Myasthenia gravis is a disease that leads to a marked decrease in the number of

Question

Myasthenia gravis is a disease that leads to a marked decrease in the number of acetylcholine (Ach) receptors at the neuromuscular junction. As a result, suppose only about 200 (instead of 2000) Ach receptor-channels are opened by each quantum of Ach. The Ach-gated channels that survive operate normally and each cause a depolarization of about 0.25 x 10-3 mV when open. The function of the presynaptic terminal is normal and an action potential will cause the release of 140 quanta of neurotransmitter. For a patient with myasthenia gravis, what would be:

a) the magnitude of the depolarization (in mV) associated with opening of one Ach-gated channel?

b) the size (in mV) of a miniature excitatory post-synaptic potential (or that associated with one quantum of Ach)?

c) the size (in mV) of the full excitatory post synaptic potential consequent to the entry of an action potential into the presynaptic terminal of the neuromuscular junction

d) the total number of Ach-gated channels opened associated with a full excitatory post synaptic potential?

Explanation / Answer

a) the magnitude of the depolarization = 0.25 x 10-3 * 200

                                                              = 0.05 mV

b) the size (in mV) of a miniature excitatory post-synaptic = 0.25 x 10-3 * 140

                                                           = 0.035 mV

c) the size (in mV) of the full excitatory post synaptic =   0.25 x 10-3 * (200/140)

                                                                                   = 0.357 * 10-3 mV

d)   the total number of Ach-gated channels   = (200/140) * 3

                                                                      = 4

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