Skeletal System 1

Skeletal System 1

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Skeletal System 1...

Skeletal System 1:

Imagine for a moment that people did not have skeletons. What comes to mind? Probably that each of us would be a heap on the floor, much like a jellyfish out of water. This image is accurate and therefore reflects the most obvious function of the skeleton; to support the body.

FUNCTIONS OF THE SKELETAL SYSTEM

¨ Provides a framework that supports the body; the muscles that are attached to bones move the skeleton.

¨ Protects some internal organs from mechanical injury; the rib cage protects the heart and lungs.

¨ Contains and protects the red bone marrow, one of the hemopoietic (blood-forming) tissues.

¨ Provides a storage site for excess calcium. Calcium may be removed from bone to maintain a normal blood calcium level, which is essential for blood clotting and proper functioning of muscles and nerves.

TYPES OF BONE TISSUE

Bone cells are called osteocytes, and the matrix of the bone is made of calcium salts and collagen. The calcium salts give bones the strength for its supportive and protective functions. The function of osteocytes is to regulate the amount of calcium that is deposited in or removed from the bone matrix.

Bone is an organ, it has its own blood supply and is made up of two types of tissue; compact and spongy bone.

Compact bone is made of haversian systems or cylinders of bone matrix with osteocytes in concentric rings around central haversian canals. Each haversian system consists of mature osteocytes arranged in concentric circles around large blood vessels. The area surrounding the osteocytes is filled with protein fibers, calcium, and other minerals. Each haversian system looks like a long cylinder. Compact bone consists of many haversian systems running parallel to each other. The network of blood vessels ensures that the bone tissue receives an adequate supply of blood. Blood supplies tissues with oxygen and necessary nutrients.

Spongy or cancellous bone has a much different structure from compact bone. Unlike compact bone, spongy bone does not contain haversian systems. In spongy bone, the bone tissue is arranged in plates called trabeculae. These bony plates are separated by irregular spaces, or holes, and give spongy bone a punched out “Swiss cheese” appearance. The spaces in the bone are important for two reasons: they decrease the weight of the bone, making it lighter, and they contain red bone marrow. The red bone marrow richly supplies the spongy bone with blood and also produces cells for use throughout the body.

ANATOMY OF A LONG BONE

The arrangement of compact and spongy tissue in long bone accounts for its strength. Long bones contain sites of growth and reshaping and structures associated with joints. The parts of a long bone include the following:

¨ Diaphysis- The diaphysis is the long shaft of the bone. It is composed primarily of compact bone and therefore provides considerable strength.

¨ Epiphysis- The enlarged ends of the long bone are the epiphyses. The epiphyses of a bone articulates, or meets, with a second bone at a joint. Each epiphysis consists of a thin layer of compact bone overlying spongy bone. The epiphyses are covered by cartilage.

¨ Epiphyseal disc or plate- A growing bone contains a band of cartilage located at the ends of long bones, between the epiphyisis and the diaphysis. This band of cartilage is the epiphyseal plate. It is here that longitudinal bone growth occurs.

¨ Medullary cavity- The medullary cavity is the hollow center of the diaphysis. In infancy, the cavity is filled with red bone marrow for blood cell production. In the adult, the medullar cavity is filled with yellow bone marrow and functions as a storage site for fat; at this stage it is not associated with blood cell production. The inside of the medullary cavity is lined with connective tissue called the endosteum.

¨ Periosteum- The periosteum is a tough fibrous connective tissue membrane that covers the outside of the diaphysis. It is anchored firmly to the outside of the bone on all surfaces except the articular cartiglage. The periosteum protects the bone, serves as a point of attachment for muscle, and contains blood vessels that nourish the underlying bone. Because the periosteum carries the blood supply to the underlying bone, any injury to this structure has serious consequences to the health of the bone. Like any other organ the loss of blood supply can cause its death.

¨ Articular cartilage- The articular cartilage is found on the outer surface of the epiphysis. It forms a smooth, shiny surface that decreases friction within a joint. Because a joint is also called an articulation, this cartilage is called articular cartilage.

OSSIFICATION

How does bone form? The 3-month-old fetus has an early skeleton-like framework composed of cartilage and connective tissue membrane. As the fetus matures, the cartilage and connective tissue change into bone. the formation of bones is called ossification. Ossification occurs in different ways in flat and long bones.

Ossification of flat bones

In the fetus, the flat bones consist of a thin connective tissue membrane. Ossification begins when osteoblasts or bone forming cells, migrate to the region of the flat bones. The osteoblasts secrete calcium and other minerals into the spaces between the thin connective tissue membranes thereby forming bone. This type of ossification involves replacement of thin membrane with bone.

Ossification of long bones

Ossification of long bones occurs as bone tissue replaces cartilage. The fetal skeleton is comprised largely of cartilage, and the layout of the cartilage in the fetus provides a model for bone formation. As the baby matures, osteoblasts invade the cartilage and gradually replace the cartilage with bone. This process continues in each long bone until all but the articular cartilage and the epiphyseal plate have been replaced by bone. By the time the fetus has fully matured, most cartilage of the body has been replaced by bone. Only isolated pieces of cartilage such as the bridge of the nose and the parts of the ribs, remain.

GROWING BONES

Maturation from infancy to adulthood is characterized by two types of bone growth. Bones grow longitudinally and determine the height of an individual. Bones also grow thicker and become wider so as to support the weight of the adult body.

Growing taller

Longitudinal bone growth occurs at the epiphyseal plate or growth plate. Longitudinal bone growth ceases when the growth plate becomes ossified or hardened. This plate or disc is sensitive to the effects of certain hormones, especially growth hormone and sex hormones. GH stimulates growth at the plate, making the child taller. The sex hormones estrogen and testosterone, however, cause the plate to seal or fuse, thereby inhibiting further longitudinal growth.

Why are females generally shorter than males?

The growth plates or epiphyseal plates are generally more sensitive to the effects of estrogen than to those of testosterone. During puberty in the female, the rising levels of estrogen seal the epiphyseal plate earlier than testosterone does in males. The effects of the male hormone, testosterone, are felt at a later stage. Thus, females stop growing earlier than males do.

Because the epiphyseal disc or plate plays such a crucial role in longitudinal bone growth, injury to the plate can severely retard bone growth. A child who injures the plate in a tibia, for instance, may end up with that leg considerably shorter than the non-injured leg.

BUMPS AND GROOVES

The surface of bone appears irregular and bumpy. This appearance is due to numerous ridges, projections, depressions, and grooves called bone markings. The projecting bone marking serves as points of attachment for muscles, tendon, and ligaments. The grooves and depressions form the routes traveled by blood vessels and nerves as they pass over and through the bones and joints. The projections and depressions also help to form joints. The head of the upper arm bone, for instance, fits into a depression in a shoulder bone, forming the shoulder joint. Please refer to handout.

TYPES OF BONES

Bones come in many shapes and sizes, from the pea sized bones in the wrist to the 24 inch long femur in the thigh. The size and shape of the bone reflects its function.

¨ Long bones. Long bones are longer than they are wide. They are found in the arms and legs.

¨ Short bones. Short bones are shaped like cubes and are found primarily in the wrist and ankles.

¨ Flat bones. Flat bones are thin, flat, and curved. They form the ribs, breastbone, and skull.

¨ Irregular bones. Irregular bones are different shaped and are not classified as long, short, or flat. They include the hip bones, vertebrae, and various bones in the skull.

¨ Sesamoid bones. Sesamoid bones are small round bony masses embedded in certain tendons that may be subjected to compression and tension. The largest sesamoid bone is the patella, which is embedded in the tendon of the quadriceps femoris at the knee.

Short, flat, and irregular bones are all made of spongy bone covered with a thin layer of compact bone. Red bone marrow is found within the spongy bone.

The joint surfaces of bones are covered with articular cartilage, which provides a smooth surface. Covering the rest of the bone is the periosteum.

DIVISIONS OF THE SKELETAL SYSTEM

The skeleton is divided into the axial skeleton and the appendicular skeleton. The axial skeleton includes the bones of the skull, the hyoid bone, the bones of the middle ear, the vertebral column, and the bony thorax.

The appendicular skeleton includes the bones of the extremities and the bones of the hip and shoulder girdles.

AXIAL SKELETON

Skull

The skull is formed by two groups of bones; the cranium and the facial bones. These bones also contain air spaces called sinuses.

The cranium is a bony structure that encases and protects the brain. It is composed of eight bones:

1. The frontal bone forms the forehead, part of the nose, and part of the bony structure surrounding the eyes.

2. The two parietal bones form most of the top of the head and part of the sides of the head.

3. The two temporal bones are on the sides of the head, close to the ears. Several important bone marking are found on the temporal bones. They include

¨ external auditory meatus- the opening of the ear

¨ zygomatic process- which forms part of the cheekbone

¨ styloid process- a sharp projection used as a point of attachment for several muscles associated with the tongue and larynx

4. The occipital bone is located at the base of the skull. The large hole in the occipital bone is called the foramen magnum. On either side of the foramen magnum.

5. The sphenoid bone is a butterfly shaped boned that forms part of the floor and sides of the cranium. The sphenoid bone also forms part of the orbits surrounding the eyes. In the midline of the sphenoid bone is a depression called the sella turcica; it forms the seat for the pituitary gland.

6. The ethmoid bone is an irregular shaped bone that helps form the bony structure of the nasal cavity.

FACIAL BONES

There are 14 facial bones, most ofwhich are paired. Only the mandible and the vomer are single bones.

1. The mandible, the lower jaw bone, forms the only freely movable joint in the skull. The anterior portion of the mandible forms the chin. Two posterior upright projections on the mandible have bony processes. These articulate with the temporal bones at the temporomandibular joint. These processes are points of attachment for chewing muscles. The lower teeth are located in this bone. Tension or stress often causes pain in this joint. Teeth grinding or bruxism during sleep may also cause TMJ syndrome.

2. Two maxillary bones fuse to form the upper jaw. The maxilla carries the upper teeth. An extension of the maxilla, the palatine process, forms the anterior portion of the hard palate or the roof of the mouth. These bones also form part of the nasal cavity and the eye orbits.

3. Two palatine bones form the posterior part of the hard palate and the floor of the nasal cavity. Failure of the palatine bones to fuse causes a cleft palate, making suckling very difficult for an infant. Fortunately, a cleft palate can be surgically repaired.

4. The zygomatic bones are the cheekbones. They also form a part of the orbits of the eyes.

5. There are several other facial bones that complete the facial structure. These bones include the lacrimal bones, the nasal bones, the vomer bone and the inferior nasal conchae.

SINUSES

Sinuses, air filled cavities located in several of the bones of the skull, perform two important functions. First, they lessen the weight of the skull because they are empty cavities rather than solid bone. Second, the amplify an increase the sound of the voice. There are four sinuses. They are called the paranasal sinuses because they surround and connect with the nasal structures. The names of the four sinuses reflect their location within the various skull bones: frontal sinus, ethmoidal sinus, sphenoidal sinus, and maxillary sinus. The mastoid sinuses are air cavities in the mastoid process of each temporal bone. They open into the middle ear. Before the availability of antibiotics, middle ear infections often caused mastoiditis, infection of these sinuses.

The lining of the sinuses also have ciliated epithelium or cells that have tiny finger like projections on them so that they can sweep away debris and mucus that accumulates in the nasal cavities.

Because the sinuses connect with the nasal passages and the throat, infections may spread from the nose and throat into the sinuses. Such infections are called sinusitis.

HOW THE SKULL BONES HOLD TOGETHER

The bones of the adult skull form a unique kind of joint called a suture. The suture joins the bones of the skull much like a zipper. The major sutures include the coronal suture (meaning crown), the lamboidal suture, and the squamosal suture. Unlike other bones in the body, no significant movement occurs between cranial bones.

The infant skull

The two major differences between the infant skull and the adult skull are fontanels and unfused sutures. The infant skull has areas that have not yet been converted to bone. Instead, they are covered by fibrous membrane. Because these are areas are soft to touch, they are called the baby’s soft spot. Also, the rhythm of the baby’s pulse can be felt in these soft spots and so they are called the fontanels, meaning little fountains.

The two major fontanels are larger diamond shaped anterior fontanel and a smaller posterior triangular occipital fontanel. By the time the child reaches 2 years of age, these fontanels have been gradually converted to bone and can no longer be felt.

The fontanels are one reason that the infant skull bones are more movable than those of the adult skull. Another reason is that the sutures of the infant skulls are not fused. Unfused sutures allow the skull to be compressed during birth. They also allow for the continued growth of the brain and skull after birth and throughout infancy.

Occasionally, the sutures of the infant skull fuse too early, preventing the growth of the brain. This condition is called microcephalia and is characterized by a small skull and impaired intellectual functioning. Sometimes, the skull expands too much. For instance, if excessive fluid accumulates within the brain of an infant, the bones are forced apart, and the skull enlarges. This condition is called hydrocephalus or what the laypersons call “water on the brain”.

What does a bulging or sunken fontanel indicate?

If an infant suffers a severe head injury, the brain may swell. Because the fontanel is soft tissue, it will bulge outward in response to increasing pressure within the skull. Observation of the fontanels can therefore provide valuable information regarding the degree of brain swelling. Conversely, the fontanels may flatten out. If the infant is dehydrated, blood volume is low. In response to dehydration, the fontanels appear sunken. Thus the fontanels can indicate increases or decreases in pressure within the skull.

Hyoid bone

The hyoid bone is a U-shaped bone located in the neck. It anchors the tongue and is associated with swallowing. It is the only bone in the body that does not articulate with another bone.

Bones in the middle ear

Each ear contains three small bones; the tiny ear bones are called ossicles. (incus, stapes, malleus…as Michelle pointed out. Nice job)

VERTEBRAL COLUMN

The vertebral column or the backbone, extends from the skull to the pelvis. The vertebral column consists of a series of bones called vertebrae. The vertebrae are stacked in a column hence the term vertebral column. Sitting between each vertebra is a cartilaginous disc that acts as a shock absorber. The column performs four major function; it forms a supporting structure for the head and thorax; it forms an attachment for the pelvic girdle, it encases and protects the spinal cord as it extends from the brain into the lumbar area; and it provides flexibility for the body.

The vertebrae are named according to their location in the body. For instance, the seven cervical vertebrae (C1-C7) are located in the neck region; the 12 thoracic vertebrae (T1-T12) are located in the chest region; and the five lumbar vertebrae (L1-L5) are located in the lower back region. In addition, five sacral vertebrae fuse into one sacrum. The sacrum forms the posterior wall of the pelvis. Four to five small coccygeal vertebrae fuse into one coccyx, the tailbone. The adult has 26 vertebrae.

CURVATURES

When viewed from the side, the vertebral column has four normal curvatures. The cervical curve, the thoracic curve, the lumbar curve, and the sacral curve. The directions of the curvatures are important. The cervical and the lumbar curves are convex so that they bend forward. The thoracic and sacral curves are concave so that they bend backward, away from the front of the body. These curves center the head over the body thereby providing the balance needed to walk in an upright position.

The fetal spine is different its single curvature is concave and turned away from the front of the body. The cervical curvature develops about 3-4 months after birth as infants start to hold their heads up. The lumbar curve develops at about one year of age when the child begins to walk.

Scoliosis refers to a lateral curvature, usually involving the thoracic vertebrae. If severe, a lateral curvature can compress abdominal organs. It can also diminish.

MALE vs. FEMALE

A man’s skeleton and a woman’s differ:

1. a man’s skeleton is larger

2. the pelvic inlet and the pelvic outlet are usually much wider in a female