Chemical Coordination and Integration

Chemical Coordination and Integration

• Neural system provides point-to-point coordination among organs, which is fast but short-lived
• Hormones are responsible for a special kind of coordination and integration that is continuously regulated
• The neural and endocrine systems work together to coordinate and regulate physiological functions

Endocrine Glands and Hormones

• Endocrine glands, lacking ducts, are called ductless glands
• Hormones, secreted by endocrine glands, are non-nutrient chemicals acting as intercellular messengers produced in trace amounts
• Invertebrates have simple endocrine systems, while vertebrates rely on many chemicals acting as hormones for coordination

Human Endocrine System

• Endocrine glands and hormone-producing tissues/cells constitute the endocrine system.
• Major endocrine glands: pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads
• Other organs like the gastrointestinal tract, liver, kidney, and heart also produce hormones

The Hypothalamus

• It is the basal part of the diencephalon, forebrain and regulates many body functions
• Neurosecretory cells (nuclei) produce hormones that regulate pituitary hormones
• Releasing hormones stimulate pituitary hormone secretion, inhibiting hormones inhibit it
• Gonadotrophin Releasing Hormone (GnRH) stimulates pituitary synthesis and release of gonadotrophins
• Somatostatin inhibits growth hormone release
• Hypothalamic hormones reach the anterior pituitary through a portal circulatory system
• The posterior pituitary is under direct neural regulation

The Pituitary Gland

• Pituitary is located in sella tursica and attached to the hypothalamus by a stalk
• Divided into adenohypophysis and neurohypophysis
• Adenohypophysis has pars distalis (anterior pituitary) and pars intermedia

Anterior Pituitary Hormones

• Growth Hormone (GH)
• Prolactin (PRL)
• Thyroid Stimulating Hormone (TSH)
• Adrenocorticotrophic Hormone (ACTH)
• Luteinizing Hormone (LH)
• Follicle Stimulating Hormone (FSH)
• Pars intermedia secretes Melanocyte Stimulating Hormone (MSH)

Posterior Pituitary Hormones

• Oxytocin
• Vasopressin
• Synthesized by the hypothalamus and transported axonally

Growth Hormone (GH)

• Oversecretion causes gigantism
• Low secretion causes pituitary dwarfism
• Excess secretion in adults leads to Acromegaly
• Acromegaly causes severe disfigurement

Prolactin (PRL)

• Regulates mammary gland growth and milk formation

Thyroid Stimulating Hormone (TSH)

• Stimulates thyroid hormone synthesis and secretion

Adrenocorticotrophic Hormone (ACTH)

• Stimulates synthesis and secretion of glucocorticoids from the adrenal cortex

Luteinizing Hormone (LH) and Follicle Stimulating Hormone (FSH)

• Stimulates gonadal activity (gonadotrophins)
• In males, LH stimulates androgen synthesis
• In males, FSH and androgens regulate spermatogenesis
• In females, LH induces ovulation and maintains the corpus luteum
• In females, FSH stimulates ovarian follicle growth and development

Melanocyte Stimulating Hormone (MSH)

• Regulates skin pigmentation by acting on melanocytes

Oxytocin

• Stimulates smooth muscle contraction
• Causes uterine contraction during childbirth
• Milk ejection from mammary glands

Vasopressin

• Acts at the kidney to stimulate water and electrolyte reabsorption
• Reduces water loss through urine (antidiuretic hormone, ADH)
• ADH impairment results in Diabetes Insipidus

The Pineal Gland

• Located on the dorsal side of the forebrain
• Secretes melatonin
• Regulates the 24-hour (diurnal) rhythm of sleep-wake cycle, body temperature
• Influences metabolism, pigmentation, menstrual cycle, and defense capability

Thyroid Gland

• Composed of two lobes on either side of the trachea connected by the isthmus
• Consists of follicles and stromal tissues
• Follicular cells synthesize tetraiodothyronine (T4) or thyroxine and triiodothyronine (T3)
• Iodine is essential for normal thyroid hormone synthesis

Thyroid Deficiency

• Hypothyroidism results in goitre
• Hypothyroidism during pregnancy causes cretinism
• Other effects include stunted growth and mental retardation

Thyroid Disorders

• Hyperthyroidism is caused by increased hormone synthesis and secretion, which adversely affects body physiology
• Exophthalmic goitre involves thyroid gland enlargement, bulging eyeballs, increased metabolic rate, and weight loss (Graves' disease)

Thyroid Hormones

• Regulates basal metabolic rate
• Supports red blood cell formation
• Controls metabolism of carbohydrates, proteins, and fats
• Influences water and electrolyte balance
• Secretes thyrocalcitonin (TCT) which regulates blood calcium levels

Parathyroid Gland

• Four glands on the back side of the thyroid gland
• Secretes parathyroid hormone (PTH)
• PTH secretion is regulated by calcium ion levels
• PTH increases blood calcium levels
• Stimulates bone resorption, reabsorption of Ca2+ by renal tubules, and Ca2+ absorption from digested food
• Hypercalcemic hormone; regulates calcium balance

Thymus Gland

• Lobular structure located behind the sternum
• Plays a major role in the development of the immune system
• Secretes thymosins
• Thymosins aid in T-lymphocyte differentiation
• Thymosins promotes production of antibodies for humoral immunity
• Degenerates later in life, leading to decreased thymosin production and weaker immune responses

Adrenal Gland

• A pair located on the anterior part of each kidney, composed of the adrenal medulla and adrenal cortex
• Underproduction of adrenal cortex hormones causes Addison's disease, leading to weakness and fatigue
• Adrenal medulla produces adrenaline (epinephrine) and noradrenaline (norepinephrine), known as catecholamines

Catecholamines

• Secreted during stress and emergencies ("fight or flight" hormones)
• Increase alertness, pupillary dilation, piloerection, sweating, heart rate, heart contraction strength, and respiration rate
• Stimulate glycogen breakdown

Adrenal Cortex

• Divided into zona reticularis, zona fasciculata, and zona glomerulosa
• Secretes corticoids involved in carbohydrate metabolism (glucocorticoids) like cortisol
• Regulates water and electrolyte balance (mineralocorticoids) such as aldosterone

Glucocorticoids

• Stimulate gluconeogenesis, lipolysis, and proteolysis
• Inhibit cellular amino acid uptake
• Maintains cardiovascular system and kidney functions
• Produces anti-inflammatory reactions and suppresses immune response

Mineralocorticoids

• Acts on renal tubules to stimulate reabsorption of Na⁺ and water and excretion of K+ and phosphate ions
• Aldosterone helps maintain electrolytes, fluid volume, osmotic pressure, and blood pressure

Androgenic Steroids

• The adrenal cortex also secretes small amounts of androgenic steroids, playing a role in the growth of axial, pubic, and facial hair during puberty

Pancreas

• Functions as both exocrine and endocrine gland
• Endocrine pancreas consists of Islets of Langerhans (1 to 2 million in a normal human)
• Alpha cells secrete glucagon, beta cells secrete insulin

Glucagon

• Peptide hormone that maintains normal blood glucose levels
• Acts on liver cells (hepatocytes)
• Stimulates glycogenolysis, resulting in increased blood sugar (hyperglycemia)
• Stimulates gluconeogenesis and reduces glucose uptake
• It is a hyperglycemic hormone

Insulin

• Peptide hormone which regulates glucose homeostasis
• Acts on hepatocytes and adipocytes.
• Enhances cellular glucose uptake
• Stimulates glycogenesis, leading to decreased blood glucose levels (hypoglycemia)

Glucose Homeostasis

• Blood maintains a balance between Insulin and glucagon.
• Prolonged hyperglycemia leads to diabetes mellitus characterized by loss of glucose through urine and formation of ketone bodies.

Testis

• A pair of Testes are present in the scrotal sac outside the abdomen of male individuals
• It performs dual functions as a primary sex organ and an endocrine gland
• Composed of seminiferous tubules and stromal or interstitial tissue.
• Leydig cells (interstitial cells) produce androgens (mainly testosterone).

Androgens

• Regulates the development, maturation, and functions of male accessory sex organs.
• Stimulates muscular growth, facial hair, aggressiveness, and voice pitch reduction.
• Major stimulatory role in spermatogenesis
• Acts on the central nervous system in influencing the male libido

Ovary

• A pair of ovaries is located in the abdomen
• The primary female sex organ which produces one ovum during each menstrual cycle
• They produce two groups of steroid hormones: estrogen and progesterone.
• Estrogen is primarily synthesised and secreted by ovarian follicles
• After ovulation, the ovarian follicles are then converted to the corpeus luteum, which secretes mainly progesterone.

Estrogen

• Produces wide ranging actions such as the stimulation of growth and activity of female secondary sex organs
• Development of growing ovarian follicles and appearance of female secondary sex characteristics

Progesterone

• Supports pregnancy and acts on the mammary glands
• Stimulates the formation of alveoli and milk secretion

Hormones of Heart, Kidney and Gastrointestinal Tract

• The atrial wall of the heart secretes atrial natriuretic factor (ANF) to decrease blood pressure by dilating blood vessels
• Juxtaglomerular cells of the kidney produce Erythropoietin, which helps in the formation of RBCs
• Endocrine cells in the gastro-intestinal tract secretes four major peptide hormones.

Gastro-intestinal Tract Hormones:

• Gastrin: Acts on the gastric glands to stimulate hydrochloric acid and pepsinogen
• Secretin: Acts on the exocrine pancreas to stimulate secretion of water and bicarbonate ions
• Cholecystokinin (CCK): Acts on the pancrease and gall bladder to stimulate secretion of pancreatic enzymes and bile juice
• Gastric Inhibitory Peptide (GIP): Inhibits gastric secretion and motility

Growth Factors

• Several non-endocrine tissues secrete growth factors to support growth, repair and regeration.

Mechanism of Hormone Action:

• Hormones produce their effects on target by binding to hormone receptors.
• Membrane-bound receptors are present on the cell membrane
• Intracellular receptors are present inside the target cells

Hormone Receptors

• The receptors are specific to one hormone
• Hormone-Receptor complex formation leads to biochemical changes in target issue
• Target tissue metabolism and physiological functions are regulated by hormones
• Divided into peptide, polypeptide, and protein.

Hormones that interact with membrane-bound receptors normally do not enter the target cell

• Generate other messengers, which regulate cellular metabolism
• Those that interact with intracellular receptors usually regulate gene expression.