Muscle Tissues and Sliding Filament Model

Muscle tissue is composed of muscle fibers, which are muscle cells designed for specific functions. There are three primary types of muscle tissue: cardiac, smooth, and skeletal.

Found in the heart, cardiac muscle fibers are branched, striated, and typically have one or two nuclei. Intercalated discs facilitate organized, wave-like contractions. Cardiac muscle control is involuntary.

Smooth muscle lacks striations, has a single nucleus per fiber, and is spindle-shaped. It's found in organs like the digestive system, blood vessels, and bladder, and its contractions are involuntary.

Skeletal muscle attaches to bones or skin and is responsible for voluntary movements. These fibers are long, cylindrical, striated, and multinucleated.

All muscle tissues exhibit extensibility (stretching), elasticity (returning to original length), excitability (responding to stimuli and producing action potentials), and contractility (ability to shorten).

Skeletal muscles are often named by their location or shape, using Latin or Greek roots. They attach to bones with an insertion (movable) and an origin (fixed). Muscles can be agonists (prime movers) or antagonists (performing opposite actions).

Skeletal muscles are made of muscle fibers, which contain myofibrils. Myofibrils are composed of repeating units called sarcomeres, which give skeletal muscle its striated appearance. Sarcomeres contain thin filaments (actin) and thick filaments (myosin).

Sarcomeres are bordered by Z lines, where thin filaments attach. Thick filaments are held by the M line. Muscle contraction occurs when sarcomeres shorten, not by the shortening of filaments themselves, but by thin and thick filaments sliding past each other, pulling the Z lines closer.

Myosin heads, initially bound to ATP, hydrolyze it into ADP and phosphate, allowing them to bind to actin, forming a cross-bridge. The release of ADP and phosphate causes a 'power stroke,' sliding the actin filament. A new ATP molecule is needed for the myosin head to detach from actin. Lack of ATP causes rigor mortis.

Muscle contraction is regulated by two proteins on actin: tropomyosin and troponin. Tropomyosin blocks myosin binding sites on actin. When a neuron stimulates the muscle, calcium ions (Ca2+) are released, bind to troponin, causing a conformational change that moves tropomyosin away from the binding sites, allowing myosin to bind and contraction to occur.