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The skeletal, smooth & cardiac muscles: a comparison

Multi-cellular organisms, especially those with advanced organ systems, are in fact collections of many physiological miracles and complexities. Taking three fundamental types of muscles of the human body—the skeletal muscle, the cardiac muscle, and the smooth muscle, one may find some interesting comparable characteristics as well as the unique features of each. For purpose of definition, the smooth muscle is the tissue that is found as layers on the outside as well as the inside of walls of many major organs of the body. Similarly, the skeletal muscle is made up of striated fibres and provides the skeletal structure of complex vertebral organisms, whereas the cardiac muscle is a specialised tissue found only around the heart. It is made of special muscle cells. As stated earlier, a comparative study of these three muscles is the purpose of this document, wherein unique features of these muscles shall be stated before the essay moves on to discussing the similarities.

The fundamental difference lies in the structure of the three muscles' structure and fundamental functions. The smooth muscle is made up of unstriated spindly mononucleated cells that combine in layered forms to create the walls of the internal organs including the blood vessels, the intestines, and the stomach but not the heart (Bevan and Bayliss, 1996). In contrast, the skeletal muscle is formed by long cells in the shape of fibres acting as threads that are virtually always jointed to bones of the body (van Baak 2000). Also, the cardiac muscle is very unique in its structure when compared with the other two muscles. It is made of a special type of striated cells which can be considered on a mediating point in comparison with the other two types of cells discussed above in terms of metabolism, structure and appearance (Bevan and Bayliss, 1996).

The function of these studied muscles also proposes a vivid distinction among them. The smooth muscle is responsible for the involuntary movement of the internal organs for purposes such as facilitating transport of changing fluid levels in the body (Tang et al., 1996). With the exception of the heartbeat and pulmonary function of respiration, all the involuntary movement within the body is controlled through the smooth muscle. On the other hand, the skeletal muscle functions solely for provision of physical movements of the body which are voluntary in nature, including locomotion etc. Arguably, the only involuntary movement of this muscle is what we consider a “reflex action”, wherein the body responds to any accident with astonishing speed (Pakdell, 2009). In contrast, the cardiac muscle with help from its cells, the cardiac myocites, functions only to facilitate the cardiac movements assisting the heart in performing its critically important function of keeping the supply of blood to the body according to the changing needs of varying states of movement (Griffin Press, 1999).

As a result, the movement of these muscles is also distinct when each is compared to the other two. The smooth muscle, being layered, helps the organs dilate and contract as needed. The thin cells in the layers are able to expand and contract due to their

flexibility, resulting from the presence of a single nucleus in the cell (Alexander, 2000; Tang et al., 1996). The skeletal muscles usually join any two bones of the body in mammals. The biceps muscle in the arms of a human is a good example for this demonstration. The movement is done for the purpose of moving the arm for varying purposes. This movement is limited and though voluntary, it cannot be done in every direction (van Baak, 2000). On a completely different scale, the cardiac muscle is a constantly moving muscle, which usually moves at a constant rate of the heartbeat. Though the heartbeat rate differs in varying states of movement of the body, the adaptability of the cardiac muscle enables it to facilitate the changing requirements of the organ (Bevan and Bayliss, 1996, Griffin Press,1999).

While studying the similarities between these three muscles, it is often thought that there are certain similarities that exist between any two of them, but there is hardly any physiological characteristic that all of these muscle types share in amongst. It is noted by scholars that in many ways, the smooth muscle is quite distinct among the three studied muscle types due to its very unique structure and function of involuntary movement, whereas the cardiac muscle resembles the skeletal muscle with regard to its cellular structure, flexibility vis-à-vis physical labour and contraction. Aidley (2001), discussing this similarity states that though the two types of muscles are able to react flexibly under stress of labour, the number of active fibres in the cardiac muscles remains unchanged, which is not true for the skeletal muscles. Similarly, the both these muscle types are covered with a considerably thick layer of endomysium; however, the nervous infrastructure of the skeletal muscle is more comprehensive during action as compared to the cardiac muscle, which has a distinctly limited function (Aidley, 2001).

Table I: Brief Comparison of Smooth, Cardiac and Skeletal Muscles (Source: Aidley, 2001; Bevan and Bayliss (1996); van Baak (2000).

Property

Smooth

Cardiac

Skeletal

Muscle appearance

Unstriped

Irregular stripes

Regular stripes

Cell shape

Spindle

Branched

Spindle or cylindrical

Number of Nuclei

One per cell

Many per cell

Many per cell

Speed of contraction

Slow

Intermediate

Slow to rapid

Contraction Caused by

Spontaneous, stretch, nervous system, hormones

Spontaneous

Nervous system

Function

Control movement of substances through hollow organs

Pumps blood

Moves the skeleton

Voluntary control

Usually no

Usually no

Yes

Table I provides a brief yet comprehensive overlook on the comparative nature of the three studied muscle types. It can be observed that in all of the noted characteristics the cardiac muscle is similar to any one of the other two. While in cellular appearance, cellular structure, nucleic nature, and contraction, it is similar to the characteristics of the skeletal muscle, in the cause of contraction of the muscle and the voluntary control of the body, it resembles the reactions of the smooth muscle. It is also clear that among the three, the smooth muscle is the most distinguishable. It has a number of properties that are not considerably similar to that of the other two. In short, it is visibly clear that between the two extreme muscle types of the smooth and skeletal nature, the cardiac muscle is arguably the middle form of muscle with some characteristics of one end and others of the other end.

Conclusively, the study provides clear evidence that the complex nature of the human body eludes human wisdom and though it is observantly clear that the three studied types of the body muscles seem similar due to their combined functions for the overall working of the human body, the three are quite distinct in their characteristics and functions, showing that though in the big picture of the physiology they may look similar, their specific jobs and properties are unique too. The cardiac muscle, with its very specific job of serving the heart takes characteristics of both the skeletal and the smooth muscle types, which are very distinct from each other.



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