Download HUBS 191 MODULE 5 NOTES MODULE 5 : ENDOCRINE SYSTEM

HUBS 191 MODULE 5 NOTES
MODULE 5 : ENDOCRINE SYSTEM
Endocrine System
-
consists of endocrine (gland) cells that secrete hormones (chemical messengers)
that are carried in the bloodstream to target cells upon which they act
Endocrine glands = ductless glands ! secrete products (hormones) directly into
blood
made of glandular epithelium whose cells manufacture & secrete hormones
some made of neurosecretory tissues which are modified neurons that secrete
chem messengers that diffuse into bloodstream rather than across synapse (here
chem messenger is hormone not NT)
Exocrine glands = secrete products (eg enzymes for digestion) into ducts
Functions of Endocrine system
-
Regulates internal environment by secreting hormones that travel through bloodstream
to target cells ! homeostasis
4 main functions = regulation of internal environment (homeostasis), growth, energy
metabolism, reproduction & development
Remember!
- Homeostasis = maintenance of a relatively constant internal environment despite
changes in the internal or external environment
- set point range = normal range of values for that variable
Major Endocrine Glands
-
hypothalamus = hypothalamus links nervous system to endocrine system & controls
secretion of many endocrine glands ! principle regulator / master gland
pituitary gland
thyroid gland
parathyroid gland
adrenal gland
pancreas
(pineal, thymus, testis, ovaries)
NOTE :
- hypothalamus = brain tissue that secretes hormones (neurons that secrete hormones
into blood)
- many of the hypothalamic hormones control function of the pituitary gland which itself
secretes hormones that affect other parts of the body
Hormones
-
chemical messengers secreted in 1 location and transported (usually via
bloodstream) to a second location (target cells) where they exert their effects
hormones usually have secondary functions in body which modulate activity of other
regulatory mechanism
unused hormones are usually quickly excreted by kidneys or broken down by metabolic
processes ! so that they wont stimulate target cells when not required
-
SYNERGISM = some hormones work together to enhance each other's influence on
target cell
PERMISSIVENESS = combined hormone actions may exhibit permissiveness eg when
a small amount of one hormone allows a second hormone to have its full effect on a
target cell
ANTAGONISM = antagonism between hormones ! fine tune activity of target cells with
great accuracy, signaling cell when to increase or decrease a certain cellular process
Specificity of Hormone Action
- hormones only affect cells with specific receptors that hormones can bind to
- Signal transduction = binding of hormones to receptors which activates mechanism of
action / pathway (depending on type of hormone & receptor)
- Signal transduction may :- initiate synthesis of new proteins
- trigger activation or inactivation of certain enzymes
- open / close specific ion channels in the PM
Regulation of Target Cell Sensitivity
- sensitivity depends on number of receptors expressed
- receptors constantly degraded & synthesised to ensure efficiency
- up-regulation = synthesis > degradation ! cells become more sensitive
- down-regulation = degradation > synthesis ! cells become less sensitive
Control of Hormone Secretion
- negative feedback mechanism (inhibitory, stabilise physiological variables – keep them
within set range)
- endocrine reflexes = responses that result from operation of feedback loops within
endocrine system
- endocrine cells are sensitive to the physiological changes produced by their target
cells
- secretion by many endocrine glands is regulated by hormone produced by
another gland eg secretion by anterior pituitary can be regulated by releasing
hormones/inhibiting hormones secreted by hypothalamus
- target gland may be able to adjust its own output but additional controls exerted by long
feedback loops involving APG & hypo allow more precise regulation of hormone
secretion and hence internal env
- input from nervous system also influence secretion of hormones eg secretion by
posterior pituitary is regulated by direct nervous input from hypo, sympathetic nerve
impulses reaching medulla of adrenal glands trigger secretion of E & NE
Properties
Water-soluble hormones
Chem
- peptides (75% of all
classification hormones) ! some aren’t
very soluble in water
- catecholamines ! involved
in stress response
Storage
- made & stored until
required (released by
exocytosis)
Lipid-soluble hormones
- steroid hormones
- thyroid hormones (modified peptide hormone
that gives it lipid-soluble properties)
- steroids made only when required (cannot
store them cuz hormones will dissolve in the
lipid membranes wrapping them)
- thyroid hormones made in thyroid cells &
3. Activated G protein reacts with GTP which activates membrane-bound enzyme
adenyl cyclase
4. Adenyl cyclase removes 2 phosphates from ATP converting it into cAMP (second
messenger)
5. cAMP activates/inactivates protein kinases
6. Protein kinases activate specific intracellular enzymes which influence specific
cellular reactions, producing target cell's response to hormone
Mobile receptor Model
- for lipid-soluble hormones
- amount of steroid hormone present determines magnitude of target cell's
response
- more hormones ! more hormone-receptor complexes ! more mRNAs ! more
proteins
- steroid hormones may also have additional secondary effects such as influencing
signal transduction pathways at PM
1. Lipid-soluble hormones attach to soluble plasma protein & travel in blood
2. Lipid-soluble hormones detach from carrier protein & passes through PM
3. Hormone molecules enter cell where they bind with a mobile receptor either in
cytoplasm / nucleus to form a hormone-receptor complex
4. Complex acts as transcription factor & activates a certain gene sequence to begin
transcription of mRNA
5. Newly-formed mRNA moves out of nucleus into cytosol where they associate with
ribosomes, undergoes translation & begins synthesis of protein molecules
6. New protein (usually enzyme or channel protein) produces specific effects in
target cells
Comparison of Nervous & Endocrine Systems
Nervous system
- Neuronal (AP in axons, NT released at synapse)
- Fast ! minimise response delay, tissue damage
- Good for brief responses
Endocrine System
- Hormonal (hormones released into bloodstream)
- Slow
- Good for widespread, sustained responses
BUT BOTH CAN ONLY INTERACT WITH CELLS WITH SPECIFIC RECEPTORS
A) Pituitary Glands