Compare and contrast the anatomy and physiology of skeletal and smooth muscle.
It was expected answers would describe in detail the role of troponin, tropomyosin and calmodulin in mediating muscle contraction. Detail on the structure (histology) of the skeletal and smooth muscle cells was often lacking. Many answers omitted the mechanism of muscle relaxation.
Faced with the task of expressing over ten minutes a topic that textbooks take multiple chapters to explore, most trainees likely froze, and produced inelegant results. The best approach here would be a tabular format with some minimum information in the largest number of cells, to cover the possible marking criteria as broadly as possible.
| Skeletal muscle | Smooth muscle | |
| Anatomy | ||
| Macro organisation | Fascicles and motor units | Sheets and bands |
| Innervation | ||
| Central control | Voluntary motor | Autonomic |
| Innervation | Every cell | Not every cell |
| Automaticity | No automaticity | Automaticity |
| Metabolism | ||
| Fatiguability | Fatiguable | Non-fatiguable |
| Energy requirements | High | Low |
| Speed of contraction | Very fast | Slow |
| Histology | ||
| Arrangement of myofilaments | Sarcomeres | Irregular |
| Cell size | Huge and long | Very small |
| Nuclei | Multinucleated | Single nucleus |
| Sarcolemma invaginations | T tubules | Caveolae |
| Contraction physiology | ||
| Mechanism of contraction | Calcium-induced conformational change of tropomyosin and troponin, leading to exposure of actin active sites which permits the binding of myosin and cross-bridge cycling | Calcium induces calmodulin to activate MLCK, which phosphorylates myosin light chain and allows cross-bridge cycling |
| Mechanism of relaxation | Calcium dissociation away from troponin and tropomyosin, which occurs when the calcium is taken up into the SR | Dephosphorylation of myosin light chain by myosin light chain phosphatase (the removal of calcium by itself is not enough) |
| Role of calmodulin | Minor, regulatory | Central to excitation-contraction coupling |
Sweeney, H. Lee, and David W. Hammers. "Muscle contraction." Cold Spring Harbor Perspectives in Biology 10.2 (2018): a023200.
Mukund, Kavitha, and Shankar Subramaniam. "Skeletal muscle: A review of molecular structure and function, in health and disease." Wiley Interdisciplinary Reviews: Systems Biology and Medicine 12.1 (2020): e1462.
Webb, R. Clinton. "Smooth muscle contraction and relaxation." Advances in physiology education 27.4 (2003): 201-206.
Walsh, Midiael P. "Review Article Calmodulin and its roles in skeletal muscle function." Canadian Anaesthetists’ Society Journal 30.4 (1983): 390-398.