Endocrinology

• Endocrinology


  Communication within the human body involves the transmission of signals to control and coordinate actions to maintain homeostasis
• Nervous system (NS) and endocrine system


  • They are the two major organ systems responsible for providing communication pathways, and are directly or indirectly under the control of the CNS
• Response by the NS

  Rather localized & more rapid through faster neural impulses and by the release of a local transmitter
• Endocrine system

  Works along with the nervous system to regulate the functions of the human body to maintain homeostasis
• Hypothalamus


  Its endocrine functions include the production and release of hormones, so it is considered to be a neuroendocrine organ

  
• Endocrine system


  Responds through the release of chemicals (hormones) directly into the circulation, the influence of which is rather prolonged & may be on distinct target organs
• Hormones


  Highly potent, specialized organic chemicals produced by endocrine cells in response to specific stimuli that exert their actions on specific target cells

  
• Functions controlled by hormones

  • Cellular metabolism
  • Growth
  • Osmoregulation
  • Reproduction
  • Cardiovascular function
  • Digestion
• Exocrine glands vs Endocrine glands


  Difference between them

  
• Cell signaling


  Process by which cells communicate with each other using chemical signals
• Ligand


  Chemical messenger released by one cell to signal either itself or a different cell
• Stages of cell signaling

  1. Receptors

  2. Transduction

  3. Amplification

  4. Integration

  5. Cell response

  6. Termination

  
• Types of cell signaling

  • Endocrine
  • Paracrine
  • Autocrine
  • Synaptic
  • Direct signaling across gap junctions
• Neurotransmitters


  Substances which neurons use to communicate with one another and with their target tissues in the process of synaptic transmission

  
• Neurotransmitters


  • Located in the axon terminal
  • Stored within synaptic vesicles
  • Released into the synaptic junction
  • Bind to specific receptors on the target cell
  • Trigger a change or action in the target cell

  
• Neurohormones


  Secreted by neurosecretory cells, translate neural signals into chemical stimuli, travel along the neuron axon and are typically released into the bloodstream at neurohemal organs

  
• Hormones have no direct effect on target cells, they first bind to a receptor on the target cells, forming a hormone-receptor complex</b>

  
• Hormone receptors


  Proteins that are not static cell components, their number grows or falls under different conditions, each receptor recognises only one hormone
• Feedback control of hormone production

  Can be positive (low levels stimulate release) or negative (high levels inhibit release), self-regulating and prevent hormonal overproduction
• Areas of endocrine control

  • Metabolic function
  • Growth
  • Development
  • Reproduction
  • Homeostasis
• Chemical Nature of Vertebrate Hormones

  • Steroids
  • Peptides & proteins
  • Tyrosine-derived hormones

  
• Steroid hormones


  All are derived from cholesterol. E.g. androgens, estrogens, progesterones, & corticosteroids (cortisone & aldosterone)
• Peptide & protein hormones

  • Several hormones from the hypothalamus exert their action by causing the release of other hormones from their respective endocrine glands
  • Releasing hormones (RH) are all peptides that contain from 3 to 14 amino acids
  • Major hormones originating from the adenohypophysis (AH) are protein in nature & may contain several hundred amino acids. E.g., growth hormone contains 191 amino acids

  
• Tyrosine-derived hormones


  • Two common catecholamines are norepinephrine (noradrenalin) & epinephrine (adrenaline)
  • Tyrosine is also the raw material for the syntheses of the thyroid hormones triiodo-thyronine & thyroxine
• Brain Control of Endocrine Function

  • Constant interaction between endocrine organs & CNS
  • Brain directly or indirectly influences & controls endocrine function
  • Hormones also have profound effects on the function of the CNS
  • The hypothalamus plays a completely dominant role in the control of endocrine function
  • Adjacent parts of the brain, the supraoptic & paraventricular nuclei, are part of this system

  
• Hypothalamus


  • Located at the base of the brain, immediately above the hypophysis (also known as pituitary gland)
  • Is the seat of several nervous control functions, notably temperature regulation & regulation of intake of water & food
  • Controls the function of the hypophysis - a two-lobed organ i.e., adenohypophysis (AH) and neurohypophysis (NH)
  • Control is mediated to NH via neural connections & to the AH via special blood vessels, known as the portal circulation

  
• Neurohypophysis (NH)


  • Contains mostly nerve fibers and neuroglial cells, which support axons that extend from the hypothalamus
  • Receives, stores, and releases hormones (oxytocin and antidiuretic hormone/vasopressin) made in the hypothalamus and transported to the NH via axons
  • Serves as a storage & release organ (neurohemal organ) for the hormones that are produced in the hypothalamus of the brain
  • Contains 2 hormones, ADH or vasopressin & oxytocin

  
• ADH (vasopressin)


  • Responsible for absorption of water in the kidney & is necessary for the formation of concentrated urine
  • If injected in large amounts, it causes a marked rise in blood pressure owing to constriction of arterioles
• Oxytocin


  Causes contraction of the smooth muscle of the uterus in the pregnant female at term
• Adenohypophysis (AH)

  • Anterior lobe, made up of glandular tissue
  • Synthesizes and secretes a number of hormones
  • Surrounded by a capillary network that extends from the hypothalamus to AH
  • Capillary network is a part of the hypophyseal portal system that carries substances from the hypothalamus to AH then into the circulatory system
  • Has three regions: pars distalis, pars intermedia, and pars tuberalis

  
• Hypothalamic Control System


  • AH produces hormones, & their release into the blood is regulated by the hypothalamus
  • The secretion of hormones from the AH is regulated by two classes of hormones: releasing hormones (RH) that stimulate the secretion of hormones from the AH and the release inhibiting hormones (RIH) that inhibit secretion
  • Hypothalamic hormones are secreted by neurons, but enter AH through blood vessels
  • Within the infundibulum is a bridge of capillaries that connects the hypothalamus to the AH, called the hypophyseal portal system
  • Allows hypothalamic hormones to be transported to the AH without first entering the systemic circulation
  • Hormones produced by the AH (in response to RH) enter a secondary capillary plexus, and from there drain into the circulation

  
• Hypothalamic regulating hormones involved in the system that controls the AH

  • 10 hormones (see table slide 23)

  
• Tropic hormones


  • ACTH, TSH, LH & FSH have as their targets the adrenal cortex, the thyroid & the gonads, respectively
  • These glands when stimulated release the appropriate hormones into the blood & the hormones (corticosteroids, thyroxine & reproductive steroids) in turn inhibit, by negative feedback, the secretion from the AH of the tropic hormones

  
• The major portion of the entire endocrine system is under nervous control, acting through the central role of the hypothalamus in the control system

  
• Hormones


  Produce their specific effects on target tissues via specialized receptor proteins located either inside the cell or on the surface of the cell

  
• Lipid-soluble (hydrophobic) hormones (steroid & thyroid hormones)


  • Readily penetrate the plasma membrane and bind to receptors in the cytoplasm of target cells

  
• Lipid-insoluble (hydrophilic) hormones (catecholamines and peptides)


  • Cannot penetrate the plasma membrane, bind to receptors on the cell surface

  
• Intracellular mechanism of action of a hormone

  Depends on whether it binds to cytoplasmic or cell-surface receptors

  
• Lipid-soluble hormones


  1. Bind to cytoplasmic receptors, forming hormone-receptor complexes

  2. Translocate to the nucleus

  3. Act directly on the DNA of the cell to effect long-term changes lasting hours or days

  
• Lipid-insoluble hormones


  1. Bind to cell-surface receptors

  2. Often leading to production of one or more second messengers

  3. Amplify the signal and mediate rapid, short-lived responses via various effector proteins