Neurotransmitters can be loosely defined as natural chemical
signals which effect the behavior of neuron
s. 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 function
Composed of a catechol
nucleus and an amine
group. Generally broadcast transmitters, and can have hormonal
(DA): Acts as a precursor for norepineprhine in the peripheral nervous system
, and has an important functional role in the kidney
transmitter in frogs.
transmitter in mammals.
Chemically similar to catecholamines, also broadcast transmitters.
(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
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 is the principal inhibitory
transmitter in the brain.
(D or L)-glutamate
: Glutamate is the principal excitatory
transmitter in the brain.
: An inhibitory transmitter.
: An excitatory transmitter.
: 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
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
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
A quick note on neurotransmitter receptors. They come in two general types:
: Have a fast action, resuting in the opening of ion channels, affecting membrane voltage.
: 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.
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 Mg2+
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 Ca2+
to enter. Since Ca2+
is known to have chemical effects in neurons, this has been proposed as a mechanism for hebbian learning
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