1. Introduction to the endocrine system Flashcards
Define the endocrine system
System of ductless glands which secrete chemical messengers (hormones) into the blood. the hormones are then carried in the blood and are able to act on cells possessing the appropriate hormone receptors (target tissues). Each hormone may act on several target tissues.
Where are endocrine glands located
distributed throughout the body
where is the pituitary gland located
below the brain, encased in skull
what is the master gland?
the pituitary gland is sometimes called the master gland
where are the thyroid and parathyroid glands are what do they do?
- neck
- important in controlling metabolic rate and calcium homeostasis
Describe the pancreas
- secretes digestive enzymes directly into the GI tract (exocrine function)
- it also secretes the hormone insulin into the blood
- insulin regulates blood glucose concentrations and glucose utilisation
describe the adrenal glands
- lie on top of the kidneys
- divided into outer cortex and inner medulla
- medulla is concerned with stress response
- cortex concerned with stress, sodium and glucose homeostasis
Describe the gonads
- ovaries or testes
- secrete the female and male reproductive hormones respectively
- responsible for development of secondary sexual characteristics and reproduction
Mechanisms of hormone action
hormones produce effects by interaction with receptors which may be intracellular or membrane-bound major receptor types are: -steroid hormone receptors -G-protein coupled receptors -Tyrosine kinase receptors
Mechanism of steroid hormone receptors
hormone crosses cell membrane and binds to a receptor (cytoplasmic or nuclear)
hormone receptor complex then binds to hormone response element of the DNA to influence gene transcription
receptor may be constitutively active
Describe mechanism of GPCR
- hormone binds to 7tm receptor to cause synthesis of a second messenger e.g. inositol triphosphate or cyclic AMP
- Second messengers phosphorylate intracellular, regulatory protein to influence cellular activity (e.g. smooth muscle contraction)
Tyrosine kinase receptor mechanism
- membrane-bound enzyme which is a single strand of protein
- often two receptors must be present for hormone action (dimerised receptor)
- one hormone molecule binds to each receptor
- the receptor then acts as an enzyme to phosphorylate ‘tyrosine’ components of intracellular regulatory proteins
Two broad classes of hormones
- peptide hormones
- steroid hormones (+ thyroid hormone)
peptide hormones
- chains of amino acids, highly water soluble, susceptible to protease attack
- unable to cross the cell membrane without a carrier protein
- act on membrane-bound receptors
Steroid & thyroid hormones
- highly lipid soluble
- poorly water soluble
- must be transported in blood bound to plasma protein
- only unbound hormone biologically active
- able to cross the cell membrane and act on intracellular receptors
What controls the secretion of a hormone?
Typically (not in all cases though) the anterior pituitary gland controls hormone secretion from a gland
What does the anterior pituitary gland secrete?
Stimulating (or trophic) hormones
Thyroid stimulating hormone (TSH) released by anterior pituitary
acts on thyroid gland to stimulate thyroxine secretion
Adrenocorticotrophic hormone (ACTH) released by anterior pituitary
acts on adrenal cortex to stimulate secretion of cortisol
Follicle stimulating hormone (FSH)
made by anterior pituitary
acts on ovary
stimulates secretion of oestrogen
Luteinising hormone (LH)
made by anterior pituitary, acts on ovary to stimulate secretion of progesterone
Hypothalamus in controlling endocrine system
hypothalamus secretes releasing hormones which control the secretion of anterior pituitary gland stimulating hormones
Thyrotrophin releasing hormone (TRH)
- secreted by hypothalamus
- stimulates secretion thyroid stimulating hormone (TSH)
Corticotrophin releasing hormone (CRH)
Secreted by the hypothalamus to stimulate secretion of adrenocorticotrophic hormone (ACTH)
Gonadotrophin releasing hormone (GnRH)
- secreted by hypothalamus
- stimulates secretion of follicle stimulating hormone (FSH)
Growth hormone releasing hormone (GHRH)
Released from hypothalamus to stimulate secretion of growth hormone
What controls hormonal secretion
Negative feedback
Thyroid hormone acts to
decrease secretion of TRH and TSH
Cortisol acts to
decrease secretion of CRH and ACTH
Oestrogens act to
decrease secretion of GnRH and FSH
Common consequence of negative feedback
cyclical variation in hormone concentration
