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Learning Objectives
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Define neurotransmitters
Enlist the neurotransmitter of locomotor system. Give their
structure.
Enlist different neurotransmitter receptors
Give the synthesis of these neurotransmitters.
Justify that acetylcholine has sympathetic & parasympathetic
functions.
Explain the mechanism of action of acetylcholine in modulating
muscle contraction.
Give the receptors through which these neurotransmitters carry out
these functions.
Lecture outline
Definition
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Neurotransmitters are substances that are produced by
neurons, stored in the synapses, and released into the
synaptic cleft in response to a stimulus.
Important neurotransmitters
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Neurotransmitters can be classified into several groups
according to their chemical structure.
Types of Receptors
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Like all signaling substances, neurotransmitters act via receptor
proteins. The receptors for neurotransmitters are integrated into the
membrane of the postsynaptic cell, where they trigger ion inflow or
signal transduction processes
The receptors for neurotransmitters are divided into two large groups
according to the effect produced by binding of the transmitter
Two large groups according to their mode of action.
1  Ionotropic receptors
2  Metabotropic receptors
Ionotropic receptors
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They are ligand-gated ion channels. The receptors for stimulatory
transmitters mediate the inflow of cations (mainly Na+). When these
open after binding of the transmitter, local depolarization of the
postsynaptic membrane occurs.
By contrast, inhibitory neurotransmitters (GABA and glycine) allow Cl– to
flow in. This increases the membrane’s negative resting potential and
hinders the action of stimulatory transmitters (hyperpolarization)
Metabotropic receptors
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They are coupled to G proteins, through which they influence
the synthesis of second messengers. Receptors that work
with type Gs proteins increase the cAMP level in the
postsynaptic cell (cAMP), while those that activate Gi proteins
reduce it. Via type Gq proteins,other receptors increase the
intracellular Ca2+ concentration.
Neurotransmitters of Locomotor System
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Acetylcholine (ACh) was the neurotransmitter first discovered,
at the beginning of the last century. It binds to two types of
receptor.
The nicotinic ACh receptor responds to the alkaloid nicotine
contained in tobacco this). The nicotinic receptor is ionotropic
The muscarinic ACh receptors are metabotropic
Acetylcholine, the acetic acid ester of the cationic alcohol
choline acts at neuromuscular junctions, where it triggers
muscle contraction and in certain parts of the brain and in the
autonomous nervous system.
The muscarinic ACh receptors influence the cAMP level in the
postsynaptic cells
Acetylcholine receptors
Synthesis
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Acetylcholine is synthesized from acetyl-CoA and choline in the cytoplasm of
the presynaptic axon and is stored in synaptic vesicles, each of which
contains around 1000–10 000 ACh molecules.
After it is released by exocytosis, the transmitter travels by diffusion to the
receptors on the postsynaptic membrane. Catalyzed by acetylcholinesterase,
hydrolysis of ACh to acetate and choline immediately starts in the synaptic
cleft and within a few milliseconds, the ACh released has been eliminated
again. The cleavage products choline and acetate are taken up again by the
presynaptic neuron and reused for acetylcholine synthesis.