Neurotransmitters and their Functions

Neurotransmitters are chemicals that facilitate the transmission of nerve impulses from one neuron to another neuron or body cell. Some of the important neurotransmitters are discussed in this article.

The transmission of signals from one neuron to another, across the synapse was earlier thought to be electrical. In 1921, it was confirmed that neurons mostly communicate by releasing certain chemicals. These chemicals are called neurotransmitters.

So, neurotransmitters allow the nerve impulses or signals to travel across the synapse. A synapse is a small gap or junction between two neurons, or a neuron and a muscle cell. The credit for discovering the chemical transmission of nerve impulses goes to Sir Henry Dale (an English pharmacologist and physiologist) and Otto Loewi (a German pharmacologist). Both of them shared the Nobel Prize in Physiology or Medicine in 1936 for their work on neurotransmitters.

Types of Neurotransmitters:

There are several types of neurotransmitters, and each one of them is responsible for some specific functions. Neurotransmitters are usually classified as amino acids, peptides, and monoamines.

The neurotransmitters that fall into the category of amino acids are:
  • Glutamate
  • Aspartate
  • Glycine
  • D-serine
  • Gamma-aminobutyric acid (GABA)
The most important monoamines or other biogenic amines that are considered as neurotransmitters are:
  • Serotonin
  • Norepinephrine
  • Epinephrine
  • Histamine
  • Melatonin
A large number of neurotransmitters fall into the category of peptides, of which a few important ones are:
  • Beta-endorphin
  • Opioid peptides
  • Somatostatin
  • Calcitonin
  • Vasopressin
  • Oxytocin
  • Glucagon
Apart from these, there are several other important neurotransmitters, such as acetylcholine, dopamine, adenosine, and nitric oxide. So far, about 50 neuroactive peptides have been discovered.

Sometimes, neurotransmitters are also classified as excitatory and inhibitory. This classification is based on their actions on the neurons. Excitatory neurotransmitters are those that excite the neurons and stimulate the brain, while inhibitory neurotransmitters are known for having a calming effect on the brain.

Neurotransmitters like, GABA and serotonin come under the category of inhibitory neurotransmitters, while epinephrine and norepinephrine are the excitatory neurotransmitters. Dopamine on the other hand, can act as an excitatory, as well as an inhibitory neurotransmitter.

However, the effect of a neurotransmitter on a postsynaptic cell depends on the receptors present in it. For some neurotransmitters, like glutamate, the important receptors have excitatory properties. On the other hand, most of the important receptors produce an inhibitory effect for GABA. But there are some neurotransmitters, for which both types of receptors exist.

Neurotransmitters and the Transmission of Nerve Impulses:
Nerve cells or neurons communicate messages by transmitting nerve impulses. The impulses travel from one neuron to another neuron or body cell by the movement of the neurotransmitters. A nerve impulse traveling through the axon (a long, slender projection arising from the nerve cell) eventually reaches the axon terminal, and then the synaptic knob or terminal button.

The synaptic knobs are the slightly swollen tips of the branches that arise from an axon. They contain the neurovesicles, which store and release neurotransmitters. For transmitting impulses, neurons form a specialized structure, called synapse. It serves as a junction where impulses or information can flow from one neuron to another.

There are basically three elements of a synapse, a presynaptic membrane of the signal-passing neuron (usually found in the synaptic knob of an axon), the postsynaptic membrane found in a dendrite or a target cell, and a synaptic cleft. A synaptic cleft is the space between the presynaptic and post synaptic membranes.

When an impulse reaches the synaptic knob of the signal-passing or presynaptic neuron, neurotransmitter molecules are released into the synaptic cleft by the neurovesicles. The neurotransmitter molecules then diffuse and float across the synaptic cleft, in order to bind to the receptors embedded in the postsynaptic membrane. The neurotransmitter molecules are then internalized by the postsynaptic cell, and thus the nerve impulse is carried forward.

Important Neurotransmitters & Their Functions:
As mentioned already, about 50 neurotransmitters have been discovered so far. Out of these, a few of the most important neurotransmitters and their functions are discussed below.

This neurotransmitter was discovered in the year 1921, by Otto Loewi. It is mainly responsible for stimulating muscles. It activates the motor neurons that control the skeletal muscles. It is also concerned with regulating the activities in certain areas of the brain, which are associated with attention, arousal, learning, and memory. People with Alzheimer's disease are usually found to have a substantially low level of acetylcholine.

Dopamine is the neurotransmitter that controls voluntary movements of the body, and is associated with the reward mechanism of the brain. In other words, dopamine regulates the pleasurable emotions.

Drugs like cocaine, heroin, nicotine, opium, and even alcohol increase the level of this neurotransmitter. A significantly low level of dopamine is associated with Parkinson's disease, while the patients of schizophrenia are usually found to have excess dopamine in the frontal lobes of their brain.

Serotonin is an important inhibitory neurotransmitter, which can have a profound effect on emotion, mood, and anxiety. It is involved in regulating sleep, wakefulness, and eating. It plays a role in perception as well. The hallucinogenic drugs like LSD actually bind to the serotonin receptor sites, and thereby block the transmission of nerve impulses, in order to alter sensory experiences.

A significantly low level of serotonin is believed to be associated with conditions like depression, suicidal thoughts, and obsessive compulsive disorder. Many antidepressants work by affecting the level of this neurotransmitter.

Gamma-aminobutyric Acid (GABA):
GABA is an inhibitory neurotransmitter that slows down the activities of the neurons, in order to prevent them from getting over excited. When neurons get over excited, it can lead to anxiety. GABA can thus help prevent anxiety.

GABA is a non-essential amino acid, that is produced by the body from glutamate. A low level of GABA can have an association with anxiety disorders. Drugs like Valium work by increasing the level of this neurotransmitter. Alcohol and barbiturates can also influence GABA receptors.

Glutamate is an excitatory neurotransmitter that was discovered in 1907 by Kikunae Ikeda of Tokay Imperial University. It is the most commonly found neurotransmitter in the central nervous system. Glutamate is mainly associated with functions like learning and memory. An excess of glutamate is however, toxic for the neurons. An excessive production of glutamate may be related to the disease, known as amyotrophic lateral sclerosis (ALS) or Lou Gehrig's disease.

Epinephrine and Norepinephrine:
Epinephrine (also known as adrenaline) is an excitatory neurotransmitter, that controls attention, arousal, cognition, and mental focus. Norepinephrine is also an excitatory neurotransmitter, and it regulates mood and physical and mental arousal. An increased secretion of norepinephrine raises the heart rate and blood pressure.

Endorphins are the neurotransmitters that resemble opioid compounds, like opium, morphine, and heroin in structure. The effects of endorphins on the body are also quite similar to the effects produced by the opioid compounds. In fact, the name 'endorphin' is actually the short form for 'endogenous morphine'.

Like opioids, endorphins can reduce pain, stress, and promote calmness and serenity. The opioid drugs produce similar effects by attaching themselves to the endorphin receptor sites. Endorphins enable some animals to hibernate by slowing down their rate of metabolism, respiration, and heart rate.

It is the hormone produced by the pineal gland that also acts as a neurotransmitter. It basically controls the sleep-wake cycle. It is also associated with controlling mood and sexual behavior. The production of melatonin is dependent on light. Light to the retina inhibits the production of melatonin, while darkness has a stimulating effect on its production.

Nitric Oxide:
It is a gas that acts both as a hormone and neurotransmitter, depending on the specific requirement. It can cause the blood vessels to dilate, besides preventing the formation of clots. This in turn, can promote the circulation of blood. Nitric oxide can increase the level of oxygen in the body, and improve memory, learning, alertness, and concentration. It is also responsible for causing the smooth gastrointestinal muscles to relax.

To sum up, neurotransmitters are chemicals that allow the nerves to communicate with each other, and thus, regulate the various functions of the body. A substantially high or low level of these chemicals can alter the functions of the entire nervous system.

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