The Human Body

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TYPES OF NEURON

Posted by Prakash
Wednesday, August 31, 2011
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Neurons may be classified into three groups: sensory neurons, motor neurons, and interneurons.

Sensory neurons (or afferent neurons) carry impulses from receptors to the central nervous system. Receptors detect external or internal changes and send the information to the CNS in the form of impulses by way of the afferent neurons. The central nervous system interprets these impulses as a sensation. Sensory neurons from receptors in skin, skeletal muscles, and joints are called somatic; those from receptors in internal organs are called visceral sensory neurons.

Motor neurons (or efferent neurons) carry impulses from the central nervous system to effectors. The two types of effectors are muscles and glands. In response to impulses, muscles contract or relax and glands secrete. Motor neurons linked to skeletal muscle are called somatic; those to smooth muscle, cardiac muscle, and glands are called visceral. Sensory and motor neurons make up the peripheral nervous system. Visceral motor neurons form the autonomic nervous system, a specialized subdivision of the PNS that will be discussed later in this chapter.

Interneurons are found entirely within the central nervous system. They are arranged so as to carry only sensory or motor impulses, or to integrate these functions. Some interneurons in the brain are concerned with thinking, learning, and memory. A neuron carries impulses in only one direction. This is the result of the neuron’s structure and location, as well as its physical arrangement with other neurons and the resulting pattern of synapses. The functioning nervous system, therefore, is an enormous network of “one-way streets,” and there is no danger of impulses running into and canceling one another out.

FACTORS AFFECTING BONE GROWTH

Posted by Prakash
Tuesday, August 30, 2011
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1. 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.

2. 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.

3. 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. The hormones and their specific functions are listed in Table.

4. 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.

EMBRYONIC GROWTH OF BONES

Posted by Prakash
Saturday, August 27, 2011
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During embryonic development, the skeleton is first made of cartilage and fibrous connective tissue, which are gradually replaced by bone. Bone matrix is produced by cells called osteoblasts (a blast cell is a “growing” or “producing” cell, and osteo means “bone”). In the embryonic model of the skeleton, osteoblasts differentiate from the fibroblasts that are present. The production of bone matrix, called ossification, begins in a center of ossification in each bone.

The cranial and facial bones are first made of fibrous connective tissue. In the third month of fetal development, fibroblasts (spindle-shaped connective tissue cells) become more specialized and differentiate into osteoblasts, which produce bone matrix. From each center of ossification, bone growth radiates outward as calcium salts are deposited in the collagen of the model of the bone. This process is not complete at birth; a baby has areas of fibrous connective tissue remaining between the bones of the skull. These are called fontanels, which permit compression of the baby’s head during birth without breaking the still thin cranial bones. The fontanels also permit the growth of the brain after birth. You may have heard fontanels referred to as “soft spots,” and indeed they are. A baby’s skull is quite fragile and must be protected from trauma. By the age of 2 years, all the fontanels have become ossified, and the skull becomes a more effective protective covering for the brain.

The rest of the embryonic skeleton is first made of cartilage, and ossification begins in the third month of gestation in the long bones. Osteoblasts produce bone matrix in the center of the diaphyses of the long bones and in the center of short, flat, and irregular bones. Bone matrix gradually replaces the original cartilage.

The long bones also develop centers of ossification in their epiphyses. At birth, ossification is not yet complete and continues throughout childhood. In long bones, growth occurs in the epiphyseal discs at the junction of the diaphysis with each epiphysis. An epiphyseal disc is still cartilage, and the bone grows in length as more cartilage is produced on the epiphysis side. On the diaphysis side, osteoblasts produce bone matrix to replace the cartilage. Between the ages of 16 and 25 years (influenced by estrogen or testosterone), all of the cartilage of the epiphyseal discs is replaced by bone. This is called closure of the epiphyseal discs (or we say the discs are closed), and the bone lengthening process stops.

Also in bones are specialized cells called osteoclasts (a clast cell is a “destroying” cell), which are able to dissolve and reabsorb the minerals of bone matrix, a process called resorption. Osteoclasts are very active in embryonic long bones, and they reabsorb bone matrix in the center of the diaphysis to form the marrow canal. Blood vessels grow into the marrow canals of embryonic long bones, and red bone marrow is established. After birth, the red bone marrow is replaced by yellow bone marrow. Red bone marrow remains in the spongy bone of short, flat, and irregular bones. For other functions of osteoclasts and osteoblasts, 

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