Know The Hardware - V (Nervous System)

Neurotransmitters and Neuroactive Peptides

Communication of information between neurons is accomplished by movement of chemicals across a small gap called the synapse. Chemicals, called neurotransmitters, are released from one neuron at the presynaptic nerve terminal. Neurotransmitters then cross the synapse where they may be accepted by the next neuron at a specialized site called a receptor.

Neurotransmitter Criteria

Neuroscientists have set up a few guidelines or criteria to prove that a chemical is really a neurotransmitter. 

1.  The chemical must be produced within a neuron.
2.  The chemical must be found within a neuron.
3. When a neuron is stimulated (depolarized), a neuron must release the chemical
4. When a chemical is released, it must act on a post-synaptic receptor and cause a biological effect.
5. After a chemical is released, it must be inactivated. Inactivation can be through a reuptake mechanism or by an enzyme that stops the action of the chemical
6. If the chemical is applied on the post-synaptic membrane, it should have the same effect as when it is released by a neuron

Neurotransmitter Types

There are many types of chemicals that act as neurotransmitter substances. Below is a list of some of them.

Small Molecule Neurotransmitter Substances

Acetylcholine (ACh), Dopamine (DA), Norepinephrine (NE), Serotonin (5-HT), Histamine, Epinephrine

Amino Acids

Gamma-aminobutyric acid (GABA), Glycine, Glutamate, Aspartate

Neuroactive Peptides

bradykinin	beta-endorphin	bombesin	calcitonin
cholecystokinin	enkephalin	dynorphin	insulin
gastrin	substance P	neurotensin	glucagon
secretin	somatostatin	motilin	vasopressin
oxytocin	prolactin	thyrotropin	angiotensin II
sleep peptides	galanin	neuropeptide Y	thyrotropin-releasing hormone
gonadotropnin-releasing hormone	growth hormone-releasing hormone	luteinizing hormone	vasoactive intestinal peptide

Soluble Gases

Nitric Oxide (NO), Carbon Monoxide

Functions of Neurotransmitters:

Acetylcholine (ACh)

Distributed widely throughout the central and peripheral nervous system, where it is involved in arousal, attention, memory, motivation, and movement. Involved in muscle action through presence at neuromuscular junctions (specialized type of synapse where neurons connect to muscle cells). Degeneration of neurons that produce ACh have been linked to Alzheimer’s disease. Too much can lead to spasms and tremors; too little, to paralysis.

Dopamine

Involved in a wide variety of behaviors and emotions, including pleasure. Implicated in schizophrenia and Parkinson’s disease.

Serotonin

Involved in the regulation of sleep, dreaming, mood, eating, pain, and aggressive

behavior. Implicated in depression.

Norepinephrine

Affects arousal, wakefulness, learning, memory, and mood.

Endorphins

Involved in the inhibition of pain. Released during strenuous exercise. May be

responsible for “runner’s high.” Glutamate Involved in long-term memory and the perception of pain.

GABA

A largely inhibitory neurotransmitter distributed widely throughout the central (Gamma aminobutyric acid) nervous system. Implicated in sleep and eating disorders. Low levels of GABA have also been linked to extreme anxiety.

Glycene

Principally responsible for inhibition in the spinal cord and lower brain centers.