Download Neuromuscular junctions

Neuromuscular Junction
(NMJ)
Neuromuscular junction
• Neuromuscular junction : the synapse between
motor neuron and muscle fibre
• Motor neurons : are the nerves that innervate
(power) muscle fibres
• Motor unit : single motor neuron and the muscle
fibre it innervates
• Axon: nerve fibre that is a long, slender projection of
a nerve cell, or neuron
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• As an axon approaches muscle, it divides into
many terminal branches
• Each of these axon terminal forms special
junction  a neuromuscular junction with
one or more muscle fibre
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Synaptic Terminal
• Contains molecules of Acetylcholine
(ACh)
• ACh is a neurotransmitter  chemical
released by neurons to change the
membrane properties of other cells.
Synaptic Cleft
• A narrow space that separates the
synaptic terminal of the neuron from
the sarcolemma surface.
• Contains Acetylcholinesterase (AChE)
which breaks down Ach
Motor End Plate
• The surface of the sarcolemma
that contains membrane
receptors that bind ACh
Nerve Stimulus to Muscles
• Neuromuscular junctions – association site of
nerve and muscle
Nerve Stimulus to Muscles
• Synaptic cleft – gap
between nerve and
muscle
– Nerve and muscle
do not make
contact
– Area between
nerve and muscle
is filled with fluid
Sequence of events
1. An action potential arrives at the end of a
motor neurone, at the neuromuscular
junction.
2. This causes the release of the
neurotransmitter acetylcholine.
Sequence of events
3. This initiates an action potential in the
muscle cell membrane (Sarcolemma).
4. This action potential is carried quickly into
the large muscle cell by invaginations in the
cell membrane called T-tubules.
Sequence of events
5. The action potential causes the sarcoplasmic
reticulum to release its store of calcium into
the myofibrils.
6. Ca2+ causes tropomoysin to be displaced
uncovering myosin binding sites on actin.
7. Myosin cross bridges can now attach and the
cross bridge cycle can take place.
8. Relaxation is the reverse of these steps
Neuromuscular junction: Note Ach = Acetylcholine
In Summary…
Sequence Of Events At Neuromuscular Junction
Action potential
Ca2+
Presynaptic
terminal
Voltage-gated
Ca2+ channel
Action potentials arrives at the presynaptic terminal 
causes voltage-gated Ca2+ channels to open.
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Sequence Of Events At Neuromuscular Junction
Ca2+
Synaptic
vesicle
Acetylcholine
Ca2+
diffuse
the cellcauses
and cause
synaptic
to release
Ca2+ uptake
into
the into
terminal
release
of vesicles
the neurotransmitter
acetylcholine, a neurotransmitter molecule.
acetylcholine into synaptic cleft, which has been synthesized & stored
into synaptic vesicles
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Sequence Of Events At Neuromuscular Junction
Ca2+
Presynaptic
terminal
Synaptic cleft
Acetylcholine
• Ach travels across the synaptic cleft to postsynaptic membrane
which is alsodiffuses
known as
motor
end plate. terminal across the 31
Acetylcholine
from
the presynaptic