Muscle tissue, made of up highly specialized cells for contraction, is one of the four basic tissue types in multicellular organisms. There are three types of muscle tissue: skeletal, smooth and cardiac. However, this essay will concentrate on comparing skeletal and smooth muscle, firstly in the way they are structured and secondly in their function.
Skeletal muscle is found in vertebral organisms and attaches to bone via tendons. Smooth muscle is found in blood vessel walls and lining the walls of visceral organs. Each type of muscle is structured to best provide the body with the movements it needs internally and externally. Almost every essential function in the body, whether it is breathing, digesting, controlling blood pressure or simply walking requires muscle tissue.
Both skeletal and smooth muscle use actin and myosin to build their contractile elements, however their arrangement is different. In both muscle types there are two types of filaments: thick and thin. Within skeletal muscle, actin and myosin are arranged in myofibrils. Thin filaments in skeletal muscle are formed from filamentous actin, nebulin, tropomyosin and troponin. The length of thin filaments is defined by nebulin to form filaments of 1µm in length (Martini). Thick filaments are composed of “about 300 myosin molecules, each made up of a pair of myosin subunits twisted around one another”. Myosin molecules bind to one another via their long tail, leaving the head free to bind to the nearest thin filament. Thick filaments also have a specific length of 1.6µm and between 10 and 12 µm in diameter (Martini). The arrangement of these myofilaments in myofibrils and repeating sarcomeres, gives skeletal muscle its striated and regular appearance, as shown in figure… to the right. Overall, skeletal muscle has the following characteristics: up to 100µm in diameter and up to 30 cm in length and containing multiple nuclei located near the sarcolemma.
This contrasts with smooth muscle that has much smaller fibres – between 5 and 10 µm and only ranging from 30 to 200 µm – and which have a single nucleus situated centrally in the cell. (Martini, 2012). As can be seen in figure… , smooth muscle lacks striations. This is due to the fact that it is not organised in sarcomeres and myofibrils. The sarcoplasm of smooth muscle contains a network of desmin intermediate filaments throughout which dense bodies are dispersed and serve as anchors for the thin filaments. The thin filaments only contain actin and tropomyosin. The myosin thick filaments are distributed in the sarcoplasm and have more heads.
Skeletal muscle also has a structure called a T-tubule, which is not present in smooth muscle. These are invaginations of the sarcolemma (“plasma membrane of a muscle fibre”, Martini) that encircle myofibrils to ensure the spread of the action potential throughout an entire fibre in a synchronised manner. Smooth muscle uses gap junctions between its tightly connected cells to electrically...