Neurotransmitters
Definition
Neurotransmitters are chemicals that allow the movement of information from one neuron across the gap between it and the adjacent neuron. The release of neurotransmitters from one area of a neuron and the recognition of the chemicals by a receptor site on the adjacent neuron causes an electrical reaction that facilitates the release of the neurotransmitter and its movement across the gap.
Description
The transmission of information from one neuron to another depends on the ability of the information to traverse the gap (also known as the synapse) between the terminal end of one neuron and the receptor end of an adjacent neuron. The transfer is accomplished by neurotransmitters.
In 1921, an Austrian scientist named Otto Loewi discovered the first neurotransmitter. He named the compound "vagusstoff," as he was experimenting with the vagus nerve of frog hearts. Now, this compound is known as acetylcholine.
Neurotransmitters are manufactured in a region of a neuron known as the cell body. From there, they are transported to the terminal end of the neuron, where they are enclosed in small membrane-bound bags called vesicles (the sole exception is nitric oxide, which is not contained inside a vesicle, but is released from the neuron soon after being made). In response to an action potential signal, the neurotransmitters are released from the terminal area when the vesicle membrane fuses with the neuron membrane. The neurotransmitter chemical then diffuses across the synapse.
At the other side of the synapse, neurotransmitters encounter receptors. An individual receptor is a transmembrane protein, meaning part of the protein projects from both the inside and outside surfaces of the neuron membrane, with the rest of the protein spanning the membrane. A receptor may be capable of binding to a neurotransmitter, similar to the way a key fits into a lock. Not all neurotransmitters can bind to all receptors; there is selectivity within the binding process.
When a receptor site recognizes a neurotransmitter, the site is described as becoming activated. This can result in depolarization or hyperpolarization, which acts directly on the affected neurons, or the activation of another molecule (second messenger) that eventually alters the flow of information between neurons.
Depolarization stimulates the release of the neuro-transmitter from the terminal end of the neuron. Hyperpolarization makes it less likely that this release will occur. This dual mechanism provides a means of control over when and how quickly information can pass from neuron to neuron. The binding of a neurotransmitter to a receptor triggers a biological effect. However, once the recognition process is complete, its ability to stimulate the biological effect is lost. The receptor is then ready to bind another neurotransmitter.
Neurotransmitters can also be inactivated by degradation by a specific enzyme (e.g., acetylcholinesterase degrades acetylcholine). Cells known as astrocytes can remove neurotransmitters from the receptor area. Finally, some neurotransmitters (norepinephrine, dopamine, and serotonin) can be reabsorbed into the terminal region of the neuron.
Since Loewi's discovery of acetylcholine, many neurotransmitters have been discovered, including the following partial list:
- Acetylcholine: Acetylcholine is particularly important in the stimulation of muscle tissue. After stimulation, acetylcholine degrades to acetate and choline, which are absorbed back into the first neuron to form another acetylcholine molecule. The poison curare blocks transmission of acetylcholine. Some nerve gases inhibit the breakdown of acetylcholine, producing a continuous stimulation of the receptor cells, and spasms of muscles such as the heart.
- Epinephrine (adrenaline) and norepinephrine: These compounds are secreted principally from the adrenal gland. Secretion causes an increased heart rate and the enhanced production of glucose as a ready energy source (the "fight or flight" response).
- Dopamine: Dopamine facilitates critical brain functions and, when unusual quantities are present, abnormal dopamine neurotransmission may play a role in Parkinson's disease, certain addictions, and schizophrenia.
- Serotonin: Synthesized from the amino acid tryptophan, serotonin is assumed to play a biochemical role in mood and mood disorders, including anxiety, depression, and bipolar disorder.
- Aspartate: An amino acid that stimulates neurons in the central nervous system, particularly those that transfer information to the area of the brain called the cerebrum.
- Oxytocin: A short protein (peptide) that is released within the brain, ovary, and testes. The compound stimulates the release of milk by mammary glands, contractions during birth, and maternal behavior.
- Somatostatin: Another peptide, which is inhibitory to the secretion of growth hormone from the pituitary gland, of insulin, and of a variety of gastrointestinal hormones involved with nutrient absorption.
- Insulin: A peptide secreted by the pancreas that stimulates other cells to absorb glucose.
As exemplified above, neurotransmitters have different actions. In addition, some neurotransmitters have different effects depending upon which receptor to which they bind. For example, acetylcholine can be stimulatory when bound to one receptor and inhibitory when bound to another receptor.
