Chemical Coordination and Integration

Introduction

Our body produces special chemical messengers called hormones, which are imperative in ensuring an efficient system of coordination and control, along with the nervous system. Different types of hormones are produced by a variety of different endocrine glands, across the body. These hormones ensure normal and efficient functioning. What are these hormones, and how are they produced? Are they always present in the blood, at all times, or is their production regulated?

What is an Endocrine System?

• The endocrine system is an essential regulatory system of the body. It comprises glands that lack ducts, i.e., the endocrine system is made up of ductless glands.

• These glands secrete very crucial chemicals known as hormones. These ductless glands are also referred to as endocrine glands.

• The endocrine system comprises the hypothalamus (the main control centre) and pituitary gland, thyroid gland, parathyroid gland, thymus gland, pancreas, adrenal glands, and the primary reproductive organs.

Hormones and their Functions

• Hormones are special signal molecules of the body, aka chemical messengers of the body.

• These molecules are released into the blood by the endocrine glands and broadcast to the different targeted parts of the body.

• The target cells bear specific receptors to which the particular hormones bind, and initiate a response.

• Once recognised, a response pathway is triggered, wherein the signal is relayed and amplified by a signal transduction process which culminates with the cellular response.

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• Hormones regulate various processes of the body including but not limited to metabolism, growth and development, sexual development, reproduction, and homeostasis, etc.

• Hormones are a crucial part of our coordination and control. The body releases different kinds of hormones, with each hormone having a different function.

Regulation – Feedback Mechanism

• A question that may occur in the mind is, how is the secretion of hormones controlled? That is, how does the body know when to stop the secretion of a hormone?

The endocrine system has a very interesting way of regulating hormones - the feedback mechanism.

Negative Feedback Mechanism

• Sometimes, a feedback signal is sent by a hormone to the gland, prompting its own discontinuation. This is known as negative feedback.

• An often cited example of a negative feedback mechanism is that of the thyroid hormone. The thyrotropin-releasing hormone (TRH) is secreted by the hypothalamus.

• The thyroid-stimulating hormone (TSH) (pituitary gland) is stimulated by TRH

• Finally, thyroid hormones (T₃ and T₄) are stimulated by TSH.

• When a certain level is reached, these hormones send a feedback signal to the hypothalamus, asking it to stop producing the TRH, and to the pituitary, thereby discontinuing the secretion of TSH.

• Naturally, when TSH is absent, there’s nothing to stimulate the secretion of thyroid hormones.

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In this way, the secretion of thyroid hormones is regulated by the thyroid hormones themselves.

Positive Feedback Mechanism

• Contrary to what happens in negative feedback, a hormone tends to signal an increase in its production.

• Although rare, positive feedback mechanisms are quite important in some cases, such as during lactation.

• As the baby suckles, the pituitary gland is stimulated to secrete more prolactin when it receives signals from the nerve cells of the mother’s nipples.

• Once prolactin is released, the mammary glands can secrete more milk.

Common Disorders

• Hormonal disorders are a result of hormonal imbalances- hyposecretion or hypersecretion.

• Defective hormonal receptors or inadequate numbers of receptors can also cause defects in the coordination system.

Pituitary Gland Disorders

• Hyposecretion of the growth hormone can lead to pituitary dwarfism, characterised by short stature and retarded skeletal growth.

• Alternatively, hypersecretion of growth hormone leads to gigantism in children and acromegaly in adults.

Pancreatic Disorders

• One of the most well-known hormonal disorders, diabetes mellitus is a disease in which glucose is insufficiently metabolised.

  Diabetes is of two major types, type 1 (aka insulin-dependent diabetes) and type 2 diabetes mellitus (insulin-independent diabetes).

Thyroid Gland Disorders

• Hypothyroidism and hyperthyroidism are the two major disorders of the endocrine system.

• Hypothyroidism causes a reduction in metabolic activity, along with a tendency to gain weight and fatigueness.

• Hypothyroidism at birth leads to cretinism, characterised by dwarfism and mental retardation.

  Goitre is another disease that results from the enlargement of the thyroid gland due to overstimulation of the gland.

Adrenal Gland Disorders

• Cushing’s syndrome is a disorder caused by the hypersecretion of cortisol.

• Another disorder of the adrenal gland is Addison’s disease, caused due to hyposecretion of both glucocorticoids and mineralocorticoids.

Conclusion

The endocrine system comprises ductless glands that are involved in the production of chemicals called hormones. Hormones are special messenger molecules of the body, released by endocrine glands, that are involved in generating a cellular response. Each hormone performs a specific function, and its production is regulated tightly by a feedback mechanism. Feedback mechanisms are of two kinds - the negative feedback mechanisms as seen in the secretion of thyroid hormones, and the positive feedback mechanisms as seen in the case of oxytocin production during labour, and prolactin production during lactation. The endocrine system is subject to several kinds of disorders, which mainly affect hormone secretion and sometimes the hormone receptors as well. Some significant hormonal disorders include hyper- and hypothyroidism, gigantism, diabetes, pituitary dwarfism, Cushing’s syndrome, Addison’s disease, etc.

FAQs

Q1. What is Hormone Replacement Therapy?

Ans. Hormone replacement therapy involves the use of specific medications that contain estrogen and progesterone. This therapy is especially employed by postmenopausal women and women who have undergone hysterectomy.

Q2. Does the endocrine system consist only of glands?

Ans. Most of the hormone-secreting structures of the endocrine system are glands. However, some organs such as the hypothalamus and the thymus, although not classified as endocrine glands, contain cells that can secrete hormones.

Q3. What is the connection between Grave’s disease and hyperthyroidism?

Ans. Grave’s disease is the most common cause of hyperthyroidism. It is an autoimmune disorder, wherein the thyroid-stimulating immunoglobulin is produced by the body. Its target is the TSH receptors of the thyroid cells. This then stimulates the growth of the thyroid. However, TSI, unlike TSH, is not subject to negative-feedback inhibition by the thyroid hormone. This results in excessive secretion of the thyroid hormone, or hyperthyroidism.

Q4. What are somatotropic cells?

Ans. These are the cells of the pituitary gland that secrete the human growth hormone.

Q5. How is oxytocin regulated via a positive feedback mechanism?

Ans. During labour, the uterine walls send a signal to the pituitary gland, as the baby pushes against the walls. The posterior pituitary is stimulated, thereby secreting oxytocin aka the pregnancy hormone. Oxytocin causes the walls to contract, thereby initiating delivery of the baby. Once the baby is delivered, the walls don’t contract anymore, thereby terminating oxytocin secretion.