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Synapse and synaptic transmission steps
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What are synapse and synaptic transmission? 

Synapse and synaptic transmission steps: The junction between two neurons is called Synapse. And these are the junctions where the axon or some other portion of one nerve cell (the presynaptic cell) terminates on the dendrites, soma, or axon of another neuron or, in some cases, a muscle or gland cell (the postsynaptic cell). 

Synaptic transmission is the process by which information from a presynaptic neuron (cells) passes to the postsynaptic neuron(cells) through the synapse.

Classification of the synapse (@Synapse and synaptic transmission)

The classification of synapse varies depend on the following conditions such as;

On the basis of the nature of transmission

  • Chemical synapse 
  • Electrical synapse
  • Conjoint synapse

Chemical synapse

In these synapses, the first neuron secretes a chemical substance (compounds) called a neurotransmitter at the synapse that acts on receptor proteins in the membrane of the next neuron cell to excite, inhibit or modify its sensitivity.
Some of the transmitters are acetylcholine, norepinephrine, epinephrine, histamine, GABA, glycine, serotonin, and glutamate, etc. Chemical synapse is found in most synapses of the central nervous system (CNS).

Electrical synapse

These synapses are characterized by direct open fluid channels that conduct electricity from one cell to the next cell in the body.
They consist of small protein tubular structures which are known as gap junctions and that allow free movement number of ions from the interior of one cell to the interior of the next cell. Mainly these are found in cardiac & skeletal muscle in the body.

Conjoint synapse

The conjoint synapse is between partly electrical and partly chemical synapses.

On the basis of part of neurons involved

  • Axodendritic
  • Axosomatic
  • Axoaxonic
  • Dendrodendritic
Synapse and synaptic transmission steps
Synapse and synaptic transmission steps

Axodendritic

It is a junction between the axon of the Presynaptic neuron & the dendrites of postsynaptic cells in the body. It is the commonest synapse in the central nervous system (CNS). The axon terminal may synapse with the spinous process of the dendrite neuron called axospinous synapse or with the shaft of the dendrite neuron called shaft synapse.

Axosomatic

It is a junction between the axon of the presynaptic Neuron& the body of the postsynaptic neuron in the body. It is also a common form of synapse in the central nervous system (CNS).

Axoaxonic

The junction between axons of 2 neurons in the body is called axoaxonic synapse.

Dendrodendritic

It is a junction between presynaptic neuron and postsynaptic neuron of dendrites

On the basis of the number of neurons involved in the body

  • One to one: Found in the neuromuscular junction
  • Many to one: Found in CNS
  • One to many: Found in the sympathetic system

Steps in synaptic transmission

(@Synapse and synaptic transmission)

The following steps are involved in synaptic transmission. They are following

  • Presynaptic mechanism
  • Synaptic cleft
  • Postsynaptic mechanism
  • Synaptic potentials

Presynaptic mechanism

In these steps, the action potential that arrives at the presynaptic axon of the neuron terminal depolarizes the presynaptic membrane.

Then the opening of the voltage-gated Ca 2+ channel allows a large amount of Ca 2+ to enter from the extracellular fluid (ECF).

Exocytosis of vesicles occurred and the quantity of transmitter (neurotransmitter) substance that is then released from the terminal into the synaptic cleft is directly related to the number of calcium ions (Ca 2+ ) that enter.

What are neurotransmitters?

The neurotransmitters (transmitters) are the chemical substances released at nerve endings that help to transfer messages in the form of nerve impulses from the presynaptic membrane to the postsynaptic membrane in the neuron.

There are two types of excitatory neurotransmitters (Acetylcholine, Glutamate, Aspartate, etc) and inhibitory neurotransmitters (for example, Glycine, GABA, etc).

Synapse and synaptic transmission steps

Synaptic Cleft

The synaptic cleft is a gap between presynaptic and postsynaptic membranes in the cells. The width is about 20 to 50 nm and this space is filled with extracellular fluid.

The neurotransmitter molecules released from the presynaptic terminal end diffuse across the cleft to reach the postsynaptic receptors of the cells.

Postsynaptic mechanism

When a neurotransmitter binds with receptors present in the postsynaptic membrane of the cell. which have two important components including;

    • A binding component: It protrudes outward from the membrane into the synaptic cleft where it binds the neurotransmitter in the cell.
    • An ionophore component: That passes all the way through the postsynaptic membrane to the interior of the postsynaptic neuron in the body. This component in turn is divided into two types:
      • An ion channel that allows the passage of specified types of ions through the membrane
      • “Second messenger” activator is not an ion channel but instead is a molecule that protrudes into the cell cytoplasm and activates one or more substances inside the postsynaptic neuron in the body.
      • These substances in turn serve as “second messengers” to increase or decrease specific cellular functions in the cells.

Synaptic potential

When ions move inside the postsynaptic membrane it brings a graded change in membrane potential in the cell which is defined as synaptic potential. It is divided into 2 types, such as:

    • Excitatory postsynaptic potential (EPSP)
    • Inhibitory Postsynaptic Potential (IPSP)

Excitatory postsynaptic potential (EPSP)

When a positive voltage increase above the normal resting neuronal potential, that is, to a less negative value is called the excitatory postsynaptic potential (or EPSP).
The rapid influx of positively charged sodium ions (Na+) to the interior neutralizes part of the negativity of the resting membrane potential (RMP).
Thus, the resting membrane potential (RMP) has increased in the positive direction from −65 to −45 millivolts.
The discharge of a single presynaptic terminal can never increase the neuronal potential from -65 millivolts all the way up to -45 millivolts.
An increase of this magnitude requires simultaneous discharge of many terminals around 40 to 80 for the usual anterior motor neuron at the same time or in rapid succession. This occurs by the process of summation in the body.

Generation of Action Potentials

When the EPSP rises high enough in the positive and crosses the threshold, it will elicit an action potential in the postsynaptic neuron in the body.

It begins in the initial segment of the axon of the neuron and once the action potential begins, it travels peripherally along the axon and usually also backward over the soma in the neuron. 

Inhibitory Postsynaptic Potential (IPSP)

An increase in negativity beyond the normal resting membrane potential (RMP) level is called an inhibitory postsynaptic potential (IPSP).

It is due to the potassium ion efflux, chloride ion influx, or both in the membrane of neurons.

 

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