The Skeletal System

2. The Skeletal System • Bones of the skeleton are divided into two main parts: the axial skeleton, which includes the skull, spine, ribs, and sternum, and the appendicular skeleton, which includes the two limb girdles along with the attached limbs. • Male bones tend to be larger and heavier than the female skeleton, and the female pelvic cavity is wider to accommodate childbirth. • The skeleton plays a part in movement, support, and protection of internal organs; in blood cell production; and in storing minerals, such as calcium and phosphorous.

4. Bones • Bones consist of microscopic cells called osteocytes. Bone is made up of 35% organic material, 65% inorganic mineral salts, and water. • The organic part derives from a protein called bone collagen. • Bone formation • The embryonic skeleton initially consists of collagenous protein fibers secreted by the osteoblasts • During the embryonic development, ossification begins. That is, mineral matter starts to replace previously formed cartilage, creating bone.

6. Bones cont. • Structure of the long bone contains a shaft, or diaphysis. • At each end (extreme) of the diaphysis is an epiphysis. • In the center of the shaft is the broad medullary canal or cavity, filled with yellow bone marrow, mostly made of fat cells. • The medullary canal is surrounded by compact or hard bone that carries blood vessels to nourish the osteocytes. • Where less strength is needed in the bone, some of the hard bone is dissolved away, leaving spongy bone. • The outside of the bone is covered with the periosteum, which is necessary for bone growth, repair, and nutrition.

8. Bones cont. • Growth • Osteoblasts are bone cells that deposit the new bone. • Osteoclasts are immense bone cells that secrete enzymes that digest the bony material, splitting off the bone minerals calcium and phosphorus, and enabling them to be absorbed by the surrounding fluid. • The length of a bone shaft continues to grow until all the epiphyseal plate cartilage is ossified.

10. Bones cont. • Bone Types • Bones are classified as one of four types on the basis of their shape: long bones, flat bones, irregular bones, and short bones. • The degree of movement at a joint is determined by bone shape and joint structure.

17. Injuries to Bones • Fractures • A simple or closed fracture can be either incomplete or complete, but the broken ends of the bones do not penetrate the skin. • A compound or open fracture results in the fractured end penetrating the skin, greatly increasing the chances of wound and bone infection.

21. Injuries to bones cont. • A greenstick fracture is an incomplete break and is more common among children whose bones are more pliable that adults. • A comminuted fracture occurs when forces are great enough to shatter the bone into at least three pieces. • A stress fracture is a small, incomplete break due to overuse, poor muscle balance, lack of flexibility, weakness in soft tissues, or biomechanical problems. Lower leg and foot bones are particularly susceptible.

23. Injuries to bones cont. • Eppiphyseal plate fractures are breaks that occur near the epiphyseal (growth plate), especially in the wrist and ankle, mostly in individuals between 10 and 16 years of age, usually as a result of sports participation. • Treatment involves the natural processes of bone remodeling, occurring with the broken bones held in their normal position relative to each other (reduction). • External fixation refers to the use of a cast to keep bones aligned. • Internal fixation occurs when surgery is necessary to keep bones aligned, often using wires, plates, and screws.

32. Muscles • Muscle cells, like most cells of the body, are surrounded by a membrane called the sarcolemma and are filled with a protein-rich cytoplasm called the sarcopolasm. Skeletal muscles are very long and, unlike most of the body’s cells, often have many nuclei. • Skeletal muscle is attached to the bone of the skeleton and is responsible for moving the bones relative to one another, under voluntary control. • Smooth muscle, which is not under conscious control and is not attached to the skeleton, is often found in the walls of hollow organs like the stomach, intestines, bladder, blood vessels, and uterus.

33. Muscles cont. • Cardiac muscle is found only in the heart, is not under conscious control, and even has the ability to contract without nerve stimulation. • Sphincter, or dilator, muscles are special circular muscles that open and close to control the passage of substances in various parts of the body.

34. Characteristics of Muscles • Muscles are characterized by contractibility, the ability to shorten, unlike other body tissues. • Like nerve cells, muscle cells are excitable (irritable), meaning they are able to respond to electrical signals called action potentials. • Muscles also exhibit characteristics of extensibility, the ability to be stretched, and elasticity, the ability to return to their original length after being stretched.

36. Muscle Attachments and Functions • Skeletal muscles are attached to a bone that does not move (origin) and a bone that does move (insertion), and when the muscle contracts, since it volume cannot change, its belly gets thicker. • Muscles are usually arranged in pairs, with the prime mover causing the main motion and the antagonist opposing the main motion. Often other muscles, called synergists, are present to steady the movement of the larger muscles or to stabilize joint activity.

43. Sources of Heat and Energy • Muscles provide the body with heat from the contractions of the muscles utilizing energy.

44. Contraction of Skeletal Muscle • Contraction of skeletal muscles within a motor unit occurs when the motor neuron, after receiving a nerve impulse from the central nervous system, releases a chemical neurotransmitter, acetylcholine, which diffuses across a gap known as the synaptic cleft neuromuscular junction, to the muscle cell, which is stimulated to contract.

46. Muscle Fatigue • Is caused by an accumulation of lactic acid in the muscle. • If the muscle, which is always under a state of partial contraction (muscle tone), contracts too often, it will use up its oxygen reserves and start producing lactic acid, which causes muscle fatigue, or the reduction in a muscle’s ability to contract and the generation of muscle pain.

48. Muscle Tone • Muscle atrophy is the wasting away or loss of muscle tissue resulting from the disease or lack of use. Vigorous exercise will reverse this type of problem unless it has gone to far. For athletes, immobilization because if injury is a common reason for lack of use. • Muscle hypertrophy, which is an increase in the mass (size) of a muscle (the number of muscle fibers do not increase, but the fibers already present do increase in size), is commonly caused by exercise. For muscles to be enlarged beyond their normal size, they must be exposed to a training stimulus that is sufficient to cause overcompensation in the muscle.

50. Injuries to Muscles • A strain is caused by twisting or pulling a muscle or tendon. • An acute strain is caused by trauma or an injury, improperly lifting heavy objects, or overstressing muscles. • Chronic strains are usually the result of overuse-prolonged, repetitive movement of the muscles and tendons. • Symptoms include pain, muscle spasm, muscle weakness, perhaps localized swelling, cramping, or inflammation, and some loss of muscle function.