W1 L1: Intro to Endocrinology

- hormone regulating process

Physiological Importance:

• Controls many aspects of physiology, via secretion of hormones
• Endocrine and neuronal (nervous) systems are considered the 2 major control systems
  • although there is substantial interaction between them (emergence of ‘neuroendocrinology’ as a discipline).
• Endocrine systems also interact with; cardiovascular, digestive and immune system

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• Endocrine disruptors - natural or man-made chemicals that may mimic or interfere with the body's hormones

  • e.g. feminisation of fish near business-dense areas due to incr. micro plastic in the water

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What is a hormone?

• chemical messenger secreted into blood to act on distant targets
• present at v. low conc in the body, as highly specific mechanisms of action in target tissues
• grouped by biochem & physiological function
• dynamics of secretion and action vary greatly, depending on the hormone and physiological role

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How does a hormone reach its target (signalling)?

• A substance produced by 1 tissue in the body that travels in the bloodstream to act on another tissue.
• Autocrine - cell releases substance and has receptors itself to respond to the signal
• Endocrine - cell releases hormone which then travels in blood stream to distant targets (could be cm’s to long distance)
• Paracrine - releases hormone and neighbouring cell responds due to receptor on cell surface
• Neuroendocrine - neurotransmitter released from synapse

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Biochem of hormones

- more accurate way of categorising hormones compared to physiology

• 3 main gps
• AA derivatives
  • tryptophan (melatonin)
  • tyrosine (others, e.g. adrenaline)
  • water soluble
• peptides
  • TRH (3AA)
  • Growth hormone (200AA)
  • water soluble
• Steroids - used to generate cortisol and vit D3
  • lipid soluble

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Anatomy of endocrine system

• endocrine glands/organs

  • ductless glands that secrete hormones into the blood
  • (NB exocrine gland: secretes onto an epithelial surface, such as of the gut lumen, usually via a duct)

• Pancreas acts as both an endocrine and an exocrine organ

  • located throughout the body
  • often contain multiple cell types, which have different function (e.g. the pituitary gland) - referred to as heterogeneous

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Control of hormone secretion

1. Stimulus changes composition of blood

   • Blood components
   • Nutrients
   • Ions

2. Endocrine organs & glands – there are quite a few

   • pineal gland - secretes melatonin → important for bio rhythms
   • pituitary gland
   • hypothalamus - portal blood supply important in supplying to the anterior part
   • adipose tissue (fat) & GI tract - secrete hormones involved in E balance & metabolism
   • placenta - secrete hormones involved in foetal and maternal development

3. Hormones released from a gland are transported in blood to target cell

   • Receptors on target cells effect changes
   • Receptors on cell sense changes and initiate further changes
   • Tropic hormones control the activity of a target endocrine cell. E.g. Adrenocorticotropic hormone (ACTH)

4. Metabolic/other effect observed

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‘Heterogeneity’ of Endocrine Organs

• Heterogeneity: amount of variation within a subject

  • cf homogeneity: uniformity of a subject
  • Different cell types are subject to different control mechanisms (inputs) and secrete different hormones

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Feedback networks

• Maintain homeostasis
• Most pathways consist of these components: sensor, comparator, transducer and receptor
• Feedback is where systems limit one another's activity around a pre-set oscillator (physiological range).
• Communication between the receptor, control centre and effector is essential for normal operation of the system.

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Hormonal feedback

• System 1 responds to stimulus by increasing hormone 1 output, which then stimulates system 2 to increase hormone 2 output.

• E.g. Oestrogen from the ovary increases Luteinising hormone (LH) production from the anterior pituitary gland which results in ovulation

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If hormones cause changes in physiology, how do they do this?

• They bind to receptors on target cells
• Receptors may be located at diff. conc in diff. parts of the body
• receptor conc dictate the strength of reaction to the hormone:
  • high receptor conc → big reaction
  • low receptor conc → small reaction

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Effects of hormone-receptor binding

• Change in transporter activity
• Activation of secondary messengers (signalling cascade) e.g. Ca2+ release or cAMP
• Effects are long acting (hours, days) because hormones influence protein synthesis (through gene expression)

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Patterns of hormone secretion

Episodic secretion → up & down around a normal point

stress response → due physical or mental stress

Circadian rhythm → follows a pattern (night & day)

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QUIZ

1. Are hormones found at
   (a) high
   or
   %%(b) low concentration in the blood?%%
2. %%True%% or false?
   Some hormones may require days to exert their physiological effects.
3. What best describes a paracrine signal?
   (a) It is secreted by a nerve cell
   %%(b) It acts on an adjacent cell%%
   (c) It’s a steroid
4. True or %%false%%?
   All endocrine organs are made up of a single cell type.

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