Myotonia or paralysis may result from impaired inactivation of Na+ channels in skeletal muscle. A. The electrical signature of myotonia (muscle stiffness) is
a rapid burst of action potentials in response to a single stimulus. The action potentials, shown here from extracellular recordings, vary in amplitude and
wax and wane in frequency. Such a burst may be triggered by a voluntary muscle contraction or a mechanical stimulus such as percussion of the muscle.
B. Cell-attached patch recordings from cultured human muscle cells. In normal muscle the Na+ channels open early and briefly in response to a voltageclamp depolarization from –120 mV to –40 mV. In muscle from patients with hyperkalemic periodic paralysis (M1592V Na+ channel mutation) the
prolonged openings
reopenings
impaired
inactivation.
Theofprobability
of channel
Source: and
Diseases
of the indicate
Nerve and
Motor Unit,
Principles
Neural Science,
Fifth opening
Editon (obtained by averaging individual records) remains
elevated in the hyperkalemic muscle following inactivation. (Reproduced, with permission, from Cannon, Brown, and Corey 1991.)
Citation: Kandel ER, +Schwartz JH, Jessell TM, Siegelbaum SA, Hudspeth AJ, Mack S. Principles of Neural Science, Fifth Editon; 2012 Available
C. Even modest disruption
of Na channel inactivation
sufficient
to produce bursts of myotonic discharges or depolarization-induced loss of excitability.
at: http://mhmedical.com/
Accessed: is
July
31, 2017
These computer
simulation
records
show
muscle
voltage
in
response
to an injected depolarizing current (dashed line). The fraction of mutant channels
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that actually fails to inactivate normally (f) varies within a population over time. In these simulations f was varied from normal to values appropriate for