Human body is controlled by the brain and the nervous system.
This nervous system has two divisions as central and peripheral nervous system.
The stimulation and nerve impulses are carried by neurotransmitters.
These are chemical messengers present in the brain, spinal cord and nerve junctions.
They have the name due to their presence and function in relation to nervous systems.
These substances as the name indicates are meant for transfer of signals from place to another other in the body.
They are released into the junction of two neurons (neuron synapse or synaptic cleft) and then they act on the neurotransmitter receptors present on the next neuron to continue the signal.
They help in control of the functions, actions and also movements of the body by the brain.
Their study is important in subjects like anatomy & physiology, patho-physiology of disease and also in pharmacology.
There are different types of neurotransmitter with diverse functions and roles in the body.
They are organic substances which have shortest span of action. Their action lasts for fraction of seconds at times.
They are so quick in action and retraction. They can be compared with hormones as they have similar properties and mode of actions. Hence we can conveniently call them as neuro-hormones.
They are mostly synthesized within the neuron cell and stored in vesicles (sac). A change in voltage potential along the nerve membrane, helps release neurotransmitter.
Neurotransmitters are many and even some of the other substances may be discovered or classified in future as neurotransmitters.
But based on the wider role they play in the body, they can be studied as
Classical neurotransmitters and non-classical ones.
Classical neurotransmitters: These are ones which have wide distribution in the body and have a larger role in physiology. Any small change in their quantities or functions will lead to disorders. These classical neurotransmitters include
- Acetyl choline
- Noradrenalin (Nor-epinephrine)
- Adrenalin (Epinephrine)
- Serotonin (5HT)
- GABA (gamma amino benzoic acid)
Other neurotransmitters: These are less widely distributed and also few functions in the body. Also minute changes in their quantities will not affect or lead to major disorders.
- Nitric oxide.
- Adenosine triphosphate.
- beta endorphin
- angiotensin II
Neurotransmitters carry many functions in the body and some of them are specifically designated for certain roles like
Acetyl choline: One of the major neurotransmitter in the brain, spinal cord and peripheral nerves. It also found in RBC’s and other cells in the body.
It is synthesized in the neurons and released at nerve ending to pass on the nervous stimuli post synaptically.
It is a part of parasympathetic system and released at synapse. It exerts actions opposite to that of sympathetic system. Acetyl choline is involved in almost all the body functions like heart beat, respiration, digestion, excretion, reproduction etc.
It acts through receptors like Muscarnic receptors like M1, M2 and also through nicotinic receptors in the skeletal muscles like N1, N2. Deficiency and rise in the levels of acteylcholine leads to many diseases and also toxic effects. Examples of diseases include parkinsonism, Alzheimer disease, glaucoma etc.
Dopamine: Another important neurotransimtter related sympathteic sysmte.
But it is mostly confined to brain and spinal cord and few nerves. It has functions of mood and sympathetic system. It acts through receptors like D1, D2, D3. Disorders due to its deficiency or rise include depression, parkinsonism.
Norepinpehrine: This also widely distributed like acteyl choline.
It covers all of the sympathetic functions and has actions opposite to acetyl choline. It acts through receptors like α1, α2 and β1, β2.
Epinephrine: This is similar to nor-epinephrine in terms of actions and also chemistry. But it is present in more quantities in blood and peripheral body organs and to a small extent in brain. In periphery it is released by adrenal medulla. It is involved in stress regulation and is also called as flight or fight hormone. It acts through similar receptors as that of epinephrine. Its quantity levels rise is in the body is indicative of stress or struggle.
Serotonin: This is found in large quantities in intestine, platelets and also brain. It is recently found to be important transmitters in stress, mood and also coagulation. It acts through 14 types of serotonin receptors like 5HT1, 5HT2, 5HT3, 5HT4, 5HT7 and their sub-types. Its deficiency or change is seen in disorders like depression, schizophrenia etc.
Gluatamate: This is a neurotransmitter found predominantly in brain and also as amino-acid in rest of the body. It acts through NMDA, AMPA receptors. It is involved in memory and learning. When the brain undergoes oxygen deficient stress or physical injury its release in the extracellular space can be devastating leading to nervous tissue damage (excito-toxicity). This phenomenon is mostly is seen in asphyxia babies.
GABA (gamma amino benzoic acid): GABA is another neurotransmitter present predominantly in brain. It acts to control nerve conduction and electric potential in the brain and also muscle tone. Its deficiency leads to epilepsy. It acts through GABA receptors. This neurotransmitter plays a key role in epilepsy or convulsion disorder.
Histamine: Unlike other, this substance is widely distributed in tissues. It is called as an afferent neurotransmitter as it initiates the sense of itch and pain at sensory nerve endings in skin and other regions.
In the brain it controls functions like wakefulness, body temperature, thirst etc.
Besides these, there are also neurotransmitters like
Exitatory amino acids: l-Glutmate, l-Aspartate, l-Cystein, l-Homocycttein
Inhibitory amino acids: GABA, Glycine, b-alanine and taurine