Neurotransmitter : History, Classification

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NEUROTRANSMITTER DEFINITION

Neurotransmitter is a chemical substance that acts as a mediator for the transmission of nerve impulse from one neuron to another neuron through a synapse.

If you go through basics than it will be easier to understand further topics. If you want to understand this topic, you must go through our previous blog synapse.

In our body, neurons are not anatomically connected. It is physiological continuation. There is gap between two neurons. Message pass from one neuron to another neuron, although there is gap in between. It is because of chemical named as neurotransmitter.

HISTORY

In 1921, Australian scientist named Otto Loewi discover existence of neurotransmitter. 

Loewi Experiment

frog experiment by Otto Loewi

Otto Loewi used two frogs for this experiment. Heart of frog A was with intact vagus nerve and put in a saline-filled chamber. Heart of frog B was with removal of nerve & put in another saline filled chamber. 

As you can see in image that connection between chamber in such a way that fluid from chamber A can flow into chamber B.

When he electrically stimulate the vagus nerve of frog A, he finds that there is slowing of heart rate. After a short delay, the heart rate of frog B also comes down. It happens after short delay because fluid from chamber A comes to chamber B with short delay.

It directly means that there is something releases from vagus nerve of frog A. He named it as ‘vagusstoff’. Later this chemical is known as a neurotransmitter.

Creteria For Neurotransmitter

Nowadays, many substances are categorise as neurotransmitter. To consider a substance as a neurotransmitter, it should fullfill certain criteria as mention below.

It must be found, produce and release by neuron.

After release, it must act on a target area and produce some biological effect.

After the action, it must be inactivated.

CLASSIFICATION OF NEUROTRANSMITTERS

  1. Depending upon chemical nature
  2. According to function

Depending Upon Chemical Nature

According to chemical nature, there are three types mention as below.

Amino Acids

Neurotransmitter of this group have fast synaptic transmission and are excitatory and inhibitory in action. Below chemicals are of aminoacid group.

  • GABA (gamma aminobutyric acid)
  • Glutamate (glutamic acid)
  • Glycine
  • Aspartate (aspartic acid)

Amines

Amines are the modified amino acids. These neurotransmitter involve in slow synaptic transmission. These neurotransmitters are also excitatory and inhibitory in action. Below chemical is neurotransmitter of amines group.

  • Adrenalin
  • Noradrenalin
  • Histamine
  • Dopamine
  • Serotonin

Others

Some neurotransmitters do not fit into catagory of aminoacids or amines. That are mention in others group.

  • Acetylcholine
  • Nitric oxide (NO)

Depending Upon Function

  1. Excitatory
  2. Inhibitory

Excitatory Neurotransmitter

Excitatory neurotransmitter is a chemical substance, which is responsible for the conduction of impulse from presynaptic neuron to postsynaptic neuron. Simply it means that it passes message from one neuron to another neuron. So, this chemical is called excitatory neurotransmitter.

When nuerotransmitter releases from synaptic vesicles in presynaptic neuron, it causes effect in post synaptic neuron. There is opening of sodium channels and influx sodium ion from ECF. Due to sodium ion entry message passes to postsynaptic neuron.

Inhibitory Neurotransmitter

Inhibitory neurotransmitter is a chemical substance, which inhibits the conduction of impulse from the presynaptic neuron to the postsynaptic neuron. It simply means that after releasing of this type of chemical, message can not pass further. So, this chemical is called inhibitory neurotransmitter.

When inhibitory neurotransmitter like GABA releases in presynaptic neuron, it causes chloride ion entry in postsynaptic neuron. Chloride ion means Cl- & it makes negetivity there. Once negetivity develops, message can not pass further. This process is known as IPSP.

IMPORTANT NEUROTRANSMITTER

Acetylcholine (Ach)

It is cholinergic neurotransmitter. Ach produces the excitatory functions by opening the ligand sodium channel.

Acetylcholine is not live for long time in neuron. Within 1 millisecond after release from vesicles, it hydrolysed into Acetate + Choline.

GABA

Gamma-aminobutyric acid (GABA) is an inhibitory neurotransmitter in synapses particularly in central nervous system. It means that this chemical blocks or inhibits certain brain signals and decreases the activity in your nervous system.

GABA causes synaptic inhibition by opening potassium channels and chloride channels. So, potassium comes out of synapse and chloride enters in. This leads to hyperpolarization, which is known as IPSP (Inhibitory postsynaptic potential).

Dopamine

It is type of neurotransmitter which have Inhibitory action. Dopamine helps us in many ways, it controls many functions of our body mentioning below.

  • Motivation
  • Mood
  • Movement
  • Attention
  • Learning

If deficiency of this chemical occurs than we may suffer from disease like Parkinson’s disease in which rigidity, tremoring, stiffness occurs.

Serotonin

It is very important neurotransmitter for our body. Other name of serotonin is                  5-hydroxytryptamine (5-HT). Large amount of serotonin (90%) is found in enterochromatin cells of GI tract & small amout is found in platelets and nervous system.

Serotonin maintain our mood. Whenever this chemical decreases, it causes mood changes, anxiety, and depression.

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