Neurotransmitters can be loosely defined as natural
chemical signals which effect the behavior of
neurons. For a more rigid definition, see the above
criteria. Neurotransmitters can be released at specific synapses, to facilitate neuron-neuron communication, or they can be released by
broadcast transmission, in which they are released over a wider area, and have a more modulatory role. Different neurotransmitters can have different effects in different regions of the nervous system. Neurotransmitters are often divided into
classical neurotransmitters and
nonclassical neurotransmitters. These distinctions are biologically arbitrary, being based only on when the transmitters were discovered. Thus, I will be ignoring that distinction. What follows is a list of known transmitters, their structural type, and their common
functions.
Catecholamines
Composed of a
catechol nucleus and an
amine group. Generally broadcast transmitters, and can have
hormonal functions.
Dopamine(DA): Acts as a precursor for norepineprhine in the
peripheral nervous system, and has an important functional role in the
kidney.
Epinephrine(E):
Sympathetic transmitter in frogs.
Norepinephrine(NE):
Sympathetic transmitter in mammals.
Indolamines
Chemically similar to catecholamines, also broadcast transmitters.
Serotonin(5HT or 5-hydroxytryptamine): Neurotransmitter which acts to increase aggresiveness in
invertebrates. Has the opposite sort of effect in
vertebrates. Has been implicated to have roles in sleep and modulation of
circadian rhythm.
Amino Acids
These are specific transmitters (not broadcast). Absorbed and broken down by
glial cells. They sometimes act to
modulate the ongoing function of a neuron.
GABA: GABA is the principal
inhibitory transmitter in the brain.
(D or L)-
glutamate: Glutamate is the principal
excitatory transmitter in the brain.
glycine: An inhibitory transmitter.
aspartate: An excitatory transmitter.
Cholines
Acetylcholine: The neuromuscular transmitter of the
peripheral nervous system. (It's the chemical that triggers
muscle contraction.) In the
central nervous system it has been associated with learning and
Alzheimer's
Peptides
Neurotransmitters with widespread, occasionally dramatic action. They are usually co-released with
classic transmitters. Inactivation occurs through diffusion or enzymatic action (not reabsorption). Typically, peptides have long effects (on the order of minutes). Or they can act on the genome via
secondary messengers.
Endorphins
Gases
Gases are unique as neurotransmitters in that they can diffuse anywhere, even through
cell membranes. They are broken down quickly, and can be thougt of as having a 'sphere of influence' centered at their point of
synthesis.
Carbon monoxide(CO)
Nitrous oxide(NO)
Neurotransmitter receptors
A quick note on neurotransmitter receptors. They come in two general types:
ionotropic: Have a fast action, resuting in the opening of ion channels, affecting membrane voltage.
metabotropic: Have a slower action, and trigger a cascade of chemical actions. Ion channels can open as a result, and there can also be metabolic effects which are more long term.
The
NMDA receptor is one of the more interesting receptors. It is responsive to glutamate, but requires activation of both the pre-synaptic and post-synaptic cells to activate. When glutamate binds to the receptor, the channel opens, but is blocked by Mg
2+ ions which are too big to fit through. If the post-synaptic is then
depolarized, the Mg
+2 is forced out and the cell allows Ca
2+ and Na
+ to enter. Since Ca
2+ is known to have chemical effects in neurons, this has been proposed as a mechanism for
hebbian learning.
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how your brain works.