Chapter 5. Muscle Tissue
What is the function of the sarcoplasmic reticulum in skeletal muscle?
A. Ca2+ release from the transverse tubules during muscle relaxation
B. Cellular storage of Ca2+
C. Cellular storage of glycogen
D. Degradation of cellular glycogen
E. Transport of Ca2+ into the terminal cisternae during muscle contraction
The answer is B: Ca2+ is responsible for the coupling of excitation and contraction in skeletal muscle. The sarcoplasmic reticulum is a modified endoplasmic reticulum. Ca2+ is concentrated in the lumen of the sarcoplasmic reticulum (B). Glycogen is stored as droplets in the cytoplasm (C). The transverse tubule system, or T system, is an extension of the cell membrane of the myofiber (sarcolemma). The T system allows for simultaneous contraction of all myofibrils because it encircles the A-I bands in each sarcomere of every myofibril (A). It is important to note that cardiac muscle also has a T system, although it is not as elaborate and well organized as that found in skeletal muscle (e.g., dyads are present rather than the triads of skeletal muscle, and there are fewer T tubules in the atrial versus ventricular muscle).
A 66-year-old man who lives alone has a severe myocardial infarction and dies during the night. The medical examiner's office is called the following morning and describes the man's body as being in rigor mortis. The state of rigor mortis is due to which of the following?
A. Absence of adenosine triphosphate (ATP) preventing detachment of the myosin heads from actin
B. Enhanced retrieval of Ca2+ by the sarcoplasmic reticulum
C. Failure to disengage tropomyosin and troponin from the myosin active sites
D. Increased lactic acid production
E. Inhibition of Ca2+ leakage from the extracellular fluid and sarcoplasmic reticulum
The answer is A: There is a small amount of production of adenosine triphosphate (ATP) after death through anaerobic and phosphagen pathways. However, there is insufficient ATP to induce the detachment of the myosin heads from actin. Ca2+ continues to leak from the extracellular fluid and the sarcoplasmic reticulum (E); however, the sarcoplasmic reticulum is no longer able to retrieve the Ca2+ (B). Tropomyosin and troponin are disengaged from the myosin active sites (C). Lactic acid is produced during rigor mortis through anaerobic pathways. The high levels of lactic acid cause deterioration of the skeletal muscle and end the state of rigor mortis (D).