Page 316 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Diseases of the Motor Unit and Muscles
The motor unit consists of the motoneuron (α- Genetic ionic channel defects (→ B) are the
motoneuron in the spinal cord or the cranial cause of a group of functional muscle diseases.
nerves), the associated axon, and all the mus- Normally (→ B1) depolarization of the muscle
cle fibers innervated by its collaterals. The cell membrane is triggered on excitation by a
function of the motor unit can be affected by voltage-gated Na + channel that causes the
Systems disease of the motoneuron, by interruption or opening of a voltage-gated Ca + channel (→
2+
2+
p. 304) and a Ca
delay of axonal conduction, or by disease of
channel of the sarcoplasmic
2+
reticulum. As a result, intracellular Ca
is in-
the muscle (→ A).
The α-motoneurons can be infected by po-
creased, mediating muscular contraction. Re-
Neuromuscular and Sensory in spinal muscular atrophy, a group of degen- Na channels, by Cl influx, and K efflux. This
polarization is achieved by inactivation of the
liovirus and irreversibly destroyed by it. Also
+
–
2+
channels so
causes the inactivation of the Ca
erative diseases largely of unknown cause,
2+
that the intracellular Ca
concentration again
these cells are destroyed. Amyotrophic lateral
falls and the muscle relaxes.
sclerosis (ALS) may be caused primarily by a
+
Delayed inactivation of the Na channel due
disorder, partly genetic, of axonal transport
that secondarily leads to the death of spinal
to mutation in the gene for the channel protein
can lead to delayed relaxation, increased excit-
α-motoneurons and supraspinal motoneurons
+
congenital paramyotonia; → B2). Cold further
Damage to or death of axons may, among
+
slows Na
channel inactivation such that
others causes, be due to autoimmune diseases,
10 (→ A1). ability, and cramps (Na channel myotonia and
a deficiency of vitamin B 1 or B 12 , diabetes mel-
cramps occur, particularly in paramyotonia
litus, poisoning (e.g., lead, alcohol), or genetic when the muscle gets cold. An additional de-
+
+
defects (e.g., Charcot–Maire–Tooth syndrome; fect of the Na channel or a defective K chan-
→ p. 302) (→ A2). nel (?) can cause paralysis when the extracel-
+
The musculature (→ A3) can also be affec- lular concentration of K is high (hyperkalemic
ted by autoimmune diseases (e.g., dermato- periodic paralysis). A genetic defect of the volt-
myositis). In addition, genetic defects may age-gated Ca 2+ channel also leads to hypokale-
involve the musculature, for example, in myo- mic periodic paralysis. If there are defects in the
-
tonia or dystrophy (see below). Cl channels, myotonia occurs. Depending on
Lesion of a motor unit causes paralysis of the severity of the molecular defect, inheri-
the affected muscles, regardless of whether it tance of the disease is dominant (congenital
is localized in an α-motoneuron, axon, or the myotonia, Thomsen’s disease) or recessive
muscle itself (→ A). In primary death of an α- (Becker’s myotonia). In certain defects of sarco-
motoneuron fasciculations typically occur. plasmic Ca 2+ channels the volatile halogenated
They are the result of synchronous stimulation anesthetics may bring about potential-inde-
and contraction of the muscle fibers of a motor pendent activation of these channels with an
2+
unit. In ALS the destruction of the supraspinal increase in intracellular Ca . The resulting
neurons may result in hyperreflexia and spas- massively increased energy metabolism cau-
ticity (→ p. 310), as long as some of the α-mo- ses hyperthermia (malignant hyperthermia;
toneurons are still intact. A lesion of a periph- → p. 22).
eral nerve which has reduced the thickness of In Duchenne’s or Becker’s degenerative
the myelin layer will result in a slowing of the muscular dystrophy (→ C; → p. 307) dystro-
nerve’s conduction velocity (→ p. 302). As a phin, an element of the cytoskeleton, is defec-
rule, sensory parts of the nerve are also affect- tive. The responsible gene is on the short arm
ed. This leads to abnormal sensory functions as of the X chromosome. The disease occurs
well as spontaneous action potentials in the practically only in males, because in females
damaged nerves, resulting in corresponding with one defective gene the dystrophin
sensations (paresthesias). In primary death of formed from the normal gene is sufficient. In
306 muscles fibrillations often occur, i.e., uncoordi- Duchenne’s dystrophy only short, completely
nated contractions of individual muscle fibers. functionless dystrophin fragments are formed
"
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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