Subject - General Science
Chapter : Muscular & Skeletal System

The Skeleton and Muscles

• The skeleton and muscles function together as the musculoskeletal system. This system (often treated as two separate systems, the muscular, and skeletal) plays an important homeostatic role: allowing the animal to move to more favorable external conditions.
• Certain cells in the bones produce immune cells as well as important cellular components of the blood.
• Bone also helps regulate blood calcium levels, serving as a calcium sink. Rapid muscular contraction is important in generating internal heat, another homeostatic function.

Types of Skeletons

• The axial skeleton consists of the skull, vertebral column, and rib cage.

• The appendicular skeleton contains the bones of the appendages (limbs, wings, or flippers/fins), and the pectoral and pelvic girdles.

Bone

Although bones vary greatly in size and shape, they have certain structural similarities. Bones have cells embedded in a mineralized (calcium) matrix and collagen fibers. Compact bone forms the shafts of long bones; it also occurs on the outer side of the bone. Spongy bone forms the inner layer.

Compact bone has a series of Haversian canals around which concentric layers of bone cells (osteocytes) and minerals occur. New bone is formed by the osteocytes. The Haversian canals form a network of blood vessels and nerves that nourish and monitor the osteocytes.

Types of Joints

There are three types of joints: immovable, partly movable, and synovial.

• Immovable joints, like those connecting the cranial bones, have edges that tightly interlock.

• Partly movable joints allow some degree of flexibility and usually have cartilage between the bones; example: vertebrae.

• Synovial joints permit the greatest degree of flexibility and have the ends of bones covered with a connective tissue filled with synovial fluid; example: hip.

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Skeletal Disorders

• Injury, degenerative wear and tear, and inflammatory disorders affect joints. Sprains are common injuries that cause ligaments to rip of separate from the bone. Tendinitis (such as tennis elbow) and bursitis are inflammations of the tendon sheaths.

• Osteoarthritis is a degenerative condition associated with the wearing away of the protective caps of cartilage covering the bone-ends. Bony growths or spurs develop as the cartilage degenerates, causing restriction of movement and pain. The cause is not known and may just be wear-and-tear associated with aging.

Skeletal Muscle Structure

• Muscle fibers are multinucleated, with the nuclei located just under the plasma membrane. Most of the cell is occupied by striated, thread-like myofibrils. Within each myofibril there are dense Z lines. A sarcomere (or muscle functional unit) extends from Z line to Z line. Each sarcomere has thick and thin filaments. The thick filaments are made of myosin and occupy the center of each sarcomere. Thin filaments are made of action and anchor to the Z line.

• Muscles contract by shortening each sarcomere. The sliding filament model of muscle contraction has thin filaments on each side of the sarcomere sliding past each other until they meet in the middle. Myosin filaments have club-shaped heads that project toward the actin filaments.

MUSCLES AND JOINTS

The fundamental property of muscle tissue is contraction.

Muscle are categorised into three varieties

• Cardiac

• Smooth

• Skeletal

Cardiac muscle is the muscle of the heart. Smooth muscle in the muscle of internal organs such an stomach, intestines, urinary bladder blood vessels or uterus. Skeletal muscle are also known as voluntary muscles as they are under the control of will power.

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