The Neuromuscular Junction

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The Neuromuscular Junction

• Anatomy and Physiology
• Liberty Senior High School

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Neuromuscular Junction(NMJ)
Communication b/w the nervous system skeletal
muscle fiber
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Soma
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Soma
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Neuromuscular Junction(NMJ)
As an axon enters a muscle, it branches into a
number of synaptic terminals, each of
which forms a NMJ with a single muscle fiber
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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

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Synaptic Cleft

• A narrow space that separates the synaptic
  terminal of the neuron from the sarcolemma
  surface
• Contains Acetylcholinesterase (AChE) which breaks
  down ACh

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Motor End Plate

• The surface of the sarcolemma that contains
  membrane receptors that bind ACh

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When a neuron stimulates a muscle fiber,
Heeeeeres what happens . . .
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The arrival of an action potential

• Sudden change in the transmembrane potential
  along the axon
• This stimulates the release of ACh at the
  synaptic terminal

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The release of ACh

• Vesicles that contain ACh, in the synaptic
  terminal, fuse with the membrane of the neuron
  are released by exocytosis into the synaptic cleft

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ACh binding at the motor end plate

• ACh molecules diffuse across the synaptic cleft
  bind to ACh receptors on the motor end plates
  surface
• When ACh binding occurs, sodium ions (Na) now
  can rush into the sarcoplasm

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Appearance of an action potential in the
sarcolemma

• The Na results in an action potential in the
  sarcolemma
• The action potential sweeps across the entire
  membrane surface travels down each of the T
  tubules which triggers the release of Ca2 at the
  terminal cisternae
• While this is occurring, AChE removes the ACh

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Return to the initial state

• Even before the action potential has spread
  across the entire membrane, the ACh has been
  broken down by AChE.
• This sequence of events can now be repeated,
  should another AP arrive at the synaptic terminal

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Normal
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Show me the action! Potential, that is!
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What happens at the neuromuscular junction when
exposed to botulinum toxin?
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Normal
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Nerve gas
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Sarcoplasmic Reticulum

• Pumps Ca2 out of the sarcoplasm and stores it.
• The resting cell has very little Ca2 in the
  sarcoplasm. 1000X more Ca2 in the SR than in
  the sarcoplasm.
• The SR is made up of terminal cisternae that lie
  on above the junction of thin and thick filaments
  in the sarcomeres.

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Sarcoplasmic Reticulum

• As an Action Potential passes down the T-tubule,
  it stimulates the terminal cisternae to release
  enough Ca2 to saturate all the troponin in the
  sarcomere.

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