“Muscles are in a most intimate and peculiar sense the organs of the will. They have built all the roads, cities and machines in the world, written all the books, spoken all the words, and, in fact done everything that man has accomplished with matter.” (Hall, 1906.) The fundamental importance of muscular tissue in human beings is irrefutable as eloquently illustrated by G. Stanley Hall above. Muscle forms the largest group of tissue in the body, comprising of approximately 36% and 10% of body weight in skeletal and smooth muscle respectively. (Sherwood, 2009: 257) Fundamentally, there are a few similarities between skeletal and smooth muscle but these basic similarities aside, skeletal and smooth muscle are highly divergent. Skeletal muscle is striated in appearance, with several levels of muscular organisation providing structure. Functionally, muscle contraction in skeletal muscle occurs with the “sliding filament mechanism”, of which “excitation-contraction coupling” plays a pivotal role. (Sherwood, 2009: 261,264). Conversely, smooth muscle cells are unstriated, with elongated nucleus and arranged in sheets as seen in Figure 1. (Sherwood, 2009: 289) Smooth muscle function is diverse and its uses of Ca2+ ions in “excitation-contraction coupling” unique.
Elongated nucleusDespite so many differences between smooth and muscle cells, an understanding of their similarities will provide insights into muscle’s role in the human body.
Both skeletal and smooth muscle cells consist of thick myosin and thin actin filaments, their cross-bridges are activated by Ca2+ ions and both muscles require adenosine triphosphate (ATP) for activation. (Sherwood, 2009: 290-291) These similarities are fundamental to both types of muscle cells but recent research has begun to question the presence of troponin in skeletal muscle only, one of many differences. Moran et al (2008) assayed directly for troponin transcripts as seen in Figure 2 and found significant levels of troponin in aortic smooth muscle and blood vessels of all sizes. More recently, Ju et al (2013) studied Troponin T3, believed to only be present in fast skeletal muscle cells, in gene knockout mice. X-gal staining for transgene expression revealed positive staining in smooth muscle cells in the aorta, bladder and bronchus. The study surmised that “troponins are expressed in smooth muscle cells and required for normal growth and breathing for postnatal survival.” (Ju et al, 2013) These studies may raise doubts about troponin’s presence in skeletal muscle cells but the differences in structure and contraction mechanisms are crucial to the function of both types of muscle cells.
A skeletal muscle cell comprises of numerous myofibrils and along the myofibril are dark A and light I bands. One section of these striated bands is known as the sarcomere and a sarcomere consists of thick myosin filaments and thin actin filaments. The thin actin filaments in skeletal muscle cells have three troponin...