Role of Hormones as Messengers and Regulators:
Ø Secretion of endocrine glands is called hormone. Hormones were first discovered by Starling and Bayliss in 1902.
Ø Hormone can be defined as a chemical substance produced in one part of the body and carried by blood to a target organ where it regulates certain processes.
Ø Hormones are also known as chemical messengers (by Huxley) or autocoids because they carry information and transmit it to the target organ and regulate different biological processes of the body.
Ø Hormones are informational molecule which is secreted inside or outside the body in response to changes in the environment.
Ø They are released into the Extracellular fluid, where they are diffused into the blood stream. The latter carries them from the site of production to the site of action. They act on specific organs called target organs.
Ø Hormones are low molecular weight substances which are effective in low concentration. They catalyze physiological processes in both direction i.e. they can accelerate or inhibit a physiological processes.
Ø The blood contains all the hormones but the cells of target organ can pick up the specific required hormones only and ignore all the others.
Ø Hormones may stimulate or inhibit specific biological processes in the target organs to modify their activities, so they act as regulators. There is coordination between nerves and hormones. Nerve regulates synthesis and release of some hormones. Thus, hormonal coordination plays an important role in regulating body functions. For example, Calcitonin secreted by thyroid gland regulates the concentration of Ca and P in the blood.
Hormones as Messengers:
On the ventral side of diencephalon hypothalamus is present; it consists of number of scattered masses of grey matter in the white matter. Masses of grey matter containing neurons form hypothalamus nuclei. The neurons of hypothalamus nuclei secrete several hormones called neuro-hormones into the blood. Blood carries these neuro-hormones to the anterior pituitary where the neuro-hormones stimulate pituitary to release various hormones. Thus, neuro-hormones act as messengers. It is of two types:
i. Positive feedback control
ii. Negative feedback control
i. Positive feedback control: Thyroxine hormones are secreted by thyroid gland. The thyrotropin releasing hormone (TRH) from the hypothalamus stimulates the anterior pituitary to secrete thyroid stimulating hormone (TSH). TSH stimulates the thyroid gland to secrete thyroxine. If the level of thyroxine in blood is less than normal, this low thyroxine level stimulates hypothalamus to secrete more thyroid releasing hormone (TRH). This results in increased secretion of TSH which in turn stimulates increased secretion of thyroxine.
ii. Negative feedback control: If the level of thyroxine in blood is more than the normal this high thyroxine level produces an inhibitory effect on hypothalamus. As a result less TRH and less TSH are produced by hypothalamus and pituitary respectively. This results in decrease in thyroxine. This is called negative feedback control.
Hormones as Regulator:
Hormones regulate metabolic activities in various tissues. They are one kind of mechanism for signalling among cells and tissues. Hormones are distinguished from communication mechanism that depends on direct cell-cell contact through gap junctions. Hormones act by binding to receptors which are usually protein molecule. Receptors have two functions; first they bind the hormone and secondly they transducer the signal to affect the metabolism of the recipient cell. The ability of a cell to respond to hormones depends on two properties of the receptor molecule; how many of them are on a particular cell and how well they bind the hormone. The first property is called receptor number and second is called the affinity of the receptor for the hormone. The biochemical responsiveness of a cell to a hormone depends on the number of occupied receptors on the responsive cell.
Suppose that a hormone binds to a receptor with a dissociation constant given by the following equation:
Kd=[R] [H]/ [RH]
In the receptor, R is the receptor, H is the hormone and RH is the receptor hormone complex. If 50 occupied receptors trigger the appropriate metabolic response, you can achieve the responsibility having 55 receptors on a cell with 90% of them occupied. How can this be achieved? If the second set of receptors had a tenfold greater affinity for the hormone, the same concentration of hormone would result in 50 bound receptors.
Rearranging the previous equation to solve for [H], the levels of circulating hormone yields
[H] =K [RH] / [R]
If two receptors exist, type 1 and type2, each of which is responding to a constant concentration of hormone [H], then
[H]=K₁ [RH₁] [R₁]= K₂ [RH₂] /[R₂]
So, higher Kd means lower affinity.
If you set the number of occupied receptors [RH₁] = [RH₂] = 50, we can solve the number of unoccupied receptors of each type [R₁] =50 and [R₂]= 50. Suppose the hormone concentration increased by 50%. In this case, structure receptor system R₁ would be more responsive R₂ would be close to saturation, complete saturation of R₂ would yield only 5 more occupied receptors. This means that the concentration of the occupied receptors can change when the receptor is about half occupied. The previous equation shows that the maximum responsiveness to a change in hormone concentration is possible when the associate constant of the receptor for a hormone is near the physiological concentration of the hormone.
The compounds that bind to the receptor can modulate the action of that receptor. Agonists act to reinforce the activity of a receptor by binding to it and mimicking the action of the receptor. Drugs can be either agonists or antagonists; for example, Isoproteranol is agonists for a receptor that increases blood pressure, while propanoid- a commonly used drug to decrease blood pressure and is an antagonist for another class of recipients. Both of these compounds are structurally related to the natural hormone epinephrine.
Endocrine glands are ductless glands and their secretions are known as Hormones. They are chemical messengers which are secreted in trace amounts and affects the target organ of the body.
Pituitary gland release several hormones which regulate the functioning of different parts of the body. Growth hormone, PRL, TSH, ACTH, LH, FSH, MSH, oxytocin and vasopressin are the hormones released by the pituitary gland.
Thyroid glands are located in our neck region and secretes thyroxine, TCT and calcitonin hormone. On the other hand Parathyroid gland is located on the posterior side of thyroid gland and release PTH.
Adrenal glands are supra-renal glands which consists of two parts; cortex and medulla which releases certain hormones to regulate the process of human body.
Gonads,testes in male and ovary in female are the reproductive glands which secretes hormones that helps in the reproduction of male and female. Testes secrete androgen, inhibin,AMF and ovary release estrogen and progesterone.
Pancreas is a composite gland which function as both exocrine and endocrine gland. The endocrine pancreas consists of islets of langerhans. Alpha cells of pancreas secretes glucagon and beta cells release insulin.
Thymus gland is a lymphoid organ that secretes the peptide hormone thymosin. Thymus gland promotes the development of specific cell of the immune system called as T- lymphocytes.
Pineal gland is an endocrine gland found in the cerrebral ventricle in the brain midline. Pineal gland release melatonin hormone which controls circadian rhythm of the body.
Hypothalamus is the basal part of the brain which contain neuro-secretory cells called nuclei These nuclei produce hormones which regulate the functioning of pituitary hormones.
Hormones are natural organic substances that regulate growth, metabolism of the body. Protein hormones perform their action through extracellular receptors while mode of action of steroid hormones is through intracellular receptor.
Dwarfism is a condition in which the growth of the individual is very slow resulting in short stature. There are two types of dwarfism; Proportionate and Disproportionate dwarfism.
Acromegaly is a hormonal disorder that results from the excess of Growth hormone (GH)in the body. In Acromegaly pituitary produces excessive amounts of growth hormone.It is often diagnosed in middle aged adults and can result in serious illness and premature death.
Cretinism refers to severe hypothyroidism which is due to the deficiency of thyroid hormone. This is a medical condition present at birth and characterized by physical deformity, dwarfism and mental retardation..
Goitre is an enlarged thyroid gland that causes the neck to swell. The thyroid gland is controlled by the pituitary gland which prompts the thyroid to secrete hormones T4 and T3 by releasing TSH. Excessive stimulation of the thyroid gland by TSH can result in goitre.
It is preferably known as Diabetes Mellitus. It is a group of metabolic disorders in which the person has high blood glucose due to inadequate insulin production. There are three types of diabetes; type-1, type-2, Gestational diabetes.
Exopthamic goitre is also known as Grave's disease. It is the most common form of hyperthyroidism and thyrotoxicosis. It results in an abnormal over activity of the thyroid gland which produce too much of thyroid hormone; thyroxine.
Addison's disease also called as primary adrenal insufficiency, the adrenal glands do not produce enough hormone named cortisol and aldosterone. Cortisol helps in maintaining body response towards stress.