The document discusses neurotransmitters and synaptic plasticity. It defines neurotransmitters as chemicals that are released from the presynaptic terminal and elicit a normal depolarization. It lists several common neurotransmitters including acetylcholine, biogenic amines, amino acids, and peptides. The document also discusses how acetylcholine is synthesized and recycled at cholinergic neurons. Finally, it covers several forms of synaptic plasticity that underlie learning and memory, including post-tetanic potentiation, habituation, sensitization, long-term potentiation, and long-term depression.

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NEUROTRANSMITTERS
Neurotransmitter is a chemical:
I. it must be released from presynaptic terminal,
ii. must elicit the normal depolarization,
iii. The effect of the substance must be blocked by the
same agents that block synaptic transmission.
Neuromodulators alter or modify the functioning of
synapses.
Agonists are the chemicals that have the same effect as a
neurotransmitter.
Anatagonists are the chemicals that reduce or prevent the
synaptic transmission.

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Table 4-1. Selected Neurortransmitters and Neuromodulators
Compound Site of Action of Neurons
Acetylcholine Neuromuscular Junction, autonomic endings,
autonomic ganglia, sweat glands, brain, retina, GI
tract
Biogenic amines
Epinephrine
Norephinephrine
Dopamine
Serotonin
Histamine
Brain, spinal cord
Sympathetic endings, brain, spinal cord, GI tract
Brain, Sympathetic ganglia, retina
Brain, spinal cord, retina, GI tract
Brain, GI tract

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Compound Site of Action of Neurons
Amino acids
GABA
Glutamate
Aspartate
Glycine
Brain, retina
Brain
Spinal cord, brain?
Spinal cord, brain, retina
Prines/Purine
nucleotides
Adenosine
ATP
Brain
Autonomic ganglia, brain
Gas
Nitric oxide Brain, spinal cord, GI tract

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Compound Site of Action of Neurons
Peptides
Activins
Angiotensin II
Atrial natriuretic
peptide
Calcitonin gene-
related peptide
Cholecystokinin
β-Endorphines
Enkephalins
Endothelins
FMRF amide
Galanin
Brain
Brain, spinal cord
Brain
Spinal cord, brain,
Brain, Retina
Brain, retina, GI tract
Brain, retina, GI tract
Brain, pituitary gland
Brain
Brain, spinal cord

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Compound Site of Action of Neurons
Peptides
Gastrin
Gastrin-releasing
peptide
Gonadotropin-
relating hormone
Inhibins
Motilin
Neuropeptide Y
Neurotensin
Oxytocin
Brain
Brain
Brain, autonomic ganglia, retina
Brain
Brain, pituitary gland
Brain, autonomic nervous system
Brain, retina
Pituitary gland, brain, spinal cord

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Compound Site of Action of Neurons
Peptides
Secretin
Somatostatin
Vasoactive intestinal
polypeptide
Brain
Brain, retina, GI tract
Autonomic nervous system, spinal cord, brain,
retina, GI tract

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Synthesis and metabolism of
Neurotransmitters
Synthesis and recycling of neurotransmitters is
very important phenomenon.
Let us take One Example
ACETYLCHOLINE
Acetylcholine is the principal neurotransmitter at
neuromuscular junctions.
The neurons that synthesize and release
acetylcholine are called “cholinergic neurons”.

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Active uptake

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SYNAPTIC PLASTICITY AND LEARNING
• Depending on history of discharge at a
synapse, short term or long-term changes
can occur at a synapse.
• These are very important as these are
the bases for learning and memory.

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POSTTETANIC POTENTIATION
• It is the production of enhanced
postsynaptic response to an incoming
stimulus (coming again and again).
• It acts through accumulation of Ca++ in
the terminals.
• It lasts for 60 seconds and occur after a
brief period of stimulus at persynaptic
neuron.

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Stimulus
Neuron
 More Ca++
 More neurotransmitter release
 More response
Ca++ ions

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HABITUATION
When a stimulus is benign (have no
effect) and is repeated over and over,
response to it gradually disappears.
This is called habituation.
This inactivation is due to decreased
response of Ca++ channel and resulting
decrease of intracellular Ca++. It could be
short-term or long-term.

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SENSITIZATION
• Sensitizaion is a the prolonged occurrence of
augmented post-synaptic responses after a
stimulus to which an animal become habituated
is paired once or several times with a noxious
stimulus.
• It may be short-term and long-term.
• In case of short term, it occur by Ca++ mediated
change in adenylyl cyclase that leads to greater
production of cAMP and hence more response.

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• Long-term potentaiton, however, also
involve protein synthesis and growth of
pre- and postsynaptic neurons and their
connections.

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LONG-TERM POTENTIATION
(LTP)
 LTP is the enhancement of the response
of a postynaptic neuron to a presynaptic
stimulation.
 This occur after a repeated stimulus of the
presynaptic neuron for a brief period.
 It is due to increased Ca++ conductance in
postsynaptic neurons. It occurs at many
places in brain.

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LONG TERM DEPRESSION
• It is opposite response to the LTP.
• It is characterized by synaptic decrease in
many ways.
• It is produced by slower stimulation of
presynaptic neurons and is associated
with a smaller rise in intracellular Ca++.

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EPSP and IPSP
• Excitatory Postsynaptic Potentials are
those which cause excitation of the
terminal.
• Inhibitory postsynaptic Potentials are
those which cause a reverse potential or
hyperpolarization to stop or inactivate a
depolarizing current.

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