Study Notes on Chemical Coordination and Integration
Study Notes on Chemical Coordination and Integration
CHEMICAL COORDINATION AND INTEGRATION
- Cellular functions need to be continuously regulated. Hence, there is a need for another special kind of regulatory and Coordinating system which can fulfil these conditions.
- This cellular regulation is carried out by chemical messengers called hormones, released by endocrine glands.
- Together all endocrine glands and hormones released by them form the endocrine system.
- The endocrine glands release their secretions directly into the blood and through blood, hese secretions reach each and every cell of the body and regulate their functions.
- ‘Endo’ means within and ‘krinein’ means to secrete, therefore the endocrine system is a system of isolated glands which secrete chemicals called ‘hormones’ directly into lymph or venous blood for transport to target tissues, often located away from the site of secretion.
- The endocrine system comprises endocrine glands which are also called ductless glands as they lack ducts and release their secretions directly into the venous blood or lymph.
- These chemicals (hormones) are then taken up by the specific organs from the blood circulation to initiate a particular metabolic change.
- Hormones were classically defined as chemicals produced by endocrine glands and released into the blood which transports them to distantly located target organ.
- Nowadays hormones are defined as non-nutrient chemicals which act as intercellular messengers that are produced in trace amounts
- The new definition covers a number of new molecules in addition to the hormones secreted by the organised endocrine glands.
HUMAN ENDOCRINE SYSTEM
- The endocrine glands and hormone-producing diffused tissues/cells located in different parts of our body constitute the endocrine system.
- It includes the pituitary gland, pineal gland, thyroid gland, adrenal glands, pancreas, parathyroid glands, thymus and gonads (testis in males and ovary in females).
- In addition to these glands, certain other organs, e.g., gastrointestinal tract, liver, kidney, the heart also produce hormones.
- Hypothalamus is an anatomical and physiological connection between the nervous system and the endocrine system.
- Hypothalamus is connected to the pituitary gland via Infundibulum
- Hypothalamus is the basal part of the diencephalon, forebrain.
- The hypothalamus contains several groups of neurosecretory cells called nuclei which produce hormones for regulating a wide spectrum of body functions.
- There are several hormones released by neurosecretory cells of the hypothalamus which regulate the synthesis and secretion of anterior pituitary hormones, i.e., hormones secreted by the pituitary gland.
- The hormones produced by the hypothalamus are of two types: releasing hormones (which stimulate the secretion of pituitary hormones) and inhibiting hormones; (which inhibit the secretion of pituitary hormones
- Some of the neurohormones released by the neurosecretory cells of the hypothalamus are:
For example :
(i) Gonadotrophin-Releasing Hormone (GNRH) : GnRH is a hypothalamic hormone which stimulates the pituitary gland to synthesise and release the gonadotrophins. Therefore, GnRH is a releasing hormone.
(ii) Somatostatin: Somatostatin is a hypothalamic hormone which inhibits the release of growth from the pituitary gland. Hence, somatostatin is an inhibitory hormone.
- These hormones originating in the hypothalamic neurons, pass through axons and are released from their nerve endings.
- These hormones reach the pituitary gland through a portal circulatory system and regulate the functions of the anterior pituitary.
- The posterior pituitary is under the direct neural regulation of the hypothalamus
Pituitary Gland (Hypophysis)
- The pituitary gland is located in a bony cavity of the sphenoid bone of the skull called sella turcica.
- It is attached to the hypothalamus (inferior surface) by a stalk called the infundibulum.
- Anatomically the pituitary gland (hypophysis) is divided into adenohypophysis and neurohypophysis.
- It comprises of 75% part of the pituitary gland. It is highly cellular & vascular.
- It consists of two portions –pars distalis and pars intermedia. However, in humans, the pars intermedia is almost merged with the pars distalis.
- The pars distalis region of the pituitary is commonly called anterior pituitary.
- It produces GH (growth hormone),PRL(prolactin), TSH (thyroid-stimulating hormone), ACTH (adrenocorticotrophic hormone), LH (luteinizing hormone), FSH (follicle-stimulating hormone).
- Pars intermedia releases only one hormone called melanocyte-stimulating hormone (MSH)
- The functions of above-mentioned hormones are under the control of the hypothalamus.
- The hormones released from the hypothalamus are carried to the adenohypophysis through the hypophyseal portal veins.
The hormones synthesised and released from the adenohypophysis (anterior pituitary) are :
- Growth hormone: The growth hormone is also known as somatotropin or somatotrophic hormone.
Over and under secretion of growth hormone leads to many disorders such as :
Dwarfism: The low secretion of growth hormone before puberty causes pituitary dwarfism.
Gigantism: Over secretion of growth hormone before puberty causes gigantism.
- Prolactin (PRL): It was previously called luteotrophic hormone (LTH). Prolactin regulates the growth and secretion of milk from mammary glands after childbirth of mammary glands and milk production in them. It activates the growth of breasts during pregnancy
- Thyroid-stimulating hormone (TSH): TSH stimulates the synthesis and secretion of thyroid hormones like thyroxine from the thyroid gland. It also controls the growth of thyroid gland.
- Adrenocorticotrophic hormone (ACTH): ACTH controls the structure and functioning of the adrenal cortex, especially secretions of glucocorticoids from it.
- Gonadotrophic hormones: They are also called the gọnadotrophins as they stimulate gonadal activity. The anterior pituitary (pars distalis) releases following gonadotrophic hormones:
(a) Follicle-stimulating hormone (FSH) :(In males. FSH regulates spermatogenesis (formation of sperms) and ( females, FSH stimulates growth and development of ovarian follicles and secretion of estrogen hormone
(b) Luteinizing hormone (LH) (In males) which stimulates testes to synthesise and secrete androgens (testosterone).
6)Melanocyte Stimulating Hormone (MSH) It acts on melanocytes (which contain melanin pigment) and regulates the pigmentation of the skin
- Neurohypophysis comprises of 25% portion of the pituitary gland.
- It is also called pars nervosa or posterior pituitary.
- It does not produce or secrete any hormone of its own. It receives neurohormones directly secreted from the neurosecretory cells (hypothalamic neurons) of the hypothalamus.
The two_neurohormones synthesised in the hypothalamus and secreted by neurohypophysis are:
Oxytocin (Pitocin): It is a hormone which acts on smooth muscles and stimulates their contraction)
- In females, it stimulates a vigorous contraction of the uterus at the time of childbirth
- Widening of the uterus near the completion of pregnancy and suckling of nipples by young infant stimulates the hypophysis to release oxytocin.
Vasopressin or Antidiuretic hormone (ADH)
- Vasopressin acts mainly on kidneys and the reabsorption of water and electrolytes, ADH acts on nephrons of kidneys and stimulates the reabsorption of water from the distal tubules ie, distal convoluted tubules (DCT) and collecting tubules.
- It thereby reduces the water loss from the body It prevents diuresis (water loss). therefore it is called anti-diuretic hormone (ADH).
- Failure of secretion of vasopressin leads to reduced renal reabsorption of water and consequent elimination of a large volume of very dilute (hypotonic) urine; this disease is known as Diabetes Insipidus
- It is also called Epiphysis. The pineal gland is located on the dorsal side of the forebrain, It has a stalk called the pineal
- Stalk and a small, rounded, reddish structure called the pineal body.
- The pineal gland secretes a hormone called melatonin. Melatonin plays a very important role in the regulation of the 24-hour (diurnal) rhythm of our body.
- The secretion of melatonin follows a diurnal cycle, i.e., it rises in the evening and throughout the night and drops around the noon. It has its maximum concentration at midnight and minimum during noon.
- Light reduces melatonin formation and hence maintains the normal rhythm of the sleep-wake cycle. In addition, melatonin influences metabolism, body temperature, pigmentation of the skin, menstrual cycle as well as defence capability of an individual. It acts like a biological clock.
- The thyroid gland is the largest endocrine gland which is butterfly-shaped or H-shaped. It is composed of two lateral lobes which are located on either side of the trachea
- These lobes are interconnected with a thin flap of connective tissue called the isthmus.
- The thyroid gland is composed of follicles and stromal tissues.
- These follicles are held together by areolar connective tissue. Each thyroid follicle is composed of follicular cells, enclosing a cavity.
- The follicular cells present in the follicles synthesise two hormones, namely, tetraiodothyronine or thyroxine (T₄) and tri-iodothyronine (T₃).
- When necessary, the hormones are released from the colloid to the blood. These have similar effects on the target cells and hence are together grouped as thyroid hormones (TH). The synthesis of thyroid hormones requires iodine.
- These hormones are released under the influence of TSH, released by the anterior pituitary.
Functions of Thyroid Hormones (Thyroxine)
- They control the metabolism of proteins, fats and carbohydrates and hence regulate the growth of the body uses.
- These hormones play an important role in the regulation of the basal metabolic rate (BMR).
- Maintenance of water and electrolyte balance is also influenced by thyroid hormones.
Deficiency or under secretion of thyroid hormones results in hypothyroidism.
- Simple Goitre: Iodine is essential for the normal rate of hormone synthesis in the thyroid. Deficiency of iodine in the diet results in the low secretion of thyroid hormone (hypothyroidism) and enlargement of the thyroid gland (simple goitre).
- Cretinism: Failure of thyroid secretion from infancy or childhood slows body growth and mental development and reduces metabolic rate markedly. The child remains physically stunted and mentally retarded low intelligence quotient, abnormal skin deaf-mutism.
- Myxedema (Gull’s diseases): Deficiency of thyroid hormones produces myxedema in adults. The patient has a puffy appearance and lacks alertness, intelligence and initiative.
Due to cancer of the thyroid gland or due to the development of nodules in the thyroid gland, the rate of
synthesis and secretion of the thyroid hormones are increased to abnormally high levels leading to a condition called hyperthyroidism which adversely affects the body physiology.
- Exophthalmic goitre is a form of hyperthyroidism, characterised by enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate, and weight loss also called Graves’ disease.
- The parathyroid glands are present on the backside of the thyroid gland.
- Four parathyroid glands are present on the surface of two lobes of the thyroid gland.
- One pair of the parathyroid gland is present on each lobe of the thyroid gland.
- Though parathyroid glands are present on the surface of the thyroid gland, these are functionally and developmentally independent from the thyroid gland.
- The parathyroid glands secrete a peptide hormone called parathyroid hormone collins hormone or parathormone (PTH)
- The release of parathormone is regulated by the circulating levels of calcium (Ca2″) iôns in the blood.
- Along with TCT, PTH plays an important role in calcium balance of the body. It maintains the calcium balance between the blood and other tissues.
- When the calcium level in the blood decreases. parathyroid hormone is released in order to increase the calcium level in the blood.
- The thymus gland is a lobular structure located between lungs behind sternum on the ventral side of the aorta.
- The thymus plays a major role in the development of the immune system.
- This gland secretes the peptide hormones called thymosins.
- Thymosins play a major role in the differentiation of T-lymphocytes, which provide cell-mediated immunity.
- In addition, thymosins also promote the production of antibodies to provide humoral immunity.
- The thymus is degenerated in old individuals resulting in a decreased production of thymosins. As a result, the immune responses of old persons become weak.
- Also called suprarenal glands or glands of Emergency.
- These are paired glands located at the anterior part of the kidneys,
- Each kidney has an adrenal gland on top of it. These glands have dual origin i.e., from Mesoderm and Ectoderm of the embryo.
- Each adrenal gland is composed of two types of tissues, adrenal medulla and adrenal cortex.
Adrenal Cortex is derived from mesoderm of embryo, It is the external or outer part of the adrenal gland. It is divided into three layers:
(i)Zona reticularis: It is the innermost layer of the adrenal cortex.
(ii) Zona fasciculata : It is the middle layer of the adrenal cortex.
(iii) Zőna glomerulosa : It is the outermost layer of the adrenal cortex.
The adrenal cortex secretes over 20 steroid hormones, commonly called cortical steroids or corticoids.
These are grouped into three major categories, namely, mineralocorticoids, glucocorticoids and sex corticoids.
Mineralocorticoids: These are the steroid hormones which are released from the zona glomerulosa layer of the adrenal cortex. These regulate the water and electrolyte balance in the body, e.g. aldosterone.
- These are also the steroid hormones which are produced by zona fasciculata layer of the adrenal cortex. As the name suggests, these are the corticoids which are involved in carbohydrate metabolism. These are also involved in the regulation of protein and fat metabolism.
- Cortisol is also involved in maintaining the cardiovascular system as well as the kidney functions.
- Cortisol produces anti-inflammatory reactions and suppresses the immune response.
- The cortisol retards phagocytic activities of WBCS and thus suppresses “inflammation reactions”.
- The cortisol stimulates the formation of RBC and thereby increases RBC count (but decreases WBC count in the blood).
- Aldosterone acts mainly at the renal tubules and stimulates the reabsorption of Na+ and water and excretion of K+ and phosphate ions. Thus, aldosterone helps in the maintenance of
electrolytes, body fluid volume, osmotic pressure and blood pressure.
- Small amounts of androgenic steroids are also secreted by the adrenal cortex which plays a role in the growth of axial hair, pubic hair and facial hair during puberty.
- Adrenal Medulla develops from Neuroectoderm of the embryo. The adrenal medulla is the internal or inner part of the adrenal gland.
- It releases two hormones called epinephrine/and norepinephrine/ The epinephrine is also called adrenaline and norepinephrine is also called noradrenaline.
- These two hormones are commonly called catecholamines. These two hormones are released during the stress of any kind or emergency situations and are therefore called as emergency hormones and adrenal gland is called emergency gland.
- These hormones also prepare the body during the flight, fright and fight hence are also called as 3F. These have different effects on different organs of the body.
- These hormones increase the heartbeat, the strength of heart contraction and the rate of respiration.
- They increase the sweating, piloerection (raising of hair), alertness and pupillary dilation.
- Catecholamines also știmulate the breakdown of glycogen resulting in an increased concentration of glucose in the blood.
- In addition, they also stimulate the breakdown of lipids and proteins.
Origin: Endoderm of the embryo.
- The pancreas is a composite (heterocrine) gland. It acts as both exocrine and endocrine gland.
- The endocrine part of the pancreas consists of over one million (i.e., about 1 to 2 million) patches of cells located around blood capillaries called Islets of Langerhans.
- These constitute only 1 to 2% of the pancreatic tissue.
- The two main types of cells in the Islet of Langerhans are called a-cells and B-cells which produce endocrine secretions.
(i)Alpha cells (a-cells) :
- These are also called A-cells. The a-cells secrete a hormone called glucagon.
- It is a peptide hormone which helps in maintaining the normal glucose level in the blood.
- Glucagon acts mainly on the liver cells (hepatocytes) and stimulates the conversion of stored glycogen into glucose (i.e., glycogenolysis).
- It is controlled by negative feedback, If the level of glucose in the blood is low glucagon stimulates hepatocytes to undergo glycogenolysis which increases the level of glucose in the liver.
- The increased glucose is then transferred into the blood which increases the blood glucose level. Glucagon also stimulates the process of gluconeogenesis, i.e., the formation of glucose from deaminated acids and lactic acid (non-carbohydrate sources), This all leads to hyperglycemia.
- Glucagon also reduces cellular glucose uptake and utilisation. Thus, glucagon is a hyperglycemic hormone.
(ii) Beta cells (B-cells) :
- These are also called B-cells. The B-cells produce a peptide hormone called insulin which regulates the glucose homeostasis.
- Insulin acts mainly on adipocytes (cells of adipose tissue) hepatocytes and muscle cells and enhances cellular uptake and utilisation of glucose.
- Whenever the level of glucose in the blood increases, then B-cells of the pancreas get stimulated and release insulin which increases
- Uptake of glucose by cells for respiration.
- Uptake of glucose by liver and muscle cells for glycogenesis or conversion of glucose into glycogen.
- Uptake of amino acids by cells and synthesis of proteins (effect on protein metabolism).
- Fat synthesis in adipose tissue (effect on fat metabolism).
- This would decrease the glucose concentration in blood and would increase the rapid movement of glucose from blood tó hepatocytes and adipocytes resulting in hypoglycemia (decrease in blood glucose level).
- Therefore, it could be concluded that both glucagon and insulin help in the homeostasis of glucose where both maintain the blood glucose level. (When the glucose level in the blood increases, the pancreas secretes insulin which causes glycogenesis and decreases the level of glucose in the blood but when the glucose level decreases
- In the blood, then pancreas secretes glucagon which causes glycogenolysis and gluconeogenesis and increases the level of glucose in the blood.
- Hyposecretion of insulin results in the presence of excess glucose in the blood. This condition is known as hyperglycemia.
- Prolonged hyperglycemia leads to a complex disorder called diabetes mellitus. This is characterised by loss of excess glucose through urine and also the formation of complex harmful compounds known as ketone bodies.
- As the glucose is excreted in the urine, water also moves out along with the glucose and causes excessive urination and dehydration of body tissues.
- A pair of the testis is present in the scrotal sac (outside abdomen) of male individuals
- Testis performs dual functions as a primary sex organ as well as an endocrine gland.
- The testis is composed of seminiferous tubules and stromal or interstitial tissue.
- The Leydig cells or interstitial cells, which are present in the intertubular spaces produce a group of hormones called androgens mainly testosterone
- Androgens regulate the development, maturation and functions of the male accessory sex organs like epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc.
- These hormones stimulate muscular growth, growth of facial and axillary hair, aggressiveness, low pitch of voice etc.
- Androgens play a major stimulatory role in the process of spermatogenesis (formation of spermatozoa).
- Androgens act on the central neural system and influence male sexual behaviour (libido).
- These hormones produce anabolic (synthetic) effects on protein and carbohydrate metabolism.
- Females have a pair of ovaries located in the abdomen
- The ovary is the primary female sex organ which produces one ovum during each menstrual cycle.
- In addition, ovary also produces two groups of steroid hormones called estrogen and progesterone.
- The ovary is composed of ovarian follicles and stromal tissues.
- The estrogen is synthesised and secreted mainly by the growing ovarian follicles.
- After ovulation, the ruptured follicle is converted to a structure called corpus luteum, which secretes mainly progesterone.
- Estrogens produce wide-ranging actions such as stimulation of growth and activities of female secondary sex organs, development of growing ovarian follicles, the appearance of female secondary sex characters (e.g., the high pitch of voice, etc.), mammary gland development.
- Estrogens also regulate female sexual behaviour.
- Progesterone supports a pregnancy. Progesterone also acts on the mammary glands and stimulates the formation of alveoli (sac-like structures which store milk) and milk secretion.
MECHANISM OF HORMONE ACTION
- 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 functions are regulated by hormones.
- On the basis of their chemical nature, hormones can be divided into groups :
(i) peptide, polypeptide, protein hormones (e.g., insulin, glucagon, pituitary hormones, hypothalamic hormones, etc.)
(ii) steroids (e.g., cortisol, testosterone, estradiol and progesterone)
(iii) iodothyronines (thyroid hormones)
(iv) amino-acid derivatives (e.g., epinephrine).
- Hormones which interact with membrane-bound receptors normally do not enter the target cell, but generate second messengers (e.g., cyclic AMP, IP3, Ca++etc) which in turn regulate cellular metabolism
- Hormones which interact with intracellular receptors (e.g., steroid hormones, iodothyronines, etc.) mostly regulate gene expression or chromosome function by the interaction of the hormone-receptor complex with the genome.
- Cumulative biochemical actions result in physiological and developmental effects
Diagramatic representation of the mechanism of hormone action