Movement is the temporary or permanent displacement of a body or its parts from its original position. Humans possess a unique physical structure that enables them to stand up against the pull of gravity. Humans and animals utilize contact forces to create movement and motion. The biggest part of the human body is the trunk; comprising on the average 43% of total body weight. Head and neck account for 7% and upper limbs 13% of the human body by weight. The thighs, lower legs, and feet constitute the remaining 37% of the total body weight. The frame of the human body is a tree of bones that are linked together by ligaments in joints called articulations. There are 206 bones in the human body.Muscles are connected to the bones through cable-like structures called tendons or to other muscles by flat connective tissue sheets called aponeuroses. About 40% of the body weight is composed of muscles.
Notation for Human Movement
Spatial positions of various parts of the human body can be described referring to a Cartesian coordinate system that originates at the center of gravity of the human body in the standing configuration (Fig. 1.). The directions of the coordinate axis indicate the three primary planes of a standing person.
The transverse plane is made up of the x1 and x3 axes. It passes through the hip bone and lies at a right angle to the long axis of the body, dividing it into superior and inferior sections. Any imaginary sectioning of the human body that is parallel to the (x1, x3) plane is called a transverse section or cross section. The frontal plane is the plane that passes through the x1 and x2 axes of the coordinate system (see Fig. 1.). It is also called the coronal plane.
The frontal plane divides the body into anterior and posterior sections. The sagittal plane is the plane made by the x2 and x3 axes. The sagittal plane divides the body into left and right sections. It is the only plane of symmetry in the human body.
FIGURE 1. The three primary planes of a standing person.
The sagittal plane is the only plane of symmetry. This plane divides the body into left- and right-hand sides. The frontal plane separates the body into anterior and posterior portions. The transverse (horizontal) plane divides the body into two parts: superior and inferior
Frontal plane divides the body into anterior and posterior sections. The sagittal plane is the plane made by the x2 and x3 axes. The sagittal plane divides the body into left and right sections. It is the only plane of symmetry in the human body.
Anatomists have also introduced standard terminology to classify movement configurations of the various parts of the human body (Fig. 2). Most movement modes require rotation of a body part around an axis that passes through the center of a joint, and such movements are called angular movements. The common angular movements of this type are flexion, extension, adduction, and abduction. Flexion and extension are movements that occur parallel to the sagittal plane. Flexion is rotational motion that brings two adjoining long bones closer to each other, such as occurs in the flexion of the leg or the forearm. Extension denotes rotation in the opposite direction of flexion; for example, bending the head toward the chest is flexion and so is the motion of bending down to touch the foot. In that case, the spine is said to be flexed. Extension reverses these movements. Flexion at the shoulder and the hip is defined as the movement of the limbs forward whereas extension means movement of the arms or legs backward. Flexion of the wrist moves the palm forward, and extension moves it back. If the movement of extension continues past the anatomical position, it is called hyperextension.
FIGURE 2. Anatomical notations used in describing the movements of various body parts.
In figure 2 above,
(a) Represents Rotation
(b) Represents Flexion
(c) Represents Extension
Abduction and adduction are the movements of the limbs in the frontal plane. Abduction is movement away from the longitudinal axis of the body whereas adduction is moving the limb back. Swinging the arm to the side is an example of abduction. During a pull-up exercise, an athlete pulls the arm toward the trunk of the body, and this movement constitutes adduction. Spreading the toes and fingers apart abducts them. The act of bringing them together constitutes adduction.
Another example of angular motion is the movement of the arm in a loop, and this movement is called circumduction. The rotation of a body part with respect to the long axis of the body or the body part is called rotation. The rotation of the head could be to the left or right. Similarly, the forearm and the hand can be rotated to a degree around the longitudinal axis of these body parts.
There are other types of specialized movements such as the gliding motion of the head with respect to the shoulders or the twisting motion of the foot that turns the sole inward.
SKELETAL SYSTEM (TREE)
The skeletal system is composed of bones, cartilages, and ligaments tightly joined to form a strong, flexible framework for the body. Cartilage, the embryonic forerunner of most bones, covers many joint surfaces in the mature skeleton. Ligaments hold bones together at the joints and Tendons are structurally similar to ligaments but attach muscles to bones.
The human skeleton is divided into two parts: the axial and the appendicular (Fig 3). The axial skeleton shapes the longitudinal axis of the human body. It is composed of 22 bones of the skull, 7 bones associated with the skull, 26 bones of the vertebral column, and 24 ribs and 1 sternum comprising the thoracic cage. It is acted on by approximately 420 different skeletal muscles. The axial skeleton transmits the weight of the head and the trunk and the upper limbs to the lower limbs at the hip joint. The muscles of the axial skeleton position the head and the spinal column, and move the rib cage so as to make breathing possible. They are also responsible for the minute and complex movements of facial features. The vertebral column begins at the support of the skull with a vertebra called the atlas and ends with an insert into the hip bone. The average length of the vertebral column among adults is 71 cm. The vertebral column protects the spinal cord. In addition, it provides a firm support for the trunk, head, and upper limbs. From a mechanical viewpoint, it is a flexible rod charged with maintaining the upright position of the body . The vertebral column fulfills this role with the help of a large number of ligaments and muscles attached to it. A typical vertebra is made of the vertebral body (found anteriorly) and the vertebral arch (positioned posteriorly). The vertebral body is in the form of a flat cylinder. It is the weight-bearing part of the vertebra. Between the vertebral bodies are 23 intervertebral disks that are made of relatively deformable fibrous cartilage. These disks make up approximately one-quarter of the total length of the vertebral column. They allow motion between the vertebrae. The shock absorbance characteristics of the vertebral disks are essential for physical activity. The compressive force acting on the spine of a weight lifter or a male figure skater during landing of triple jumps peak at many times the body weight. Without shock absorbants, the spine would suffer irreparable damage.
FIGURE 3. Frontal view of the human skeleton.
The skeleton is composed of 206 bones. It is divided into two parts: the axial skeleton and appendicular skeleton. The numbers in parentheses indicate the number of bones of a certain type (or in a certain subgroup). The names of the major bones of the skeleton are identified in the figure.
Skeletal system functions
Support. Bones provide support for the soft organs of the body against gravity. Large bones of the lower limbs hold up the body when standing
Movement . Bones provide points of attachment for muscles that move the body. Bones and muscles work together as different types of mechanical levers to produce motion. Different types of joints determine the type and range of movement.
Protection of delicate organs . The skeleton protects some internal organs through surrounding them and cover them. For example, the skull protects the brain; ribs protect the heart and lungs; the spine protects the spinal cord.
Figure 4. Bones Protect Brain The cranium completely surrounds and protects the brain from non-traumatic injury.
Electrolyte balance. The skeleton is the body’s main reservoirs for fats and other minerals for example fat is stored in the internal cavities of bones also the bones which form the skeleton preserve important minerals like calcium and phosphate. In times of dietary shortage, as an emergency measure, these stored minerals are released from bones into the bloodstream, to be distributed to where they are needed. Other minerals contained in bones include magnesium and potassium.
Formation of blood cells . Most blood cell formation (hematopoiesis) takes place in the red marrow of bones, in the skull, ribs, sternum, clavicles, vertebrae, and pelvis. Fat is stored in the yellow bone marrow.
Acid-base balance . Bone buffers the blood against excessive pH changes by absorbing or releasing alkaline salts such as calcium phosphate.
Detoxification. Bone tissue removes heavy metals and other foreign elements from the blood and thus reduces their toxic effects on other tissues. It can later release these contaminants more slowly for excretion. The tendency of bone to absorb foreign elements can, however, have terrible consequences
BONE, CARTILAGE, AND LIGAMENTS
Bones are the parts of the human body that are most resistant to deformation. Unless they are broken or fractured, bones do not undergo significant shape changes during short periods. As such, they can be considered as rigid bodies in the analysis of movement and motion. In a rigid body neither the distance between any two points nor the angle between any three point’s changes during motion. The bones that compose the adult skeleton 206 and are divided into six categories based on their shapes
Bone is not a homogeneous material; that is, its physical properties vary with location. In a long bone, compact bone tissue (relatively dense and solid) forms the walls of the cylindrical shaft . The compact bone constitutes the surface layer of other bones. An internal layer of spongy bone, an open network of struts and plates, surrounds the marrow cavity. Spongy bone is also present at the expanded areas (heads) of long bones. Both compact and cancellous (spongy) bone have the same matrix composition but they differ in weight density and three-dimensional microstructure. In general, spongy bone is found where bones are not heavily stressed or where stresses arrive in many different directions. On the other hand, compact bone is thickest where the bone is stressed extensively in a certain direction.
Classification of human bones.
Using shape as a criteria, bones of the human body have been classified into six categories. Long bones are found in the upper arm and forearm, thigh and lower leg, palms, soles, fingers, and toes. They play a crucial role in movement, functioning as lever systems. Short bones such as those found in wrists and ankles are boxlike in appearance. Flat bones form the roof of the skull, sternum, the ribs, and the scapula. They protect the underlying soft tissues from the forces of impact. They also offer an extensive surface area for the attachment of skeletal muscles. Irregular bones such as the vertebrae of the spinal column have complex shapes with short, flat, and irregular surfaces. Sutural bones are small, flat, and oddly shaped bones of the skull in the suture line. Finally, sesamoid bones such as the patellae are usually small, round, and flat. They develop inside tendons.
Bone is a living tissue. Cells constitute approximately 2% of the mass of a typical bone. Among the bone cells, osteoblasts excrete collagen and control the deposition of inorganic material on them. They are responsible for the production of new bone. Osteoclasts, on the other hand, secrete acids that dissolve the bony matrix and release the stored minerals of calcium and phosphate. During this activity, osteoclasts are tightly sealed to the bone surface. They dissolve bone mineral by active secretion of hydrogen ions. Bone degradation products are then transported within vesicles across the cell and emptied out to the extracellular space. This process, called resorption, is fundamental to the regulation of calcium and phosphate concentration in body fluids. In the human body, regardless of age, osteoblasts are adding to the bone matrix at the same time osteoclasts are removing from it. The balance between the activities of these two cell types is important: if too much salt is removed, bones become weaker. When osteoblast activity predominates, bones become stronger and more massive.
Human bones classification.
||Cylinder-like shape, longer than it is wide||Leverage||Femur, tibia, fibula, metatarsals, humerus, ulna, radius, metacarpals, phalanges|
|Short||Spongy bone covered by a thin all-over layer of compact bone, which is thicker in some parts to resist greater stresses||Cube-like shape, approximately equal in length, width, and thickness||Provide stability, support, while allowing for some motion||Carpals, tarsals|
||Thin and curved||Points of attachment for muscles; protectors of internal organs||Sternum, ribs, scapulae, cranial bones|
||Complex shape||Protect internal organs||Vertebrae, facial bones|
|Sesamoid||They are small, round, and flat.||Small and round; embedded in tendons||Protect tendons from compressive forces.|
|Accessory||They look like extra bones or broken on X-ray.||. They usually occur in the developing bone and do not fuse completely.||Movement.||Sutural(wormian) bones|
Gross anatomy of a typical long bone.
The structure of a long bone allows for the best visualization of all of the parts of a bone (Figure 6). A long bone has two parts: the Diaphysis and the Epiphysis. The diaphysis is the tubular shaft that runs between the proximal and distal ends of the bone. The hollow region in the diaphysis is called the medullary cavity, which is filled with yellow marrow. The walls of the diaphysis are composed of dense and hard compact bone.The folling is an example of long bone which founds in the leg known as Tibia .
Figure 6. Anatomy of a Long Bone.
In adults it have:
Diaphis, the tubular shaft, hallow cylindrical with walls of compact bone tissue. The center of the cylinder is the medullary cavity, which is filled with marrow.
Epiphysis is roughly spherical end of the bone. It is wider than the shaft. Flat and irregular bones of the trunk and limbs have many epiphysis and the long bones of the finger and toe have only one epiphysis.
Metaphysis is the part separating diaphysis from epiphysis. It is made up of epiphyseal plate and adjacent bony trabeculae of cancellous bone tissue.
Epiphyseal plate is a thick plate of hyaline cartilage, which provides the framework of synthesis of the cancellous bone tissue within metaphysis.
The medullary cavity running through the length of the diaphysis contains Yellow marrow.
The porous lattice work of the spongy epiphyses is filled with red bone marrow.
The red marrow also known as myeloid tissue
Endosteum is the lining the medullary cavity of compact bone tissue and covering the trabeculae of spongy bone tissue.
Periosteum : it is covering the outer surface of the bone. It is absent at joints and replaced by articular cartilage.
The surface features of bones vary considerably, depending on the function and location in the body. There are three general classes of bone markings which are, articulations, projections, and holes.
Articulation is where two bone surfaces come together. These surfaces tend to conform to one another, such as one being rounded and the other cupped, to facilitate the function of the articulation.
A projection is an area of a bone that projects above the surface of the bone. These are the attachment points for tendons and ligaments. In general, their size and shape is an indication of the forces exerted through the attachment to the bone.
A hole is an opening or groove in the bone that allows blood vessels and nerves to enter the bone. As with the other markings, their size and shape reflect the size of the vessels and nerves that penetrate the bone at these points.
General Features of Bones.
Bones have an outer shell of dense white osseous tissue called compact (dense) bone, usually enclosing a more loosely organized form of osseous tissue called spongy bone. The skeleton is about three-quarters compact bone and one-quarter spongy bone by weight. Compact and spongy bone are described later in more detail.
The principal features of a long bone are its shaft, called theDiaphysis, and an expanded head at each end, called the Epiphys. The diaphysis consists largely of a cylinder of compact bone enclosing a space called the medullary cavity. The epiphysis is filled with spongy bone. Bone marrow occupies the medullary cavity and the spaces amid the spongy bone of the epiphysis. The diaphysis of a long bone provides leverage, while the epiphysis is enlarged to strengthen the joint and provide added surface area for the attachment of tendons and ligaments.
In children and adolescents, an epiphyseal plate of hyaline cartilage separates the marrow spaces of the epiphysis and diaphysis (as shown in figure below). On X rays, it appears as a transparent.
Figure 7.Features of long bornes
The epiphyseal plate is a zone where the bones grow in length. In adults, the epiphyseal plate is depleted and the bones can grow no longer, but an epiphyseal line on the bone surface marks the former location of the plate.
Externally, most of the bone is covered with a sheath called the periosteum.This has a tough, outer fibrous layer of collagen and an inner osteogenic layer of bone-forming cells. Some collagen fibers of the outer layer are continuous with the tendons that bind muscle to bone, and some penetrate into the bone matrix as perforating fibers.
The periosteum thus provides strong attachment and continuity from muscle to tendon to bone. The osteogenic layer is important to the growth of bone and healing of fractures. Blood vessels of the periosteum penetrate into the bone through minute holes called nutrient foramina. The internal surface of a bone is lined with endosteum, a thin layer of reticular connective tissue and cells that deposit and dissolve osseous tissue.
At most joints, the ends of the adjoining bones have no periosteum but rather a thin layer of hyaline cartilage, the articular10 cartilage. Together with a lubricating fluid secreted between the bones, this cartilage enables a joint to move far more easily than it would if one bone rubbed directly against the other. Flat bones have a sandwichlike construction, with two layers of compact bone enclosing a middle layer of spongy bone (fig. 6). In the skull, the spongy layer is called the diploe.A moderate blow to the skull can fracture the outer layer of compact bone, but the diploe can sometimes absorb the impact and leave the inner layer of compact bone unharmed.
Figure 8. Two flat bones of the cranium, joined at a suture.
Bone (Osseous) Tissue
Bone tissue is composed of cells embedded in a matrix of ground substances and fibers. It is more rigid than other tissues because it contains inorganic salts mainly calcium phosphate & calcium carbonate. A network of collagenous fibers in the matrix gives bone tissue its strength and flexibility. Most bones have an outer sheet of compact bone tissue enclosing an interior spongy bone tissue.
Compact Bone: Compact bone is the denser, stronger of the two types of bone tissue.It can be found under the periosteum and in the diaphyses of long bones, where it provides support and protection.
Compact bone tissue contains cylinders of calcified bone known asosteons . Osteons are made up of concentric layers called lamellae, which are arranged seemingly in wider and wider drinking straws. In the center of the osteons are central canals which are longitudinal canals that contains blood vessels, nerves and lymphatic vessels. Central canals, usually have branches called perforating canals that run at right angle to central canal extending the system of nerves and vessels out ward to periosteum and to endosteum. Lacunae (Little spaces) that houses osteocytes (bone cells) are contained in lamella. Radiating from each lacuna are tiny canaliculi containing the slender extensions of the osteocytes where nutrients and wastes can pass to and from central canal.
Spongy Bone :They are also known as cancellous bone, contains osteocytes housed in lacunae, but they are not arranged in concentric circles. Instead, the lacunae and osteocytes are found in a lattice-like network of matrix spikes called trabeculae Spongy Bone consists of a lattice of thin plates (trabeculae) and rods and spines called spicules. Although calcified and hard, spongy bone is named for its spongelike appearance; it is permeated by spaces filled with bone marrow. The matrix is arranged in lamellae like those of compact bone, but there are few osteons. Central canals are not needed here because no osteocyte is very far from the blood supply in the marrow. Spongy bone is well designed to impart strength to a bone with a minimum of weight. Its trabeculae are not randomly arranged as they might seem at a glance, but develop along the bone’s lines of stress. In this frontal section of the femur (thighbone), the trabeculae of spongy bone can be seen oriented along lines of mechanical stress applied by the weight of the body.
Figure 9.Spongy Bone Structure in Relation to Mechanical Stress.
Bone Marrow: Bone marrow is a general term for soft tissue that occupies the medullary cavity of a long bone, the spaces amid the trabeculae of spongy bone, and the larger central canals. In a child, the medullary cavity of nearly every bone is filled with red bone marrow (myeloid tissue). This is a hemopoietic tissue that is, it produces blood cells. Red bone marrow looks like blood but with a thicker consistency. It consists of a delicate mesh of reticular tissue saturated with immature blood cells and scattered adipocytes.
Bone contain five types of cells.
a. Osteogenic (osteoprogenitor) cells: these are small spindle shaped cell. They found mostly in the deepest layer of periosteum and endosteum. They have high mitotic potential and can be transformed into bone forming cells (osteoblasts).
b. Osteoblasts are found in the growing portion of bone including periosteum. They are able to synthesize secrete un-mineralized ground substance, act as pump cell to move calcium and phosphate in and out of bone tissue.
c. Osteocytes are the main cell of fully developed bones. They have a cell body that occupies a lacuna. Osteocytes are derived from osteoblasts. They together with osteoclasts play an important role of homeostasis by helping to release calcium.
d. Osteoclasts are multinuclear giant cell, which are found where bone is resorbed during its normal growth. Osteoclasts are derived from white blood cells called monocytes.
e. Bone – lining cells are found on the surface of most bones in the adult skeleton. They are believed to be derived from osteoblast that ceases their physiological activity.
Bone Formation and Development
Bones develop through a process known as Ossification.In the early stages of embryonic development, the embryo’s skeleton consists of fibrous membranes and hyaline cartilage. By the sixth or seventh week of embryonic life, the actual process of bone development, ossification (osteogenesis), begins. There are two osteogenic pathways which are intramembranous ossificationand endochondral ossification but bone is the same regardless of the pathway that produces it.
Endochondral ossification is a process in which a bone develops from hyaline cartilage. Most bones form by this method, including the vertebrae, pelvic bones, and limb bones. In endochondral ossification, embryonic mesenchyme condenses into a hyaline cartilage model that resembles the shape of the bone to come. The cartilage is then broken down, reorganized, and calcified to form a bone (fig. 8).
FIGURE 10. Stages of Endochondral Ossification.
(a) Chondrocyte hypertrophy at the center of the cartilage model and formation of a supportive bony collar. (b) Invasion of the model by blood vessels and creation of a primary marrow space. (c) Typical state of a long bone at the time of birth, with blood vessels growing into the secondary marrow space and well-defined metaphyses at each end of the primary marrow space. (d) Appearance of a long bone in childhood. By adulthood, the epiphyseal plates will be depleted and the primary and secondary marrow spaces will be united.
The primary Ossification center.
In the cartilage model, the first sign of endochondral ossification is the multiplication and swelling of chondrocytes near the center, forming a primary ossification center. As the lacunae enlarge, the matrix between them is reduced to thin walls and the model becomes weak at this point. It soon gets reinforcement, however. Some cells of the perichondrium become osteoblasts, which produce a bony collar around the model. This collar acts like a splint to provide temporary support for the model, and it cuts off the diffusion of nutrients to the chondrocytes, hastening their death. Once the collar has formed, the fibrous sheath around it is considered periosteum rather than perichondrium.
Buds of connective tissue grow from this periosteum into the cartilage and penetrate the thin walls between the enlarged lacunae. They break down the lacunae and transform the primary ossification center into a cavity called the primary marrow space. Osteogenic cells invade the cartilage model by way of the connective tissue buds, transform into osteoblasts, and line the marrow space. The osteoblasts deposit an organic matrix called osteoid tissue soft collagenous tissue similar to bone except for a lack of minerals and then calcify it to form a temporary framework of bony trabeculae. As ossification progresses, osteoclasts break down these trabeculae and enlarge the primary marrow space. The ends of the bone are still composed of hyaline cartilage at this stage.
At the boundary between the marrow space and each cartilaginous head of a developing long bone, there is a transitional zone called the metaphysis it exhibits five histological zones of transformation from cartilage to bone (fig. 9):
FIGURE 11. Zones of the Metaphysis
Zone of reserve cartilage. In this zone, farthest from the marrow space, the resting cartilage as yet shows no sign of transforming into bone.
a) Zone of cell proliferation. A little closer to the marrow space, chondrocytes multiply and become arranged into longitudinal columns of flattened lacunae.
b) Zone of cell hypertrophy. Next, the chondrocytes cease to divide and begin to hypertrophy, just as they did in the primary ossification center. The cartilage walls between lacunae become very thin. Cell multiplication in zone 2 and hypertrophy in zone 3 continually push the zone of reserve cartilage toward the ends of the bone and make the bone grow longer.
c) Zone of calcification. Minerals are deposited in the matrix between columns of lacunae and calcify the cartilage for temporary support.
d) Zone of bone deposition. Within each column, the walls between lacunae break down and the chondrocytes die. This converts each column into a longitudinal channel, which is quickly invaded by marrow and blood vessels from the primary marrow space. Osteoclasts dissolve the calcified cartilage while osteoblasts line up along the walls of these channels and begin depositing concentric layers of bone matrix. The channel therefore grows smaller and smaller as one layer after another is laid down, until only a narrow channel remains in the middle now a central canal. Osteoblasts trapped in their own matrix become osteocytes and stop producing matrix.
The Secondary Ossification Center
Around the time of birth, secondary ossification centers begin to form in the epiphyses. Here, too, chondrocytes enlarge, the walls of matrix between them dissolve, and the chondrocytes die. Vascular buds arise from the perichondrium and grow into the cartilage, bringing osteogenic cells and osteoclasts with them. The cartilage is eroded from the center of the epiphysis outward in all directions. Thin trabeculae of cartilage matrix calcify to form spongy bone. Hyaline cartilage persists in two places-on the epiphyseal surfaces as the articular cartilages and at the junction of the diaphysis and epiphysis, where it forms the epiphyseal plate.Each side of the epiphyseal plate has a metaphysis, where the transformation of cartilage to bone occurs.
Intramembranous ossification produces the flat bones of the skull and most of the clavicle (collarbone). It begins when some of the embryonic connective tissue (mesenchyme) condenses into a sheet of soft tissue with a dense supply of blood capillaries. The cells of this sheet enlarge and differentiate into osteogenic cells, and some of the mesenchyme transforms into a network of soft trabeculae. Osteogenic cells gather on the trabeculae, become osteoblasts, and deposit osteoid tissue.
Functions of bones.
- Supportive and protection of internal organs.
- The store house and main supply of reserve calcium and phosphate.
- he manufacture of red and white blood cell.
Bone degradation products are then transported within vesicles across the cell and emptied out to the extracellular space. This process, called resorption, is fundamental to the regulation of calcium and phosphate concentration in body fluids. In the human body, regardless of age, osteoblasts are adding to the bone matrix at the same time osteoclasts are removing from it. The balance between the activities of these two cell types is important: if too much salt is removed, bones become weaker. When osteoblast activity predominates, bones become stronger and more massive.
Bone Growth and Remodeling.
The bone growth and remodeling appear to be tightly regulated in the human body by hormones and steroids. Electrical fields are known to stimulate bone repair and stimulate the self-repair of bone fractures. Heavily stressed bones become thicker and stronger, whereas bones not subjected to ordinary stresses become thin and brittle.
Bones continue to grow and remodel themselves throughout life, changing size and shape to accommodate the changing forces applied to the skeleton. The bone growth and remodeling appear to be tightly regulated in the human body by hormones and steroids. Electrical fields are known to stimulate bone repair and stimulate the self-repair of bone fractures. Heavily stressed bones become thicker and stronger, whereas bones not subjected to ordinary stresses become thin and brittle. Regular exercise serves as a stimulus that maintains normal bone structure. Growth plates are the sites of bone growth during childhood and early adulthood. They are positioned at the spongy ends of the long bones. Osteoblasts proliferate on the surface of the growth plate and make new bone.
The long bones of the average infant lengthen by 50% during the first year after birth. The bone growth rate drops to about 7% per year by age 3. The bone growth stops around 30 years of age, and between 35 and 40 the osteoblast activity begins to decline gradually while osteoclast activity continues at previous levels. Nevertheless, among all the mature tissues and organs of adult body, only one has the ability to remake itself and that is bone. When broken, bone reconstructs itself by triggering biological processes reminiscent of those that occur in the embryo. The repair begins when a class of stem cells travel to the damaged site and undertake specific tasks such as producing a calcified scaffolding around the break. Thus, a break or a fracture uncovers the remaking characteristics of bone tissue in adulthood. Regular exercise serves as a stimulus that maintains normal bone structure
Factors affecting bone growth and maintenance.
Heredity. Each person has a genetic potential for height, that is, a maximum height, with genes inherited from both parents. Many genes are involved, and their interactions are not well understood. Some of these genes are probably those for the enzymes involved in cartilage and bone production, for this is how bones grow.
Nutrition. Nutrients are the raw materials of which bones are made. Calcium, phosphorus, and protein become part of the bone matrix itself. Vitamin D is needed for the efficient absorption of calcium and phosphorus by the small intestine. Vitamins A and C do not become part of bone but are necessary for the process of bone matrix formation (ossification). Without these and other nutrients, bones cannot grow properly. Children who are malnourished grow very slowly and may not reach their genetic potential for height.
Hormones .endocrine glands produce hormones that stimulate specific effects in certain cells.
Several hormones make important contributions to bone growth and maintenance. These include growth hormone, thyroxine, parathyroid hormone, and insulin, which help regulate cell division, protein synthesis, calcium metabolism, and energy production. The sex hormones estrogen or testosterone help bring about the cessation of bone growth.
Exercise or “stress” for bones, exercise means bearing weight, which is just what bones are specialized to do. Without this stress (which is normal), bones will lose calcium faster than it is replaced. Exercise need not be strenuous; it can be as simple as the walking involved in everyday activities. Bones that do not get this exercise, such as those of patients confined to bed, will become thinner and more fragile.
Achondroplastic dwarfism is a condition in which the long bones of the limbs stop growing in childhood, while the growth of other bones is unaffected. As a result, a person has a short stature but a normal-sized head and trunk (fig.10). As its name implies, achondroplastic dwarfism results from a failure of cartilage growth specifically, failure of the chondrocytes in zones 2 and 3 of the metaphysis to multiply and enlarge. This is different from pituitary dwarfism, in which a deficiency of growth hormone stunts the growth of all of the bones and a person has short stature but normal proportions throughout the skeletal system. Achondroplastic dwarfism results from a spontaneous mutation that can arise any time DNA is replicated. Two people of normal height with no family history of dwarfism can therefore have a child with achondroplastic dwarfism. The mutant allele is dominant, so the children of a heterozygous achondroplastic dwarf have at least a 50% chance of exhibiting dwarfism, depending on the genotype of the other parent.
Nutritional and Hormonal Factors
The balance between bone deposition and resorption is influenced by nearly two dozen nutrients, hormones, and growth factors. The most important factors that promote bone deposition are as follows.
1. Calcium and phosphate are needed as raw materials for the calcified ground substance of bone.
2. Vitamin A promotes synthesis of the glycosaminoglycans (GAGs) of the bone matrix.
3. Vitamin C (ascorbic acid) promotes the cross-linking of collagen molecules in bone and other connective tissues.
4. Vitamin D (calcitriol) is necessary for calcium absorption by the small intestine, and it reduces the urinary loss of calcium and phosphate. Vitamin D is synthesized by one’s own body. The process begins when the ultraviolet radiation in sunlight acts on a cholesterol derivative (7-dehydrocholesterol) in the keratinocytes of the epidermis. The product produced here is picked up by the blood stream, and the liver and kidneys complete its conversion to vitamin D.
5. Calcitonin, a hormone secreted by the thyroid gland, stimulates osteoblast activity. It functions chiefly in children and pregnant women; it seems to be of little significance in nonpregnant adults.
6. Growth hormone promotes intestinal absorption of calcium, the proliferation of cartilage at the epiphyseal plates, and the elongation of bones.
7. Sex steroids (estrogen and testosterone) stimulate osteoblasts and promote the growth of long bones, especially in adolescence. Bone deposition is also promoted by thyroid hormone, insulin, and local growth factors produced within the bone itself. Bone resorption is stimulated mainly by one hormone:
8. Parathyroid hormone (PTH) is produced by four small parathyroid glands, which adhere to the back of the thyroid
Figure 12.The image of normal person and Dwarf person.
The Aging Skeletal System The predominant effect of aging on the skeleton is a loss of bone mass and strength. After age 30, osteoblasts become less active than osteoclasts. The imbalance between deposition and resorption leads to osteopenia, the loss of bone; when the loss is severe enough to compromise physical activity and health, it is called osteoporosis (discussed in the next section). After age 40, women lose about 8% of their bone mass per decade and men lose about 3%. Bone loss from the jaws is a contributing factor in tooth loss. Not only does bone density decline with age, but the bones become more brittle as the osteoblasts synthesize less protein. Fractures occur more easily and heal more slowly. Arthritis, a family of joint disorders associated with aging
STRUCTURAL DISORDERS OF BONE
A fracture is a broken bone. It will heal whether or not a physician resets it in its anatomical position. If the bone is not reset correctly, the healing process will keep the bone in its deformed position.
There are multiple ways of classifying bone fractures. A stress fracture is a break caused by abnormal trauma to a bone, such as fractures incurred in falls, athletics, and military combat. A pathologic fracture is a break in a bone weakened by some other disease, such as bone cancer or osteoporosis, usually caused by a stress that would not normally fracture a bone. Fractures are also classified according to the direction of the fracture line, whether or not the skin is broken, and whether a bone is merely cracked or is broken into separate pieces.
(a) closed fracture, (b) open fracture, (c) transverse fracture, (d) spiral fracture, (e) comminuted fracture, (f) impacted fracture, (g) greenstick fracture, and (h) oblique fracture.
The Repair Process
Even a simple fracture involves significant bone damage that must be repaired if the bone is to resume its normal function. Fragments of dead or damaged bone must first be removed. This is accomplished by osteoclasts, which dissolve and reabsorb the calcium salts of bone matrix. Imagine a building that has just collapsed; the rubble must be removed before reconstruction can take place. This is what the osteoclasts do. Then, new bone must be produced. The inner layer of the periosteum contains osteoblasts that are activated when bone is damaged. The osteoblasts produce bone matrix to knit the broken ends of the bone together. Because most bone has a good blood supply, the repair process is usually relatively rapid, and a simple fracture often heals within 6 weeks. Some parts of bones, however, have a poor blood supply, and repair of fractures takes longer. These areas are the neck of the femur (the site of a “fractured hip”) and the lower third of the tibia. Other factors that influence repair include the age of the person, general state of health, and nutrition. The elderly and those in poor health often have slow healing of fractures. A diet with sufficient calcium, phosphorus, vitamin D, and protein is also important. If any of these nutrients is lacking, bone repair will be a slower process.
Osteoporos is literally, “porous bone “is a disease in which the bones lose mass and become increasingly brittle and subject to fractures. It involves loss of proportionate amounts of organic matrix and minerals, and it affects spongy bone in particular, since this is the most metabolically active type .The bone that remains is histologically normal but insufficient in quantity to support the body’s weight.
The most serious consequence of osteoporosis is pathologic fractures, which occur especially in the hip, wrist, and vertebral column and under stresses as slight as sitting down too quickly. Among the elderly, hip fractures often lead to fatal complications such as pneumonia. For half of those who survive, a hip fracture involves a long, costly recovery.
Is the branch of medicine that deals with the prevention and correction of injuries and disorders of the bones, joints, and muscles. As the word suggests, this field originated as the treatment of skeletal deformities in children, but it is now much more extensive. It includes the design of artificial joints and limbs and the treatment of athletic injuries.
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