L1 - Introduction to the endocrine system Flashcards
What is a hormone
- The endocrine system is a 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)
What is the pituitary gland also sometimes called
- The master gland
Role and location of thyroid and parathyroid glands
- Situated in the neck
- Important in the control of metabolic rate and calcium homeostasis
Role of 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
Location of the adrenal glands
- Lie on top of the kidneys
What are the adrenal glands divided into
- Outer cortex
- Inner medulla
What is the inner medulla of the adrenal glands concerned with
- Stress response, cortex with stress, sodium and glucose homeostasis
Role of the gonads
- Secrete the female and male reproductive hormones respectively
- These are responsible for the development of secondary sexual characteristics and reproduction
How do hormones produce their effects
- By interaction with receptors which may be intracellular or membrane-bound
What are the major types of receptors involved in hormone action
- Steroid Hormone Receptors
- G-Protein Coupled Receptors
- Tyrosine Kinase Receptors
How do steroid hormone receptors function
- The hormone crosses the cell membrane and binds to a receptor which may be cytoplasmic or nuclear.
- The hormone receptor complex then binds to the hormone-response element of the DNA to influence gene transcription.
- The receptor may be constitutively active
How do g-protein coupled receptors work
- The hormone binds to the 7TM receptor to cause synthesis of a second messenger, eg inositol trisphosphate or cyclic AMP.
- Second messengers phosphorylate intracellular, regulatory protein to influence cellular activity (eg smooth muscle contraction).
How do tyrosine kinase receptors function
- Membrane-bound enzyme which is a single strand of protein.
- Often two receptors must be present for hormone action (dimerized receptor).
- One hormone molecule binds to each receptor, the receptor then acts as an enzyme to phosphorylate ‘tyrosine’ components of intracellular, regulatory proteins.
What are the two broad classes of hormones
- Peptide hormones
- Steroid hormones (+ thyroid hormone)
What are 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.
What are steroid and 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.
Features of peptide hormones
- Water soluble
- Not orally active
- Rapid onset of action
- Short plasma half-life
- Short duration of action
Features of steroid and thyroid hormones
- Poorly water soluble
- Orally active
- Slow onset of action
- Long plasma half-life
- Long duration of action
What is the secretion of a hormone by a given gland typically controlled by
- Anterior pituitary gland
What does the anterior pituitary gland secrete
- Stimulating(or trophic) hormones
Action of thyroid stimulating hormone (TSH)
- TSH acts on the thyroid gland to cause the release of thyroxine
Action of adrenocorticotrophic hormone (ACTH)
- ACTH acts on the adrenal cortex to cause the release of cortisol
Action of follicle stimulating hormone (FSH)
- FSH acts on the ovary to release oestrogens
Action of luteinising hormone (LH)
- LH acts on the ovary to cause the release of progesterone
What is the secretion of a hormone by the anterior pituitary gland controlled by
- Hypothalamus
Effect of thyrotrophin releasing hormone (TRH)
- Secreted by hypothalamus to stimulate secretion of thyroid stimulating hormone (TSH)
Effect of corticotrophin releasing hormone (CRH)
- Secreted by hypothalamus to cause release of adrenocorticotrophic hormone (ACTH)
Effect of gonadotrophin releasing hormone (GnRH)
- Secreted by hypothalamus to stimulate secretion of follicle stimulating hormone (FSH)
Effect of growth hormone releasing hormone (GHRH)
- Secreted by the hypothalamus to stimulate the secretion of growth hormone
What is hormonal secretion also controlled by
Hormonal secretion is also controlled by feedback, usually negative feedback.
Thus:
- 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
What is a common consequence of negative feedback
- Cyclical variation in hormone concentration
