Neurotransmitters

What Are Neurotransmitters?

Neurotransmitters are endogenous chemical messenger which transmits signals across a chemical synapse from one neuron (nerve cell) to another 'target' neuron, muscle cell, or gland cell. Many neurotransmitters are synthesized from simple and plentiful precursors such as amino acids, which are readily available and only require a small number of biosynthetic steps for conversion. 

Neurotransmitters are released from synaptic vesicles in synapses into the synaptic cleft, where they are received by neurotransmitter receptors on the target cell. They are essential to the function of complex neural systems. The exact number of unique neurotransmitters in humans is unknown, but more than 200 have been identified.

Neurotransmitters are often referred to as the body’s chemical messengers. They are the molecules used by the nervous system to transmit messages between neurons, or neurons to muscles.

Neuromodulators are a bit different, as they are not restricted to the synaptic cleft between two neurons, and so can affect large numbers of neurons at once. They, therefore, regulate populations of neurons, while also operating over a slower time course than excitatory and inhibitory transmitters.

NB: A neurotransmitter influences a neuron in one of three ways: excitatory, inhibitory or modulatory.

Whether a neurotransmitter is excitatory or inhibitory depends on the receptor it binds to. An excitatory neurotransmitter promotes the generation of an electrical signal called an action potential in the receiving neuron, while an inhibitory transmitter prevents it. 

NB: There are 4 main criteria for identifying neurotransmitters:

1. The chemical must be synthesized in the neuron or otherwise be present in it.
2. When the neuron is active, the chemical must be released and produce a response in some targets.
3. The same response must be obtained when the chemical is experimentally placed on the target.
4. A mechanism must exist for removing the chemical from its site of activation after its work is done

What Are The 7 Key neurotransmitters?

There are about a dozen known small-molecule neurotransmitters and more than 100 different neuropeptides, and neuroscientists are still discovering more about these chemical messengers. It is important to note that most neurotransmitters are either small amine molecules, amino acids, or neuropeptides. 

There are 7 key neurotransmitters working together in the human nervous system, we will carefully outline them all in the paragraphs below.

1) Acetylcholine.

Acetylcholine is an organic chemical that functions in the brain and body of many animals (and humans) as a neurotransmitter of chemical messages released by nerve cells to send signals to other cells, such as neurons, muscle cells, and gland cells.

Acetylcholine is a neurotransmitter used by neurons in the PNS and CNS in the control of functions ranging from muscle contraction and heart rate to digestion and memory.

Its name comes from its chemical structure: it is an ester of acetic acid and choline. Parts in the body that use or are affected by acetylcholine are referred to as cholinergic.

This neurotransmitter is used at the neuromuscular junction. It is the chemical that motor neurons of the nervous system release in order to activate muscles. This property means that drugs that affect cholinergic systems can have very dangerous effects ranging from paralysis to convulsions. 

2) Norepinephrine 

Norepinephrine, also known as "noradrenaline" is a substance that is released predominantly from the ends of sympathetic nerve fibers and that acts to increase the force of skeletal muscle contraction and the rate and force of contraction of the heart. 

The general function of norepinephrine is to mobilize the brain and body for action. The actions of norepinephrine are vital to the fight-or-flight response, whereby the body prepares to react to or retreat from an acute threat.

Noradrenalin is an organic chemical in the catecholamine family that functions in the brain and body as a hormone and neurotransmitter. It is also the international nonproprietary name given to the drug. Regardless of which name is used for the substance itself, parts of the body that produce or are affected by it are referred to as noradrenergic.

3) Serotonin.

Serotonin is a chemical that has a wide variety of functions in the human body. Serotonin is sometimes called a happy chemical because it contributes to wellbeing and happiness among other things. Its actual biological function is complex and multifaceted, modulating cognition, reward, learning, memory, and numerous physiological processes such as vomiting and vasoconstriction.

Serotonin is used to transmit messages between nerve cells, it is thought to be active in constricting smooth muscles. As the precursor for melatonin, it helps regulate the body’s sleep-wake cycles and the internal clock.

The scientific name for serotonin is 5-hydroxytryptamine or 5-HT. It is mainly found in the brain, bowels, and blood platelets. It is stored in blood platelets and is released during agitation and vasoconstriction, where it then acts as an agonist to other platelets.

Serotonin is primarily found in the enteric nervous system located in the gastrointestinal tract (GI tract). Serotonin is also produced in the central nervous system, specifically in the Raphe nuclei located in the brainstem. 

4) Dopamine.

Dopamine is an organic chemical of the catecholamine and phenethylamine families. It functions both as a hormone and a neurotransmitter and plays several important roles in the brain and body. 

Dopamine is usually seen as the main chemical of pleasure, but the current opinion in pharmacology is that dopamine instead confers motivational salience.

Dopamine plays a role in how we feel pleasure. It's a big part of our uniquely human ability to think and plan. It helps us strive, focus, and find things interesting. Dopamine is also synthesized in plants and most animals.

Dopamine is made in the brain through a two-step process. Firstly, it changes the amino acid tyrosine to a substance called dopa, and then into dopamine.

5) GABA (Gamma-Aminobutyric Acid).

GABA (Gamma-Aminobutyric Acid) is the chief inhibitory neurotransmitter in the developmentally mature mammalian central nervous system. Its principal role is reducing neuronal excitability throughout the nervous system. GABA is sold as a dietary supplement.

GABA is blocks impulses between nerve cells in the brain. Researchers suspect that GABA may boost mood or have a calming, relaxing effect on the nervous system.

GABA acts at inhibitory synapses in the brain by binding to specific transmembrane receptors in the plasma membrane of both pre- and postsynaptic neuronal processes.

6) Glutamate.

Glutamate in neuroscience refers to the anion of glutamic acid in its role as a neurotransmitter: a chemical that nerve cells use to send signals to other cells. Glutamate is said to be the most abundant excitatory neurotransmitter in the vertebrate nervous system.

The acid can lose one proton from its second carboxyl group to form the conjugate base, the singly-negative anion glutamate ⁻OOC-CH(NH+3)-(CH2)₂-COO. This form of the compound is prevalent in neutral solutions. 

The glutamate neurotransmitter plays the principal role in neural activation. This anion is also responsible for the savory flavor (umami) of certain foods and used in glutamate flavorings such as MSG. In Europe. It is classified as a food additive E620. The radical corresponding to glutamate is called glutamyl.

In addition, Glutamate is used by every major excitatory function in the vertebrate brain, accounting in total for well over 90% of the synaptic connections in the human brain. It also serves as the primary neurotransmitter for some localized brain regions, such as cerebellum granule cells.

7) Endorphin.

Endorphin contracted from ("endogenous morphine")are chemicals produced by the bodies of both humans and animals to relieve stress. The classification of molecules as endorphins is based on their pharmacological activity, as opposed to a specific chemical formulation. They work similarly to a class of drugs called opioids. Opioids relieve pain and can produce a feeling of euphoria. 

The endorphin class consists of α-endorphin, β-endorphin, and γ-endorphin. All 3 preferentially bind to μ-opioid receptors. The principal function of endorphins is to inhibit the communication of pain signals. Endorphins may also produce a feeling of euphoria very similar to those produced by other opioids.

Examples of important neurotransmitter actions.

As explained above, the only direct action of a neurotransmitter is to activate a receptor. Therefore, the effects of a neurotransmitter system depend on the connections of the neurons that use the transmitter, and the chemical properties of the receptors that the transmitter binds to.

Here are a few examples of important neurotransmitter actions:

• Glutamate is used at the great majority of fast excitatory synapses in the brain and spinal cord. It is also used at most synapses that are "modifiable", i.e. capable of increasing or decreasing in strength.
  
  Excessive glutamate release can overstimulate the brain and lead to excitotoxicity causing cell death resulting in seizures or strokes.
  
  Excitotoxicity has been implicated in certain chronic diseases including ischemic stroke, epilepsy, amyotrophic lateral sclerosis, Alzheimer's disease, Huntington disease, and Parkinson's disease.
• GABA, on the other hand, is used at the great majority of fast inhibitory synapses in virtually every part of the brain. Many sedative/tranquilizing drugs act by enhancing the effects of GABA.
  
  
• Norepinephrine is synthesized in the central nervous system and sympathetic nerves, modulates the responses of the autonomic nervous system, the sleep patterns, focus, and alertness. Norepinephrine is synthesized from tyrosine.
  
• Acetylcholine was the first neurotransmitter discovered in the peripheral and central nervous systems. Acetylcholine also operates in many regions of the brain but using different types of receptors, including nicotinic and muscarinic receptors.
  
  Acetylcholine activates skeletal muscles in the somatic nervous system and may either excite or inhibit internal organs in the autonomic system.
• Dopamine has a number of important functions in the brain, some of which include regulation of motor behavior, pleasures related to motivation, and also emotional arousal. Dopamine plays a critical role in the reward system; Parkinson's disease has been linked to low levels of dopamine and schizophrenia has been linked to high levels of dopamine.
  
• Epinephrine which is also synthesized from tyrosine is released in the adrenal glands and the brainstem. It plays a role in sleep, with one's ability to become and stay alert, and the fight-or-flight response.
  
• Serotonin is a monoamine neurotransmitter. Most are produced by and found in the intestine (approximately 90%), and the remainder in central nervous system neurons. Serotonin is speculated to have a role in depression, as some depressed patients are seen to have lower concentrations of metabolites of serotonin in their cerebrospinal fluid and brain tissue.
  
  Serotonin works to regulate appetite, sleep, memory and learning, temperature, mood, behavior, muscle contraction, and function of the cardiovascular system and endocrine system.