The Properties, Functions and Properties of Smooth Muscles - Assignment Example

The Properties, Functions and Properties of Smooth Muscles Smooth muscles are the non-voluntary muscles in our body. The term smooth is given due to its non-striated visualization under the microscope. Smooth muscle cell is relatively short, about 30-200 micrometer in length, with fusiform shape. The cross sectional diameter is about 5-10 micrometer and unlike other types of muscles, smooth muscle cell has a single nucleus. The arrangement of these cells to form a muscle tissue is also unique as they do not form a syncytium but they contract individually or as group of cells. This property of smooth muscle allows it to contract slowly in a wave like motion making it a suitable for gastrointestinal tract. Circular and longitudinal muscles are two smooth muscles found throughout the GIT tract and are responsible for peristalsis. Smooth muscles also line the walls of the blood vessels. They play an important role, especially in the arterioles, by constricting or dilating the blood vessel thereby controlling the flow of blood. Smooth muscles are also crucial in bigger blood vessels, such as aorta, and enable them to withstand high pressure generated during systole or ventricular contraction. (Clark, 2005, p. 139) Skeletal Muscle: These are the muscles under voluntary control. Skeletal muscle is also sometimes referred to as striated muscle but this term should be avoided as cardiac muscle is also a striated muscle and leads to ambiguity. A cell of skeletal muscle is very long, up to 30cm in length, and has a cylindrical shape. The cross section size of these cells is about 10-100 micrometer. It is not surprising that cell of this length has multiple nuclei for support and survival. But these nuclei are not located at the center and rather aligned at the periphery. This is because the contractile components in the cell cytoplasm push these nuclei towards the call border. Skeleton muscles form bulk of a human body and perform various functions. They are essential for locomotion and to perform any movement of the body. Although, skeletal muscles are mostly under voluntary control they also take part in a reflex arc that does not involve the higher centers and, therefore, is an involuntary process. The contractions produced by skeletal muscles are forceful and quick as compared to the smooth muscle. (Clark, 2005, p. 139) Cardiac Muscle: Cardiac muscle is a specialized tissue perfectly adapted to perform its function. It is the only muscle that is found in the heart. A very special property of a cardiac muscle is that is can contract on its own which mean it does not require an external stimulation. Certain specialized cardiac cells found in the sinus node, atrioventricular node and conducting fibers are capable of initiating an action potential and set up a particular rhythm for contraction of surrounding muscle cells. Due to these properties they are referred to as pace makers of the heart. Cardiac cells are arranged in a syncytium that helps them in conducting the electrical signals more efficiently. Moreover, these cells are joined to each other by special intercalated disks. These intercalated disks are actually fused cell membranes with gap junctions. Gap junctions allow fast sharing of cytoplasmic contents and help in the progression of electrical signals across this syncytium. The function of cardiac muscle is to set up its own rhythm and provide force required to pump blood throughout body’s circulation. Sliding filament hypothesis of muscle contraction: The cytoskeleton and its arrangement are unique in myofibers as compared to other places in our body. The entire length of myofiber is studded in the center with large structures called myofibrils. These myofibrils are composed of two types of myofilaments; thick myofilaments and thin myofilaments. Thick myofilaments are made up of a protein called myosin. This protein has a special shape that is vital for its function. The thin myofilament is composed of many proteins most importantly tropomyosin, troponin and actin. Actin is important because it is the myosin binding molecule. The arrangement of these myofilaments is very precise and it is mandatory that to understand how both thick and thin filaments are in present in close proximity before understanding the sliding filament hypothesis. (Clark, 2005, p. 142-143) The sliding filament theory can be explained as a multi step process. When the muscle is at rest an ATP binds to the myosin side arm and takes it to a higher energy state. At this point all the calcium is in the terminal cistern and not present in the sarcoplasmic reticulum. Therefore, no further process takes place. When action potential reaches the muscle cell and eventually at the sarcolemma it travels down the T-tubules and causes opening of the voltage gated calcium channels. Now calcium is present in the sarcoplasmic reticulum. This calcium binds to troponin causing it to change its shape and pulls tropomyosin away from actin. Now actin is available to bind with myosin. Myosin short arm binds to actin and forms a bond. The energy from ATP used initially to charge myosin is used to form this bond. The formation of a bond further initiates a conformational change in the side arm of myosin or thick filament. This is called the power stroke. During a power stroke the side arm moves in such way that it slides over the thin filament. This sliding causes shortening of sarcomere and contraction of muscle cell. ADP and phosphate that was initially cleaved from ATP are dissociated at the end of power stroke. Another ATP binds to myosin and initiate release of actin from its side arm. ATP is again cleaved to ADP and phosphate resulting in high energy state of the side arm and its return to the starting position. The cycle will repeat if calcium is still present because then actin will be again available for binding. But if all calcium is back in the terminal cistern, then tropomyosin covers actin and cycle halts at a resting stage. During all this process it is worth noticing that ATP is required for both initiating contraction and also to terminate the process. Even return of calcium to terminal cistern requires ATP derived pumps. (Clark, 2005, p. 144-146) Explain the properties and function of tendons, ligaments and cartilage. Tendons, ligaments and cartilages are important component of connective tissues that supports the skeletal system of the body. Tendons It is important to explain both tendons and ligaments together because they have a lot of structural similarities and functions. Tendons can be defined as specialized tough cords or bands that are continuous with the periosteum of the bone. Periosteum is a double layer membrane that completely surrounds the bone with the exception of atricular surfaces. Major function of tendon is to connect muscle to a bone. The structure of tendon makes it suitable to perform its respective function. Tendon is mostly composed of collagen which is a fibrous protein that accounts for one third of total proteins in a human body. Histological analysis of a tendon tissue will reveal relatively few cells, called fibroblasts. The rest of the structure is the extracellular matrix. The cellular component is about 20%, the remaining 80% is composed of extracellular matrix which consists of water, collagen, ground substance and small amount of elastin. Collagen content is usually greater in tendons and the specific arrangement of these collagen fibers gives it a very high tensile strength. The arrangement is more parallel in tendons giving them a high unidirectional tensile strength to withstand brutal force during vigorous exercise or weight lifting (Nordin & Frankil, 2001, p.103-104). Moreover, tendon behaves in a nonlinear way, meaning that it is very compliant when low load is applied but it gets stiffer when carrying heavy loads. This property has a major physiological function. It ensures that at low load it allow sufficient muscle contraction by stretching and at heavy loads change its compliance to prevent any damage to the muscle tissue. Ligaments Ligaments are also tough band like tissues but their major function is to link or join two bones together. Ligaments are made up of fibroblasts that are surrounded by extracellular proteins. Just like most of the connective tissues, collagen constitutes most of the ECM. The specific arrangement of collagen fibers vary in ligaments depending on their function and location. This specific arrangement has an impact on the shape of a ligament. For example, medial collateral ligaments in knee are broader like a ribbon as compared to more cylindrical cruciate ligaments. Ligaments provide necessary support to all important articulating regions such as knee, ankle and shoulder joints. They not only stabilize the joints but maintain a normal range of motion by preventing excessive bone displacement in any particular direction (Webster, 2011, p.118-119). They are present along the entire length of vertebral column and prevent herniation of vertebral disk or nucleus pulposis. Cartilages Cartilages are specialized structures and can be seen as precursor of bone formation. During the fetal development bones are derived from a special cartilage called hyaline cartilage. Cartilages are more smooth and flexible than a normal bone which makes them suitable as articulation surfaces. In a joint where two bones are held together, hyaline articular cartilage can serve two important purposes. Firstly, it distributes the load on a joint over a wide area reducing the stress at any particular articulating point. Secondly, due to its smooth surface it allows movement of two opposite surfaces with minimal friction. This reduces the wear and tear that would result otherwise due to constant friction. The composition of cartilage makes it suitable to perform this function. As in any other connective tissue the cellular component of a cartilage, chondrocytes, only constitute about 10%. But these well separated cells still play a major role in organizing and maintaining the extracellular matrix. (Nordin & Frankil, 2001, p.61-62). Other important locations where cartilage plays an important role includes trachea, where they are present as C shaped cartilages and nasal septum. The C shaped cartilages in trachea supports the structure and ensures that it does not collapse during pressure changes associated with breathing. Reference: CLARK, R. K. (2005). Anatomy and physiology: understanding the human body. Sudbury, Mass, Jones and Bartlett Publishers. NORDIN, M., & FRANKEL, V. H. (2001). Basic biomechanics of the musculoskeletal system. Philadelphia, Lippincott Williams & Wilkins WEBSTER, T. J. (2011). Nanotechnology enabled in situ sensors for monitoring health. New York, Springer. Read More