NEUROMUSCULOSKELETAL SYSTEMS
LECTURE 7
NEUROMUSCULAR
JUNCTION
Medical Science
Dr R G Luckwill
Figure 7.5
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Membrane Potential (mV)
Muscle
+30
0
-90
1
2
3
4
Time (ms)
Muscle
Membrane Potential (mV)
+ Curare
+30
0
Threshold
EPP
-90
1
2
3
4
Time (ms)
Axon of motor neuron
Action potential
propagation
in motor neuron
Myelin sheath
Axon terminal
Terminal button
Voltage-gated
calcium channel
Vesicle of acetylcholine
Acetylcholine
receptor site
Action potential
propagation
in muscle fiber
Acetycholinesterase
Plasma membrane
of muscle fiber
Voltage-gated
Na+ channel
Chemically gated
cation channel
Motor end plate
Contractile elements within muscle fiber
Figure 7.6
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Neuromuscular Junction Differences from Nerve - Nerve Synapse
 Scale : the NMJ is much larger than a nerve-nerve synapse with
a wider synaptic cleft
 Folding of postsynaptic membrane gives larger surface area
 Potential is therefore higher - always suprathreshold
 NMJ is always excitatory
 Only one transmitter substance - Acetylcholine
 Transmission ratio is 1:1 (no summation needed)
Miniature End Plate Potentials - MEPPs
TOXINS AFFECTING NEUROMUSCULAR TRANSMISSION
Curare
Blocks Acetylcholine receptors
Binds irreversibly to receptors
Botulinum toxin
Blocks acetylcholine release
Therefore blocks neuromuscular transmission
Black Widow venom
Excess release of acetylcholine
Prolonged depolarisation & continuous contraction
Organophosphates
(Pesticides & Nerve Gas)
Blocks acetylcholine-esterase
Binds irreversibly to acetylcholine-esterase preventing
breakdown of Ach. Prolonged depolarisation &
continuous contraction
Myasthenia gravis - an autoimmune disorder
The body produces antibodies which attack nicotinic acetylcholine receptors
at skeletal neuromuscular junctions .
∴ Acetylcholine is destroyed by acetylcholine esterase before it has had a chance to find
and attach to a remaining receptor.
∴ The normal release of ACh in response to an Action Potential produces a smaller End Plate
Potential which may be sub-threshold.
∴ Fewer Action potentials in muscle for the same motor drive (transmission no longer 1 : 1).
∴ Muscle contractile force is reduced.
∴ Muscular weakness develops.
The MOTOR UNIT
A motor Unit consists of an α−motorneuron and all the muscle
fibres which it innervates.
A motor Unit is therefore the functional unit of Skeletal (Somatic)
Muscle
A large Motor Unit may have more > 1000 muscle fibres e.g. Gastrocnemius
A small Motor Unit may have as few as 10 muscle fibres e.g. Ocular muscles
Spinal cord
= Motor unit 1
= Motor unit 2
= Motor unit 3
Figure 8.15
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