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Hormones described so far are called circulating hormones, because these circulate in whole body with blood. When stimulated by physical or chemical stimuli, all body cells, except red blood corpuscles (RBCs), secrete certain such compounds which transmit coded informations of metabolic adjustments between neighbouring cells and hence remain ECF instead of diffusing into the blood. These compounds are called local tissue hormones or autocoids. These are short-lived, because various enzymes present in ECF continue degrading these at a fast rate. Local hormones are of two main categories-paracrine and autocrine. Paracrine hormones affect metabolism of cells located in the neighbourhood of those which secrete them. Autocrine hormones affect metabolism of the every cells which secrete them. Most local hormones are paracrine. These belong to the following categories :  (1) Eicosanoids : These are a category of lipids derived from a fatty acid, arachidonic acid, synthesized in the plasma membrane of cells, and released in ECF. These are of four categories, viz. Prostaglandins, prostacyclins, thromoboxanes and leukotrienes. (i) Prostaglandins (PGs) : In 1935, Ulf von Euler discovered that human semen contains a very active compound presumably secreted by prostate gland and, hence, named as such. He found that after the semen is discharged in woman’s vagina, this compound contracts uterine muscles to facilitate the sperms to ascend into fallopian tubes and reach ova to fertilize these. (ii) Prostacyclins : These are found in walls of blood vessels and induce vasodilation. These also facilitate flow of blood in vessels and prevent thrombosis by inhibiting aggregation of platelets. (iii) Thromboxanes : These are secreted by blood platelets. These help in blood clotting by instigating aggregation of platelets at the place of injury. These also instigate vasoconstriction at places of injury to prevent excessive loss of blood. (iv) Leukotrienes : These are secreted by eiosinophils of blood and mast cells of connective tissues. These serve as mediators in inflammatory and allergic reactions, induce bronchoconstriction (constriction of bronchioles), constrict arterioles and induce migration of neutrophils and eosinophils towards the places of inflammation. These can cause asthma, arthritis, colitis, etc. (2) Neuroregulators : These are a category of proteins which function as paracrine hormones in nervous tissues. These can be classified in three categories as follows : (i) Neurotransmitters : These are synthesized in nerve cells and are secreted by exocytosis by axon terminals of these cells. These serve to transmit nerve impulses from one neuron to other neighbouring neurons, or muscles, or glands across synapses. About 60 to these have so far been discovered, but the most common of these are acetylcholine, norepinephrine, dopamine, serotonin and histamine. (ii) Neuromodulators : In nervous tissues, the neurons secrete such paracrine hormones which modulate (increase or decrease) the excitability of other neighbouring neurons. These hormones are called neuromodulators. The main positive neuromodulators which increase the excitability of other neurons are the amino acids glutamate and aspartate, and polypeptide named ‘P’ substance. Contrarily, the main negative modulators which decrease the excitability of neighbouring neurons are the amino acid glycine and more...

Position and Structure : Hypothalamus is the floor of diencephalon. It is formed of masses of grey matter, called hypothalmic nuclei, containing neurosecretory cells. It is connected with anterior pituitary lobe by blood capillaries of hypophyseal portal system and with the posterior pituitary lobe by axons of its neurons, both passing through the pituitary stalk. Hormones of hypothalamus : Neurosecretory cells of hypothalamus secrete neurohormones called releasing factors (RF) or inhibiting factors (IF). These neurohormones are carried by hypophyseal portal system to adenohypophysis (primary target organ) and stimulate or inhibit the release of trophic hormones from adenohypophysis. These neurohormones are proteinous in nature and formed of \[320\] amino acids.   Neurohormones of Hypothalamus
S.No. Neurohormones Physiological effects
1. TRH (Thyroid Releasing Hormone - Releasing Factor) Increased TSH secretion from adenohypophysis.
2. ACTH-RF (Adrenocorticotrophic Hormone-Releasing Factor) Increased ACTH secretion from adenohypophysis.
3. STH-RF  
  more...
Hormones are informational molecules secreted by the endocrine cells in one part of the body and carried by blood to another part where they stimulate or inhibit specific physiological process. Discovery : First hormone discovered was secretin. It was discovered by two English physiologists : William M Bayliss and Ernest H. Starling in 1903. Term hormone was coined by starling (1905) from Greek word Homone means to excite. It is a mishomer because a number of hormones are known to have inhibitory effect (e.g., Somatostatin). General function of hormones (1) Some hormones control Basic Metabolic Rate (BMR) e.g., thyroxine of thyroid gland. (2) Some hormones control the secretion of other endocrine glands, e.g., Tropic hormones of Anterior pituitary control Thyroid, Adrenal cortex, gonads, etc.  (3) Some hormone control blood pressure e.g., Aldosterone, Atrial Natriuretic Hormone (ANH) of heart, Vasopressin or ADH’, oxytocin and Renin of kidney. (4) Increase production of RBC e.g., erythropoietin of kidney. Properties of hormones (1) These are secreted by endocrine gland (biogenic in origin). (2) Their secretions is released directly into blood (except local hormones e.g., gastrin). (3) These are carried to distantly located specific organs, called target organ. (4) These have specific physiological action (excitatory or inhibitory). These co-ordinate different physical, mental and metabolic activities and maintain homeostasis. (5) The hormones have low molecular weight e.g., ADH has a molecular weight of \[6002000\] daltons. (6) These act in very low concentration e.g., around \[{{10}^{10}}\] molar. (7) Hormones are non antigenic. (8) These are mostly short-lived. So have a no cumulative effect. (9) Some hormones are quick acting e.g., adrenalin, while some acting slowly e.g., oestrogen of ovary. (10) Some hormones secreted in inactive form called Prohormone e.g., Pro-insulin. (11) Hormones are specific. They are carriers of specific information to their specific target organ. Only those target cell respond to a particular hormone for which they have receptors. (12) Hormones after their action destroyed in liver and kidney. Classification of hormones (1) On the basis of chemical nature : On the basis of chemical composition hormones are classified into three categories. (i) Amine hormones : These are derived form tyrosine amino acid and have amino \[(-N{{H}_{2}})\] group e.g., Thyroxine, Epinephrine, Nor-epinephrine. (ii) Steroids : These are fat soluble and have sterol group. These are derived from cholesterol e.g., hormones of adrenal cortex (cortisol, cortisone, corticosterone, aldosterone) testes (testosterone) and ovaries (estrone, estradiol, progesterone etc.) (iii) Proteinous and peptide hormones : These are formed of \[3200\] amino acids interlinked by peptide bonds and are water soluble e.g., (a) Proteinous hormones like STH, TSH, FSH, LH etc. Out of these FSH and LH are glycoproteins. (b) Long peptide hormones like insulin and glucagon, ACTH, Paratharmone. (c) Short peptide hormones like oxytocin, ADH, MSH. These hormones formed of a few amino acids. (2) On the basis of mode of action (i) Quick acting hormones : These hormones initiate immediate response from their target cells. There receptor is always located on the outer surface of plasma membrane of target cell more...

The gonads are the sex glands, the testes and the ovary. Testes is the male gonad and ovary is the female gonads. Besides producing gametes, the gonads secrete sex hormones from the onset of puberty (sexual maturity) to control the reproductive organs and sexual behaviour. The sex hormone were discovered by Adolf Butenononal in 1929 and 1931. He won the 1939 Nobel prize jointly with Leopold Ruzicka. Testes Location and structure : In testes between the siminiferous tubules, special types of cells are present called interstitial cells or cells of leydig. These cells secrete male hormones (androgens) derived from cholesterol. The main androgen is testosterone other less important androgens include androstenedione and dehydroepiandrosterone (DHEA). It is a masculinizing hormone. From puberty to the age of about twenty year i.e. adolescence or the period of sexual maturation or attainment of adult hood.       Function (1) It stimulates the male reproductive system to grow to full size and become functional. (2) It stimulates the formation of sperms (spermatogenesis) in the seminiferous tubules. (3) It stimulates the development of male accessory sex characters such as hair on the face (beard and moustaches), growth and distribution of hair on the body, thickening of skin, deepening of voice, enlargement of larynx, broadening of shoulders, Narrow lips enlarged and stronger bones and muscles. It also maintains these characters. (4) It also determines the male sexual behaviour sex urge, aggressive behaviour.   (5) Under its effect protein anabolism increases. (This function is obvious in the heavier muscle and bone mass of most men as compared to women). (6) Grythropoisis in bone marrow increases. (7) In brief, testosterone determines libido or sex drive. It is also required, together with the follicle stimulating hormone (FSH) of pituitary, for initiation and completion of spermatogenesis. All androgens are also secreted in traces from adrenal glands in both boys and girls. (8) Increased sebaceous gland secretion. Development of testis : Under the effect of chorionic gonadotropic hormone, secreted by placenta during pregnancy, the testes of eight to nine months old fetus start secreting testosterone. The latter regulates differentiation and development of urinogenital system, accessory genital organs and external genitalia in the embryo. During childhood i.e. from birth to puberty (age of 11 to 13 years), testes remain quiescent, so that androgens are not secreted. At puberty, the gonadotropic hormones (FSH and ICSH) of pituitary reactivate the testes which, therefore, start producing sperms and resume secreting androgens. Upto the age of about 40 years, androgens are secreted in sufficient amounts. There after, their secretion starts gradually declining, but the capability of reproduction still continues for many years. Castration : Surgical removal of testes is called castration or orchidectomy. Castration, or deficient secretion of testosterone (hypogonadism) before puberty (due to congenital defects or injury to testes) retards growth of genitalia, muscles and bones, as well more...

(1) Gastro-intestinal mucosa : Inner most layer of the wall of the alimentary canal is called mucosa. Certain cell of the mucosa of the stomach and intestine secrete important hormones. Gastro-intestinal mucosa is endodermal in origin. (i) Stomach : The mucosa of the pyloric stomach near the duodenum secretes a hormone called gastrin. Presence of food in the stomach provides a stimulus for gastrin secretion. Gastrin stimulates the gastric glands to produce the gastric juice. It also stimulates the stomach movements. (ii) Intestine : The intestinal mucosa secretes six hormones : secretin, cholecystokinin, enterogastrone, enterocrinin, duocrinin and villikinin. Entry of acidic food from the stomach into the duodenum serves as a stimulus for the release of these hormones. (a) Secretin : It is produced by the small intestinal mucosa. It causes the release of sodium bicarbonate solution from the pancreas for pancreatic juice and from the liver for bile. It also inhibits the secretion and movements of stomach. (b) Cholecystokinin-pancreozymin (CCK-PZ) : This hormone is secreted by the mucosa of entire small intestine. The actions of cholecystokinin and pancreozymin were discovered independently. But it has been discovered that both hormones have similar effects and hence it is considered one hormone. As the name suggest \[CCK-PZ\] has two main functions. The word cholecystokinin is derived from three roots : Chol meaning bile, Cyst meaning bladder, and kinin meaning to remove. The word pancreozymin is derived from pancreas and Zymin, which means enzyme producer. This hormone stimulates the gall bladder to release the bile and also stimulates the pancreas to release its enzymes. (c) Enterogastrone : It is secreted by the duodenal mucosa. It shows gastric contractions and stops the secretion of gastric juice. (d) Enterocrinin : It is secreted by duodenal mucosa. It stimulates crypts of Lieberkuhn to secrete the enzymes in the intestinal juice. (e) Duocrinin : It is secreted by the duodenal mucosa. It stimulates the release of viscous mucus from Brunner’s glands into the intestinal juice. (f) Villikinin : It is secreted by the mucosa of the entire small intestine. It accelerates the movements of villi to quicken absorption of food. (2) Placenta : When the early embryo reaches into the uterus from fallopian tube, it becomes implanted with uterine wall by a placenta for support and nutrition. The cells of placenta secrete two steroid hormones (estradiol and progesterone) and two protein hormones (human chorionic gonadotropin-hcG and human chorionic somato mammotrophin-HCS). Early placenta secretes so much of chorionic gonadotropin that the latter starts being exerted in mother’s urine just after about two weeks of pregnancy. Its presence in urine is used for pregnancy test. It serves to maintain the corpus luteum, and to stimulate it for secretion. Due to its effect, the corpus luteum continues secreting estrogens, progesterone and relaxin. It also serves to maintain pregnancy by preventing contraction of uterine wall. After about three months of pregnancy, secretion of progesterone by the placenta increases. Hence, importance of corpus luteum decreases, and it starts degenerating. If therefore, ovaries more...

Adrenal gland was first reported by Eustachius. Origin and position : The adrenals are paired glands placed on the top or superior of the kidneys as cap. Hence, they are also called suprarenal glands. Adrenals have a dual origin, they are originated from ectoderm and mesoderm both. Like thyroid, adrenals also highly vascular in nature.     Structure : Each adrenal is a small ( 5 cm long, 3 cm broad and 1 cm thick), triangular and yellowish cap like structure. Its weight in humans is about 3.5 to 5.09 gm. At birth Adrenal gland best developed. Each gland has two parts – Outer cortex and inner medulla. (1) Outer cortex : The cortex is derived from mesoderm and forms about 80% part of the gland. Cortex consists of fatty, cholesterol rich cells. These cells distinguish the cortex into three zones or regions. (i) Zona glomerulosa : It is the outer part of the cortex (15% of the gland), which consists of small polyhedral cells. It secrets mineralocorticoids e.g., Aldosterone. (ii) Zona fasciculata : It is the middle part of the cortex (50% of the gland). Which consists of large polyhedral cells. This part secrets gluco-corticoids. e.g., Cortison, Corticosterone. (iii) Zona Reticularis : It is the inner part of the cortex (7% of the gland). In which the parallel cell cords of the zona fasciculata branched to form a loose anastomosing network. It secrets sex hormones. (2) Inner medulla : The medulla is derived from ectoderm and forms about 20% part of the gland. Adrenal medulla is reddish brown in colour and colourless of rounded groups of short cords of relatively large and granular cells. These cells are modified postganglionic cells of sympathetic nervous system. These are called chromaffin cells or phaeochromocytes. Adrenal medulla secrets adrenalin and nor-adrenalin which are collectively called as catecholamines.  Hormones of adrenal cortex About 20 steroids (steroidogenic) compounds have secreted from adrenal cortex. These are called adrenocorticoids (corticosteroids). Only few of them are biologically active as hormone. These hormones are steroid in nature. The later, however account about 80% of the secretion of adrenal cortex and are classified into three categories.     (1) Mineralo-corticoids : The principal mineralocorticoid is aldosterone. It is also called salt-retaining hormone. It promotes reabsorption of sodium ions from kidney and excretion of potassium ions in urine. It also reabsorb Cl– ions from kidney. Thus aldosterone has a important contribution in homeostasis by controling osmatic pressure of ECF (Extra cellular fluid). Remember that doctors administer saline drip to the patients who lose excessive water and salts due to diarrhoea, cholera, etc. Aldosterone also helps in maintaining acid-base equilibrium and blood pH (7.35) by promoting reabsorption of \[HC{{O}^{}}_{3}\] and regulating excretion of \[{{H}^{+}}\]by kidneys. It also promotes absorption of water and salt in intestine, mainly in colon. The Renin-angiotensin-aldosterone or RAA pathway controls more...

(1) Exteroreceptors : Receive stimulation directly from external environment. These may be of following type (i) Photoreceptor : Sensitive to light (Eye)                            (ii) Thigmoreceptor : Sensitive to touch. (iii) Tectoreceptor : Sensitive to touch. (iv) Tangoreceptor : Sensitive to touch pressure. (v) Phonoreceptor : Sensitive to sound (Ear). (vi) Olfactoreceptor : Sensitive for smell (Nose). (vii) Gustoreceptor : Sensitive to taste (Tongue). (viii) Thermoreceptor : Sensitive to temperature. (ix) Calo receptor : Sensitive to heat. (x) Frigido receptor : Sensitive to cold. (xi) Galvano receptor : Sensitive to electric current. (xii) Rheoreceptor : Sensitive to water or air current. (xiii) Geo receptor : Sensitive to gravity. (xiv) Telero receptor : Sensitive to distance. (Receptors of vision, hearing and smell receive stimuli from a distance hence called teleroreceptor). (2) Proprioceptors : Proprioceptors are located in skeletal muscles, joints, tendons etc. It is from these receptors that we know the position of our arm or leg without having to look at it. (3) Intero receptor : These are present in internal organ. Ex. receptor for hunger, thirst, pain and balancing.

Also known as stato-acuostic organ. It is the receptor for balancing and hearing which is sensitive for gravity and sound waves. It is also sensitive in orientation of body. It is also known as mechano receptor because of it change mechanical energy of sound waves in to action potential. Structure of Ear Ear of mammal is divided in to 3 parts - (1) External ear : It is made up of pinna and auditory meatus. Pinna is found in only mammals. Its upper rounded part is helix and lower is ear lobe. It is made up of adipose connective tissue and elastic cartilage and has ear muscles which are vestigeal in case of human beings. Pinna collect the sound waves and drive towards auditory meatus. Auditory meatus is 25 mm. long canal lined by simple columnar epithelia and made up of fibro elastic cartilage. It possesses ceruminus gland which secrete cerumin (ear wax). Cerumin trap the dust particles and microbes. Tympanic membrane : It is also called ear drum and present at the junction of auditory meatus and tympanic cavity. (2) Middle ear : The cavity of middle ear is known as tympanic cavity which is enclosed by tympanic bulla bone of skull and filled with air. Middle ear separated from external ear by ear drum and from internal ear by thin bony portion or partition with two openings known as oval and round window.     (i) Ear ossicle : A chain of three small, movable bones, the auditory or ear ossicles crosses the tympanic cavity. The outer ossicle is attached to the inner surface of the tympanic membrane.   Ear ossicles
Ear ossicle Shape Modification of
M - Malleus Hammer shaped Articular bone of lower jaw.
I - Incus more...
(1) Important tangoreceptor (i) Merckel's corpuscles : Found in epidermis (stratum malphighi) of skin. (ii) Merckel's disk : Found in epidermis (stratum malphighi) of skin. (iii) Meissner's corpuscles : Present in skin around the base of hair and feather. These are sensitive for touch and pressure both. (iv) Genital corpuscles : These are sensitive cells with nerve endings in skin around the genital organ. (v) Grandey's corpuscles : Found in birds at the base of the beak. These are kidney shaped in structure. (vi) Herbert corpuscles : Found in buccal cavity of birds. (vii) Capsulated corpuscles : These are sensitive cells encapsulated  and found in skin. (viii) Paccinian corpuscles : Found in deep layer of dermis and sensitive to touch, pain and pressure. (x) Golgi corpuscles and Mazzoni corpuscles : These are sensitive to touch and found in subcutaneous region. (2) Important Olfactoreceptor Jacobson's organ (Vomero-nasal organ) : It is concerned with smell. These were 1st appear in amphibians and well developed in snake, lizzard and sphenodon that is reptile. These are less developed in birds and mammal. Structure is blind sac like and lined by olfactory epithelium (Shneiderian membrane). Jacobsons organ is not found in rabbit. In man it is vestigeal organ. In human foetus Jacobson organ present. Jacobson organ also found in marsupiales, Rodents, and Insectivora. (3) Important Thermoreceptor (i) Krause end bulb : These are sensitive for temperature and pain and freidgo in nature. Found in lips, tongue, conjunctiva of eyes and corium of fingure. (ii) Organ of Ruffeni : Sensitive to temperature and mainly related with heat. (iii) Ampulla of Lorenzini : Found in snout region of cartilagenous fishes these are helpful in detecting the temperature of water.

Human have binocular vision. The eye can discriminate colour, appraise length, width and depth visually and form true inverted image. Structure of eye The eyes are two in number and lodged in orbits (bony socket) of skull. The eye is a hollow, spherical organ, about 2.5 cm in diameter and about 6 to 8 gram in weight. It has two parts - (1) Protective devices : Eye has four protective devices. (i) Eye brows : The outwardly directed hair of the eyebrows carry the sweat and rain drops trickling down the forehead to the sides to prevent their falling into the eyes. (ii) Eye lids (Palpebrae) : In man two eyelids are present, upper is movable. They are regularly closed at short intervals to clean the cornea. This is called blinking. In frog out of two upper eyelid is immovable and lower eyelid is movable. Nictitating membrane is present in frog which protect eye in water. Movement of nictitating membrane takes place by retractor bulbi. It becomes folded by levator bulbi. A nonfunctional vestigeal nictitating membrane, called plica semilunaris, occurs in human eyes. It remains permanently retracted at the inner angle of each eye. (iii) Eye lashes : The eyelids bear at the free edge a row of stiff hair, the eye lashes. These check the entry of dust particles, tiny insects and rain drops into the eyes.     (iv) Eye glands (a) Meibomian gland : The eye-lids bear at the free edge a row of meibomian gland that is modified sebaceous gland. (Act as a lumbricant). (b) Lacrimal gland or Tear gland : It lies in the upper outer part of the orbit and  secretes a slightly saline, watery fluid that contains a bacteriolytic enzyme named lysozyme. This secretion moistens the surface of the eyeball. The excess of this secretion passes through nasolacrimal duct. It is modified sweat gland. (c) Harderian gland : Some aquatic mammals (whale) possess harderian gland which lubricate nictitating membrane. It is also found in frog and birds. (d) Glands of zeis (zis) : These are modified sebaceous gland, found at base of hair follicle of eye lashes, pour lubricating fluid in hair follicle. Infection of these glands is sty. (e) Glands of Moll : It is modified sweat gland and open into the follicles of eyelashes. In human meibomian, lachrymal, Moll's glands, and zeis glands are present. (v) Connective tissue : A layer of fatty connective tissue surrounds the eyeball. It serves as a soft shockproof pad. (2) Eye ball : Eye ball is made up of 3 coats or tunic. (i) Sclerotic layer (Fibrous tunica) : Outer most and opaque, fibrous and non-vascular layer easily seen as white of the eye. It is a coat of dense connective tissue made up of collagen fibers and fibroblasts. Sclera covers entire eye ball except cornea, gives shape to eye ball. Sclera in frog is cartilaginous. (a) Cornea : In the more...



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