Tutorial 7 - endocrine system Flashcards
What are the principle differences between the endocrine and nervous systems?
Endocrine system:
- Usually slower, more prolonged response – though exceptions exist – e.g. adrenalin and cortisol are released rapidly
- Uses chemical messengers or hormones
- Affects most body cells
Nervous system:
- Rapid response
- Uses electrochemical impulses & neurotransmitters
- Mainly affects muscle fibres, smooth muscle (in blood vessel and organs), and glands
Define ‘exocrine glands’
- Exocrine glands secrete their products into ducts.
(none of these are hormones) - These glands include sudoriferous (sweat) glands, sebaceous (oil) glands, mucous glands, digestive glands and several other throughout the body.
Define ‘endocrine glands’
- Endocrine glands secrete hormones into the blood. They do not have ducts and secrete their hormones directly into the interstitial fluid that surrounds them.
- The hormones diffuse into the blood stream through capillaries and are carried to target cells throughout the body.
- “classic” or primary Endocrine tissue includes hypothalamus, pituitary, thyroid, parathyroid glands, adrenal glands, pineal gland, pancreas, ovaries and testes – hormone secretion is one of the primary functions of these tissues
Where are hormones produced?
- Hormones are a diverse group of chemical substances that act on specific target cells.
- Hormones are produced by endocrine glands specific to the hormone produced
e. g. insulin is produced by the pancreas and released directly into the bloodstream.
Characteristics of ‘hormone activity’
- Hormones traveling throughout the body will only affect target cells that possess specific protein receptors.
- Receptors are continually being synthesised and broken down.
- Receptors may be down-regulated in the presence of high concentrations of hormone.
- Receptors may be up-regulated in the presence of low concentrations of hormone.
- Hormones that don’t circulate are local hormones (paracrine). Those that act on the same cell that secretes them are autocrine.
Define ‘lipid-soluble hormone’ and ‘water-soluble hormone’
- Hormones are either lipid-soluble (steroid hormones, thyroid hormones, nitric oxide) or water-soluble (amine hormones, peptide and protein hormones, eicosanoid hormones).
- Water-soluble hormones circulate freely in the plasma.
- Lipid-soluble hormones circulate bound to transport proteins (e.g. most serum cortisol binds to transcortin or albumin).
What will be the possible responses on the target cells to hormonal stimuli?
- Changes in the expression of particular genes (increases and/or decreases) (estrogen & testosterone)
Changing permeability of the cell membrane (aldosterone)
Stimulating transport of a substance into or out of the cell (renin-angiotensin-aldosterone)
Altering the rate of metabolic actions (thyroid T3&4)
Causing contraction of smooth or cardiac muscle (thyroid T3&4)
What factors influence the way a target cell responds to a hormone?
- The hormone’s concentration in the blood
- The number of hormone receptors on the target cell
- Influences exerted by other hormones
- Some hormones work more effectively when a second hormone is present to assist them (synergistic effect).
- Some hormones oppose the action of others (antagonistic effect).
Hormone secretion and duration of effectiveness.
Hormones are secreted in short bursts – and often follow a pulsatile release pattern rather than continuous (e.g. sex hormones), a diurnal (e.g. cortisol – high in morning and low at night) or ultradian pattern (e.g. every 20 min)
Secretion is regulated by:
- Signals from the nervous system
- Activation of chemoreceptors
- Chemical changes in the blood
- Other hormones
Control of hormone secretion
- Most hormone regulation is achieved via negative feedback.
- A few hormones operate via positive feedback (e.g. oxytocin in childbirth).
Describe how hormones that are proteins exert their effect on target cells. Most hormones are amino acid based.
- Protein hormones are water soluble and therefore cannot directly pass through the plasma membrane of the target cell.
- Protein hormones bind to receptors on the plasma membrane causing a structural change in the cell membrane and the activation of intracellular second messengers, which exert changes e.g. changes on the cell membrane, gene transcription, etc.
Why is insulin not given orally?
Insulin is a small peptide hormone which would be broken down in the stomach.
Which organs synthesise and release steroid hormones?
- Steroid hormones are synthesised from cholesterol and are therefore lipid soluble.
- The adrenal cortex and the gonads (testes and ovaries):
a. Hormones that are synthesised and released from the adrenal cortex:
- Glucocorticoids such as cortisol.
- Mineralocorticoids such as aldosterone.
- Gonadocorticoids such as weak androgens. Androgens are converted to testosterone and oestrogens.
b. Hormones that are synthesised and released from the gonads:
- Testosterone (from testes in males)
- Oestrogens and progesterone (from ovaries in females)
Describe how steroid hormones exert their effect on target cells.
Steroid hormones are synthesised from cholesterol and are therefore lipid soluble. Lipid soluble steroid hormones generally diffuse across both the cell and nuclear membranes and exert their effects directly on the nucleus.
Explain the term ‘tropic hormone’ and list the four tropic hormones produced by the anterior pituitary.
- A tropin is a hormone that is produced in one endocrine gland and regulates the secretion of a hormone in another endocrine gland.
- TSH (thyroid stimulating hormone) stimulates the thyroid gland to produce thyroid hormone (T3 and T4).
- ACTH (adrenocorticotropic hormone) stimulates the adrenal cortex to produce cortisol.
- FSH (follicle stimulating hormone) and LH (leuteinising hormone) stimulate the gonads to produce gametes (ova and sperm) as well as the sex hormones - oestrogen (estrogen) and progesterone in the ovaries and testosterone in the testes.
