INTRODUCTION:
There are several systems of
neurotransmitters that are found at various synapses in the nervous
system. These groups refer to the chemicals that are the
neurotransmitters, and within the groups are specific systems. The
first group, which is a neurotransmitter system of its own, is the
cholinergic system. It is the system based on acetylcholine. This
includes the NeuroMascular
Junction
(NMJ) as an example of a
cholinergic synapse, but cholinergic synapses are found in other
parts of the nervous system. They are in the autonomic nervous
system, as well as distributed throughout the brain.
Muscarine,
a product of certain mushrooms, will bind to the muscarinic receptor.
However, nicotine will not bind to the muscarinic receptor and
muscarine will not bind to the nicotinic receptor.
AMINO
ACIDS AS NEUROTRANSMITTERS: Another
group of neurotransmitters are amino acids. This includes glutamate
(Glu), GABA (gamma-aminobutyric acid, a derivative of glutamate), and
glycine (Gly). These amino acids have an amino group and a carboxyl
group in their chemical structures. Glutamate is one of the 20 amino
acids that are used to make proteins. Each amino acid
neurotransmitter would be part of its own system, namely the
glutamatergic, GABAergic, and glycinergic systems. They each have
their own receptors and do not interact with each other.Amino
acid neurotransmitters are eliminated from the synapse by reuptake. A
pump in the cell membrane of the presynaptic element, or sometimes a
neighboring glial cell, will clear the amino acid from the synaptic
cleft so that it can be recycled, repackaged in vesicles, and
released again.
BIOGENIC
AMINES AS NEUROTRANSMITTERS: Another
class of neurotransmitter is the biogenic amine, a group of
neurotransmitters that are enzymatically made from
amino acids. They have amino groups in them, but no longer have
carboxyl groups and are therefore no longer classified
as amino acids. Serotonin is made from tryptophan. It is the basis of
the serotonergic system, which has its own specific receptors.
Serotonin is transported back into the presynaptic cell for
repackaging. Other biogenic amines are made from tyrosine, and
include dopamine, norepinephrine, and epinephrine. DopamineDopamine
is part of its own system, the dopaminergic system, which has
dopamine receptors. Dopamine is removed from the synapse by transport
proteins in the presynaptic cell membrane. Norepinephrine and
epinephrine belong to the adrenergic neurotransmitter system. The two
molecules are very similar and bind to the same receptors, which are
referred to as alpha and beta receptors. Norepinephrine and
epinephrine are also transported back into the presynaptic cell. The
chemical epinephrine
is also known as adrenaline, and norepinephrine is sometimes referred
to as noradrenaline.
EFFECTS
OF NEUROTRANSMITTERS ON THE POSTSYNAPTIC MEMBRANES: The effect
of a neurotransmitter on the postsynaptic element is entirely
dependent on the receptor protein. First, if there is no receptor
protein in the membrane of the postsynaptic element, then the
neurotransmitter has no effect. The depolarizing
or hyperpolarizing effect is also dependent on the receptor. When
acetylcholine binds to the nicotinic receptor, the postsynaptic cell
is depolarized. This is because the receptor is a cation channel and
positively charged Na+ will rush into the cell. However,
when acetylcholine binds to the muscarinic receptor, of which there
are several variants, it might cause depolarization or
hyperpolarization of the target cell. The amino acid
neurotransmitters, glutamate, glycine, and GABA, are almost
exclusively associated with just one effect. Glutamate is considered
an excitatory amino acid, but only because Glu receptors in the adult
cause depolarization of the postsynaptic cell. Glycine and GABA are
considered inhibitory amino acids, again because their receptors
cause hyperpolarization. The biogenic amines have mixed effects. For
example, the dopamine receptors that are classified as D1 receptors
are excitatory whereas D2-type receptors are inhibitory. Biogenic
amine receptors and neuropeptide receptors can have even more complex
effects because some may not directly affect the membrane potential,
but rather have an effect on gene transcription or other metabolic
processes in the neuron.
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