Classification of Synapse & Function of Synapse

In this blog, we learn about classification and Function of Synapse.

DEFINITION OF SYNAPSE

Synapse is the junction between two neurons. It is not an anatomical continuation. But, it is only a physiological continuity between two nerve cells.

This means that synapse is junction between two neuron, which not anatomically connected. Here we learn, classification and function of synapse.

CLASSIFICATION OF SYNAPSE

It is classified by two methods:

1. Anatomical classification
2. Functional classification

ANATOMICAL CLASSIFICATION

To understand this classification, first we understand neuron. Neuron is structural and functional unit of nervous system.

As you can see in image that neuron have main 3 parts names as

• Axon ( pipe like structure ) which ends as a axon terminal
• Soma ( cell body)
• Dendrites ( hair like structure)

There are three possibilities that axon terminal of one neuron can attach to either axon, soma or dendrites of other neuron.

1. Axoaxonic synapse
   

   

     
   In which axon of the one neuron terminates on axon of another neuron.

2. Axosomatic synapse
   

   

   In this synapse, axon of one neuron ends on soma (body) of another neuron.

3. Axodendric synapse
   

   

   In which one neuron’s axon attach to dendrites of other neuron.

FUNCTIONAL CLASSIFICATION

Functional Classification of synapse, is on the basis of mode of impulse transmission. According to that synapse is classified into two categories:

• Electrical synapse
• Chemical synapse

ELECTRICAL SYNAPSE

It is very simple, as you can see in image that two neuron are connected by gap junction. There is direct exchange of ions between the two neuron through the gap junction. Because of this reason, the action potential reaches to end part of presynaptic neuron directly enters into postsynaptic neuron.

Due to direct flow of current, synaptic delay is very less. Moreover, the impulse is transmitted in either direction through electrical synapse.

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

CHEMICAL SYNAPSE

Chemical synapse is the junction between two nerve fiber or between nerve fiber and muscle fiber. 

Chemica is very interesting and important, you just need to understand the image.

In this synapse, there is no continuity between two neurons. There is space between two neurons is known as synaptic cleft. Before this synaptic cleft, neuron is known as presynaptic neuron. In same way, after synapse neuron is known as postsynaptic neuron.

You can see that there are synaptic vesicles in presynaptic neuron. When action potential reaches to the presynaptic terminal, neurotransmitter substance release from this vesicles.

When this chemical named neurotransmitter releases, it has ability to tear the presynaptic membrane and reaches to the postsynaptic membrane. In simple language message reaches from one neuron to other neuron, although there is space.

Now, neurotransmitter and receptor protein make complex. This causes potential changes in neuron, this way message passes from one neuron to other neuron.

FUNCTIONS OF SYNAPSE

On basis of the functions, synapse are divided into two types.

1. Excitatory synapse
   This synapse transmit the impulse, so it is called excitatory synapse.
2. Inhibitory synapse
   This synapse inhibit the impulse, so it is called inhibitory synapse.

EXCITATORY FUNCTION

EXCITATORY POSTSYNAPTIC POTENTIAL (EPSP)

Simply, Excitatory word means to pass action potential (message) from one neuron to other neuron. 

It is the same process as we learn in chemical synapse. In EPSP neurotransmitter (chemical) have excitatory property, so it can transmit the impulse. We learn the mechanism of EPSP by below chart.

Properties of EPSP

EPSP has two properties:

1. It is non-propagated
2. It does not obey all-or-none law.

INHIBITORY FUNCTION

Inhibition of synaptic transmission is calssified into total five types, we discuss here only 1st one IPSP.

1. Postsynaptic or direct inhibition (IPSP)
2. Presynaptic or indirect inhibition
3. Negetive feedback or Renshaw cell inhibition
4. Feedforward inhibition
5. Reciprocal inhibition

POSTSYNAPTIC INHIBITION

It inhibit the impulse from one neuron to other neuron. It is also known as inhibitory postsynaptic potential (IPSP). 

Postsynaptic inhibition is the type of synaptic inhibition that occurs due to the release of inhibitory neurotransmitter (chemical) from presynaptic neuron. Due to release of inhibitory neurotransmitter, it has function of inhibition.

Name of inhibitory neurotransmitter are gamma-aminobutyric acid (GABA), dopamine and glycine. We learn the mechanism of IPSP by below chart.

Why painkillers are effective or how anaesthesia works? All questions have answer in this IPSP. As after giving medicine like GABA, there is postsynaptic inhibition occurs. Which not allow action potential (message) to pass further. This way it works.

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