- 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

• 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

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

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

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

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

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

‘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

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.

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

  

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

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)

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)

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.