1. ATP is needed for both muscle contraction and relaxation. In contraction, ATP must activate the myosin heads before they can bind to the active sites of actin. One ATP is needed for each cycle of cross-bridge formation and detachment. In relaxation, ATP is needed for the active transport of Ca²⁺ from the cytosol into the sarcoplasmic reticulum and for the detachment of myosin heads from actin. In addition, muscle would never be stimulated to contract at all if not for its resting membrane potential, and this is maintained by the Na⁺-K⁺ pump, which consumes ATP.

2. One factor in muscle fatigue is the accumulation of lactic acid, which lowers the pH of the sarcoplasm. This change in pH could alter the conformation of myosin heads and actin molecules and interfere with such processes as cross-bridge formation, the sliding filament process, and the hydrolysis of ATP by myosin ATPase.

3. Stretching of the bladder wall would stretch the sarcomeres of the muscle to the point that there would be little overlap between the thick and thin myofilaments. In accordances with the length-tension relationship, when the muscle was stimulated, relatively few myosin-actin cross-bridges could form and the bladder would contract weakly. Smooth muscle is better able to contract when highly stretched for three reasons: (1) the absence of Z discs imposes less limitation on its contraction; (2) the thick filaments of smooth muscle have myosin heads over their entire lengths, so cross-bridges can form anywhere along the filament; and (3) smooth muscle filaments have variable degrees of overlap, so even when smooth muscle is highly stretched, some myofilaments overlap enough to form cross-bridges and get the contraction process started. Thus the molecular organization of smooth muscle is the key to understanding its functional effectiveness in organs that become highly stretched, such as the bladder and stomach.

4. The A bands remain the same width because they are defined by the length of the thick filaments, and that does not change when muscle contracts. The H and I bands become narrower and may disappear during contraction. In relaxed muscle, these are the regions where actin and myosin do not overlap (H bands are myosin only, and I bands are actin only.) During contraction, actin and myosin overlap more and more, so these bands become narrower.

5. Weakness of the skeletal muscles and depressed somatic reflexes are early signs of botulism, resulting from inability of the somatic motor neurons to stimulate the muscles. Suffocation is a common cause of death in botulism, resulting from complete failure of the nerve fibers to stimulate the respiratory muscles.

Copyright ©1998 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use and Privacy Policy.
McGraw-Hill Higher Education is one of the many fine businesses of The McGraw-Hill Companies.
For further information about this site contact mhhe_webmaster@mcgraw-hill.com.