The term “muscle” is derived from Latin word “Musculus” diminutive of “mus” meaning mouse. They were named so because their belly resembles body of the mouse and their tendons resemble mouse’s tail. Muscles are contractile tissues that bring about movements of different body parts. They can be regarded as motors of human body because they provide all the force necessary to perform different types of movements. Whether it is the locomotion of body as a whole or just motion of its parts, its all because of muscles.
Types of muscles:
Muscles are of three types; skeletal, smooth and cardiac. Characteristic features of each of them are described below.
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- They are also known as striped, striated, somatic and voluntary muscles
- They are the most abundant type and are found attached to the skeleton. For this reason they are called skeletal muscles.
- They are innervated by somatic nervous system and are therefore under voluntary control. They obey the will of human beings.
- They respond quickly to stimuli and are capable of rapid contractions. They get fatigued easily because of their rapidity
- Each muscle fiber is multinucleated cylindrical cell containing groups of myofibrils. The myofibrils are in turn made up of myofilaments of three types namely actin, myosin, and tropomyosin. Thus the skeletal muscles have three structural levels namely muscle fibers, myofibrils and myofilaments.
- Examples of skeletal muscles include all muscles of body wall.
- They are also known as plain, unstriped, visceral and involuntary muscles.
- Unlike skeletal muscles, they do not exhibit cross striations under the microscope and thus they got the name “smooth”.
- They are supplied by autonomic nervous system and therefore they are involuntary in their action. They do not obey the will of human being.
- They respond slowly to stimuli but are capable of long time sustained contractions. They do not get fatigued easily because of their slowness of response.
- They provide motor power for regulating internal environment related to digestion, circulation, secretion and excretion.
- Each smooth muscle fiber is an elongated spindle shaped cell with a single nucleus placed at the center. They also possess actin and myosin filaments but the structural arrangement of these filaments is very different as compared to the skeletal muscles.
- Examples of smooth muscles include muscles of blood vessels, and muscles of the gut etc.
- They form the myocardium of human heart.
- Cardiac muscle is intermediate in structure lying between the skeletal and smooth muscles. They are striated like skeletal muscles but at the same time they are involuntary and have uninuclear cells like smooth muscles.
- They are meant for automatic rhythmic contractions for long period of time.
- Each muscle fiber has a single centrally placed nucleus. The fibers branch and anastomoses with each other to form a syncitium. Neighboring cells are joined by intercalated discs which provide conductive pathways from one cell to another.