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Answer Key to Short Answer Questions for
“Overheated: A Case Study on Skeletal Muscle Physiology”
1. David’s body temperature rises above normal during the surgery (hyperthermia).
How does skeletal muscle tissue contribute to body temperature?
Skeletal muscle produces heat when it contracts, which is an essential function in
maintaining normal core body temperature.
2. During malignant hyperthermia, there is an increased amount of calcium released into
the sarcoplasm of skeletal muscle cells.
a. What organelle stores calcium in muscles cells?
Sarcoplasmic reticulum
b. Describe the events that must occur in the muscle cell before calcium is
released from this organelle.
The junctional fold of the motor end plate must be depolarized and a wave of
depolarization travels along the sarcolemma.
The transverse tubules carry the depolarizing current to the interior of the muscle cell
where they activate voltage-sensitive channels to change shape.
This shape change opens calcium-release channels in the terminal cisterns of the
sarcoplasmic reticulum.
Once open a massive amount of calcium floods into the sarcoplasm.
3. Jaw muscle contraction (masseter spasm) is one of the key physical findings seen in
David’s case of malignant hyperthermia. Explain how calcium functions to cause
contraction of a skeletal muscle cell.
Calcium binding to troponin triggers it to change shape and pivot. This action pulls
tropomyosin off the actin binding sites for myosin on the thin myofilament. This sets
the stage for cross bridge formation. At this point, the myosin heads have already been
activated by the hydrolysis of ATP before cross bridges can form.
4. Dr. Hodges notes that David’s exhaled carbon dioxide levels are elevated. List all the
metabolic pathways that function to synthesize ATP for skeletal muscle contraction.
Which of these pathways produces carbon dioxide as a by-product?
Direct phosphorylation of ADP by creatine phosphate
Anaerobic metabolism
Aerobic metabolism: has the highest yield of ATP (32) and produces CO2.
5. Malignant hyperthermia causes a hypermetabolic state in skeletal muscle, which is
triggered by high demands for ATP during uncontrolled muscle contractions.
a. What is the role of ATP in cross bridge cycling?
Activation of the myosin head is required for contraction and uses one ATP per cross
bridge cycle.
b. What is the role of ATP in generating a resting membrane potential?
The Na+/K+ pump helps maintain resting membrane potential, which is a prerequisite
for depolarization and action potential propagation. The pump hydrolyzes one ATP
molecule in the transport of three Na+ out of the cell and two K+ into the cell.
c. What is the role of ATP in maintaining calcium concentration gradients?
Calcium must be actively pumped back into the terminal cisterns of the sarcoplasmic
reticulum to terminate the contraction and relax the skeletal muscle. For every Ca2+
transported, one ATP molecule is hydrolyzed.
6. Dantrolene is the only drug available to treat malignant hyperthermia. It works by
inhibiting calcium channels of the terminal cisterns of the sarcoplasmic reticulum.
Explain how this inhibition helps to terminate skeletal muscle contraction. What
effect would this have on David’s body temperature?
Once the outflow of calcium stops, the active transport of calcium back into the SR
"catches up," causing a net reduction in the concentration of calcium in the
sarcoplasm. In the absence of calcium, troponin reverts back to its original shape. This
pulls tropomyosin back over the myosin binding sites on the thin myofilament which
blocks the formation of cross bridges between actin and myosin.
Once the uncontrolled skeletal muscle contractions stop, David's body temperature
would begin to fall back toward normal.