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Neuromuscular Junction
• Contraction of muscles is under the control of
the nervous system:
– via the motor end plate
– which, together with the muscle cell, makes the
neuromuscular junction
• Structurally the neuromuscular junction is
similar to, and operates in a similar manner to
the synapse.
• The light micrograph
shows a motor end
plate and the
muscle fibre that it
innervates.
• Striations are visible
in the muscle fibre
at the top of the
image.
Motor end plate
Nervous control
Skeletal muscle is under the control of the voluntary nervous
system. Each muscle is controlled by a motor neurone.
motor neurone
Motor neurones interact
with muscles at a
neuromuscular
junction, sometimes
called a motor endplate.
This is a specialized form
of synapse that forms
between a neurone and
muscle fibre.
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neuromuscluar
junction
muscle fibre
© Boardworks Ltd 2009
The neuromuscular junction
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© Boardworks Ltd 2009
Summary – controlling movement
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© Boardworks Ltd 2009
Motor end plate with
vesicles containing
neurotransmitter
Neurotransmitter diffuses
across the gap
Depolarisation
spreads to the T
tube system
Sarcolemma contains
receptors for
neurotransmitter
Ca2+
Ca2+
Ca2+ ions released from
the sarcoplasmic
reticulum
Ca2+ ions bind to the troponin and
so exposes the myosin binding
sites on the actin, allowing
contraction (next lesson)
Ca2+
Ca2+
Ca2+
• When an action potential arrives in the motor end plate
– vesicles containing neurotransmitter migrate to and fuse
with the membrane
• release transmitter into the gap
• The neurotransmitter
– diffuses across the gap
– binds to receptors in the sarcolemma
• the membrane surrounding the muscle
– causes depolarisation
– depolarisation spreads to the T or tubule system which
extends deep within the muscle cell
• Depolarisation causes release of Ca2+ ions from the
sarcoplasmic reticulum
– bind to the muscle myofibrils and cause their contraction
• Note: In common with synaptic transmission
there is a minimum depolarisation needed to
bring about an action beyond the nerve
– there is a threshold level
• However, at the neuromuscular junction,
stimuli above threshold bring about an
increased or graded response in a whole
muscle.
• Examine the following graphs which show the
effect of stimulus intensity, summation and
tetanus. Then, complete the description and
interpretation of them.
response
Stimulus – larger
stimuli are indicated
by larger arrows
response
time
Stimulus
time
Stimulus intensity
If the stimulus is below threshold, there
is no contraction of the muscle. If
threshold is exceeded, contraction
increases with increased stimulus
intensity, i.e. there is a larger response.
Note: Individual fibres have an ‘all or
nothing response’, but the more fibres
that are stimulated, the greater the
overall contraction.
Summation
If an impulse arrives before the
previous contraction has faded, a new
contraction is started which is added to
the effect of the previous one.
Consequently, the overall contraction is
greater than a single contraction would
have been if the stimuli were further
apart. This is summation.
response
Stimulus
Tetanus
Repeated large stimuli results in
summation of the individual stimuli and
produces a sustained and powerful
contraction. This is tetanus.
Clostridium tetani produces a toxin that
causes sustained contraction of
voluntary muscle, including those used
in breathing. This can lead to death.
time
response
Stimulus
time
Fatigue
If stimulation is continued for a period
of time, the contraction may fade as
the muscle becomes fatigued.
The muscle tires because respiration
cannot keep pace with ATP demands,
substrates are consumed and wastes
like lactic acid accumulate.
Neuromuscular junction
Events in the nerve ending:
Transmitter substance
Cleft or gap
Effect of action potential
arriving
Events in post synaptic membrane:
Receptors in postjunction membrane
Enzyme to remove
transmitter
Effect of depolarisation
Key features
Minimum depolarisation
required for effect
Effect of increasing
stimulus intensity
Summation
Fatigue
Acclimatisation
Synapse