Neurotransmitters
Neurotransmitters - Pass messages from a nerve cell to a target cell
Neurotransmitters are chemicals in the body that transmit
signals from a nerve cell (neuron) to a target cell across a synapse
A neurotransmitter
is released from a nerve cell by the arrival of an electrical impulse.
The impulse is transferred across the synapse to a target cell when
the neurotransmitter binds to a receptor in the membrane on the postsynaptic side
of the synapse.
The receiving
target cell can be:
An adjacent nerve cell
A muscle fiber (to stimulate movement)
A body organ cell
Other tissue cell
Myelin sheath. Consists of
Schwann cells that encircle axon like a jelly roll, act as insulators and
are separated by gaps of unsheathed axon called Nodes of Ranvier.
Instead of a continuous traveling down the axon, the action potential jumps from node to node (called saltatory conduction), thereby speeding up
propagation of impulse.
Some well-known neurotransmitters
E.g.
ACETYLCHOLINE , DOPAMINE ,
GABA ,
GLUTAMATE ,
NITRIC
OXIDE ,
SEROTONIN
See
CHART of neurotransmitters for the different categories of the more common NTs
Release of neurotransmitters
Neurotransmitters are usually released following arrival
of an action potential at the synapse. This causes the voltage
across the cell membrane (membrane potential) of a cell (in this case a presynaptic
neuron) to rapidly rise and fall.
Neurotransmitter synthesis
Neurotransmitters are made via just a few biosynthetic
steps, from simple precursors, such as amino acids readily available from diet.
Have excitory or inhibitory effect
(or both), depending only on the type of receptors
they activate . An excitory effect increases
the probability that the target cell will fire an action potential.
Types of Neurotransmitters
(packaged in neurotransmitter vesicles, which are membrane enclosed sacks that store
and release neurotransmitters into synapse at presynaptic neuron terminal so that
they can be detected by receptors on the postsynaptic neuron)
Amino acids.
E.g. GLUTAMATE (excitatory at >90% of the brain's synapses), ASPARTATE,
GABA (inhibitory at >90% of synapses that do not use GLUTAMATE), GLYCINE.
Monoamines. DOPAMINE,
NOREPINEPHRINE
Peptides.
E.g. ß-endorphin (engages with opioid receptors in CNS). Also considered neurotransmitters (shown to be released by presynaptic
terminals to produce an action, but are not packaged in vesicles)
Single ions.
E.g. zinc
Gas molecules.
E.g. carbon monoxide,
NITRIC OXIDE
Re-uptake of neurotransmitters
For nerve cells to communicate, neurotransmitters
are secreted by one neuron and picked up by receptor proteins on the surface of
another neuron. Once the message has been delivered, a neurotransmitter
is either destroyed or reabsorbed into the cell that made it and its activity is
over. This process is known as re-uptake.
Neurotransmitter effects
Most psychoactive drugs exert their effects by
altering the actions of some neurotransmitter systems.
E.g. Addictive drugs, such as cocaine and amphetamines, primarily affect
the DOPAMINE system. Addictive opiate drugs primarily indirectly regulate DOPAMINE
levels.
Cocaine.
Blocks reuptake of DOPAMINE back into the presynaptic neuron leaving
DOPAMINE in the the synapse for a longer time to bind to receptors on postsynaptic
neuron. This elicits a pleasurable emotion, until prolonged exposure causes down-regulation
of receptors.
Prozac.
A selective SEROTONIN reuptake
inhibitor (SSRI), which blocks reuptake of SEROTONIN
by the presynaptic cell and potentiates longer effect of
SEROTONIN ;
Alpha-methyl-p-tyrosine
(AMPT).
Prevents conversion of tyrosine to the
DOPAMINE precursor (L-Dopa); can cause
depressive symptoms.