Muscular System

• All three types of muscles exhibit excitability
• While the nervous system can influence the excitability of cardiac and smooth muscle to some degree, skeletal muscle completely depends on signaling from the nervous system to work properly.
• cardiac muscle and smooth muscle can respond to other stimuli, such as hormones and local stimuli.
• The muscles all begin the actual process of contracting (shortening) when a protein called actin is pulled by a protein called myosin. This occurs in striated muscle (skeletal and cardiac) after specific binding sites on the actin have been exposed in response to the interaction between calcium ions (Ca++) and proteins (troponin and tropomyosin) that “shield” the actin-binding sites.
• Ca++ also is required for the contraction of smooth muscle, although its role is different: here Ca++ activates enzymes, which in turn activate myosin heads. All muscles require adenosine triphosphate (ATP) to continue the process of contracting, and they all relax when the Ca++ is removed and the actin-binding sites are re-shielded.
• A muscle can return to its original length when relaxed due to a quality of muscle tissue called elasticity. It can recoil back to its original length due to elastic fibers.
• Muscle tissue also has the quality of extensibility; it can stretch or extend. 
• Contractility allows muscle tissue to pull on its attachment points and shorten with force.
• contractile proteins: actin and myosin
• Differences among the three muscle types include the microscopic organization of their contractile proteins—actin
• fibers: individual muscle cells
• The striations are visible with a light microscope under high magnification
• syncytium: a single cell or cytoplasmic mass containing several nuclei, formed by fusion of cells or by division of nuclei.
• Skeletal muscle fibers are multinucleated structures that compose the skeletal muscle.
• muscle. Cardiac muscle fibers each have one to two nuclei and are physically and electrically connected to each other so that the entire heart contracts as one unit (called a syncytium).
• Because the actin and myosin are not arranged in such regular fashion in smooth muscle, the cytoplasm of a smooth muscle fiber (which has only a single nucleus) has a uniform, nonstriated appearance.
• Smooth muscle in the walls of arteries is a critical component that regulates blood pressure necessary to push blood through the circulatory system; and smooth muscle in the skin, visceral organs, and internal passageways is essential for moving all materials through the body.
• The best-known feature of skeletal muscle is its ability to contract and cause movement.
• Small, constant adjustments of the skeletal muscles are needed to hold a body upright or balanced in any position.
• Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation.
• Skeletal muscles are located throughout the body at the openings of internal tracts to control the movement of various substances. These muscles allow functions, such as swallowing, urination, and defecation, to be under voluntary control.
• Skeletal muscles also protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs.
• Skeletal muscles contribute to the maintenance of homeostasis in the body by generating heat. Muscle
• Each skeletal muscle is an organ that consists of various integrated tissues. These tissues include the skeletal muscle fibers, blood vessels, nerve fibers, and connective tissue.
• mysia: connective tissue
• ). Each muscle is wrapped in a sheath of dense, irregular connective tissue called the epimysium, which allows a muscle to contract and move powerfully while maintaining its structural integrity. The
• The epimysium also separates muscle from other tissues and organs in the area, allowing the muscle to move independently.
• Inside each skeletal muscle, muscle fibers are organized into individual bundles, each called a fascicle, by a middle layer of connective tissue called the perimysium.
• This fascicular organization is common in muscles of the limbs; it allows the nervous system to trigger a specific movement of a muscle by activating a subset of muscle fibers within a bundle, or fascicle of the muscle. Inside
• muscle. Inside each fascicle, each muscle fiber is encased in a thin connective tissue layer of collagen and reticular fibers called the endomysium.
• The endomysium contains the extracellular fluid and nutrients to support the muscle fiber. These nutrients are supplied via blood to the muscle tissue.
• In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon.
• In skeletal muscles that work with tendons to pull on bones, the collagen in the three tissue layers (the mysia) intertwines with the collagen of a tendon.
• In other places, the mysia may fuse with a broad, tendon-like sheet called an aponeurosis, or to fascia, the connective tissue between skin and bones.
• The broad sheet of connective tissue in the lower back that the latissimus dorsi muscles (the “lats”) fuse into is an example of an aponeurosis.
• Every skeletal muscle is also richly supplied by blood vessels for nourishment, oxygen delivery, and waste removal.
• In addition, every muscle fiber in a skeletal muscle is supplied by the axon branch of a somatic motor neuron, which signals the fiber to contract. Unlike
• Because skeletal muscle cells are long and cylindrical, they are commonly referred to as muscle fibers.
• Skeletal muscle fibers can be quite large for human cells, with diameters up to 100 μm and lengths up to 30 cm (11.8 in) in the Sartorius of the upper leg. During
• During early development, embryonic myoblasts, each with its own nucleus, fuse with up to hundreds of other myoblasts to form the multinucleated skeletal muscle fibers.