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Human anatomy: An overview of smooth, cardiac and skeletal muscles
Human body has around 640 muscles and it represents almost half of the human body weight. Muscles are contractile tissues, which can provide force and movement depending on its type, placement, and muscle fiber arrangement. It is made up of single muscle cells and when they are arranged or organized into bundles, it can provide a more forceful contraction than when they act in isolation. Thus, each muscle may have thousands of fibers which are made up of even smaller components known as muscle fibrils. Based on the anatomical structure, three main types of muscles are recognized in the body and these are the skeletal muscles, the smooth muscles and the cardiac muscles.
Macroscopic appearance of the skeletal muscles
These muscles contribute to bulk of the muscle mass in the body and are considered‘voluntary’ in its action. This means that the animal can control the movements or the contractile nature of these muscles via nervous stimuli. The skeletal muscles are given this name because they anchor themselves to the skeletal system or the bones in the body through a tendon on its either side. It provides stability to the skeletal system, facilitate movements, as well as supports in maintaining the posture of an animal.
Microscopic structure of the skeletal muscles
Skeletal muscles are formed of muscle cells, which contains ‘myofibrils’. The basic units giving rise to myofibrils are the actins and myosins, which are types of collagen found in animals. They form units known as the ‘sarcomeres’. These sarcomeres are bundled together by a connective tissue covering known as the ‘perimysium’ and anatomists name these bundles as the ‘fascicles’. In order to form a muscle, these fascicles are bundled together by a tissue covering known as the ‘epimysium’. Furthermore, in order to facilitate sensory feedback on the muscle movements to the central nervous system, nerve endings are distributed throughout each muscle.
According to the arrangement and the function of skeletal muscles, there are two main types. These are named as type I (red or slow twitch) and type II (fast twitching) muscles.
The type I muscles are having many capillaries and are dense in arrangement. This means that the type I muscles are able to contract for a longer time and therefore can support aerobic activity utilizing carbohydrates and fats as the sources of fuel.
Type II muscles can contract fast and with force although the movements will be short lived. It is these muscles that will give us muscle strength and following weight training, type II muscles increase in bulk.
The smooth muscles contribute towards much of the musculature in the internal organs and in the digestive system. They do not contain striation as in the case of other muscle types and contracts ‘involuntarily’ as against ‘voluntary’ contractions of the skeletal muscles. Furthermore, the microscopic appearance of the smooth muscles elaborates the presence of a single nucleus in the center of its muscle cell and it adapts a ‘spindle shape’ in its usual form. Comparatively, the smooth muscle cells contract slower than its skeletal muscle counterparts and anatomically, neither it shows the organized arrangement of ‘sarcomeres’ nor the bundle like arrangement of the muscle fibers, seen in skeletal muscles.
As its name suggest, the cardiac muscles reside in the heart and is responsible for its rhythmic and organized contractions, which enables pumping of blood.
Although the cardiac muscles appear anatomically similar in certain views, its microscopic structure elaborates several major differences between the two types of muscles. Firstly, the cardiac muscles branch themselves and connect with the nearby cardiac muscle cells in a characteristic linkage known as the ‘intercalated disk’. The intercalated disks are formed by overlapping protrusions of the ‘sarcolemmas’, or the cell membranes of the cardiac muscle cells. Secondly, these junctions contain many desmosomes and gap junctions in order to facilitate efficient adhesion between nearby cells.
Functionally, the cardiac muscles and skeletal muscles differ in that, cardiac muscles contracts involuntarily while the skeletal muscles contract voluntary. However, the contractions of the cardiac muscles differ from the smooth muscle cells as the cardiac muscle cells are able to generate and self propagate the contractions than waiting for external stimuli or sensory inputs.