Page 340 - Fundamentals of anatomy physiology
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Events at the Neuromuscular Junction.
Muscle Fiber The action potential
generated at the motor
end plate now sweeps
across the entire membrane
surface. The effects are almost
immediate because an action
potential is an electrical event that
flashes like a spark across the sarcolem-
mal surface. The effects are brief
because the ACh has been removed,
and no further stimulus acts upon the
motor end plate until another action
potential arrives at the axon terminal.
3 When the action potential 4 ACh molecules diffuse across the 5 The sudden inrush of sodium ions
reaches the neuron's axon synaptic cleft and bind to ACh results in the generation of an
terminal, permeability receptors on the surface of the action potential in the sarco-
changes in its membrane motor end plate. ACh binding lemma. ACh is removed from the
trigger the exocytosis of ACh alters the membrane’s permeabil- synaptic cleft in two ways. ACh
into the synaptic cleft. ity to sodium ions. Because the either diffuses away from the
Exocytosis occurs as vesicles extracellular fluid contains a high synapse, or it is broken down by
fuse with the neuron's plasma concentration of sodium ions, AChE into acetic acid and choline.
membrane. and sodium ion concentration This removal inactivates the ACh
inside the cell is very low, sodium receptor sites. The muscle fiber
Motor ions rush into the cytosol. pictured above indicates the
end plate propagation of the action
potential along the sarcolemma.
Na+ Action
potential
Na+ Na+ Break down AChE
ACh
receptor site of ACh
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