Integration and Control I: The endocrine system

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Chapter 40

• Integration and Control I The endocrine system

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Signaling molecules

• Diffusion in small animals and at many sites of
  larger animals
• Bulk flow for further distances
• Still too slow for numerous activities
• More rapid, direct channel of communication is
  provided by neurons
• Uses an electrical signal- the nerve impulse

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Neuroendocrine system

• Includes conduction of information by both
  chemical and electrical means
• i.e. neuron conducts information electrically but
  transmits it chemically through neurotransmitters

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Neurosecretory cells

• Nuerons that release their signalling molecules
  into the bloodstream

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Hormones

• Organic molecules secreted in one part of an
  organism that are transported by the bloodstream
• Special effect on target organs or tissues

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Hormones

• Produced by epithelial cells of the digestive
  tract, cardiac muscle cells, white blood cells,
  and injured cells
• In glandular epithelial cells and neurosecretory
  cells, secretion of hormones is the primary
  function

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Exocrine glands

• Secrete their products into ducts
• E.x. digestive glands, milk glands, sweat glands

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Endocrine glands

• Secrete their products into the bloodstream
• ductless glands
• E.x. pituitary, thyroid, adrenal cortex,
  pancreas, medulla, ovary, testis

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Endocrine glands

• Secrete three general hormone types
• Steroids
• Peptides or proteins
• Amino acid derivatives
• Hormones very potent
• Regulated by negative feedback
• Broken down by the liver

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Pituitary Gland

• Located at the base of the brain
• Consists of three lobes the anterior, the
  intermediate, and the posterior

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Anterior Lobe

• Source of at least six different hormones
• 1st Growth hormone, also called somatotropin
• Stimulates protein synthesis and promotes growth
  of bone

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Growth Hormone defects

• Deficit of growth hormone during child hood
  midget, or pituitary dwarf
• Excess of growth hormone during childhood a
  giant
• Excess of growth hormone during adulthood
  acromegaly, the increase in the size of the jaw,
  hands and feet

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Growth hormone contd.

• Affects glucose metabolism, inhibiting uptake and
  oxidation of glucose by some types of cells
• Stimulates the breakdown of fatty acids,
  conserving glucose

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Prolactin

• Produced by anterior pituitary
• Stimulates secretion of milk in mammals
• Controlled by an inhibitory hormone produced by
  the hypothalamus
• Chain reaction controlled by nerve impulses of
  suckling

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Tropic Hormones

• Four hormones secreted by anterior pituitary
• Act upon other endocrine glands to regulate their
  secretions
• One is TSH, thyroid-stimulating hormone, also
  known as thyrotropin
• Stimulates cells in thyroid gland
• Controlled by negative feedback

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Tropic Hormones

• Adrenocorticotropic hormone (ACTH) produced by
  adrenal cortex
• Other 2 tropic hormones are gonadotropins
• Act upon gonads (testes and ovaries)
• Includes follicle-stimulating hormone (FSH) and
  luteinizing hormone (LH)

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Intermediate and Posterior Lobes

• In many vertebrates, intermediate lobe is the
  source of melanocyte-stimulating hormone
• Used for camoflage in reptiles and amphibians
• Posterior lobe stores hormones produced by the
  hypothalamus

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Hypothalamus The pituitary-hypothalamus axis

• Hypothalamus is the source of at least nine
  hormones that act either to stimulate or inhibit
  the secretion of hormones by the anterior
  pituitary
• Very small and only travel few millimeters to the
  pituitary
• Never enter the general circulation uses a
  portal system

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Hypothalamic hormones

• TRH, thyrotropin-releasing hormone
• Stimulates the release of thyrotropin from
  pituitary
• Gonadotropin-releasing hormone (GnRH)
• Controls release of the gonadotropic homrones LH
  and FSH

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Hypothalamic hormones

• somatostatin
• Inhibited release of growth hormone somatotropin
• Oxytocin
• Accelerates childbirth by increasing uterine
  contractions during labor
• Also responsible for the letting down of milk

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Hypothalamic hormones

• Antidiuretic hormone (ADH)
• Decreases excretion of water by the kidneys
• Sometimes called vasopressin because it increases
  blood pressure in many vertebrates
• Oxytocin has some ADH effect and ADH some
  oxytocin effect
• Both hormones consist of only 9 amino acids and
  difference between the two hormones is only 2
  amino acids

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Thyroid gland

• Under the influence of TSH from pituitary,
  produces thyroxine
• Accelerates rate of cellular respiration
• Some animals, it plays major role in temperature
  regulation

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Thyroid disorders

• Hyperthyroidism overproduction of thyroxine
• Rseults in nervousness, insomnia, increased heart
  rate, excessive sweating, weight loss
• Hypothyroidism too little thyroxine
• In infancy, particularly of brain cells, can
  cause mental deficiency and dwarfism
• In adults, causes dry skin, intolerance to cold
  and lack of energy

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Thyroid gland

• Secretes hormone calcitonin in response to rising
  calcium levels in fluid surrounding thyroid cells
• Calcitonins major action is to inhibit release
  of calcium ion from bone

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Parathyroid Gland

• Pea-sized, smallest of known endocrine glands
• Located behind thyroid gland
• Produce parathyroid hormone, which plays an
  essential role in mineral metabolism,
  specifically in the regulation of calcium and
  phosphate ions
• Calcium deficiency can lead to severe
  disturbances in blood coagulation, muscle
  contraction, and nerve function
• Can cause death

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Parathyroid hormone

• Increases the concentration of calcium ion in
  blood in several different ways
• Stimulates the conversion of vitamin D into
  active form, which increases absorption of
  calcium ion
• Reduces excretion of calcium ions from kidneys
• Stimulates release into the bloodstream of
  calcium from bone

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Parathyroid disorders

• Hyperparathyroidism
• Caused by tumors of parathyroids
• Too much parathyroid hormone, causes blones to
  become soft and fragile, vertebrate shrinks

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Adrenal Cortex

• Outer layer of the adrenal gland
• Source of about 50 steroids
• 2 major groups of steroids glucocorticoids and
  mineralocorticoids

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Glucocorticoids

• Cortisol most important glucocorticoid in humans
• Promote the formation of glucose from protein and
  fat
• Decrease utilization of glucose by most cells
• Suppress inflammatory and immune responses
• Used in treatment of autoimmune diseases and
  severe allergic reactions
• side effects of high doses are reduced ability to
  combat infection, redistribution of body fat, and
  mental disturbances

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Cortisol

• Secreted in response to adrenocorticotropic
  hormone (ACTH)
• Secreted by pituitary gland
• Inhibited by negative feedback

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Mineralcorticoids

• Secreted by adrenal cortex
• Aldosterone
• Affect transport of ions across cell membrane

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Adrenal Cortex

• Produces small amounts of male sex hormones in
  both males and females
• Adrenal tumor may result in increased production
  of hormones
• Women get male features i.e. bearded lady

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Adrenal Medulla

• Central portion of adrenal gland
• Secrete adrenaline (or epinephrine) and
  noradrenaline (or norepinephrine)
• Increase rate and strength of heartbeat, raise
  blood pressure, stimulate respiration, and dilate
  the respiratory passages

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Pancreas

• Islet cells of pancreas source of insulin and
  glucagon- regulation of glucose metabolism
• Insulin lowers blood sugar
• Diabetes causes blood sugar to rise so high that
  not all the glucose entering kidney can be
  reabsorbed
• Loss of glucose loss of water

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Glucagon

• Produced by different islet cells of pancreas
• Stimulates the breakdown of glycogen to glucose
  in the liver
• Decreases glucose utilization

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Somatostatin

• 3rd class of islet cells in pancreas
• Released during digestion of a meal
• Exerts a variety of inhibitory effects on
  digestive tract that help regulate the rate at
  which nutrients are absorbed into the bloodstream

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Regulating blood sugar

• growth hormone, cortisol, adrenaline,
  noradrenaline, insulin, glucagon, and
  somatostatin
• Ensures that glucose is always available for
  brain cells

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Pineal gland

• Small lobe in the forebrain
• Secretes melatonin
• In many species, production of melatonin rises at
  night and falls in the day
• May be involved in sexual maturation in humans

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Prostaglandins

• First found in semen and synthesized in seminal
  vesicles
• Differ from hormones they are fatty acids,
  produced by cell membranes, target tissues
  generally the same tissues in which they are
  produced, they produce marked effects at
  extremely low concentrations

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Stimulation of smooth muscle

• Prostaglandins can induce contractions in smooth
  muscle
• Prostaglandins in semen cause the walls of the
  uterus to contract
• Also responsible for contractions during labor
• Cause dysmenorrhea right after giving birth

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Other Prostaglandin effects

• Some inhibit smooth muscle contraction
• One is a potent stimulus for platelet aggregation
  and constriction of blood cells
• One produced by endothelial cells that line the
  blood vessels is a potent inhibitory of platelet
  aggregation and dilator of vessels

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leukotrienes

• Produced by white blood cells
• Interleukins released by activated helper T cells
  as well as a variety of molecules released by
  stimulated macrophages and mast cells

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Mechanisms of action of hormones

• Porstaglandins travel only short distances
• Neurotransmitters travel short distances through
  anatomical connections
• Most hormones broadcast their messages
• Target tissues may be receptive under some
  circumstances but not others

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• Key to this specificity of hormone action lies in
  protein receptor molecules
• Intracellular and membrane receptors

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Intracelullar receptors

• Steroid hormones are small lipid soluble
  molecules
• In cytoplasm of target cells, these hormones
  encounter specific protein recptor and combine
• Thyroid hormone receptor in the nucleus
• Hormone-receptor complex binds to the chromosome
  and initiates mRNA transcription leading to
  protein synthesis

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Membrane receptors

• Receptors on membrane
• Adult diabetes results from decrease in number of
  insulin receptor sites
• After the hormone combines, it either
• Carried into cytoplasm by receptor-mediated
  endocytosis
• Sets in motion a second messenger that is
  responsible for sequence of events inside the
  cell
• 2nd messenger is usually cyclic AMP