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NeurotransmittersNeurotransmittersHandoutHandout

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the neurotransmitters fit like a key in a lockthe neurotransmitters fit like a key in a lock

Neurotransmitters are Neurotransmitters are chemicals that take chemicals that take a nerve signal across the synaptic gapa nerve signal across the synaptic gap (Figure 02a) between a sending neuron, (Figure 02a) between a sending neuron, and a receiving one. On the receiving and a receiving one. On the receiving neuron are receptors into which neuron are receptors into which the the neurotransmitters fit like a key in a lockneurotransmitters fit like a key in a lock. . Once a neuro-transmitter is bound to its Once a neuro-transmitter is bound to its specific receptor, the likelihood of the specific receptor, the likelihood of the receiving cell "firing" to send its own receiving cell "firing" to send its own message is affected. message is affected.

The excitatory neurotransmitter-receptor The excitatory neurotransmitter-receptor systems make receiving cells more likely to systems make receiving cells more likely to fire, whereas the inhibitory systems make fire, whereas the inhibitory systems make the firing less likely (see the firing less likely (see Figure 29). It all ). It all depends on the type of neurotransmitter. depends on the type of neurotransmitter. An individual nerve cell can possess both An individual nerve cell can possess both kinds of synaptic connections (with a total kinds of synaptic connections (with a total of about 50000 synapses on the surface) to of about 50000 synapses on the surface) to other nerve cells. other nerve cells.

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Figure 02a Figure 02a SynapseSynapse

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transductiontransduction

Only if the excitatory charges (positive charge) Only if the excitatory charges (positive charge) exceed a thresholdexceed a threshold does the target neuron does the target neuron starting a nerve impulse of its own and is known starting a nerve impulse of its own and is known as transduction. Figure 02b shows the various as transduction. Figure 02b shows the various components in the synapse. The vesicle components in the synapse. The vesicle contains the neuro-transmitters in the axon. contains the neuro-transmitters in the axon.

The receptor is located on the surface of the The receptor is located on the surface of the dendrite to pick up the neuro-transmitters. dendrite to pick up the neuro-transmitters.

The transporter is for recycling un-used The transporter is for recycling un-used neutrotransmitters back into the axon; while the neutrotransmitters back into the axon; while the glial cell provides nutritionglial cell provides nutrition and support for the and support for the neurons. neurons.

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Figure 02b NeurotransmitterFigure 02b Neurotransmitter

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Neurotransmitter-receptor inter actionNeurotransmitter-receptor inter action

ReleaseRelease - As the action potential - As the action potential comes down the axon, comes down the axon, the the calcium influx triggers an calcium influx triggers an exocytosis of vesicles that contain exocytosis of vesicles that contain the neurotransmittersthe neurotransmitters, which are , which are release into the synaptic cleft. release into the synaptic cleft.

BindBind - The neurotransmitters then - The neurotransmitters then drifts across , binds to the drifts across , binds to the postsynaptic receptors. postsynaptic receptors.

Transduction - Depending on the Transduction - Depending on the integration of the excitatory and inhibitory integration of the excitatory and inhibitory inputs, the receiving dendrite may inputs, the receiving dendrite may fire a fire a signal for further transmissionsignal for further transmission. .

ReuptakeReuptake - The neurotransmitter - The neurotransmitter transporters remove the un-used transporters remove the un-used neutrotransmitters in the synaptic gap neutrotransmitters in the synaptic gap back to the axon for re-use. This step is back to the axon for re-use. This step is to to prevent continuous stimulation of the prevent continuous stimulation of the postsynaptic neuron.postsynaptic neuron.

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prof. azaprof. azaFigure 02c shows the process of signal transmission across Figure 02c shows the process of signal transmission across

the synapsethe synapse

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to turn the signal offto turn the signal off

There are other ways to turn the signal off. One There are other ways to turn the signal off. One is simple is simple diffusion into the extracellular spacediffusion into the extracellular space. . Another way is Another way is to break down the neuro-to break down the neuro-transmitters with enzymestransmitters with enzymes. Then there are the . Then there are the presynaptic autoreceptors (not shown), which presynaptic autoreceptors (not shown), which terminate the release once a neutrotransmitter terminate the release once a neutrotransmitter drifts back upstream and hits one of these drifts back upstream and hits one of these receptors.receptors.

Since Since the neurotransmitters are more accessible the neurotransmitters are more accessible than the neuron itself, it can be subjected to a lot than the neuron itself, it can be subjected to a lot of internal and external manipulationsof internal and external manipulations and and abusesabuses. .

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Natural neuromodulatorsNatural neuromodulators

Natural neuromodulators Natural neuromodulators can aid the release or can aid the release or inhibit the reabsorption of neurotransmittersinhibit the reabsorption of neurotransmitters ; still ; still others delay the breakdown after reabsorption, others delay the breakdown after reabsorption, leaving them in the tip to be reused by the next leaving them in the tip to be reused by the next nerve impulse. nerve impulse.

Mood, pleasure, pain, and other mental states Mood, pleasure, pain, and other mental states are determined by particular groups of neurons are determined by particular groups of neurons in the brain that use special sets of in the brain that use special sets of neurotransmitters and neuromodulators. neurotransmitters and neuromodulators.

For example, For example, mood is strongly influenced by the mood is strongly influenced by the neurotransmitter serotoninneurotransmitter serotonin.. It is believed that It is believed that depression results from a shortage of serotonin. depression results from a shortage of serotonin.

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Figure 02d NeuromodulatorFigure 02d Neuromodulator

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Prozac inhibits the reabsorption of serotoninProzac inhibits the reabsorption of serotonin

It is difficult to treat depression directly with It is difficult to treat depression directly with serotonin because the chemical has too many serotonin because the chemical has too many other side effects. However, depression can be other side effects. However, depression can be successfully treated with drugs that act successfully treated with drugs that act as as serotornin neuromodulatorsserotornin neuromodulators (Figure 02d). (Figure 02d).

Prozac, the world's top-selling antidepressant, Prozac, the world's top-selling antidepressant, inhibits the reabsorption of serotonininhibits the reabsorption of serotonin, increasing , increasing the amount in the synapse by slowing down its the amount in the synapse by slowing down its removal. removal.

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Figure 02e Drug Figure 02e Drug

AddictionAddiction

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habituationhabituation

When a neuron cell is exposed to a When a neuron cell is exposed to a neurotransmitter for a prolonged period, it neurotransmitter for a prolonged period, it tends to lose its ability to respond to the tends to lose its ability to respond to the stimulus with its original intensity. This is stimulus with its original intensity. This is known as known as habituationhabituation, which is the result , which is the result of the cell producing fewer receptors for of the cell producing fewer receptors for that particular neurotransmitterthat particular neurotransmitter

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Drug AddictionDrug Addiction

If someone takes a drug that acts as a If someone takes a drug that acts as a neuromodulator (such as cocaine), which neuromodulator (such as cocaine), which causes abnormally large amounts of causes abnormally large amounts of neurotransmitter (dopamine in this case, Figure neurotransmitter (dopamine in this case, Figure 02e) to remain in the synapses for long periods 02e) to remain in the synapses for long periods of time, of time, it would generate more pleasure it would generate more pleasure messagesmessages. .

Such action reduces the number of receptors in Such action reduces the number of receptors in the neuron. the neuron. Next time a higher dosage is Next time a higher dosage is required to maintain the pleasurable sensationrequired to maintain the pleasurable sensation . . The result is addictionThe result is addiction. .

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CocaineCocaine

Cocaine is a stimulant discovered in the Cocaine is a stimulant discovered in the mid-1800s. mid-1800s. Many physicians at first Many physicians at first considered it a miracle drug, prescribing it considered it a miracle drug, prescribing it for all sorts of physical and mental for all sorts of physical and mental ailments; it was even added to soft drinks.ailments; it was even added to soft drinks. Today United States law forbids the Today United States law forbids the importation, manufacture, and use of importation, manufacture, and use of cocaine for nonmedical purposes, and cocaine for nonmedical purposes, and even the medical use is extremely limited. even the medical use is extremely limited.

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to the design of medicines for mental illness.to the design of medicines for mental illness.

The form of receptors for the The form of receptors for the neurotransmitters varies depending neurotransmitters varies depending on the location in the body and on the location in the body and produces different physiologic produces different physiologic symptom. Understanding the symptom. Understanding the numerous neurotransmitters, their numerous neurotransmitters, their receptors, locations and interactions receptors, locations and interactions with one another has been with one another has been central to central to the design of medicines for mental the design of medicines for mental illness.illness.

Figure 02f shows the effects of three Figure 02f shows the effects of three major neurotransmitters and the major neurotransmitters and the mental states induced by their mental states induced by their interactions. interactions. Table 01 summarizes the properties Table 01 summarizes the properties of some important neurotransmitters. of some important neurotransmitters.

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Figure 02f Types of Neuro-transmitterFigure 02f Types of Neuro-transmitter

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Classification of NeurotransmittersClassification of Neurotransmitters

Neurotransmitters can be Neurotransmitters can be broadly classified into two broadly classified into two groups - the "classical", groups - the "classical", small small molecule neurotransmitters molecule neurotransmitters and the relatively larger and the relatively larger neuropeptide neuropeptide neurotransmitters. neurotransmitters.

The small molecule types are mainly The small molecule types are mainly amino acids and amines (a nitrogen atom and amines (a nitrogen atom bonds to a maximum of three hydrocarbon bonds to a maximum of three hydrocarbon groups). The larger neurotransmitters are groups). The larger neurotransmitters are combination of two or more amino acids combination of two or more amino acids joined by peptide bonds. Some fifty joined by peptide bonds. Some fifty different neurotransmitters have been different neurotransmitters have been identified. identified.

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Table 01 NeurotransmittersTable 01 Neurotransmitters

Name TypePostsynaptic

EffectLocation(s) Function(s)

Dopamine Amine Excitatory Brain, smooth muscle Control arousal levels

Serotonin Amine Excitatory Brain, smooth muscleEffects 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)

Neuropeptide

Inhibitory Brain, spinal cordReduce stress, promote calm, natural painkiller

GABA stands for gamma aminobutyric acid, which is synthesized from glutamate by organisms.