Bones, Muscles, and JointsEvery time you walk, settle into a chair, or hug your child, you're
using your bones, muscles, and joints. Without these important body
parts, we wouldn't be able to stand, walk, run, or even sit.
Bones and What They DoFrom our head to our toes, bones provide support for our bodies and
help form our shape. The skull protects the brain and forms the shape of
our face. The spinal cord, a pathway for messages between the brain and
the body, is protected by the backbone, or spinal column.
The ribs form a cage that shelters the heart, lungs, liver, and
spleen, and the pelvis helps protect the bladder, intestines, and in
women, the reproductive organs.
Although they're very light, bones are strong enough to support our entire weight.
The human skeleton has 206 bones, which begin to develop before
birth. When the skeleton first forms, it is made of flexible cartilage,
but within a few weeks it begins the process of ossification.
Ossification is when the cartilage is replaced by hard deposits of
calcium phosphate and stretchy collagen, the two main components of
bone. It takes about 20 years for this process to be completed.
The bones of kids and young teens are smaller than those of adults
and contain "growing zones" called growth plates. These plates consist
of columns of multiplying cartilage cells that grow in length, and then
change into hard, mineralized bone. These growth plates are easy to spot
on an X-ray. Because girls mature at an earlier age than boys, their
growth plates change into hard bone at an earlier age.
Growing BonesBone-building continues throughout life, as a body constantly renews
and reshapes the bones' living tissue. Bone contains three types of
cells: osteoblasts, which make new bone and help repair damage;
osteocytes, which carry nutrients and waste products to and from blood
vessels in the bone; and osteoclasts, which break down bone and help to
sculpt and shape it.
Osteoclasts are very active in kids and teens, working on bone as it
is remodeled during growth. They also play an important role in the
repair of fractures.
Bones are made up of calcium, phosphorus, sodium, and other minerals,
as well as the protein collagen. Calcium is needed to make bones hard,
which allows them to support body weight. Bones also store calcium and
release some into the bloodstream when it's needed by other parts of the
body. The amounts of certain vitamins and minerals that you eat,
especially vitamin D and calcium, directly affects how much calcium is
stored in the bones.
The soft bone marrow inside many of the bones is where most of the
blood cells are made. The bone marrow contains stem cells, which produce
the body's red blood cells and platelets, and some types of white blood
cells. Red blood cells carry oxygen to the body's tissues, and
platelets help with blood clotting when someone has a cut or wound.
White blood cells help the body fight infection.
Bones are made up of two types of bone:
- Compact bone is the solid, hard, outside part of
the bone. It looks like ivory and is extremely strong. Holes and
channels run through it, carrying blood vessels and nerves from the
periosteum, the bone's membrane covering, to its inner parts.
- Cancellous bone, which looks like a sponge, is
inside the compact bone. It is made up of a mesh-like network of tiny
pieces of bone called trabeculae. The spaces in this network are filled
with red marrow, found mainly at the ends of bones, and yellow marrow,
which is mostly fat.
Bones are fastened to other bones by long, fibrous straps called
ligaments. Cartilage, a flexible, rubbery substance in our joints,
supports bones and protects them where they rub against each other.
Muscles and What They DoBones don't work alone — they need help from the muscles and joints.
Muscles pull on the joints, allowing us to move. They also help your
body perform other functions so you can grow and remain strong, such as
chewing food and then moving it through the digestive system.
The human body has more than 650 muscles, which make up half of a
person's body weight. They are connected to bones by tough, cord-like
tissues called tendons, which allow the muscles to pull on bones. If you
wiggle your fingers, you can see the tendons on the back of your hand
move as they do their work.
Humans have three different kinds of muscle:
- Skeletal muscle is attached to bone, mostly in the
legs, arms, abdomen, chest, neck, and face. Skeletal muscles are called
striated because they are made up of fibers that have horizontal stripes
when viewed under a microscope. These muscles hold the skeleton
together, give the body shape, and help it with everyday movements
(known as voluntary muscles because you can control their movement).
They can contract (shorten or tighten) quickly and powerfully, but they
tire easily and have to rest between workouts.
- Smooth, or involuntary, muscle is also made of
fibers, but this type of muscle looks smooth, not striated. Generally,
we can't consciously control our smooth muscles; rather, they're
controlled by the nervous system automatically (which is why they're
also called involuntary). Examples of smooth muscles are the walls of
the stomach and intestines, which help break up food and move it through
the digestive system. Smooth muscle is also found in the walls of blood
vessels, where it squeezes the stream of blood flowing through the
vessels to help maintain blood pressure. Smooth muscles take longer to
contract than skeletal muscles do, but they can stay contracted for a
long time because they don't tire easily.
- Cardiac muscle is found in the heart. The walls of
the heart's chambers are composed almost entirely of muscle fibers.
Cardiac muscle is also an involuntary type of muscle. Its rhythmic,
powerful contractions force blood out of the heart as it beats.
- Our Nonstop Muscles
Even when we sit perfectly still, muscles throughout the body are
constantly moving. Muscles enable the heart to beat, the chest to rise
and fall during breathing, and blood vessels to help regulate the
pressure and flow of blood through the body. When we smile and talk,
muscles help us communicate, and when we exercise, they help us stay
physically fit and healthy.
The movements your muscles make are coordinated and controlled by the
brain and nervous system. The involuntary muscles are controlled by
structures deep within the brain and the upper part of the spinal cord
called the brain stem. The voluntary muscles are regulated by the parts
of the brain known as the cerebral motor cortex and the cerebellum.
When you decide to move, the motor cortex sends an electrical signal
through the spinal cord and peripheral nerves to the muscles, causing
them to contract. The motor cortex on the right side of the brain
controls the muscles on the left side of the body and vice versa.
The cerebellum coordinates the muscle movements ordered by the motor
cortex. Sensors in the muscles and joints send messages back through
peripheral nerves to tell the cerebellum and other parts of the brain
where and how the arm or leg is moving and what position it's in. This
feedback results in smooth, coordinated motion. If you want to lift your
arm, your brain sends a message to the muscles in your arm and you move
it. When you run, the messages to the brain are more involved, because
many muscles have to work in rhythm.
Muscles move body parts by contracting and then relaxing. Muscles can
pull bones, but they can't push them back to the original position. So
they work in pairs of flexors and extensors. The flexor contracts to
bend a limb at a joint. Then, when the movement is completed, the flexor
relaxes and the extensor contracts to extend or straighten the limb at
the same joint. For example, the biceps muscle, in the front of the
upper arm, is a flexor, and the triceps, at the back of the upper arm,
is an extensor. When you bend at your elbow, the biceps contracts. Then
the biceps relaxes and the triceps contracts to straighten the elbow.
- Joints and What They Do
Joints occur where two bones meet. They make the skeleton flexible — without them, movement would be impossible.
Joints allow our bodies to move in many ways. Some joints open and
close like a hinge (such as knees and elbows), whereas others allow for
more complicated movement — a shoulder or hip joint, for example, allows
for backward, forward, sideways, and rotating movement.
Joints are classified by their range of movement. Immovable, or
fibrous, joints don't move. The dome of the skull, for example, is made
of bony plates, which must be immovable to protect the brain. Between
the edges of these plates are links, or joints, of fibrous tissue.
Fibrous joints also hold the teeth in the jawbone.
Partially movable, or cartilaginous, joints move a little. They are
linked by cartilage, as in the spine. Each of the vertebrae in the spine
moves in relation to the one above and below it, and together these
movements give the spine its flexibility.
Freely movable, or synovial, joints move in many directions. The main
joints of the body — found at the hip, shoulders, elbows, knees,
wrists, and ankles — are freely movable. They are filled with synovial
fluid, which acts as a lubricant to help the joints move easily.
Three kinds of freely movable joints play a big part in voluntary movement:
- Hinge joints allow movement in one direction, as seen in the knees and elbows.
- Pivot joints allow a rotating or twisting motion, like that of the head moving from side to side.
- Ball-and-socket joints allow the greatest freedom
of movement. The hips and shoulders have this type of joint, in which
the round end of a long bone fits into the hollow of another bone
Problems With the Bones, Muscles, and Joints
As strong as bones are, they can break. Muscles can weaken, and joints (as well as tendons, ligaments, and cartilage) can be damaged by injury or disease.
Problems that can affect the bones, muscles, and joints include:
* Arthritis. Arthritis is the inflammation of a joint, and people who have it experience swelling, warmth, pain, and often have trouble moving. Although we often think of arthritis as a condition that affects only older people, arthritis can also occur in children and teens. Health problems that involve arthritis in kids and teens include juvenile rheumatoid arthritis (JRA), lupus, Lyme disease, and septic arthritis (a bacterial infection of a joint).
* Fracture. A fracture occurs when a bone breaks; it may crack, snap, or shatter. After a fracture, new bone cells fill the gap and repair the break. Applying a strong plaster cast, which keeps the bone in the correct position until it heals, is the usual treatment. If the fracture is complicated, metal pins and plates can be placed to better stabilize it while the bone heals.
* Muscular dystrophy. Muscular dystrophy is an inherited group of diseases that affect the muscles, causing them to weaken and break down over time. The most common form in childhood is called Duchenne muscular dystrophy, and it most often affects boys.
* Osgood-Schlatter disease (OSD). Osgood-Schlatter disease is an inflammation (pain and swelling) of the bone, cartilage, and/or tendon at the top of the shinbone, where the tendon from the kneecap attaches. OSD usually strikes active teens around the beginning of their growth spurts, the approximately 2-year period during which they grow most rapidly.
* Osteomyelitis. Osteomyelitis is a bone infection often caused by Staphylococcus aureus bacteria, though other types of bacteria can cause it, too. In kids and teens, osteomyelitis usually affects the long bones of the arms and legs. Osteomyelitis often develops after an injury or trauma.
* Osteoporosis. In osteoporosis, bone tissue becomes brittle, thin, and spongy. Bones break easily, and the spine sometimes begins to crumble and collapse. Although the condition usually affects older people, kids and teens with eating disorders can get the condition, as can girls with female athlete triad — a combination of three conditions that some girls who exercise or play sports may be at risk for: disordered eating, amenorrhea (loss of a girl's period), and osteoporosis. Participation in sports where a thin appearance is valued can put a girl at risk for female athlete triad.
* Repetitive stress injuries (RSIs). RSIs are a group of injuries that happen when too much stress is placed on a part of the body, resulting in inflammation (pain and swelling), muscle strain, or tissue damage. This stress generally occurs from repeating the same movements over and over again. RSIs are becoming more common in kids and teens because they spend more time than ever using computers. Playing sports like tennis that involve repetitive motions can also lead to RSIs. Kids and teens who spend a lot of time playing musical instruments or video games are also at risk for RSIs.
* Scoliosis. Every person's spine curves a little bit; a certain amount of curvature is necessary for people to move and walk properly. But 3-5 people out of 1,000 have scoliosis, which causes the spine to curve too much. It can be hereditary, so someone who has scoliosis often has family members who have it.
* Strains and sprains. Strains occur when muscles or tendons are overstretched. Sprains are an overstretching or a partial tearing of the ligaments. Strains usually happen when a person takes part in a strenuous activity when the muscles haven't properly warmed up or the muscle is not used to the activity (such as a new sport or playing a familiar sport after a long break). Sprains, on the other hand, are usually the result of an injury, such as twisting an ankle or knee. A common sprain injury is a torn Achilles tendon, which connects the calf muscles to the heel. This tendon can snap, but it usually can be repaired by surgery. Both strains and sprains are common in kids and teens because they're active and still growing.
* Tendinitis. This common sports injury that usually happens after overexercising a muscle. The tendon and tendon sheath become inflamed, which can be painful. Resting the muscles and taking anti-inflammatory medication can bring relief.
Subject: Bones, Muscles, and Joints 
Wed Apr 06, 2011 1:19 pm
