Transcript Neurotransmitters

Neurotransmitters
Handout
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the neurotransmitters fit like a key in a lock
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Neurotransmitters are chemicals that take
a nerve signal across the synaptic gap
(Figure 02a) between a sending neuron,
and a receiving one. On the receiving
neuron are receptors into which the
neurotransmitters fit like a key in a lock.
Once a neuro-transmitter is bound to its
specific receptor, the likelihood of the
receiving cell "firing" to send its own
message is affected.
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The excitatory neurotransmitter-receptor
systems make receiving cells more likely
to fire, whereas the inhibitory systems
make the firing less likely (see Figure 29).
It all depends on the type of
neurotransmitter. An individual nerve cell
can possess both kinds of synaptic
connections (with a total of about 50000
synapses on the surface) to other nerve
cells.
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Figure 02a
Synapse
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transduction
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Only if the excitatory charges (positive charge)
exceed a threshold does the target neuron
starting a nerve impulse of its own and is known
as transduction. Figure 02b shows the various
components in the synapse. The vesicle
contains the neuro-transmitters in the axon.
The receptor is located on the surface of the
dendrite to pick up the neuro-transmitters.
The transporter is for recycling un-used
neutrotransmitters back into the axon; while the
glial cell provides nutrition and support for the
neurons.
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Figure 02b Neurotransmitter
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Neurotransmitter-receptor inter action
 Release
- As the action potential
comes down the axon, the
calcium influx triggers an
exocytosis of vesicles that contain
the neurotransmitters, which are
release into the synaptic cleft.
 Bind - The neurotransmitters then
drifts across , binds to the
postsynaptic receptors.
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Transduction - Depending on the
integration of the excitatory and inhibitory
inputs, the receiving dendrite may fire a
signal for further transmission.
 Reuptake - The neurotransmitter
transporters remove the un-used
neutrotransmitters in the synaptic gap
back to the axon for re-use. This step is to
prevent continuous stimulation of the
postsynaptic neuron.
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Figure 02c shows the process of signal transmission
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across the synapse
to turn the signal off
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There are other ways to turn the signal off. One
is simple diffusion into the extracellular space.
Another way is to break down the neurotransmitters with enzymes. Then there are the
presynaptic autoreceptors (not shown), which
terminate the release once a neutrotransmitter
drifts back upstream and hits one of these
receptors.
Since the neurotransmitters are more accessible
than the neuron itself, it can be subjected to a lot
of internal and external manipulations and
abuses.
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Natural neuromodulators
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Natural neuromodulators can aid the release or
inhibit the reabsorption of neurotransmitters; still
others delay the breakdown after reabsorption,
leaving them in the tip to be reused by the next
nerve impulse.
Mood, pleasure, pain, and other mental states
are determined by particular groups of neurons
in the brain that use special sets of
neurotransmitters and neuromodulators.
For example, mood is strongly influenced by the
neurotransmitter serotonin. It is believed that
depression results from a shortage of serotonin.
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Figure 02d Neuromodulator
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Prozac inhibits the reabsorption of serotonin
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It is difficult to treat depression directly with
serotonin because the chemical has too many
other side effects. However, depression can be
successfully treated with drugs that act as
serotornin neuromodulators (Figure 02d).
Prozac, the world's top-selling antidepressant,
inhibits the reabsorption of serotonin, increasing
the amount in the synapse by slowing down its
removal.
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Figure 02e Drug
Addiction
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habituation
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When a neuron cell is exposed to a
neurotransmitter for a prolonged period, it
tends to lose its ability to respond to the
stimulus with its original intensity. This is
known as habituation, which is the result
of the cell producing fewer receptors for
that particular neurotransmitter
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Drug Addiction
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If someone takes a drug that acts as a
neuromodulator (such as cocaine), which
causes abnormally large amounts of
neurotransmitter (dopamine in this case, Figure
02e) to remain in the synapses for long periods
of time, it would generate more pleasure
messages.
Such action reduces the number of receptors in
the neuron. Next time a higher dosage is
required to maintain the pleasurable sensation.
The result is addiction.
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Cocaine
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Cocaine is a stimulant discovered in the
mid-1800s. Many physicians at first
considered it a miracle drug, prescribing it
for all sorts of physical and mental
ailments; it was even added to soft drinks.
Today United States law forbids the
importation, manufacture, and use of
cocaine for nonmedical purposes, and
even the medical use is extremely limited.
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to the design of medicines for mental illness.
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form of receptors for the
neurotransmitters varies depending
on the location in the body and
produces different physiologic
symptom. Understanding the
numerous neurotransmitters, their
receptors, locations and interactions
with one another has been central to
the design of medicines for mental
illness.
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 Figure
02f shows the effects of three
major neurotransmitters and the
mental states induced by their
interactions.
Table 01 summarizes the properties
of some important neurotransmitters.
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Figure 02f Types of Neuro-transmitter
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Classification of Neurotransmitters
 Neurotransmitters
can be
broadly classified into two
groups - the "classical", small
molecule neurotransmitters
and the relatively larger
neuropeptide
neurotransmitters.
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The small molecule types are mainly
amino acids and amines (a nitrogen atom
bonds to a maximum of three hydrocarbon
groups). The larger neurotransmitters are
combination of two or more amino acids
joined by peptide bonds. Some fifty
different neurotransmitters have been
identified.
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Table 01 Neurotransmitters
Name
Type
Postsynaptic
Effect
Location(s)
Function(s)
Dopamine
Amine
Excitatory
Brain, smooth muscle
Control arousal levels
Serotonin
Amine
Excitatory
Brain, smooth muscle
Effects on mood, sleep, pain,
appetite
Noradrenaline
Amine
Excitatory
Brain, smooth muscle
Induce arousal, heighten mood
Acetylcholine
(ACh)
Acetic
acid
Excitatory
& Inhibitory
Parasymathetic nervous
system, brainstem
Role in memory, vasodilation
GABA§
amino
acid
Inhibitory
Brain
Control anxiety level
Enkephalin
(opiate)
Neurope
ptide
Inhibitory
Brain, spinal cord
Reduce stress, promote calm,
natural painkiller
GABA stands for gamma aminobutyric acid, which is synthesized from
glutamate by organisms.
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