Muscle Tissue and the Mode of Contraction Help
Muscle Tissue and the Mode of Contraction
There are three types of muscle tissue: smooth, cardiac, and skeletal. Each type has a different structure and function, and each occurs in a different location in the body.
Muscle functions include:
- Motion. Body movements such as walking, breathing, and speaking, as well as movements associated with digestion and the flow of fluids.
- Heat production.
- Posture and body support.
Microscopic Structure of Muscle
Because muscle cells resemble tiny threads, they are called muscle fibers. Each skeletal muscle fiber is a multinucleated, striated cell containing a large number of rodlike myofibrils that extend the entire length of the cell. Each myofibril is composed of still smaller units, called myofilaments (or filaments). Thin myofilaments are composed primarily of the contractile protein actin and thick myofilaments contain primarily the contractile protein myosin.
Structure of Myofilaments
Thick filament: Shaped like a golf club, each myosin protein has a long rod portion, and a globular head. The myosin head contains an actin binding site and a myosin ATPase binding site. The strands of the rod portion bind together with their globular head projecting outward to form the thick filaments that lie between the thin filaments (Figure 7-1).
Thin filament: The thin filament is composed of the proteins actin, tropomyosin, and troponin. Two long strands of actin molecules form the backbone of the thin myofilaments. Long, thin, threadlike tropomyosin proteins spiral around and cover the myosin binding sites on the actin helix. The troponin molecule fastens the ends of the tropomyosin molecule to the actin helix (Figure 7-2). In skeletal and cardiac muscle the thick and thin myofilaments overlap within the myofibril in a distinctive pattern called a sarcomere. The sarcomere is the structural and functional unit of the myofibril.
The regular overlapping pattern of thin and thick filaments within the myofibrils is responsible for the cross-banding striations seen in skeletal and cardiac muscle. The dark bands contain the thick filaments and are called A bands. The lighter bands, the I bands, are regions where only thin filaments occur. The I bands are bisected by dark Z lines where the thin filaments of adjacent sarcomeres join.
Structure of a Muscle Fiber (Cell)
The sarcolemma (cell membrane) of a muscle fiber encloses the sarcoplasm (cytoplasm). The sarcoplasm is permeated by a network of membranous channels, called the sarcoplasmic (endoplasmic) reticulum, that forms sleeves around the myofibrils. The longitudinal tubes of sarcoplasmic reticulum empty into expanded chambers called terminal cisternae. Calcium ions (Ca2+) are stored in the terminal cisternae and play an important role in regulating muscle contraction.
Transverse tubules (T tubules) are internal extensions of the sarcolemma that extend perpendicular to the sarcoplasmic reticulum. The T tubules pass between adjacent segments of terminal cisternae and penetrate deep into the interior of the muscle fiber to allow the action potential from the cell surface to be delivered into the center of the fiber.
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