Synapse

Synapse is the junction between two neurons, forming a functional connection between nerve cells and other cells.

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Synapse is the junction between two neurons, serving as the structural basis of communication between the neurons in the Central Nervous System (CNS) and between muscle cells and neurons in the Peripheral Nervous System (PNS). Based on the mode of impulse transmission, the synapse is classified into two:

Chemical Synapse

Electrical Synapse

Let’s dive deeper into understanding the purpose and significance of synapses!

Table of Contents

What is Synapse? – Synapse Meaning

Structure of Synapse

Classification of Synapse

Chemical Synapse and Its Functions

Electrical Synapse and Its Functions

Properties of Synapse

Synaptic Transmission

Frequently Asked Questions

What is Synapse? – Synapse Meaning

Synapse Definition: A synapse is a connection between two neurons or a neuron and a target or effector cell, such as a muscle cell. This connection allows for the transmission of electrical or chemical signals.

The synapse formed between presynaptic and postsynaptic neurons is known as the neuromuscular junction, connecting a neuron to a muscle.

A synapse is the medium through which nerve impulses are conducted from the axon terminal of one neuron to the dendrites of the next. This conduction can be either electrical or chemical.

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The synaptic cleft is the name for the tiny space that exists between the membrane of the postsynaptic cell and the axon terminal of the presynaptic neuron. Presynaptic terminals are specialised in mediating the rapid fusion of synaptic vesicles following calcium influx. Specialized receptors are also present in the postsynaptic terminal. In a matter of microseconds, the neurotransmitter diffuses across the synaptic cleft and binds to particular receptors.

Synapse and Neurotransmitters

Neurotransmitters are chemicals that are released from neurons after an action potential at the nerve endings. They travel across synapse, exciting or inhibiting the target neuron. It aids in transferring messages in the form of nerve impulses from the presynaptic membrane to the postsynaptic membrane in the neuron. Various types of neurons make use of different transmitters and hence impart different effects on its targets.

Neurotransmitters are of two types:

Inhibitory Neurotransmitters:

• Example: GABA
• Example: Glycine

Excitatory Neurotransmitters:

• Glutamate
• Acetylcholine
• Aspartate

Structure of Synapse

A typical synaptic structure consists of a presynaptic neuron, a postsynaptic neuron, and a synaptic cleft.

The presynaptic neuron functions in storing and releasing neurotransmitters, while the postsynaptic neuron participates in receiving neurotransmitters. Each branch of the axon terminal has several vesicles such as granular, agranular vesicles, cisternae, coated vesicles, and endosomes.

The synaptic vesicles, which are agranular and spherical with a diameter of 35-50 nm, contain small molecule neurotransmitters such as glycine, glutamate, etc.

The granular vesicles have a diametre ranging from 80 nm to 160 nm and usually contain neuropeptides. Actin filaments are necessary for the movement of synaptic vesicles towards the presynaptic membrane in order to fuse. Synapsin I, a significant phosphoprotein, is responsible for regulating neurotransmitter release in the synaptic cleft.

The presence of Fodrin and Microtubules in addition to F-Actin and Synapsin are significant in presynaptic terminal. The area in presynaptic ends that are formed by the membrane and a dense collection of proteins that communicate neurotransmitter release is called Active Zone. It is here where the initial events of synaptic transmission occurs. The postsynaptic section has a higher concentration of receptor proteins or ligand ion channels to receive neurotransmitters.

The association of excitatory neurotransmitter glutamate with its corresponding receptors induces the flow of positive ions to the post synaptic neurons, resulting in the formation of IPSP and EPSP. On the other hand, the association of inhibitory neurotransmitter with its receptor causes the opening of ion channels, leading to the flow of negative ions towards the post synaptic cell.

Classification of Synapse

Based on anatomy, synapses can be classified into three types:

1. Axoaxonic synapses are those where the axons of two different neurons come together.

2. An axodendritic synapse is where the axon of one neuron connects to the dendrite of another neuron.

3. A axosomatic synapse is where the axon of one neuron attaches to the soma (cell body) of another neuron.

Synapses can be classified into two types based on their function or physiology: electrical synapses and chemical synapses.

You May Want to Look At: Difference between Electrical and Chemical Synapses

Chemical Synapse and Its Functions

Signal transmission at synapse is a chemical process. Here, let us explore how the pre and post synaptic neurons contribute to nerve impulse transmission.

Chemical synapses are more common, and the transmission of nerve impulses through them is mediated by neurotransmitters.

The synaptic cleft is a fluid-filled space between two neurons, so the nerve impulse cannot travel directly from one neuron to the other.

Axon terminals have a knob-like structure, containing synaptic vesicles.

When the action potential reaches the terminals, neurotransmitters are released from the presynaptic neuron’s terminal into the synaptic cleft via synaptic vesicles.

Neurotransmitters bind to the receptors present in the postsynaptic membrane.

The flow of ions through voltage-gated channels leads to a change in the polarisation of the postsynaptic membrane, resulting in the conduction of the electric signal across the synapse.

Neurotransmitters can be either inhibitory or excitatory, and one neurotransmitter can result in different responses in different cells.

If there is a net flow of positively charged ions inside the cell, then the neurotransmitter is excitatory and it results in generating the action potential; this is known as an Excitatory Postsynaptic Potential (EPSP).

When the membrane potential becomes more negative, the membrane becomes hyperpolarized and the action of neurotransmitter is inhibitory, resulting in the generation of an inhibitory postsynaptic potential (IPSP).

Once the neurotransmitters attach to the receptor, they are either acted on by enzymes or taken back and recycled, thus terminating the signal after it is conducted forward.

Electrical Synapse and Its Functions

Electrical synapses are quicker than chemical synapses.

The presynaptic and postsynaptic neurons are in close proximity, forming gap junctions with a physical connection between them, mediated by protein channels.

These gap junctions allow the direct flow of ions and the transmission of an electric signal across the electrical synapse, which is similar to the conduction of impulse in an axon.

Chemical synapses are more flexible than electrical synapses, as they can turn an excitatory signal to an inhibitory signal.

In lower invertebrates, it has been discovered. In humans, it is located between glial cells.

The synapse ensures the proper conduction of nerve impulses in the correct direction and prevents any unwanted stimulation.

Properties of Synapse

The following are the 5 basic properties of a synapse:

1. One-way Conduction - At synapses, impulses can only travel in one direction, from the presynaptic neuron to the postsynaptic neuron.

2. Synaptic Delay – A brief delay in the transmission of impulses through the synapses is referred to as synaptic delay. This is caused by the time taken for the release and action of neurotransmitters.

3. Fatigue – Prolonged muscular activity can cause fatigue in synapses, as well as Betz cells in the motor area of the frontal lobe of the cerebral cortex. The depletion of acetylcholine is what causes fatigue in synapses.

4. Electrical property – Electrical properties of the synapse include the IPSP (inhibitory postsynaptic potential) and EPSP (excitatory postsynaptic potentials).

5. Summation – When multiple presynaptic excitatory terminals are stimulated simultaneously or a single presynaptic terminal is stimulated repeatedly, the effects are combined or the EPSP gradually increases.

Synaptic Transmission

Chemical synaptic transmission is a process in which a neuron communicates with a target cell across a synapse by releasing a neurotransmitter from the pre-synaptic neuron. The neurotransmitter then binds to post-synaptic receptors. Additionally, electrical synapses allow direct current flow from one neuron to another.

Electrical synapses are special ion channels that connect pre- and postsynaptic cells, providing a low-resistance pathway for the flow of electrical current between the two cells. Conversely, in chemical synapses, an action potential in the presynaptic neuron results in the release of neurotransmitters, which then induce current flow in the postsynaptic cell.

Differences between Electrical and Chemical Synapses

| Electrical Synapse | Chemical Synapse |

| Transmitting Agent | |

| Ion Current | Chemical Transmitter |

| Distance between Pre-synaptic and Post-synaptic Cell Membranes |

| 3.5 nm | 20-40 nm |

| Yes, there is cytoplasmic continuity between pre- and postsynaptic cells. |

| Yes | No |

| Transmission Direction |

| Bidirectional | Unidirectional |

| Components |

| Gap-junction Channels | Postsynaptic Receptors, Presynaptic Vesicles, and Active Zones |

Frequently Asked Questions

What is a Synapse?

A synapse is a junction between two nerve cells, or between a nerve cell and a gland or muscle cell, through which an electrical or chemical impulse passes from one to the other.

A synapse is a junction between two neurons. It is at these junctions where the axons or other parts of a neuron terminate on the dendrites, axon or soma of another neuron, or in some cases, a gland cell.

What is the Difference Between Neurons and Synapses?

Synapses are the mediators that allow neurons to transfer signals to dedicated target cells. Their function is to facilitate communication between neurons.

The synaptic cleft is the gap between the two neurons in the synapse, across which the action potentials are transmitted.

A synaptic cleft is defined as a tiny fluid-filled gap between two nerve cells, where the neurotransmitter acetylcholine diffuses.### What causes synapses in the brain?

Synapses in the brain are caused by electrical signals sent from one neuron to another.

When an electrical impulse passes through the axon, it creates a synapse by releasing neurotransmitters.

What is released from synapses?

Neurotransmitters are released from synapses.

Neurotransmitters are released from the presynaptic neurons and travel across the synapse to bind with specific receptors on the postsynaptic neuron, bringing about changes in the postsynaptic neuron. What is a synaptic end bulb?

The axon terminal tip enlarges and acquires the name synaptic end bulb near the end of the terminal, which is closest to the muscle fibre. The motor neuron’s synaptic end bulb makes up the nervous system portion of the neuromuscular junction.