  • There are two 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.
  • The chemical synapse uses neurotransmitters diffused into the synaptic cleft to bind receptors to open gates that allow ions to flow into the postsynaptic cell.
  • Depending on the neurotransmitter, synaptic transmission can act inhibitory or excitatory on the postsynaptic cell.
  • A threshold of -60mV must be reached for action potential in the postsynaptic membrane to be transmitted via the axon, and for this, the excitatory signals must summate.
  • The summation can be spatial or temporal.
  • 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-axonal (axon to axon).

Frequently Asked Questions about Process of Synaptic Transmission

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

Synaptic transmission is the main mechanism by which the nervous system communicates.

During synaptic transmission electrical charge is converted to chemicals that bridge a fluid-filled gap between the two cells, and these chemicals react with the cell membrane to create an electrical charge in the receiving cell.

The synaptic transmission allows electrical signals to be passed on throughout the nervous system.

Final Process of Synaptic Transmission 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.

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Question

What is a 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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What are the two types of synapses?

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Answer

The two types of synapses are chemical and electrical

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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

 It's fast/It's bidirectional/it enables coordination.

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Question

Which type of synapse is the most common in humans?

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Answer

The chemical synapse is the most common in humans.

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Question

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



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Answer

False. 

The postsynaptic membrane is where neurotransmitters bind to let positive or negative ions enter.



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Question

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



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Answer

True.

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Question

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


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Answer

False- this is called excitatory.

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Question

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



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 False- one action potential from the presynaptic cell leads to one of many impulses that are summed, and only if a threshold of -60mV is reached is an action potential in the postsynaptic cell initiated.

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Question

True or false- Action potential leads to Ca ++ streaming into the postsynaptic cell.

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Answer

False- Action potential leads to Ca++ streaming into the presynaptic cell.

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Question

True or false - After diffusing into the synaptic cleft, neurotransmitters enter the postsynaptic cell.



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False - neurotransmitters don't enter the postsynaptic cell, they bind to receptors which allow negative or positive ions to enter the postsynaptic cell.



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Question

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



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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

True or false - Ca++ enables exocytosis.


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Answer

True.

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