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NEUROMUSCULAR JUNCTION
DR. ZAHOOR ALI SHAIKH
LECTURE--11
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NEUROMUSCULAR JUNCTION(N.M.J)
• Neuromuscular junction is the junction between
Motor neuron and Skeletal muscle fiber. They
are linked chemically.
• Neurotransmitter at neuromuscular junction is
Acetylcholine.
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N.M.JUNCTION [CONT]
• Anatomy & Physiology
 REMEMBER
- Motor axon
- Axon terminal
- Terminal button
- Vesicles (present in terminal button) having
Neurotransmitter Acetylcholine
- Cleft or Space
- Motor end plate (Muscle cell membrane)
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N. M. JUNCTION [cont]
• An Action potential is propagated in a motor
neuron, along its large myelinated axon
(efferent nerve fiber), to the skeletal muscle
fiber.
• As the axon approaches the muscle , it gives
axon terminals and loses its myelin sheath.
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N. M. JUNCTION [cont]
• Each of the axon terminal forms a special junction
with a muscle fiber called Neuromuscular
Junction.
(one axon terminal innervates only one muscle
fiber) therefore one muscle cell has one
neuromuscular junction.
• Axon terminal branch ends in a enlarged knob like
structure called Terminal button.
• Vesicles which contain chemical transmitter are
present in terminal button.
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N. M. JUNCTION [cont]
• Axon terminal fits in the underlying muscle
fiber. This portion of muscle cell membrane is
called MOTOR END PLATE.
• There is Space or Cleft between axon terminal
and motor end plate.
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N. M. JUNCTION [cont]
• Events at a Neuromuscular Junction
1) An action potential in a motor neuron is
propagated to the axon terminal (terminal
button).
2) This local AP at the axon terminal causes the
opening of Ca2+ channels and therefore Ca2+
entry into the terminal button.
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N. M. JUNCTION [cont]
3) Ca2+ triggers the release of acetylcholine (ACh)
by exocytosis from the vesicles.
4) ACh diffuses across the space between the
nerve and muscle cells and binds with
RECEPTORS on motor end plate of the muscle
membrane
5) This binding causes opening of Na+ channels
and Na+ entry into muscle cell.
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N. M. JUNCTION [cont]
6) Na+ entry causes depolarization of Motor end
plate called END PLATE POTENTIAL ( EPP).
7) The resultant Na+ entry initiates actionpotential in the muscle fiber.
8) Acetylcholine is destroyed by enzyme
acetylcholinesterase. This enzyme is located on
the motor end plate membrane.
Destruction of ACh, causes termination of the
muscle cell response.
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N. M. JUNCTION [cont]
IMPORTANT
• Remember the name of enzyme which destroys
Acetylcholine (ACh) .
• It is called Acetylcholinesterase (AChE).
• It is located in the Motor end plate (Muscle
membrane).
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Neuromuscular Junction
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MYASTHENIA GRAVIS
• Myasthenia gravis is a disease of Neuromuscular
junction.
Q- What causes Myasthenia gravis ?
• A- Myasthenia gravis is caused by ANTIBODIES
produced against Acetylcholine receptors.
• It is an Autoimmune disease.
• Autoimmune means immunity against self.
• It is more common in young females than males.
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MYASTHENIA GRAVIS [cont]
• In Myasthenia gravis, body by mistake produces
antibodies against its own motor end-plate
acetylcholine receptor channels.
What does patient complain ?
• Patient complains of muscle weakness .
• (myasthenia means “muscle weakness”, gravis
means “severe”).
• Patient can not go up stairs.
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MYASTHENIA GRAVIS [cont]
Patient complaints (cont)
• Ptosis (dropping of eyelids).
• In sever cases difficulty in breathing.
Q- What treatment is given?
• Drug such as Neostigmine that inhibits enzyme
AChE (Acetylcholinestrase) is given.
• This drug prolongs the action of acetylcholine at
neuromuscular junction.
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EFFECTS OF CHEMICAL AGENTS AT
N.M.JUNCTION
 CURARE
• Curare blocks Ach receptors channels on motor
end plate, muscle membrane.
Therefore acetylcholine can not act to open the
Na+ channels . Muscle action potential can not
occur in response to nerve impulses.
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EFFECTS OF CHEMICAL AGENTS AT
N.M.JUNCTION
CURARE [cont]
• Curare causes muscle relaxation.
• In high dose, Muscle paralysis can occur.
• If more curare given it can cause Respiratory
muscle (Diaphragm) paralysis and death.
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SYNAPSE AND NEUROMUSCULAR
JUNCTION
‘SIMILARITIES’
• Both synapse and neuromuscular junction have
cleft (space) therefore no direct transmission of
electrical activity.
• Chemical neurotransmitter is released by Ca2+
in both.
• Neurotransmitter acts on receptors to open Na+
channels.
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SYNAPSE AND NEUROMUSCULAR
JUNCTION
‘DIFFERENCES’
• Synapse is junction between two neurons but
neuromuscular junction is present between
motor neuron and skeletal muscle fiber.
• Neuromuscular junction is always excitatory,
End Plate Potential (EPP). But Synapse may be
either excitatory (EPSP) or Inhibitory (IPSP).
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Thank you