Bone-building continues throughout life, as a body constantly renews and reshapes the bones' living tissue. Bone contains three types of cells:. Muscles pull on the joints, allowing us to move. They also help the body do such things as chewing food and then moving it through the digestive system.
Even when we sit perfectly still, muscles throughout the body are constantly moving. Muscles help the heart beat, the chest rise and fall during breathing, and blood vessels regulate the pressure and flow of blood. When we smile and talk, muscles help us communicate, and when we exercise, they help us stay physically fit and healthy. The movements that 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 ser-uh-BEL-um. When you decide to move, the motor cortex sends an electrical signal through the spinal cord and peripheral nerves to the muscles, making them 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 are where two bones meet. They make the skeleton flexible — without them, movement would be impossible. Joints allow our bodies to move in many ways. The Body Online. An Overview of the Skeletal System. Appendicular Skeleton. Joints and Ligaments.
Skeletal System Pathologies. When you select "Subscribe" you will start receiving our email newsletter. Use the links at the bottom of any email to manage the type of emails you receive or to unsubscribe. See our privacy policy for additional details. Learn Site. So, what are the different types of bones? How are they categorized? There are five types of bones in the skeleton: flat, long, short, irregular, and sesamoid.
Flat Bones Protect Internal Organs There are flat bones in the skull occipital, parietal, frontal, nasal, lacrimal, and vomer , the thoracic cage sternum and ribs , and the pelvis ilium, ischium, and pubis. Long Bones Support Weight and Facilitate Movement The long bones , longer than they are wide, include the femur the longest bone in the body as well as relatively small bones in the fingers. Short Bones Are Cube-shaped Short bones are about as long as they are wide.
The ends of long bones are supplied by the metaphyseal and epiphyseal arteries, which arise from the arteries from the associated joint Bartl and Bartl, If the blood supply to bone is disrupted, it can result in the death of bone tissue osteonecrosis.
A common example is following a fracture to the femoral neck, which disrupts the blood supply to the femoral head and causes the bone tissue to become necrotic. The femoral head structure then collapses, causing pain and dysfunction. Bones begin to form in utero in the first eight weeks following fertilisation Moini, The embryonic skeleton is first formed of mesenchyme connective tissue structures; this primitive skeleton is referred to as the skeletal template.
These structures are then developed into bone, either through intramembranous ossification or endochondral ossification replacing cartilage with bone. Bones are classified according to their shape Box 1. Flat bones develop from membrane membrane models and sesamoid bones from tendon tendon models Waugh and Grant, The term intra-membranous ossification describes the direct conversion of mesenchyme structures to bone, in which the fibrous tissues become ossified as the mesenchymal stem cells differentiate into osteoblasts.
The osteoblasts then start to lay down bone matrix, which becomes ossified to form new bone. Long bones — typically longer than they are wide such as humerus, radius, tibia, femur , they comprise a diaphysis shaft and epiphyses at the distal and proximal ends, joining at the metaphysis.
In growing bone, this is the site where growth occurs and is known as the epiphyseal growth plate. Most long bones are located in the appendicular skeleton and function as levers to produce movement.
Short bones — small and roughly cube-shaped, these contain mainly cancellous bone, with a thin outer layer of cortical bone such as the bones in the hands and tarsal bones in the feet. Flat bones — thin and usually slightly curved, typically containing a thin layer of cancellous bone surrounded by cortical bone examples include the skull, ribs and scapula. Most are located in the axial skeleton and offer protection to underlying structures.
Irregular bones — bones that do not fit in other categories because they have a range of different characteristics. They are formed of cancellous bone, with an outer layer of cortical bone for example, the vertebrae and the pelvis.
Sesamoid bones — round or oval bones such as the patella , which develop in tendons. Long, short and irregular bones develop from an initial model of hyaline cartilage cartilage models. Once the cartilage model has been formed, the osteoblasts gradually replace the cartilage with bone matrix through endochondral ossification Robson and Syndercombe Court, Mineralisation starts at the centre of the cartilage structure, which is known as the primary ossification centre.
Secondary ossification centres also form at the epiphyses epiphyseal growth plates Danning, The epiphyseal growth plate is composed of hyaline cartilage and has four regions Fig 3 :. Resting or quiescent zone — situated closest to the epiphysis, this is composed of small scattered chondrocytes with a low proliferation rate and anchors the growth plate to the epiphysis;.
Growth or proliferation zone — this area has larger chondrocytes, arranged like stacks of coins, which divide and are responsible for the longitudinal growth of the bone;. Hypertrophic zone — this consists of large maturing chondrocytes, which migrate towards the metaphysis. There is no new growth at this layer;. Calcification zone — this final zone of the growth plate is only a few cells thick.
Bones are not fully developed at birth, and continue to form until skeletal maturity is reached. In rare cases, a genetic mutation can disrupt cartilage development, and therefore the development of bone. This can result in reduced growth and short stature and is known as achondroplasia. The human growth hormone somatotropin is the main stimulus for growth at the epiphyseal growth plates. During puberty, levels of sex hormones oestrogen and testosterone increase, which stops cell division within the growth plate.
As the chondrocytes in the proliferation zone stop dividing, the growth plate thins and eventually calcifies, and longitudinal bone growth stops Ralston and McInnes, Males are on average taller than females because male puberty tends to occur later, so male bones have more time to grow Waugh and Grant, Over-secretion of human growth hormone during childhood can produce gigantism, whereby the person is taller and heavier than usually expected, while over-secretion in adults results in a condition called acromegaly.
If there is a fracture in the epiphyseal growth plate while bones are still growing, this can subsequently inhibit bone growth, resulting in reduced bone formation and the bone being shorter.
It may also cause misalignment of the joint surfaces and cause a predisposition to developing secondary arthritis later in life. A discrepancy in leg length can lead to pelvic obliquity, with subsequent scoliosis caused by trying to compensate for the difference. Once bone has formed and matured, it undergoes constant remodelling by osteoclasts and osteoblasts, whereby old bone tissue is replaced by new bone tissue Fig 4.
Bone remodelling has several functions, including mobilisation of calcium and other minerals from the skeletal tissue to maintain serum homoeostasis, replacing old tissue and repairing damaged bone, as well as helping the body adapt to different forces, loads and stress applied to the skeleton. Calcium plays a significant role in the body and is required for muscle contraction, nerve conduction, cell division and blood coagulation.
Serum calcium levels are tightly regulated by two hormones, which work antagonistically to maintain homoeostasis. Calcitonin facilitates the deposition of calcium to bone, lowering the serum levels, whereas the parathyroid hormone stimulates the release of calcium from bone, raising the serum calcium levels.
Osteoclasts are large multinucleated cells typically found at sites where there is active bone growth, repair or remodelling, such as around the periosteum, within the endosteum and in the removal of calluses formed during fracture healing Waugh and Grant, The osteoclast cell membrane has numerous folds that face the surface of the bone and osteoclasts break down bone tissue by secreting lysosomal enzymes and acids into the space between the ruffled membrane Robson and Syndercombe Court, These enzymes dissolve the minerals and some of the bone matrix.
The minerals are released from the bone matrix into the extracellular space and the rest of the matrix is phagocytosed and metabolised in the cytoplasm of the osteoclasts Bartl and Bartl,
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