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13 Dec 2019

Skeletal Muscle is innervate by axons from motor neurons whose main body is located in the spinal cord. At its termination, the motor neuron lacks a myelin sheath & divides into a number of terminals, synapsing with several muscle fibers. Impulses arriving at the terminal stumulate the realease of the neurotransmitter acetylcholine (ACh). ACh, in turn, binds to specific receptors located on the adjacent muscle membrane, causing depolarization & subsequent contraction of the muscle. The Ach receptor is a protein with a molecular weight of about 250,000. It is made up of 5 subunits which extend through the cell membrane.

Myasthenia gravis is a debilitating & sometimes fatal, autoimmune disease that causes progressive weakening of muscles. Antibodies specific to the ACh receptor on the postsynaptic membrane of the neuralmuscular junction (the motor end plate) interrupt synapatic transmission by increasing endocytosis of ACh receptors. They also form immune complexes, which causes further distruction of the post-synaptic membrane.

1. In response to bound a ACh, the ACh receptor.....

a) release calcium ions into the cytoplasm

b) hydrolyzes ATP to pump sodium ions into the cell

c) allows chloride ions to passively diffuse into the cell

d) allows sodium ions to passively diffuse into the cell

2. The antibody & the ACh receptor both.....

a) bind acetylcholine

b) are secreted proteins

c) are synthesized primarily by cells of the immune system

d) are composed of multiple polypeptides

3. Aceytcholinesterase inhibitors, such as pyridostigmine & neostigmine, increase the duration of action of released ACh, & thus are beneficial to patients with myasthenia gravis. They function by....

I. Removing anti-acetylcholine receptor antibodies

II. Preventing the degradation of acetylcholine

III. Substituting for acetylcholine in its action at the neuromuscular junction

a) I only

b ) II only

c) II & III only

d) none of the above

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Sixta Kovacek
Sixta KovacekLv2
17 Dec 2019
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