Chemical Coordination and Integration

Chemical Coordination and Integration

• Neural system provides rapid coordination among organs, but it is short-lived and does not innervate all cells
• Hormones facilitate a special kind of coordination and integration
• Neural and endocrine systems coordinate and regulate physiological functions jointly

Endocrine Glands and Hormones

• Endocrine glands, also known as ductless glands, secrete hormones
• Hormones are non-nutrient chemicals acting as intercellular messengers produced in trace amounts
• Invertebrates have simple endocrine systems, vertebrates use a large number of chemicals as hormones for coordination

Human Endocrine System

• Endocrine glands and hormone-producing tissues/cells constitute the endocrine system
• Organized endocrine bodies include the pituitary, pineal, thyroid, adrenal, pancreas, parathyroid, thymus, and gonads
• Other organs like the gastrointestinal tract, liver, kidney, and heart produce hormones
• An overview of the major endocrine glands and the hypothalamus follows

Hypothalamus

• A basal part of the diencephalon in the forebrain regulates a wide spectrum of body functions
• Contains neurosecretory cells called nuclei that produce hormones
• Hormones produced are of two types:
  • Releasing hormones stimulate pituitary hormone secretion
  • Inhibiting hormones inhibit pituitary hormone secretion
• Gonadotrophin releasing hormone (GnRH) stimulates pituitary synthesis and release of gonadotrophins
• Somatostatin from the hypothalamus inhibits the release of growth hormone from the pituitary
• Hormones from hypothalamic neurons pass through axons and are released at nerve endings
• The hormones reach the pituitary gland through a portal circulatory system, regulating the anterior pituitary's functions
• The posterior pituitary is under direct neural regulation of the hypothalamus

Pituitary Gland

• Located in the sella tursica, a bony cavity, and attached to the hypothalamus via a stalk
• Anatomically divided into the adenohypophysis and neurohypophysis
• Adenohypophysis consists of the pars distalis and pars intermedia
• Pars distalis (anterior pituitary) produces:
  • 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)
• In humans, the pars intermedia merges with the pars distalis
• Neurohypophysis (posterior pituitary) stores and releases oxytocin and vasopressin, which are synthesized by the hypothalamus and transported axonally
• Oversecretion and low secretion of GH results in gigantism and pituitary dwarfism respectively
• Excess GH secretion in middle age adults causes disfigurement called Acromegaly
• Prolactin regulates mammary gland growth and milk formation
• TSH stimulates the synthesis and secretion of thyroid hormones
• ACTH stimulates the synthesis and secretion of glucocorticoids from the adrenal cortex
• LH and FSH stimulate gonadal activity and are called gonadotrophins
• In males, LH stimulates androgens synthesis and secretion from the testis
• In males, FSH and androgens regulate spermatogenesis
• In females, LH induces ovulation of mature follicles (graafian follicles) and maintains the corpus luteum formed
• FSH stimulates growth and development of ovarian follicles in females
• MSH acts on melanocytes to regulate skin pigmentation
• Oxytocin contracts smooth muscles, stimulates uterus contraction during childbirth and milk ejection
• Vasopressin stimulates water resorption in the kidney by the distal tubules, reducing water loss through urine (diuresis); also known as anti-diuretic hormone (ADH)
• Impaired ADH synthesis or release leads to Diabetes Insipidus due to the kidneys diminished ability to conserve water

Pineal Gland

• It is situated on the forebrain's dorsal side
• Secretes melatonin
• Melatonin regulates a 24-hour diurnal rhythm of the body
• Diurnal rhythm includes sleep-wake cycle and body temperature maintenance
• Melatonin influences metabolism, pigmentation, menstrual cycle and defense capability

Thyroid Gland

• Composed of two lobes on either side of the trachea connected by the isthmus
• The thyroid gland contains follicles and stromal tissues
• Follicular cells synthesize:
  • tetraiodothyronine or thyroxine (T4)
  • triiodothyronine (T3)
• Iodine is essential for normal thyroid hormone synthesis
• Iodine deficiency causes hypothyroidism and goitre (enlarged thyroid gland)
• Hypothyroidism in pregnancy causes defective development and maturation

Diseases and Functions of the Thyroid

• Hypothyroidism can lead to cretinism, mental retardation, low intelligence quotient, abnormal skin, and deaf-mutism in babies
• In adult women, hypothyroidism can cause irregular menstrual cycles
• abnormally high hormone synthesis and secretion leads to Hyperthyroidism
• Exopthalmic goitre or Graves' disease is a hyperthyroidism form which causes enlargement of the thyroid gland, protrusion of the eyeballs, increased basal metabolic rate & weight loss
• Thyroid hormones regulate basal metabolic rate, erythropoiesis & metabolism of carbohydrates, proteins, and fats and influences water & electrolyte balance
• Thyrocalcitonin (TCT) regulates blood calcium levels

Parathyroid Gland

• Four glands are present on the backside of the thyroid gland
• Secretes parathyroid hormone (PTH) which is regulated by the circulating calcium ions level
• PTH increases blood calcium levels and stimulates bone resorption
• PTH stimulates calcium reabsorption by renal tubules and increases calcium absorption from digested food
• PTH is a hypercalcemic hormone

Thymus

• It's a lobular structure located between the lungs behind the sternum
• Thymus is present on the ventral side of the aorta
• Thymus plays a major role in the development of the immune system
• Secretes thymosins which differentiate T-lymphocytes for cell-mediated immunity
• Promotes antibodies production for humoral immunity
• Thymus degenerates in old individuals, decreasing thymosins production, weakening immune responses

Adrenal Gland

• One pair is located at the anterior part of each kidney
• Composed of the adrenal medulla centrally and the adrenal cortex outside
• Underproduction of adrenal cortex hormones leads to Addison's disease, altering carbohydrate metabolism with acute weakness and fatigue
• The adrenal medulla secretes adrenaline (epinephrine) and noradrenaline (norepinephrine)
• These are catecholamines secreted rapidly during stress and are called emergency hormones or hormones of fight or flight.
• Catecholamines increase alertness, pupilary dilation, piloerection, sweating, heart rate & the strength of heart contraction and the rate of respiration plus stimulates glycogen breakdown

Adrenal Cortex

• The cortex is divided into three layers:
  • Zona reticularis (inner layer)
  • Zona fasciculata (middle layer)
  • Zona glomerulosa (outer layer)
• Cortex secretes corticoids, which include glucocorticoids involved carbohydrate metabolism
• Cortisol is the main glucocorticoid
• Mineralocorticoids regulate water and electrolyte balance
• Aldosterone as the main mineralocorticoid
• Glucocorticoids stimulate gluconeogenesis, lipolysis, and proteolysis while inhibiting cellular amino acid uptake
• Cortisol maintains the cardiovascular system and kidney functions, produces anti-inflammatory reactions, suppresses the immune response & stimulates RBC production
• Aldosterone acts on renal tubules to stimulate reabsorption of Na+ and water plus excretion of K+ and phosphate ions to maintain the electrolytes, osmotic pressure and the blood pressure
• Small amounts of androgenic steroids from the adrenal cortex plays a role in the growth of hair during puberty

Pancreas

• Pancreas acts as both exocrine and endocrine gland
• Endocrine pancreas contains Islets of Langerhans, approximately 1 to 2 million or 1-2% of the pancreatic tissue
• Islet has ɑ-cells and β-cells, glucagon hormone is secreted by α-cells, while B-cells secrete insulin
• Glucagon is a peptide hormone maintaining blood glucose levels
• Glucagon stimulates glycogenolysis in hepatocytes, leading to hyperglycemia and promotes gluconeogenesis
• Insulin reduces cellular glucose uptake
• Glcuagon is a hyperglycemic hormone
• Insulin, a peptide hormone, enhances cellular glucose uptake in hepatocytes and adipocytes; the regulation of glucose homeostasis
• Insulin stimulates conversion of glucose to glycogen in target cells known as glycogenesis
• The glucose homeostasis in the blood is thus jointly maintained by both the insulin and glucagon hormones

Prolonged Hyperglycemia

• Prolonged hyperglycemia leads to diabetes mellitus
• Diabetes is associated with loss of glucose through urine and production of ketone bodies
• Diabetic patients require insulin therapy for treatment

Testis

• A pair of testis is present in the scrotal sac (outside abdomen) and performs the function of a primary sex organ plus an endocrine gland
• The testis contains seminiferous tubules and interstitial tissue, the Leydig cells or interstitial cells, are located in intertubular spaces which produce androgens mainly testosterone hormone
• Androgens regulate development, maturation, and functions of male accessory sex organs
• These hormones are epididymis, vas deferens, seminal vesicles, prostate gland, urethra etc
• Androgens stimulates muscular growth, growth of facial and axillary hair, aggressiveness, low pitch of voice
• Also stimulates spermatogenesis (formation of spermatozoa)
• Hormones that acts on the central neural system to influences the male sexual behavior (libido)
• Androgens produce anabolic (synthetic) effects on protein and carbohydrate metabolism.

Ovary

• A pair of ovaries is located in the abdomen region which is a primary female sex organ
• Ovary produces one ovum during each menstrual cycle
• Produces the are 2 groups of hormones estrogen and progesterone:
  • Estrogen that is synthesised and secreted mainly by the growing ovarian follicles
  • Progesterone that is secreted mainly by the corpus luteum after ovulation in females
• Estrogens are responsible for stimulation of growth & activity of female accessory sex organs, development of secondary sex characteristic
• Progesterone contributes to pregnancy and in lactation by stimulating the formation of alveoli in mammary glands

Hormones of Heart, Kidney, and Gastrointestinal Tract

• Important peptide hormone from atrial wall of heart, atrial natriuretic factor (ANF), decreases blood pressure by causing dilation of the blood vessels
• Kidney produce peptide hormone called erythropoietin produced from juxtaglomerular cells, stimulates erythropoiesis (formation of RBC or Red Blood Cells)
• Four major peptide hormones from the endocrinal cells in the gastro-intestinal tract:
  • Gastrin acts on the gastric glands and stimulates hydrochloric acid and pepsinogen secreation
  • Secretin stimulate exocrine pancreas in secreation of water and bicarbonate ions
  • Cholecystokinin(CCK) simulate the secreation of pancreatic enzymes and bile juice and act on pancreas and gall bladder
  • Gastric inhibitory peptide (GIP) which gastric secreation and motility gets inhibited
• Growth factor hormone from non-endocrine tissues are essential in growth of tissues plus repairing or regeneration

Mechanism of Hormone Action

• Hormones affect target tissues by binding to hormone receptors found on those tissues
• Membrane-bound receptors are present on the cell membrane while intracellular receptors are inside the cell, mostly in the nucleus
• Binding of hormone to the receptor produces hormone-receptor complex and is hormone specific
• Hormone-Receptor complex formation induces target tissue metabolism changes , as the physiological functions are regulated by hormones
• Chemical nature of the hormones divided into four groups are:
  • Peptide, polypeptide, protein hormones
  • Steroids
  • Iodothyronines
  • Amino-acid derivatives
• Hormones like membrane-receptors generates second messengers cyclic AMP, IP3, Ca++ etc, by not entering into the target cell
• The second messengers regulates the cellular metabolism
• Hormone interacting intracellular receptors steroid hormones and iodothyronines regulates gene expression or chromosome function by the interaction of hormone complex with genome
• The cumulative biochemical actions by the complex cause effects in physiology and development