36–1 The Skeletal System Photo Credit: © Getty Images

The Skeleton Function of the skeleton: supports the body. protects internal organs. provides for movement. stores mineral reserves. provides a site for blood cell formation.

Structure of Bones Bones are a solid network of living cells and protein fibers that are surrounded by deposits of calcium salts.

Haversian canal Spongy bone Compact bone Compact bone Periosteum Bone marrow Bones are a solid network of living cells and protein fibers that are supported by deposits of calcium salts. A typical long bone such as the femur contains spongy bone and compact bone. Within compact bone are Haversian canals, which contain blood vessels. Osteocyte Artery Vein Periosteum

The bone is surrounded by a tough layer of connective tissue called the periosteum. Blood vessels in the periosteum carry oxygen and nutrients to the bone.

Beneath the periosteum is a thick layer of compact bone. Running through compact bone is a network of tubes called Haversian canals that contain blood vessels and nerves.

Spongy bone is found inside the outer layer of compact bone. Spongy bone is also found in the ends of long bones and in the middle of short, flat bones. Spongy bone adds strength without adding mass.

Bone marrow is a soft tissue inside the cavities within bones. There are two types of bone marrow: Yellow marrow is made up of fat cells. Red marrow produces red blood cells, some kinds of white blood cells, and platelets.

Development of Bones The skeleton of an embryo is composed of cartilage. Cartilage is a strong connective tissue that supports the body and is softer and more flexible than bone.

Cartilage is replaced by bone during the process of bone formation called ossification. Many long bones have growth plates at either end. Growth of cartilage at these plates causes bones to lengthen. By early adulthood, cartilage in the growth plates is replaced by bone, the bones become ossified, and growth stops.

Types of Joints A place where one bone attaches to another bone is called a joint. Joints permit bones to move without damaging each other. Depending on its type of movement, a joint is classified as immovable, slightly movable, or freely movable.

Immovable Joints Immovable joints, called fixed joints, allow no movement. Places where bones in the skull meet are examples of immovable joints.

Slightly Movable Joints Slightly movable joints permit a small amount of restricted movement. Slightly movable joints are found in the joints between adjacent vertebrae.

Freely Movable Joints Freely movable joints permit movement in one or more directions. Four common freely movable joints are: ball-and-socket joints hinge joints pivot joints saddle joints

Ball-and-socket joints permit movement in many directions. The shoulder joint has a ball-and-socket joint.

Hinge joints permit back-and-forth motion. The knee is a hinge joint.,

Pivot joints allow one bone to rotate around another. The elbow has a pivot joint.

Saddle joints permit one bone to slide in two directions. The joint at the base of your thumb is a saddle joint.

Structure of Joints Connective tissue called ligaments hold bones together in joints and are attached to membranes that surround bones. Synovial fluid forms a thin lubricating film over the surface of the joint. Synovial fluid enables the bones to slide past each other more smoothly.

Skeletal System Disorders Excessive strain on a joint may produce inflammation, in which excess fluid causes swelling, pain, heat, and redness. Inflammation of a bursa is called bursitis. Inflammation of the joint itself is called arthritis. Another skeletal system disorder is osteoporosis. Osteoporosis is caused by a loss of calcium in the bone.

36–2 The Muscular System Photo Credit: © Getty Images

The function of the muscular system is movement. More than 40% of the mass of the average human body is muscle.

Types of Muscle Tissue There are three different types of muscle tissue: skeletal smooth cardiac

Skeletal Muscles Skeletal muscles: are usually attached to bones. are responsible for voluntary movements. have many nuclei. are sometimes called striated muscles.

Smooth Muscles Smooth muscles: are usually not under voluntary control. are spindle-shaped. have one nucleus. are not striated. are found in many internal organs and blood vessels.

Cardiac Muscle Cardiac muscle: is only found in the heart. is striated. may have one or two nuclei. Cardiac muscle cells are connected to each other by gap junctions.

Muscle Contraction Muscles are made of 2 main proteins: thick filaments =myosin. thin filaments =actin.

Bundle of muscle fibers Actin Skeletal muscle Myosin Bundle of muscle fibers Sarcomere Z line Skeletal muscles are made up of bundles of muscle fibers, which in turn are composed of myofibrils. Each myofibril contains thin filaments made of actin and thick filaments made of myosin. Muscle fibers are divided into functional units called sarcomeres. Myofibril Muscle fiber (cell)

A muscle contracts when the thin filaments in the muscle fiber slide over the thick filaments. This process is called the sliding filament model of muscle contraction.

During muscle contraction, the actin filaments slide over the myosin filaments, decreasing the length of the muscle. During muscle contraction, the actin filaments slide over the myosin filaments, decreasing the distance between the Z lines. 

Control of Muscle Contraction Impulses from motor neurons control the contraction of skeletal muscle fibers. A neuromuscular junction is the point of contact between a motor neuron and a skeletal muscle cell.

How Muscles and Bones Interact Skeletal muscles are joined to bones by tendons. Tendons pull on the bones so they work like levers. The muscles provide the force to move the lever.

Opposing Muscles Contract and Relax By contracting and relaxing, the triceps and biceps in the upper arm enable you to bend or straighten your elbow.

Opposing Muscles Contract and Relax By contracting and relaxing, the triceps and biceps in the upper arm enable you to bend or straighten your elbow.