In the hinge joint, the articular surfaces fit together in such a way as to permit motion only in one plane, forward and backward, the extent of motion at the same time being considerable. An example of a hinge joint is the elbow.
The pivot joint is formed by a process that rotates within a ring, the ring being formed partly of bone, and partly of ligament. An example of a pivot joint is the joint between the radius and ulna that allows you to turn the palm of your hand up and down.
A gliding joint, also known as a plane joint, is a joint which allows one bone to slide over another, such as between the carpels of the fingers. Gliding joints are also found in your wrists and ankles.
Not all bones are interconnected directly: There are 6 bones in the middle ear called the ossicles (three on each side) that articulate only with each other. The hyoid bone which is located in the neck and serves as the point of attachment for the tongue, does not articulate with any other bones in the body, being supported by muscles and ligaments. The longest and heaviest bone in the body is the femur and the smallest is the stapes bone in the middle ear. In an adult, the skeleton makes up around 20% of the total body weight.
Homeostatic Imbalances of Bone
Despite their great strength, bones can fracture, or break. Fractures can occur at different places on a bone, and are usually due to excessive bending stress on the bone. Fractures can be complete in which the bone is completely broken, or incomplete in which the bone is cracked or chipped, but not broken all the way, as shown in Figure below. Immediately after a fracture, blood vessels that were torn leak blood into surrounding tissues and a mass of clotted blood, called a hematoma, forms. The area becomes swollen and sore. Within a few days capillaries begin to grow into the hematoma and white blood cells clean up the dead and dying cells. Fibroblasts and osteoblasts arrive and begin to rebuild the bone. Fibroblasts produce collagen fibers which span the area of the break and connect the ends of the broken bone together. Osteoblasts begin to form spongy bone, and chondroblasts form cartilage matrix. Later, the cartilage and spongy bone are replaced by a bony growth called a callus which forms about 3 to 4 weeks after the fracture, and continues until the break is firmly sealed 2 to 3 months later. Eventually the bony callus is replaced by spongy and compact bone, similar to the rest of the bone.
Figure 21.16
Rickets is a softening of the bones in children which potentially leads to fractures and deformity; bowing of the leg bones is shown in Figure below. Rickets is among the most frequent childhood diseases in many developing countries. The most common cause is a vitamin D deficiency. Vitamin D is needed by the body to absorb calcium from foods and to form bones. However, lack of calcium in the diet may also cause rickets. Although it can occur in adults, most cases of rickets occur in children who suffer from severe malnutrition, which usually results from starvation during early childhood. Osteomalacia is the term used to describe a similar condition occurring in adults, generally due to a deficiency of vitamin D. Osteomalacia can result in bone pain, difficulty in putting weight on bones, and sometimes fractures.
Some studies show most people get enough Vitamin D through their food and exposure to ultraviolet (UV) radiation in sunlight. Vitamin D is produced by certain skin cells from a compound found inside the cells. The skin cells need UV light for this reaction to happen. However, eating foods to which vitamin D has been added or taking a dietary supplement pill is usually preferred to UV exposure, due to the increased risk of sun burn and skin cancer. Many countries have fortified certain foods such as milk, bread, and breakfast cereals with Vitamin D to help prevent deficiency.
Figure 21.17
An X ray image of a 2-year-old who shows the typical bowing of the femurs that occurs in rickets. Rickets causes poor bone mineralization, which results in the bones bending under the weight of the body.
Osteoporosis is a disease in which the breakdown of bone matrix by osteoclasts is greater than the building of bone matrix by osteoblasts. This results in bone mass that is greatly decreased, causing bones to become lighter and more porous. Bones are then more prone to breakage, especially the vertebrae and femurs. Compression fractures of the vertebrae and hip breaks, in which the top (or head) of the femur breaks are common, and can lead to further immobility, making the disease worse. Osteoporosis mostly occurs in older women and is linked to the decrease in production of sex hormones. However, poor nutrition, especially diets that are low in calcium and vitamin D, increase the risk of osteoporosis in later life. One of the easiest ways to prevent osteoporosis is to eat a healthful diet that has adequate calcium and vitamin D. For a brief animation of osteoporosis, see http://www.youtube.com/watch?v=5uAXX5GvGrI.
Figure 21.18
Total replacement of hip joint. One of the leading reasons for hip replacement is osteoarthritis of the joint in which the cartilage around the top of the femur bone deteriorates over time, and causes the bones of the joint to grind painfully against each other. This can result in a narrowing of the space in the ball-and-socket joint structure, causing limited movement of the hip and constant pain in the hip joint.
Osteoarthritis is a condition in which wearing and breakdown of the cartilage that covers the ends of the bones leads to pain and stiffness in the joint. Decreased movement of the joint because of the pain may lead to muscles that are attached to the joint to become weaker, and ligaments may become looser. Osteoarthritis is the most common form of arthritis. Some of the most common causes include old age, sport injuries to the joint, bone fractures, and overweight and obesity. Total hip replacement is a common treatment for osteoarthritis. An X ray image of a replacement hip joint is shown in Figure above. For a brief animation of osteoarthritis, see http://www.youtube.com/watch?v=0dUSmaev5b0.
Lesson Summary
The human skeleton is well adapted for the functions it must perform. Functions of bones include support, protection, movement, mineral storage, and formation of blood cells.
The adult human skeleton usually consists of 206 named bones and these bones can be grouped in two divisions: axial skeleton and appendicular skeleton.
There are two types of bone tissue: compact and spongy. Compact bone consists of closely packed osteons, or Haversian systems. Spongy bone consists of plates of bone, called trabeculae, around irregular spaces that contain red bone marrow.
Osteogenesis is the process of bone formation. Three types of cells, osteoblasts, osteocytes, and osteoclasts, are involved in bone formation and remodeling.
In intramembranous ossification, connective tissue membranes are replaced by bone. This process occurs in the flat bones of the skull. In endochondral ossification, bone tissue replaces hyaline cartilage models. Most bones are formed in this manner.
Bones grow in length at the epiphyseal plate between the diaphysis and the epiphysis. When the epiphyseal plate completely ossifies, bones no longer increase in length.
Bones may be classified as long, short, flat, or irregular. The diaphysis of a long bone is the central shaft. There is an epiphysis at each end of the diaphysis.
There are three types of joints in terms of the amount of movement they allow: immovable, partly movable, and synovial joints (which are freely movable).
Review Questions
Identify an example of a cell, a tissue, and an organ of the skeletal system.
Identify the main bones of the axial skeleton.
Identify the main bones of the appendicular skeleton.
List four functions of bones and the skeleton.
What is endochondrial ossification, and when does it occur?
Name the three main types of joints, and identify a location in the body that is an example of that type of joint.
Outline how a bone fracture is repaired.
What is the purpose of Haversian canals?
Leukemia is a type of cancer that affects bone. It is a disease in which there is an overproduction of immature white blood cells. Identify the area of bone that is affected by leukemia.
&nb
sp; Further Reading / Supplemental Links
Anatomy and Physiology © 2002 Elaine Marieb. Published by Pearson Education Inc. as Benjamin Cummings.
Biology 6th Edn. © 2002 Campbell and Reece. Published by Pearson Education Inc. as Benjamin Cummings.
http://training.seer.cancer.gov/module_anatomy/unit3_5_skeleton_divisions
http://training.seer.cancer.gov/module_anatomy/unit3_1_bone_functions
http://yucky.discovery.com/noflash/body/pg000124
http://www.estrellamountain.edu/faculty/farabee/biobk/BioBookMUSSKEL
http://en.wikipedia.org
Vocabulary
appendicular skeleton
The portion of the human skeleton that includes the bones of the limbs, scapula and the pelvis.
axial skeleton
The portion of the human skeleton that includes the bones of the head, vertebral column, ribs and sternum.
bone marrow
A soft, connective tissue found in the interior bones. Red bone marrow produces red blood cells and white blood cells are produced by yellow bone marrow.
bone matrix
A mixture of calcium salts, such as calcium phosphate and calcium hydroxide, and collagen fibers (a type of protein), which form hollow tubes that look similar to the rings on a tree.
cartilage
Dense connective tissue that is made of tough protein fibers. The function of cartilage in the adult skeleton is to provide smooth surfaces for the movement of bones at a joint.
compact bone
A type of tissue that makes up the dense outer layer of bones.
endochondrial ossification
The process of replacing cartilage with bony tissue, occurs during the gestation period and for years after birth.
endoskeleton
The sturdy internal framework of bones and cartilage that is found inside vertebrates.
epiphyseal plate
Also known as the growth plate, the area of cartilage at the end of long bones, responsible for elongation of the bone.
fracture
A break in a bone.
haversian canal
Located in the center of each osteon, serves as a passageway for blood vessels and nerves.
intramembranous ossification
The process of bone tissue developing from a fibrous membrane, usually occurs in flat bones, such as the clavicle.
joint
A point at which two or more bones make contact; also called an articulation.
ligament
A band of tough, fibrous tissue that connects a bone to another bone.
osteoarthritis
A condition in which wearing and breakdown of the cartilage that covers the ends of the bones leads to pain and stiffness in the joint.
osteoblast
A type of bone cell that secretes the organic content of bone matrix, and is responsible for the growth of new bone.
osteoclast
A type of bone cell that removes calcium salts from bone matrix.
osteocyte
A type of bone cell that is responsible for monitoring the protein and mineral content of the bone, directing the release of calcium into the blood, and directing the uptake up of calcium salts into the bone.
osteons
Cylinder-shaped units that act like strong pillars within compact bone to give strength, allow the bone to bear the weight of the attached muscles, and withstand the stresses of movement.
osteoporosis
A disease in which the breakdown of bone matrix by osteoclasts is greater than the building of bone matrix by osteoblasts.
periosteum
The tough, shiny, white membrane that covers all surfaces of bones except at the joint surfaces.
rickets
A common disease among children in developing countries; symptoms include soft bones that are prone to fractures.
spongy bone
A type of tissue that is less dense than compact bone, and is found toward the center of the bone.
synovial fluid
A thick fluid that reduces friction between the articular cartilage and other tissues in synovial (moveable) joints and lubricates and cushions them during movement.
Points to Consider
Consider how what you eat today can influence your chance of developing osteoporosis later in life.
Forensic pathologists can estimate the age of a deceased person even if only their skeleton remains. Consider how this is possible.
Lesson 21.2: Muscular System
Lesson Objectives
Outline the major role of the muscular system.
Relate muscle fibers, fascicles, and muscles to the muscular system.
Explain how muscle fibers contract.
Examine the role of ATP and calcium in muscle contraction.
Outline how muscles move bones.
Explain how muscles respond to aerobic and anaerobic exercise.
Introduction
The muscular system is the biological system of humans that allows them to move. The muscular system, in vertebrates, is controlled through the nervous system. Much of your muscle movement occurs without your conscious control and is necessary for your survival. The contraction of your heart and peristalsis, the intestinal movements that pushes food through your digestive system, are examples of involuntary muscle movements. Involuntary muscle movement is controlled by the autonomic nervous system. Voluntary muscle contraction is used to move the body and can be finely controlled, such as the pincer-type movement of the fingers that is needed to pick up chess pieces, or the gross movements of legs arm, and the torso that are needed in skating, shown in Figure below. Voluntary muscle movement is controlled by the somatic nervous system.
Figure 21.19
You need muscles to play chess. Playing chess requires fine motor movement, but not a lot of gross muscle movements. Skating on the other hand, requires a lot of gross muscle movement of the limbs and the entire body.
Muscle Tissues
Each muscle in the body is composed of specialized structures called muscle fibers. Muscle fibers are long, thin cells that have a special talent that other cells do not have—they are able to contract. Muscles, where attached to bones or internal organs and blood vessels, are responsible for movement. Nearly all movement in the body is the result of muscle contraction. Exceptions to this are the action of cilia, the flagellum on sperm cells, and the amoeboid movement of some white blood cells.
Three types of muscle tissue are in the body: skeletal, smooth, and cardiac.
Skeletal muscle is usually attached to the skeleton. Skeletal muscles are used to move the body. They generally contract voluntarily (controlled by the somatic nervous system), although they can also contract involuntarily through reflexes.
Smooth muscle is found within the walls of organs and structures such as the esophagus, stomach, intestines, bronchi, uterus, urethra, bladder, and blood vessels. Unlike skeletal muscle, smooth muscle is involuntary muscle which means it not under your conscious control.
Cardiac muscle is also an involuntary muscle but is a specialized kind of muscle found only within the heart.
Cardiac and skeletal muscles are striated, in that they contain highly-regular arrangements of bundles of protein fibers that give them a “striped” appearance. Smooth muscle does not have such bundles of fibers, and is non-striated. While skeletal muscles are arranged in regular, parallel bundles, cardiac muscle fibers connect at branching, irregular angles. Skeletal muscle contracts and relaxes in short, intense bursts, whereas cardiac muscle contracts constantly for 70 to 80 years (an average life span), or even longer.
Figure 21.20
Frontal view of the major skeletal muscles. You would not see smooth and cardiac muscles included in diagrams of the muscular system because such diagrams usually show only the muscles that move the body (skeletal muscles).
Skeletal Muscle
Skeletal muscle, which is attached to bone, is responsible for body movements and bod
y posture. There are approximately 639 skeletal muscles in the human body, some of which are shown in Figure above. These muscles are under conscious, or voluntary, control. The basic units of skeletal muscle are muscle cells that have many nuclei. These muscle cells also contain light and dark stripes called striations, which are shown in Figure below. The striations are a result of the orientation of the contractile proteins inside the cells. Skeletal muscle is therefore called striated muscle. Each muscle cell acts independently of its neighboring muscle cells. On average, adult males are made up of 40 to 50 percent skeletal muscle tissue and an adult female is made up of 30 to 40 percent skeletal muscle tissue.
Figure 21.21
Micrograph of skeletal muscle. The stripy appearance of skeletal muscle tissue is due to long protein filaments that run the length of the fibers.
Smooth Muscle
Smooth muscle is found in the walls of the hollow internal organs such as blood vessels, the intestinal tract, urinary bladder, and uterus. It is under control of the autonomic nervous system. This means that smooth muscle cannot be controlled consciously, so it is also called involuntarily muscle. Smooth muscle cells do not have striations, and so smooth muscle is also called non-striated muscle. Smooth muscle cells are spindle-shaped and have one central nucleus. The cells are generally arranged in sheets or bundles, rather than the regular grouping that skeletal muscle cells form, and they are connected by gap junctions. Gap junctions are little pores or gaps in the cell membrane that link adjoining cells and they allowing quick passage of chemical messages between cells. Smooth muscle is very different from skeletal muscle and cardiac muscle in terms of structure and function, as shown in Figure below. Smooth muscle contracts slowly and rhythmically.
CK-12 Biology I - Honors Page 102
