Chemical Control And Coordition
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* The neural system and the endocrine system
jointly co-ordinate and regulate the
physiological functions in the body.
* The neural co-ordination is fast very exact
and short lived, whereas chemical
co-ordination is usually slow, widespread and
long lasting.
* All cells of our body are not innervated by
nerve fibres but the cellular function need to
be continuously regulated so a special kind
of coordination and integration has to be
provided. This function is carried out by
endocrine system.
* The branch of biology which deals with the
study of endocrine system and its physiology
is known as "Endocrinology".
* ‘‘Thomas Addison’’ is known as father of
Endocrinology.
Endocrine glands pour their secretion directly into blood. These glands lack ducts, so these glands are called
ductless glands.
Whereas the gland with duct is called exocrine gland which secretes enzyme etc
* Substances secreted by endocrine glands are known as hormones. The meaning of word ' hormone' in Greek
is "to excite" = hormaein.
* Organised endocrine gland : Where hormone producing cells are present in cluster/tissue form called
organised endocrine gland.
Examples : Pituitary, Pancreas, Parathyroid, Pineal, Thyroid, Thymus, Adrenal and Gonads.
* Non-organised endocrine gland : Where hormone producing cells are present in scattered form called non-
organised or diffused endocrine tissue.
Example : Heart, Liver, Kidney, Gastrointestinal tract.
HORMONE
* The term hormone was coined by "Starling".
* Hormones are also called "Primary messengers" or "chemical messengers".
* Hormones are non-nutrient chemicals which act as intercellular messenger and are produced in
trace amount.
* First discovered hormone is Secretin. It was discovered by "Bayliss & Starling in 1902".
Physical & Chemical Specialities of Hormones :–
* The molecules of most of the hormones are small, and their molecular weight is low.
* Mostly hormones are soluble in water and few are soluble in fat and are easily diffusible in tissues.
* The secretion of hormone is always in very small quantity because these are very reactive substances.
* Hormones are destroyed after use i.e. hormones can not be stored in the body.
Thyroxine is exception in this regards.
* Liver and kidneys separate hormones from blood and decompose them. The product formed after decomposition
is excreted with urine. It can not be reutilized.
* Hormones are non-antigenic & non-species specific substances.
* Usually, hormones do not participate in the metabolic activities of target cells but they affect and control the
activity level of these target cells
MECHANISM OF HORMONAL REACTIONS
MECHANISM OF HORMONE ACTION –– Two types
* Once hormone enters into blood stream it can reach almost to any cell in the body.
* However, each hormone affects only certain kind of cells, which is called as target cells.
* All hormones do not act in the same way due to location of their receptor.
* Hormones are two types :
(i) Water soluble hormones (ii) Lipid soluble hormones
* Hormones produce their effects on target tissues by binding to specific proteins called hormone receptors
located in the target tissues only. Hormone receptors present on the cell membrane of the target cells are
called membrane-bound receptors and the receptors present inside the target cell are called intracellular
receptors.
* Mostly nuclear receptors (present in the nucleus). Binding of a hormone to its receptor leads to the formation
of a hormone-receptor complex.
* Each receptor is specific to one hormone only and hence receptors are specific.
* Hormone-Receptor complex formation leads to certain biochemical changes in the target tissue. Target tissue
metabolism and hence physiological function are regulated by hormone.
Water soluble hormones : Hormone which interact with membrane bound receptor normally do not enter
the target cell, but generate secondary messenger (eg. cyclic AMP, IP3 , Ca++ etc.) which in turn regulate
cellular metabolism
Lipid soluble hormone : Hormone which interact with intracellular receptor (Steroid, iodothyronine) mostly
regulate gene expression or chromosome function by the interaction of hormone receptor complex with the genome.
Cumulative biochemical action results in physiological and developmental effects
PITUITARY GLAND
On the basis of development pituitary gland is completely ectodermal.
* It is situated in the sella–turcica of sphenoid bone.
* This gland is attached to the hypothalamus through a stalk which is called as infundibulum.
* It is divided anatomically into an adenohypophysis (anterior pituitary) and a neurohypophysis (posterior
pituitary).
* The lower terminal end of infundibulum is bulging type which is called as posterior lobe or pars nervosa.
* Infundibulum & pars nervosa are collectively called as neurohypophysis.
* Adenohypophisis consists of pars distalis (anterior lobe) and pars intermedia (middle lobe).
* Hypophyseal portal vein collects the blood from hypothalamus and supplies to the anterior pituitary
HORMONES SECRETED BY ADENOHYPOPHYSIS
1. GROWTH HORMONE OR SOMATOTROPHIC
[G.H. or S.T.H] :–
Effect on growth –
(i) Promotes elongation of bones
(ii) GH promotes mitosis & increases number of cells in many visceral organs e.g. liver
(iii) GH stimulates growth of muscle and cartilage
Effect on metabolism –-
Fat : Increases lipolysis. Under the influence of growth hormone fat is used for energy in preference to
carbohydrate and protein
Carbohydrate :– - GH decreases uptake of glucose in the cells, so it is also called diabetogenic hormone.
Protein : GH increases amino acid uptake by the cells of the liver & muscles & helps in protein synthesis.
Diseases due to hyposecretion or hypersecretion of somatotropin hormone :–
(a) Hyposecretion of STH :– Due to deficiency of STH in childhood or adolescence, dwarfism is observed.
* Dwarfism due to the defect of pituitary is called Ateliosis.
* Clowns of circus are such dwarfs, they are called midgets.
(b) Hypersecretion of STH :–
(i) Due to hypersecretion of GH in childhood, the bones of legs & hands
become very long and height of that person increases very
much. Body becomes imbalanced. This disease is called Gigantism.
(ii) Excess secretion of growth hormone in adults especially in middle
age can result in severe disfigurement like excess enlargement of
bones of face, vertebral column and fore limbs, called Acromegaly,
which may lead to serious complications, and premature death if
unchecked.
2. Thyrotrophic [T.T.H or T.S.H.] or Thyroid Stimulating Hormone :–
T.S.H stimulates thyroid gland to secrete thyroxine. TSH helps in almost all steps of the thyroid hormone
synthesis & it causes growth of thyroid gland.
Secretion of TSH is stimulated by Thyrotrophin releasing factor of hypothalamus.
3. Adreno Cortico Trophic Hormone or Corticotropine [ACTH] :–
It accelerates the cortex part of adrenal gland to secrete hormones, mainly glucocorticoids.
4. Follicle Stimulating Hormone [FSH] :–
It is secreted in male and female both.
In males, it stimulates spermatogenesis and normal functioning of seminiferous tubules.
In females, it stimulates oogenesis and development of Graafian follicles in ovary.
Estrogen hormone that is secreted by Graafian follicles is also affected by FSH.
5. Luteinizing Hormone [LH or ICSH] or Interstitial Cell Stimulating Hormone :–
It stimulates ovulation in female. As a result of this corpus luteum is formed.
Hormone progesterone which is secreted by corpus luteum is also stimulated by L.H.
In men LH is called ICSH. It affects the Leydig's cells or Interstitial cells of testes and stimulates the
secretion of male hormone " Testosterone".
FSH and LH both are called gonadotrophic hormone GTH.
FSH and LH act in combined form so these are called synergesic hormone.
Gonadotrophic hormones (FSH & LH) secretion starts during puberty. Their secretion is regulated by
hypothalamus.
6. Luteotrophic or Prolactin or Lactogenic or Mammotrophin Hormone (PRL) :
Lactation (Galactopoiesis) :- Prolactin is responsible for lactation (milk formation) in postpartum (after delivery) in women.
7. Melanocyte Stimulating Hormone [MSH] :–
* It is secreted by middle lobe but in human the pars intermedia is almost merged with pars distalis.
* In man, MSH is secreted by anterior lobe.
* MSH is also called Intermedin.
* It stimulates the melanocytes to synthesize melanin in mammals, but role of MSH in determining skin color in
human is still doubtful.
* MSH is produced in all vertebrate but it is more effective in lower vertebrates.
* This hormone is related with change in the colour of skin in Amphibian and Reptiles. This phenomenon of
colour changing is known as metachrosis.
* It darkens the complexion of skin by distributing melanin pigment evenly under the skin.
* Just opposite to it, melatonin secreted by pineal body, collects the melanin pigments at one place thus
fairing the complexion of skin
Posterior Lobe or Neurohypophysis :–
Posterior lobe of pituitary gland is 1/4th part of total gland. It is just like nervous tissue, because in it, the
terminal ends of the axons of neurosecretory cells of hypothalamus are swollen. These swollen ends are called
" Herring bodies". Hormones are released in these bodies.\
* There are some large, branched fatty neuroglial supporting cells in between axons, that are called "Pituicytes".
* Posterior pituitary hormones are not synthesised in the gland itself but they are synthesized in the supraoptic
nucleus and paraventricular nucleus of hypothalamus.
(A) ADH or Vasopressin :-
* It acts mainly at the kidney and stimulates reabsorption of water and electrolytes in DCT and collecting duct of
nephrons and thereby reduces loss of H2O through urine (Diuresis), Hence, it is also called as
"Anti-diuretic hormone(ADH)"
* Hyposecretion of ADH causes "Diabetes insipidus" (taste less urine or water drinker's disease), Which is
characterised by polyuria, diluted urine, dehydration, excessive thirst (polydipsia) low BP (hypotension) etc.
* Intake of coffee, tea and excess alcohol etc. decreases the secretion of ADH.
(B) Oxytocin or Pitocin :– It is the main parturition hormone. It stimulates the fast/ rapid contractions and
expansions of non- striated muscles of the uterine wall at the last moment of gestation period (pregnancy). Due
to this uterine constrictions, labour pains start just before child birth.
* This hormone is secreted by pituitary glands of mother at the time of parturition.
* Oxytocin hormone contracts the myoepithelial cells present at all the sides of alveoli of mammary glands. Thus
it helps in milk ejection so it is also called milk let down hormone.
* In female, this hormone related with emotion.Even thought, cry or sound of baby can bring about release of
this hormone in lactating mother.
* Oxytocin acts on the smooth muscles of our body and stimulates their contraction. In females, it stimulates a
vigorous contraction of uterus at the time of child birth, and milk ejection from the mammary gland.
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