Introduction

Humans and other vertebrates have developed a highly efficient system of communication, the nervous system. It is a network of interconnected neurons that are capable of generating and transmitting nerve impulses. The communication process is based on the successful transmission of nerve impulses from one neuron to the other.

Neurons are connected via specialized structures called synapses. The transmission of a nerve impulse or action potential from one neuron to another neuron or non-neuron cell, across the synapse, is called synaptic transmission.

The process of synaptic transmission can be easily understood after studying the structure and the type of synapses involved. In this article, we will discuss the structure and types of synapses, the process of synaptic transmission, the role of neurotransmitters, the effects of drugs, and clinical conditions associated with synaptic transmission.

Types of Synapses

Not all the synapses found in the body are the same. Based on the mode of synaptic transmission, the synapses found in the human body are divided into two types; chemical synapses and electrical synapses.

Chemical Synapses

These are the most abundant synapses found in the body. The two neurons are joined via a synaptic cleft. One of the two cells secretes neurotransmitters into the cleft. These chemical messengers diffuse through the cleft and act on the receptors present in the other cell.

As chemical messengers i.e. neurotransmitters are used for the transmission of nerve impulses across a synapse, such synapses are called chemical synapses.

The detailed structure of such synapses will be discussed in detail.

Electrical Synapses

In some cases, the two cells are connected via gap junctions. The cytoplasm of these cells is connected in such a way that ions can freely diffuse among the cells. the action potential generated in one cell is transmitted to the next cell by the flow of ions.

As ions or electric currents are used for the transmission of action potential among the cells, such synapses are called electrical synapses. They are found among the cardiac muscles and some smooth muscle cells.

Structure of a Chemical Synapse

Understanding the structure of a chemical synapse will help us grab the concept of synaptic transmission.

A chemical synapse has three major components; pre-synaptic terminal, synaptic cleft, and post-synaptic terminal.

Pre-synaptic Terminal

It is the axon terminal of the pre-synaptic neuron. The axon terminal makes a dilation called axon bouton. It is connected to the post-synaptic neuron or cell via the synaptic cleft.

The pre-synaptic terminal has multiple adaptations to release neurotransmitters when the action potential reaches the axon bouton. Neurotransmitters are kept stored in the axonal terminal in the form of vesicles. Certain calcium channels are present in the plasma membrane. Certain channels for the release and uptake of neurotransmitters are also present.

Synaptic Cleft

It is the space between the pre-synaptic and post-synaptic cells. the neurotransmitters released by the pre-synaptic terminal diffuse through this space to act on the post-synaptic cells.

The size of the synaptic cleft is of the order of 20 nm. This small size allows the neurotransmitters to rapidly pile up and diffuse through the cleft.

This space between the two cells also has certain enzymes that can cause the degradation of neurotransmitters. It helps remove the neurotransmitters when they have done their action.

Post-synaptic Terminal

The post-synaptic terminal may be a neuron or non-neuronal cell. It contains receptors for neurotransmitters that are linked to some ion channels. The binding of neurotransmitters to these receptors results in the opening of ion channels, the ion diffuse across the cell, and an action potential is generated in the postsynaptic cell.

The post-synaptic receptors are usually located in an invagination of the cell membrane called synaptic gutter or junctional folds.

Read more about Nerve Impulses

Process of Synaptic Transmission

The process of synaptic transmission involves two steps; release of neurotransmitter from pre-synaptic cell and generation of an action potential in the post-synaptic cell.

Release of Neurotransmitter

Neurotransmitters are released by the pre-synaptic cell when an action potential reaches the terminal. The process is as follows;

• Specialized voltage-gated calcium channels are located in the pre-synaptic terminal.
• When the action potential reaches the axon terminal, the depolarization of axolemma (plasma membrane of axons) causes the opening of calcium channels.
• Calcium ions are present in higher concentrations in the extracellular fluid surrounding the terminal. The opening of calcium channels causes these ions to diffuse into the axonal fiber.
• Once inside the axon terminal, calcium ions bind to some specialized proteins located on the inner surface of the membrane called release sites.
• Binding of calcium ions to these release sites causes the synaptic vesicles to diffuse with the terminal membrane.
• Synaptic vesicles diffuse and the neurotransmitter molecules present in them are released into the synaptic cleft.

The number of neurotransmitters released into the cleft is proportional to the number of calcium ions diffusing into the pre-synaptic terminal.

Generation of Action Potential

The process of synaptic transmission is completed when an action potential is generated in the postsynaptic cell. It involves the following steps.

• Neurotransmitters diffuse across the synaptic cleft and reach the junctional folds on the post-synaptic cell.
• Here, the neurotransmitters bind to the membrane receptors and activate them.
• The activation of receptors causes the opening of ion channels.
• Sodium ions present abundantly in the surrounding fluid rapidly enter the cell down the concentration gradient.
• The diffusion of sodium ions into the cell causes depolarization and a receptor potential is generated.
• If the receptor potential is greater than the threshold, an action potential is generated in the post-synaptic cell.

The strength of receptor potential depends on the number of neurotransmitters binding to the receptors.

Removal of Neurotransmitters

In most of the cases, the number of neurotransmitters released during synaptic transmission is greater than the requirements. Once an action potential is generated, the excess neurotransmitters must be removed from the cell. If the extra neurotransmitters are not removed, they will cause continuous excitation of the post-synaptic cell.

There are two ways to remove extra neurotransmitters;

• Breakdown enzymes are present in the synaptic cleft. They cleave the neurotransmitters to rapidly decrease their concentration in the synapse. The breakdown products may be taken up by the pre-synaptic neuron to make new neurotransmitters.
• Certain protein channels are present in the pre-synaptic terminal. They use ATP to actively pump the neurotransmitters back into the axon. This also helps to rapidly decrease the concentration of neurotransmitters.

The reabsorbed neurotransmitters are again packed into the synaptic vesicle and are ready to be released again during the next cycle.

Neurotransmitters

These are the chemical messengers that are involved in the process of synaptic transmission in chemical synapses. Their study is necessary to completely understand the synaptic transmission.

Here we will discuss briefly the synthesis and types of neurotransmitters. 

Synthesis

Neurotransmitters are either amino acids or peptides in nature. They are made by the rough endoplasmic reticulum present in the cell body of the pre-synaptic neuron.

The neurotransmitters are then packed into excretory vesicles called synaptic vesicles. The synaptic vesicles are formed by the Golgi bodies and are stored in the pre-synaptic terminal.

Types

Not every neurotransmitter released into the synapse causes an action potential in the post-synaptic cell. Sometimes, they can also block nervous transmission.

Based on their function, there are two types of neurotransmitters.

Excitatory Neurotransmitters

These can cause the excitation of the post-synaptic cells. Binding of such neurotransmitters to the receptors causes;

• Increased conduction through the sodium channels by opening them
• Decreased conduction through the potassium channels by blocking them

Both these changes cause depolarization of the cell and an action potential is generated.

Examples of excitatory neurotransmitters include norepinephrine, acetylcholine, and dopamine, etc.

Inhibitory Neurotransmitters

These serve to block synaptic transmission. The following changes take place in a post-synaptic cell when an inhibitory neurotransmitter binds to its receptors.

• Increased conduction through the potassium channels so that potassium ions diffuse out of the cell
• Decreased conduction through the sodium ions so that no sodium ion enters the cell

All these changes make the cell hyperpolarized and no action potential is generated. As a result, synaptic transmission is blocked.

Examples of inhibitory transmitters include GABA, serotonin, and dopamine in some cases.

Drugs Action

Sometimes, there is a medical need to enhance or suppress synaptic transmission. For example, in chronic pain conditions, synaptic transmission is blocked to treat pain. Certain drugs act on synapse and are responsible for modifications in synaptic transmission. Some of these drugs are as mentioned below.

Curare Drugs

These drugs are used for complete blockage of action potential transmission at some synapses. They are the acetylcholine antagonists. Curare drugs bind to acetylcholine receptors and block their activation. The receptors are not stimulated even when abundant acetylcholine is present in the synaptic cleft.

Some examples of curare drugs are atracurium, pancuronium, and vecuronium, etc. These are the non-depolarizing muscle reactants, as they prevent the depolarization of post-synaptic cells.

Atracurium is used as a skeletal muscle relaxant during surgery or mechanical ventilation.

Morphine

It is a powerful pain-killer used in extreme situations. Morphine inhibits synaptic transmission of pain signals by activating meu-receptors.

Meu-receptors cause increased potassium efflux out of the cell and decreased influx of calcium and sodium ions. The cell becomes hyperpolarized and the synaptic transmission is blocked.

Acetylcholine esterase Inhibitors

Acetylcholine esterase is an enzyme present in the synaptic cleft that cleaves acetylcholine into choline and acetic acid. The action of this enzyme is necessary to remove excess acetylcholine and end the transmission process after one cycle.

The inhibitors of this enzyme are used to increase the concentration of acetylcholine in the synaptic cleft. These drugs are used in Myasthenia gravis, glaucoma, and to increase the motility of digestive and urinary systems.

Examples of these drugs include physostigmine, neostigmine, rivastigmine, etc.

Strychnine

It is a poisonous drug that blocks synaptic transmission at the motor endplate (synapse between skeletal muscle fiber and a motor neuron). It blocks the glycine receptors on alpha motor neurons in the spinal cord. Thus, there is no more inhibitory effect of glycine and uncontrolled muscle contractions occur. This leads to muscle spasm.

Alcohol

Alcohol plays a role in the transmission of inhibitory synaptic signals. It mimics the action of inhibitory neurotransmitter GABA, bi binding to GABA_A receptors. As a result, the inhibitory effect of GABA is enhanced. The post-synaptic neuron becomes hyperp[olarized due to this action.

Summary

Synaptic transmission is the transfer of action potential from one neuron to next neuron or non-neuronal cells at a synapse.

Two types of synapses are present in the human body.

• Chemical synapses, synaptic transmission occurs via chemical messengers
• Electrical synapses, synaptic transmission occurs via the flow of electrons

A chemical synapse consists of;

• A pre-synaptic terminal: It is the axonal terminal of a neuron
• A space between two terminal called the synaptic cleft
• A post-synaptic cell that might be a neuron or some other cell

The process of synaptic transmission involves;

• Release of neurotransmitters from the pre-synaptic terminal into the cleft, when an action potential reaches the terminal
• Diffusion of neurotransmitter across the synaptic cleft and binding to receptors on post-synaptic junctional folds
• Depolarization of cell due to activation of receptors
• Removal of excess neurotransmitters

Neurotransmitters involved in synaptic transmission can be excitatory or inhibitory. They are made in the cell body of the neuron and are stored in synaptic vesicles at the pre-synaptic terminal.

• Acetylcholine and glutamate are examples of excitatory neurotransmitters
• GABA and Glycine are examples of inhibitory neurotransmitters

Synaptic transmission is also affected by drugs.

• Curare drugs block the acetylcholine receptors
• Morphine activates the inhibitory meu-receptors
• Acetylcholine esterase inhibitors increase the half-life of acetylcholine
• Strychnine is the neurotoxin that blocks the inhibitory glycine receptors in the spinal cord
• Alcohol activates the inhibitory GABA­_A receptors

Frequently Asked Questions

What is meant by synaptic transmission?

Synaptic transmission is the process in which a chemical substance called a neurotransmitter is released by a neuron that triggers nerve impulses in another neuron or a response in a target cell.

What are the components of a synapse?

A synapse consists of a presynaptic terminal that releases neurotransmitters, a synaptic cleft into which the neurotransmitters are released, and a postsynaptic membrane where the neurotransmitters act.

What are 3 types of synapses?

The three types of synapses include axosomatic, axodendritic and axoaxonic synapses. Axosomatic synapse is between an axon and the cell body of another neuron. Axodendritic synapses are between axon and dendrite. Axoaxonic synapses are between two axons. 

What is the role of synapses?

Synapses are needed for communication between adjacent neurons or between neurons and the target cells. They are essential for the functioning of neural activity. They also play a role in cognition, learning and memory formation, etc.

References

1. Squire, Larry R.; Floyd Bloom; Nicholas Spitzer (2008). Fundamental Neuroscience. Academic Press. pp. 425–6. ISBN 978-0-12-374019-9.
2. Garber, Steven D. (2002). Biology: A Self-Teaching Guide. John Wiley and Sons. p. 175. ISBN 978-0-471-22330-6.
3. Bear, Conners, Paradiso (2007). Neuroscience: exploring the brain. Philadelphia, PA: Lippincott Williams & Wilkins. pp. 113–118
4. Van Spronsen M, Hoogenraad CC. Synapse pathology in psychiatric and neurologic disease. Curr Neurol Neurosci Rep. 2010;10(3):207–214. doi:10.1007/s11910-010-0104-8