Muscle
295
1
Only thick filaments are seen
2
Only thin filaments are seen
3
Thick filaments interconnected
by a protein mesh
4
Thin filaments interconnected
by a protein mesh
8. In the laboratory, if an isolated skeletal muscle is placed in a
solution that contains no calcium ions, will the muscle contract
when it is stimulated (1) directly by depolarizing its membrane,
or (2) by stimulating the nerve to the muscle? What would
happen if it were a smooth muscle?
9. The following experiments were performed on a single-unit
smooth muscle in the gastrointestinal tract.
a. Stimulating the parasympathetic nerves to the muscle
produced a contraction.
b. Applying a drug that blocks the voltage-sensitive sodium
channels in most plasma membranes led to a failure to
contract upon stimulating the parasympathetic nerves.
c. Applying a drug that binds to muscarinic receptors (Chapter
6), and hence blocks the action of ACh at these receptors,
did not prevent the muscle from contracting when the
parasympathetic nerve was stimulated.
From these observations, what might you conclude about
the mechanism by which parasympathetic nerve stimulation
produces a contraction of the smooth muscle?
10. Some endocrine tumors secrete a hormone that leads to
elevation of extracellular fl uid calcium concentrations.
How
might this affect cardiac muscle?
Chapter 9 Answers to Physiological Inquiries
Figure 9–8
As long as ATP is available, cross-bridges would cycle
continuously regardless of whether calcium was present.
Figure 9–16
The weight in the isotonic experiment is
approximately 14 mg. This can be estimated by determining
the time at which the isotonic load begins to move on the lower
graph (approximately 12 ms), then using the upper graph to
assess the amount of tension generated by the fi ber at that point
in time.
Figure 9–20
Unfused tetanic contractions will occur between 6.7
and 28.6 stimuli per second. In order for an unfused tetanus
to occur, the interval between stimuli must be less than 150
ms but greater than 35 ms. (If the interval was greater than
150 ms, twitches would not summate, and if less than 35 ms,
a fused tetanus would occur.) To calculate the corresponding
frequencies:
1 stimulus/150 ms × 1000 ms/sec = 6.7 stimuli/sec
1 stimulus/35 ms × 1000 ms/sec = 28.6 stimuli/sec
Figure 9–29
The force acting upward on the forearm
(85 × 5 = 425) would be less than the downward-acting force
(10
×
45 = 450), so the muscle would undergo a lengthening
(eccentric) contraction and the weight would move toward the
ground.
Figure 9–4
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