Skeletal Muscle
1.
A
patient
has
spontaneous
muscle
activity
that
is
most
observable in his hands and feet but probably occurs in most of
his distal muscles.
The movement is of very low amplitude, like
a fine tremor, but without any pattern.
Many of these muscles
are
movement.
stiffer
than
normal
to
passive
Electrical
recording (EMG) shows constant activity of one and occasionally
many, motor units.
A nerve block close to the recorded muscle
abolishes the response. A nerve block far away has little effect
upon the motor unit activity.
Direct stimulation of the motor nerve shows short latency action
potentials
potentials
of
of
many
motor
units
progressively
followed
fewer
motor
by
several
units.
action
Direct
stimulation of the muscle results in only a single action potential.
A. What are some possible sources of the spontaneous motor unit
action potentials?
B. How does the data from the nerve block narrow the possible
causes?
C.
How might you rule in/out the nerve muscle junction as the
locus of abnormal activity?
D.
He is better on
phenytoin (Dilantin) , a Na channel
blocker, what are some possibilities for the membrane basis of
his disorder?
2. Immunoglobulin G (IgG) antibodies from patients with LambertEaton syndrome reduce voltage dependent Ca channel currents (ion
flux) by about 40%.
How might this reduction in Ca channel
activity be related to the observed weakness of patients with
this syndrome?
(Data from Kim, Y.I. and E. Neher, Science.
239:405-408,1988)
3. Construct the length-tension diagram you would expect for a
muscle fiber with a 5 μm A band with bridges 2 μm in from both
ends and thin filaments 2 μm long.
4.
Ms. S.S. is brought to the emergency room in a weakened
condition.
weakness
She
which
cannot
began
sit
up
and
approximately
worsened throughout the day.
complains
12
hours
of
generalized
earlier
and
has
The patient's daughter arrives
soon thereafter; she has a milder version of the same symptoms.
They had eaten together at a local restaurant the day before.
She has double and blurred vision; light hurts her eyes; difficulty in speaking and swallowing and shortness of breath.
An
exam confirmed these symptoms and shows weakness in moving her
respiratory
muscles.
She
is
alert
and
orientated,
afebrile,
with all sensory modalities intact but a decrease in deep tendon
reflexes. (Case
narrative by Julie DuBoise, R.N.)
A. What are some possible causes of a fairly sudden general
paralysis such as this one?
B.
Neither of these patients improved with edrophonium (Tensi-
lon) (anticholinesterase) testing.
What possible causes remain?
C. Stimulation directly over the body of the muscle (as in Fig.
6-2) gave strong responses. What possible causes remain?
D.
Nerve
conduction
from
elbow
to
wrist
was
within
normal
limits. What possible causes remain?
E.
While not tried, I suspect an intraarterial injection of ACh
and an esterase inhibitor would have given a normal contraction.
What possible causes remain?
F.
How would you expect their muscle bulk to react to this
disease?
5. Consider a patient with Myasthenia Gravis.
reasons
why,
after
standard
medication,
they
might
edrophonium
(Tensilon)
be
therapy
weak?
affect
the
with
How
What are two
anticholinesterase
would
weakness
in
injection
each
of
instance?
What consequences might edrophonium have?
6.
Rapidly increasing free Ca++ in the area surrounding the foot
process
is
one
stimulus
for
the
release
sarcoplasmic reticulum of skeletal muscle.
of
Ca
from
the
This relationship
for SR from normals in shown by the dashed line-open points of
this
graph.
SR
from
persons
suffering
from
malignant
hypothermia have a different relationship shown by the solid
line-close circles.
What consequences might this altered Ca
release relationship have for the affected person? Data adapted
from MacLennan, D.H. & Phillips, M.S.
Science
256: 789-793,
1992.
7. The most common degenerative disorder of muscle in childhood
is______________________________________________.
9.
The
most
common
acquired
myopathy
of
adults
of
adults
is_______________________________.
10.
The
most
common
hereditary
myopathy
is_______________________________.
11.
The
most
common
disorder
affecting
the
postsynaptic
neuromuscular junction is__________________________.
12. Discuss how myasthenia gravis and Eaton Lambert Syndrome
differ as regards:
a) The clinical syndrome,
b) the location and nature of the underlying pathology,
c) the effects of exercise,
d) the effects of repetitive nerve stimulation,
e) treatment.