Page 318 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

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(→ C1). The disease ends in death during the An important pointer to the presence and
first 20 years of life. Hypertrophied yet weak progression of a muscle disease is provided by
calf muscles and marked spinal lordosis due the concentrations of creatine, creatinine, and
to muscular weakness are typical for this creatine kinase in blood (→ E). Creatine is
form of dystrophy. In Becker’s dystrophy the formed in the liver and is eagerly taken up by
dystrophin is also defective, but its function the intact muscles. Some of the creatine is
is less impaired and the disease therefore transformed in the muscles into the anhydride
Systems less severe (→ C2; → p. 307). creatinine which, contrary to creatine, easily
crosses the cell membrane and is quantitative-
ly excreted by the kidney. The amount of creat-
Diagnosis of Motor Unit Diseases
inine excreted in the urine per unit of time is
Neuromuscular and Sensory from a neurogenic myopathy by electromyog- mass. If, as a result of muscular dystrophy,
thus proportional to the functioning muscle
A primary myopathy can be distinguished
muscle mass is reduced, creatinine excretion
raphy (→ D).
decreases (→ E1). In acute cell destruction in-
This is carried out by putting a needle into
the muscle and measuring the potential differ-
tracellular creatine kinase and creatine are re-
leased and their plasma concentrations rise
ence from an indifferent electrode on the sur-
steeply. If there is no further cell destruction,
face of the skin. As the tip of the intramuscular
the plasma concentration of creatine kinase
electrode is largely extracellular, only a frac-
membrane is measured. The amplitude of the
tine may remain elevated, because the crea-
tine formed in the liver is now taken up by
recorded changes in potential depends on the
10 tion of the potential difference across the cell drops to normal, but the concentration of crea-
fewer muscles. However, creatine production
number of muscle fibers near the inserted
electrode that are depolarized simultaneously. also falls, as it is inhibited by creatine through
As all muscle fibers that are innervated by a feedback mechanism. As a result, plasma
one α-motoneuron are depolarized at the concentration or renal excretion of creatine do
same time, the amplitude of the recorded not parallel the reduction in muscle mass.
changes in potential is greater, the higher the
density of such fibers is near the electrode. Be-
cause the various α-motoneurons are not de-
polarized simultaneously, the frequency of the
changes in potential is a measure of the num-
ber of α-motoneurons that innervate the mus-
cle fibers near the electrode.
Normally those muscle fibers in a muscle
that are innervated by one α-motoneuron do
not lie next to each other, but are distributed
over a larger cross-sectional area (→ D1). If
muscle fibers are destroyed (myogenic myopa-
thy; → D2), the number of muscle fibers near
the electrode decreases. This results in a de-
creased amplitude of the deflection. If an α-mo-
toneuron is destroyed (neurogenic myopathy;
→ D3), the muscle fibers innervated by it do
not atrophy evenly, but some of them are tak-
en over by collaterals of neighboring α-moto-
neurons. The motor units thus get larger, as
does the amplitude of the potential changes.
However, the frequency of the deflections is re-
duced because the muscle fibers near the elec-
308 trode are now innervated by fewer motor
units.
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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