NEET MDS Lessons
Physiology
The thyroid gland is a double-lobed structure located in the neck. Embedded in its rear surface are the four parathyroid glands.
The Thyroid Gland
The thyroid gland synthesizes and secretes:
- thyroxine (T4) and
- calcitonin
T4 and T3
Thyroxine (T4 ) is a derivative of the amino acid tyrosine with four atoms of iodine. In the liver, one atom of iodine is removed from T4 converting it into triiodothyronine (T3). T3 is the active hormone. It has many effects on the body. Among the most prominent of these are:
- an increase in metabolic rate
- an increase in the rate and strength of the heart beat.
The thyroid cells responsible for the synthesis of T4 take up circulating iodine from the blood. This action, as well as the synthesis of the hormones, is stimulated by the binding of TSH to transmembrane receptors at the cell surface.
Diseases of the thyroid
1. hypothyroid diseases; caused by inadequate production of T3
- cretinism: hypothyroidism in infancy and childhood leads to stunted growth and intelligence. Can be corrected by giving thyroxine if started early enough.
- myxedema: hypothyroidism in adults leads to lowered metabolic rate and vigor. Corrected by giving thyroxine.
- goiter: enlargement of the thyroid gland. Can be caused by:
- inadequate iodine in the diet with resulting low levels of T4 and T3;
- an autoimmune attack against components of the thyroid gland (called Hashimoto's thyroiditis).
2. hyperthyroid diseases; caused by excessive secretion of thyroid hormones
Graves´ disease. Autoantibodies against the TSH receptor bind to the receptor mimicking the effect of TSH binding. Result: excessive production of thyroid hormones. Graves´ disease is an example of an autoimmune disease.
Osteoporosis. High levels of thyroid hormones suppress the production of TSH through the negative-feedback mechanism mentioned above. The resulting low level of TSH causes an increase in the numbers of bone-reabsorbing osteoclasts resulting in osteoporosis.
Calcitonin
Calcitonin is a polypeptide of 32 amino acids. The thyroid cells in which it is synthesized have receptors that bind calcium ions (Ca2+) circulating in the blood. These cells monitor the level of circulating Ca2+. A rise in its level stimulates the cells to release calcitonin.
- bone cells respond by removing Ca2+ from the blood and storing it in the bone
- kidney cells respond by increasing the excretion of Ca2+
Both types of cells have surface receptors for calcitonin.
Because it promotes the transfer of Ca2+ to bones, calcitonin has been examined as a possible treatment for osteoporosis
Functions of the nervous system:
1) Integration of body processes
2) Control of voluntary effectors (skeletal muscles), and mediation of voluntary reflexes.
3) Control of involuntary effectors ( smooth muscle, cardiac muscle, glands) and mediation of autonomic reflexes (heart rate, blood pressure, glandular secretion, etc.)
4) Response to stimuli
5) Responsible for conscious thought and perception, emotions, personality, the mind.
The cell membrane is called the sarcolemma. This membrane is structured to receive and conduct stimuli. The sarcoplasm of the cell is filled with contractile myofibrils and this results in the nuclei and other organelles being relegated to the edge of the cell.
Myofibrils are contractile units within the cell which consist of a regular array of protein myofilaments. Each myofilament runs longitudinally with respect to the muscle fiber. There are two types: the thick bands and the thin bands. Thick bands are made of multiple molecules of a protein called myosin. The thin bands are made of multiple molecules of a protein called actin. The thin actin bands are attached to a Z-line or Z-disk of an elastic protein called titin. The titin protein also extends into the myofibril anchoring the other bands in position. From each Z-line to the next is a unit called the
The sarcomere is the smallest contractile unit in the myofibril. Sarcomeres contract because the Z-lines move closer together. As the sarcomeres contract the myofibrils contract. As the myofibrils contract the muscle cell contracts. And as the cells contract the entire muscle contracts.
The arrangement of the thick myosin filaments across the myofibrils and the cell causes them to refract light and produce a dark band known as the A Band. In between the A bands is a light area where there are no thick myofilaments, only thin actin filaments. These are called the I Bands. The dark bands are the striations seen with the light microscope.
Oxygen Transport
In adult humans the hemoglobin (Hb) molecule
- consists of four polypeptides:
- two alpha (α) chains of 141 amino acids and
- two beta (β) chains of 146 amino acids
- Each of these is attached the prosthetic group heme.
- There is one atom of iron at the center of each heme.
- One molecule of oxygen can bind to each heme.
The reaction is reversible.
- Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
- Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.
Proteins:
- about 50 - 60% of the dry mass of a typical cell
- subunit is the amino acid & amino acids are linked by peptide bonds
- 2 functional categories = structural (proteins part of the structure of a cell like those in the cell membrane) & enzymes
Enzymes are catalysts. Enzymes bind temporarily to one or more of the reactants of the reaction they catalyze. In doing so, they lower the amount of activation energy needed and thus speed up the reaction
Physiology - science that describes how organisms FUNCTION and survive in continually changing environments
Functions
Manufacture - blood proteins - albumen, clotting proteins , urea - nitrogenous waste from amino acid metabolism , bile - excretory for the bile pigments, emulsification of fats by bile salts
Storage - glycogen , iron - as hemosiderin and ferritin , fat soluble vitamins A, D, E, K
Detoxification -alcohol , drugs and medicines , environmental toxins
Protein metabolism -
- transamination - removing the amine from one amino acid and using it to produce a different amino acid. The body can produce all but the essential amino acids; these must be included in the diet.
- deamination - removal of the amine group in order to catabolize the remaining keto acid. The amine group enters the blood as urea which is excreted through the kidneys.
Glycemic Regulation - the management of blood glucose.
- glycogenesis - the conversion of glucose into glycogen.
- glycogenolysis - the breakdown of glycogen into glucose.
gluconeogenesis - the manufacture of glucose from non carbohydrate sources, mostly protein