Summation. Muscle twitches summate to produce a sustained, tetanic contraction. This pattern is produced when the muscle is stimulated electrically or naturally by neurons. Tetanus, a smooth, sustained contraction, is the normal type of muscle contraction in the body.
What determines the maximum amplitude of a summated muscle contraction? Answer: The amplitude of a summated muscle contraction is determined by the number of motor units that are stimulated. Therefore, in order to obtain the maximum amplitude of muscle contraction, all of the motor neurons innervating that muscle must be stimulated.
Skeletal muscles have different proportions of fast-twitch and slow-twitch fibers. The muscles that move the eye contain mostly fast-twitch fibers, whereas the deep muscle of the leg (the soleus) contains mostly slow-twitch fibers. The calf muscle (gastrocnemius) is intermediate in its composition.
How would you determine if the calf muscle contains a mix of fast-twitch and slow-twitch fibers, or instead is composed of an intermediate form of fiber? Answer: The graph shows the contraction speed of the entire muscle. In order to determine if the calf muscle contains intermediate muscle fibers or a combination of slow- and fast-twitch muscle fibers, the contraction speed of individual motor units is measured. If some of the individual motor units trigger fast contractions (7.3 msec) and some trigger slower contractions (100 msec) the muscle contains a combination of both fibers. However, if the motor units of the calf muscle trigger contraction speeds that are between 7.3 and 100 msec and are not quick to fatigue, the muscle contains intermediate muscle fibers.
1). Given what you know about the hierarchy of chemical organization, answer the following question. If Los Angeles was a molecule, Orange County was a cell, and California was a tissue, what would the United States be?
a). a sister cell
b). an organ system
c). an organ
d). an organism
2). Which organ system is primarily responsible for coordinating, regulating, and integrating the various activities of the body?
3). Which of the following statements best describes endocrine glands?
a). Endocrine glands are ductless glands that secrete hormones.
b). Endocrine glands are ductless glands that secrete sweat, saliva, and digestive enzymes.
c). Endocrine glands secrete hormones through a duct.
d). Endocrine glands secrete sweat, saliva, and digestive enzymes through a duct.
4). Which of the following is not considered a connective tissue?
c). adipose tissue
5). Which of the following statements about nerve tissue is false?
a). Neurons transmit sensory information to the brain.
b). Both neurons and neuroglia are present in the CNS and PNS.
c). Neurons conduct electrical impulses.
d). All types of cells in nerve tissue conduct electrical impulses.
6). Exoskeletons provide excellent protection to internal organs. However, animals that utilize exoskeletons are usually relatively small. Why?
a). These animals are only able to produce a limited amount of chitin.
b). Exoskeletons are not living tissue, and therefore they cannot grow.
c). A large exoskeleton would be too heavy to move.
d). During molting, these animals are especially vulnerable to predators and therefore do no usually live long enough to grow bigger.
7). Which of the following statements best describes the sliding filament mechanism of muscle contraction?
a). Actin and myosin filaments do not shorten, but rather, slide past each other.
b). Actin and myosin filaments shorten and slide past each other.
c). As they slide past each other, actin filaments shorten, while myosin filaments do not shorten.
d). As they slide past each other, myosin filaments shorten, while actin filaments do not shorten.
8). What is the role of Ca++ in muscle contraction?
a). It binds to tropomyosin, enabling troponin to move and reveal binding sites for cross-bridges.
b). It binds to troponin, enabling tropomyosin to move and reveal binding sites for cross-bridges.
c). It binds to tropomyosin, enabling troponin to release ATP.
d). It binds to troponin, enabling tropomyosin to release ATP.
9). Motor neurons stimulate muscle contraction via the release of
10). Which of the following statements about muscle metabolism is false?
a). Skeletal muscles at rest obtain most of their energy from muscle glycogen and blood glucose.
b). ATP can be quickly obtained by combining ADP with phosphate derived from creatine phosphate.
c). Exercise intensity is related to the maximum rate of oxygen consumption.
d). ATP is required for the pumping of the Ca++ back into the sarcoplasmic reticulum.
Test Your Visual Understanding
1). Use the following terms to label the structures in the figure.
Answer: The labeling should be like that in figure 42.20a (1 and 2).
Apply Your Knowledge
1). You are planning to travel to a hot and dry desert region where little water is available. If you were able to either (a) increase the amount of keratin in your stratified squamous epithelium, or (b) increase the number of your exocrine sweat glands, which would be a better survival strategy?
Answer: If water was plentiful, it would be best to (b) increase the number of your exocrine sweat glands. In this case, a large number of sweat glands would help your body to remain cool through evaporation of sweat. However, since water is scarce, it would be best to (a) increase the amount of keratin in your stratified squamous epithelium. In this case, the increased keratin would help prevent water loss and dehydration.
2). You have just been hired as a personal trainer. One of your clients asks you if sit-ups are considered isometric or isotonic exercises. What is your response?
Answer: In isometric exercises, a muscle group is contracted without a change in length. By contrast, isotonic exercises contract a muscle through a range of motion. Sit-ups require motion to contract and relax the abdominal muscles, and therefore are considered isotonic exercises.
3). The nerve gas sarin inhibits the enzyme acetylcholinesterase, which is normally present in the neuromuscular junction and required to break down acetylcholine into inactive fragments. Based on this information, what do you think are the likely effects of this nerve gas on muscle function?
Answer: If acetylcholinesterase is inhibited, acetylcholine will continue to stimulate muscles to contract. As a result, muscle twitching, and eventually paralysis, will occur. In March 1995, canisters of sarin were released into a subway system in Tokyo. Twelve people were killed and hundreds injured.