4. Define the terms skeletal muscle fiber, motor unit, skeletal muscle twitch, electrical stimulus, and latent period. Skeletal muscle fiber: The name for an individual skeletal muscle cell
Motor unit: A motor neuron and all of the muscle fibers that it innervates.
Skeletal muscle twitch: The mechanical response (contraction) to a single action potential.
Electrical stimulus: In this simulation, represents the means of stimulating the muscle. Represents the action potential coming from the motor neuron.
Latent period: The period of time that elapses between the generation of an action potential in a muscle cell and the start of the muscle contraction.
6. Describe the process of excitation-contraction coupling in skeletal muscle fibers. (HINT: Look at page 191 in your text…and describe in detail.) Excitation-Contraction Coupling (Fig. 8.7, page 193)
Release of ACh. The nerve impulse arriving at the axon terminal in the motor neuron triggers the release of Ach. The Ach diffuses across the synapse.
ACh receptors are activated. The binding of ACh opens Na+ ion channels in the motor end plate.
Generation of muscle action potential. The AP results from inflow of Na+ ions. The muscle AP travels along the sarcolemma and through the T tubules.
ACh is broken down by acetylcholinesterase.
Release of Ca2+. Calcium ions are released from the sarcoplasmic reticulum into the sarcoplasm.
Ca2+ binds to troponin molecules in the thin filaments. The troponin changes shape exposing the binding sites for myosin.
Myosin heads bind to actin. In the presence of ATP, the myosin heads swivel and release, pulling the thin filaments toward the center of the sarcomere…the contraction.
13. How is this change in whole-muscle force achieved in vivo (eg. in the human body)?
In vivo, this process in which the number of contracting motor units is increased is called motor unit recruitment. As more and more motor neurons “fire”, more and more muscle fibers are stimulated.
15. True or False: There is no limit to the amount of force that a skeletal muscle can generate if it is stimulated by a high enough voltage at a very high frequency. FALSE
24. Describe how increasing the stimulus frequency affected the force developed by the isolated whole skeletal muscle in this activity.
Increasing the stimulus frequency caused an increase in the amount of active force generated. The amount of variability also decreased as the frequency increased.
38. Based on the unique arrangement of myosin and actin in skeletal muscle sarcomeres, explain why active force varies with changes in the muscle’s resting length.
As the length of 75mm, an “ideal” number of thin and thick filaments can overlap, contract, and cause active force. If the muscle is shortened or lengthened, there will be fewer of thin and thick filaments overlapping.
43. True or False: A muscle that is contracting isometrically will fatigue much slower than one that is contracting isotonically. FALSE
48. Describe what would happen in the following experiment: A 2.5g weight is attached to the end of the isolated whole skeletal muscle used in these experiments. Simultaneously, the muscle is maximally stimulated by 8.5 volts and the platform supporting the weight is removed.
Answer A.
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