Synapse – Definition, Parts, Types: A synapse is a small space between two neurons, where nerve impulses are relayed through a neurotransmitter from the axon of a presynaptic (to send information) neuron to the dendrite of a postsynaptic (receive Information ) neuron. The term “synaptic cleft” or “synaptic gap” refers to it. A synapse is a point of interaction where a neuron and another neuron or other cell join. Scientist Charles Sherrington coined the word “synapse” in 1897 from the Greek words “syn,” which means “together,” and “haptein,” which means “to hold.”

Synapse serve as junctions or relay centers where a nerve cell transfers its nerve information to nearby nerve cells. Neurons are cells that transfer information between your brain and other parts of the central nervous system and peripheral nervous system. The nerve cell carrying information at the synapsis is called presynaptic and the cell receiving information is called postsynaptic.

Structure of Synapsis

In a synapse, there is no physical contact between presynaptic and postsynaptic nerve cells, but there is a very narrow (20 to 40mµ = 200 – 400 Å) gap between the two. This is called the synaptic cleft. This gap is filled with tissue fluid.

Due to the synaptic cleft, nerve impulses cannot be transferred from presynaptic cells to postsynaptic cells in the form of an electrical charge, an electrical action potential. Henry Dale (1936) discovered that neural information across the synaptic cleft is transmitted by special substances that are released from presynaptic cells into the fluid of the fissure to reach the postsynaptic cell.

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

Henry Dale also discovered that in the synaptic bundles of telocentric of nerve fibers, axon fibers, there are small synaptic vesicles filled with neurotransmitter substances in the axoplasm near the art of the synaptic end (axolemma). As soon as a neural impulse reaches the synaptic knob, in the form of an action potential, many Ca ions (Ca) enter the knob from the tissue fluid. Due to their effects, many synaptic vesicles get integrated (fuse) with the art of the end of the knob. Then the fuse strands at the integration points are split and the neurotransmitter is released into the fluid (tissue fluid) of the synaptic cleft by extracellular protrusion, i.e. exocytosis. Therefore, neurotransmitters are also called neurohormones or neurohumors

The agranular vesicles are referred to as synaptic vesicles. They contain the neurotransmitter and are spherical, 35-50 nm in diameter. Small molecule neurotransmitters like glutamate and glycine are found in the synaptic vesicles.

Types of Synapse

Synapse is divided into two categories that are 

• Anatomical Classification
• Functional Classification

1. Anatomical Classification

A synapse is usually formed by the axon of one neuron terminating on the cell body, dendrite, or axon of the next neuron. Synapses are classified into three categories based on the end of the axon.

1. Axoaxonic synapse occurs when the axon of one neuron terminates on the axon of another neuron
2. Axodendritic synapse occurs when the axon of one neuron terminates on the dendrite of another neuron.
3. Axosomatic synapse occurs when the axon of one neuron ends on the soma (cell body) of another neuron\
4. Dendro-dendritic: These are dendritic connections between two different neurons.
5. Neuromuscular: The axon of one neuron connects to a muscle. These types of synapses are highly specialized. 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.

Functional Classification

The functional classification of synapses is based on the mode of impulse transmission. According to the function or physiology, synapses are of two types – electrical synapses and chemical synapses.

1. Electrical Synapse

In these synapses, there are gap junctions between the pre and postsynaptic membranes which allow the transmission of the depolarization wave directly from the pre to the postsynaptic membrane. Electrical synapses are faster than chemical synapses.

There is the direct exchange of ions between and the transmission of an electric signal across the electrical synapse is similar to the conduction of impulse in an axon. Because of this reason, the action potential reaches the end part of the presynaptic neuron directly into the postsynaptic neuron.

Due to the direct flow of current, synaptic delay is very less. Moreover, the impulse is transmitted in either direction through the electrical synapse. Electrical synapses are not as flexible as chemical synapses as they cannot turn the excitatory signal into the inhibitory signal.

This type of impulse transmission occurs in some tissues like the cardiac muscle fibers, smooth muscle fibers of the intestine, and the epithelial cells of the lens in the eye.

2. Chemical synapse

In these synapses, the transmission of signals occurs by releasing a ”chemical transmitter” from the presynaptic terminal into the synaptic cleft. Chemical synapses are more common. A chemical synapse is the junction between two nerve fibers or between a nerve fiber and a muscle fiber. There is a fluid-filled space between the two neurons called the synaptic cleft. The nerve impulse cannot jump from one neuron to another. Axon terminals have a knob-like structure, which contains synaptic vesicles.

There are molecules of such special types of proteins in the membrane of the postsynaptic synapse cells on the nucleus, which cause chemical stimulation of the nerve as soon as they bind to the molecules of the substance. As a result of this stimulation, the same changes take place in the cell membrane of the postsynaptic neuron cell which converts the resting phase potential to the action potential, that is, necessary for the generation of inspiration. 

Neurotransmitters bind to the receptors present in the postsynaptic membrane. This results in the opening of voltage-gated channels and the flow of ions. This causes a change in the polarization of the postsynaptic membrane and the electric signal is conducted across the synapse

Function of Synapsis

Excitatory postsynaptic potential (EPSP) is the non-propagated electrical potential that grows throughout the procedure of synaptic transmission. They are associated with a transmitter-induced increase in sodium and potassium conductance of the synaptic membrane, which leads to the net entrance of the positive charge carried by Na+ and membrane depolarization.

An inhibitory postsynaptic potential (IPSP) is the type of synaptic inhibition that take place during the release of an inhibitory neurotransmitter from the presynaptic terminal rather than an excitatory neurotransmitter substance. A transient hyperpolarization of the postsynaptic membrane is caused by the passage of negatively charged ions into the postsynaptic cell. Inhibitory neurotransmitters are including gamma-aminobutyric acid (GABA), dopamine, and glycin.

Properties of Synapsis

Some Basic properties of synapse are mentioned below:

• One-way conductions – At the synapse, impulses are transmitted only in one direction which is from the presynaptic neuron to the postsynaptic neuron and not vice versa.
• Synaptic delay – Synaptic delay is the term used to describe a small delay in impulse transmission through synapses. because it requires time for the transmitter to be released, diffuse across the gap, and bind to receptors on the postsynaptic membrane.
• Fatigue: When synapses are continuously stimulated, Betz cells found in the motor area of ​​the frontal lobe of the cerebral cortex as well as synapses become the site of fatigue during prolonged muscle activity. The lack of acetylcholine itself causes fatigue in the synapse so after some time, due to exhaustion of neurotransmitters at the presynaptic terminals, impulses fail to conduct.
• Convergence and divergence: A single neuron can receive input from various network neurons because of convergence. A neuron can communicate with several different neurons in the network through divergence.
• Summation – A neuromuscular junction’s response to multiple electrical impulses added together. The effects are combined or the EPSP gradually increases when several presynaptic excitatory terminals are stimulated at once or when a single presynaptic terminal is stimulated repeatedly. The two types of summing are spatial summation and temporal summation.

Frequently Asked Questions on Synapse

Q. What is the Difference Between Chemical Synapse and Electrical Synapse?

Ans: The primary difference between a chemical and an electrical synapse is that in a chemical synapse uses neurotransmitters to transfer nerve impulses chemically, whereas an electrical synapse uses channel proteins to transmit nerve impulses electrically.

Q. What are the Main Components of an Electrical Chemical Synapse?

Ans: Three components make up this structure: a presynaptic component (like an axon terminal), a synaptic cleft, and a postsynaptic component (like a dendritic spine).

Q. What is the Synaptic Cleft?

Ans: A synaptic cleft is defined as a small fluid-filled gap between two nerve cells. This is where the neurotransmitter acetylcholine circulates.

Q. What is the Function of Synapses?

Ans: The primary function of the synapse is to transmit action potentials, or impulses, from one neuron to another.