endocrine control mechanisms Flashcards
master controllers of the endocrine system
the hypothalamus and pituitary
the nervous system
- exerts point-to-point control through nerves
- control is electrical and fast
endocrine system
- transmits hormonal messages to essentially all cells in the body by secretion of specific chemicals into blood and extracellular fluid
- the body tissues that would be responsive to that hormone must Express a very specific receptor for that hormone.
how are the nervous and endocrine system connected
- connected and talk to each other to coordinate the activities of different body tissues
- neuroendocrine control
what is neuroendocrine control
information used by the endocrine system may be derived from both the external as well as the internal environment of the body
- this enables the body to adapt and respond to changes in temperature, stress, food, supply, day length etc
largest endocrine gland
- largest endocrine gland - adipose tissue
- secretes many different hormones
examples of human endocrine glands
- pineal
- hypothalamus
- pituitary
- thyroid
- parathyroids
- thymus
- adrenals
- pancreas
- ovary and testes
what are hormones
chemical messengers involved in cell communication
hormone communication through the endocrine communication system
- where a hormone-secreting cell releases its chemical messenger into blood vessels, which is then circulated throughout the body and then it is picked up by a target cell that has the specific receptor for that hormone
hormone communication - autocrine chemical communication
hormones can act on the cell from which it was secreted from
hormone communication - paracrine
rather than being transmitted through the blood vessels, the hormone will bind to a target cell to which it is adjacent or in very close proximity
hormone communication - neuroendocrine system
this is where a neuronal cell is capable of producing a hormone, which will then be released from its axon at the nerve terminal into the bloodstream and then it can be circulated
hormone communication - neurotransmitter
when a neuron will secrete a chemical messenger which binds to another neuron
- sometimes those can be hormones and would be also secreted into the bloodstream or working on adjacent cells
the axes - effect of an environmental influence
e.g. a change in the temperature in the environment
- this would be picked up by the hypothalamus, which will send a signal via neuroendocrine methods to the anterior pituitary gland
- the anterior pituitary gland will then release another chemical messenger into the blood
- will be non-tropic or tropic and cause an action on the body
non tropic chemical messenger
has a direct action on tissues in the body
tropic chemical messenger
that signal goes to another endocrine gland and then the endocrine gland will produce another hormone which then has an action on the body
hypothalamus function and anatomy
- located just below the thalamus and above the brainstem
- important for the control of basic functions in the body like hunger, thirst, sleep
- also important for the control of many different hormones
pituitary gland (hypophysis)
- the size of a pea and weighs about 0.5g
- sits in a small bony cavity (sphenoid bone), below the hypothalamus - sella turcica
- dangles down from the hypothalamus and is connected to the hypothalamus by the infundibulum
pituitary tumour
If you get a pituitary tumour it would not be able to grow downwards because of the sphenoid bone and instead it would push upwards into the space above and would press on the optic nerve.
○ So one of the first symptoms that you get is changes to the visual field because of the pressure on the optic nerve.
pituitary gland lobes
anterior and posterior pituitary gland
- These have completely different roles in the body and they don’t have any direct neuronal connections.
- They’re quite separate. It just happens that they’re joined together in this one gland
hypothalamus connection to the anterior pituitary
- parvocellular neurons in the hypothalamus produce the releasing hormones into the portal blood system
- they will then be transported down long portal vessels to the anterior pituitary to act on cells
- then those cells in the anterior pituitary will produce a different hormone, which will then be secreted to the rest of the body.
parvocellular neurons
neurons which produce and secrete the regulatory hormones, which will control cells in the anterior pituitary gland.
hypothalamus connection to the posterior pituitary gland
magnocellular neurons in the hypothalamus have long axons which extend down into the posterior pituitary lobe
- the posterior pituitary hormones are produced in the hypothalamus and then they’re transported down these long axons and are stored at the nerve terminals in the posterior pituitary.
- then when there’s a stimulus to the hypothalamus that will cause the release of those hormones from the ends of the axons into the blood supply of the posterior pituitary gland.
how does the hypothalamus exert control of anterior pituitary gland
by secreting regulatory (releasing and inhibitory) hormones
- it does not have a direct nervous connection with the hypothalamus
how do the hormones secreted by the hypothalamus reach the target cells of the anterior pituitary
by the hypothalamic portal system
hormones produced by the hypothalamus and the pituitary gland are all ………?
peptides or small proteins
- with the exception of dopamine
hormones released form hypothalamus - prolactin-releasing hormone
- hypothetical hormone
- the default system seems to be that these anterior pituitary cells which secrete prolactin just do so Automatically
what stops prolactin-releasing hormone release from the hypothalamus
they’ll be constantly producing prolactin apart from when the hypothalamus releases dopamine which inhibits the secretion of prolactin from the anterior pituitary.
function of prolactin hormone
involved in mammary development and lactation
- we don’t want that happening all of the time so under the normal basal conditions when somebody isn’t pregnant then the predominant system working here to control prolactin secretion is dopamine from the hypothalamus.
hormones produced by hypothalamus - thyrotropin-releasing hormone
will have effects on the anterior pituitary to produce thyroid-stimulating hormone
- thyroid-stimulating hormone will act on the thyroid gland to cause the thyroid gland to produce thyroid hormones.
hormones produced by hypothalamus - corticotropin-releasing hormone
will cause the secretion of adrenocorticotropic hormone/ ACTH from the anterior pituitary
- this hormone acts on the adrenal cortex and it will cause the secretion of steroids which are involved in the stress response such as cortisol.
hormones produced by the hypothalamus - growth hormone-releasing hormone
will act on the anterior pituitary to increase the production of growth hormone
- growth hormones are an example of a non tropic hormone, which will act directly on body tissues to cause growth and secretion of other growth factors, like insulin-like growth factor
hormones produced by hypothalamus - inhibitory hormone somatostatin
prevents the secretion of growth hormone
hormones produced by hypothalamus - gonadotropin releasing hormone
- will cause the secretion of follicle-stimulating hormone and luteinizing hormone from the anterior pituitary.
- And those hormones act on the gonads in males and females to cause the production
of sex hormones.
what vein are hypothalamic regulatory hormones secreted directly into
hypothalamic-pituitary portal vein
function of the anterior pituitary hormones
all of the different hormones that are released by the hypothalamus have effects on cells in the anterior pituitary
and these cells in the anterior pituitary will produce the hormones that will affect either the tissues directly or will act on other endocrine glands to cause the release of other hormones.
anterior pituitary hormones
- Follicle stimulating hormone - tropic
- luteinising hormone - tropic
- adrenocorticotropic hormone - tropic
- thyroid stimulating hormone - tropic
- prolactin - non tropic
- growth hormone - non tropic
hypothalamus pituitary axes for thyroid hormone
e.g. low temperature
- detected by neurons in the hypothalamus and will cause the release of thyrotropin-releasing hormone.
- trh will stimulate cells in the anterior pituitary to produce thyroid stimulating hormone (tsh)
- will act on thyroid gland to cause production of thyroid hormones - T3 + T4
- body temp will increase so that’s going to have a physiological feedback effect in that the hypothalamus will no longer be detecting that the body temperature is got too low. And so that will help to reduce the amount of the thyroid hormone
What will the release of T3 and T4 thyorid hormones cause
T3 will act on all of the cells of the body that have a receptor for t3 and will increase the metabolism in those cells
- by increasing metabolism that will have a by-product of increasing heat in the body as well. So that’s how you correct the low core temperature in the body.
feedback mechanisms important for control of the hypothalamus and the pituitary
the short Loop feedback which comes from T3 acting on the anterior pituitary gland to reduce the production of TSH
and then the long Loop feedback which jumps up 2 Levels on this control system so that the T3 will actually inhibit the hypothalamus from producing trh as well.
prolactin function - anterior pituitary hormone
- synthesised, stored and secreted by lactotropes
- roles and functions complex - includes effects on breast tissue for lactation
- dysregulation causes multiple problem - prolactinoma tumour
prolactinoma
most common pituitary tumour is prolactinoma
- treated using dopamine receptor agonists
e.g. bromocriptine, cabergoline
function of growth hormone - anterior pituitary hormone
synthesized and stored by the somatotropin cells in the anterior pituitary
○ growth hormone stimulates growth cell reproduction and also regeneration of some cells as well.
○ If you get over-secretion of growth hormone, it can cause two different types of syndromes.
* gigantism.
- or acromegaly
gigantism
when hypersecretion of growth hormone in childhood causes excess growth
- if you have an excess of growth hormone before the long bones have fused then you will get increased rapid growth of those long bones in which leads to the really tall stature of somebody who had this issue.
acromegaly
hypersecretion of growth hormone in adults
- when the long bones are fused and won’t grow anymore
- you’ll get other symptoms such as enlargement of the head, the hands And the tongue
treatment of acromegaly and gigantism
can be treated using somatostatin which is the natural inhibitor of growth hormone
- It will regulate the production of growth hormone and also reduce the amount of cell proliferation and growth via the somatostatin receptors
- synthetic analogues like sandostatin which have much longer Half-Life than natural Somatostatin can be used to treat those conditions.
growth hormone deficiency
where there’s insufficient production of growth hormone, which can be due to a tumour, mutation of growth hormone genes or it can be from growth hormone resistance- Laron syndrome.
- if somebody has growth hormone deficiency, you can also get growth hormone which can be injected and used to correct that deficiency.
posterior pituitary hormones
magnocellular neurons synthesis and secrete oxytocin and vasopressin (ADH)
- the hormones are released at the axon terminals where they go into this capillary Network and are then passed around the body, but they don’t go through the portal system.
how does oxytocin differ from vasopressin
differs in 2 of the 9 amino acids
function of oxytocin
- the stimulation of milk ejection from the lactating mother
- And then prior to that the stimulation of uterine muscle contraction at Birth.
- then also the establishment of maternal Behaviour.
oxytocin during childbirth
- when the baby presses on the cervix it activates Sensory neurons, which go back to the hypothalamus, where it triggers the release of more oxytocin.
- oxytocin binds to receptors that are found in the uterus to cause the uterus to contract.
- When you contract that uterine muscle it’s going to push the baby’s head on to the cervix and then trigger that sensory neuron back to the hypothalamus again to cause the release of more oxytocin
- an example of a positive feedback mechanism
- but oxytocin isn’t the initial trigger for labour
synthetic oxytocin for induction of labour
although oxytocin isn’t the initial trigger for labour. It can actually be used for induction of artificial labour.
- It can be quite dangerous though. Because if the uterine muscle contract too much and doesn’t stop Contracting it will actually stop the blood supply to the baby and that can cause Cerebral Palsy
function of vasopressin
vasopressin is involved in water regulation in the body.
- it will bind to receptors on cells in the collecting ducts of the kidney and cells in the distal convoluted tubule
- it leads to the synthesis of aquaporin 2
- aquaporin 2 is inserted into those parts of the kidney and will allow water re-absorption from the filtrate in the kidney to Go back into the blood.
absence of vasopressin
- in the absence of vasopressin those parts of the kidney tubule are virtually impermeable to water which means that more water will flow out of the body as urine.
- So it’s controlled by the osmolality of the blood as to whether vasopressin is secreted or not secreted.
what does a defect in the secretion or action of vasopressin lead to
a disease called diabetes insipidus
diabetes insipidus
- diabetes insipidus is a disease of the control of water in the body.
- somebody who has diabetes insipidus could be producing as much as 16 liters of urine per day
- If somebody’s doing that they’re going to get dehydrated very quickly. So It can be a dangerous condition to have if water isn’t available to drink.
- So the two main causes of diabetes in peace insipidus are hypothalamic (Central) or nephrogenic.
hypothalami cause of diabetes insipidus
- where you have a deficiency in vasopressin secretion and can be caused by head trauma, Infections or tumours that involve the hypothalamus and wIll stop the posterior pituitary from releasing vasopressin.
- It’s quite easy to treat this form of diabetes insipidus because all you need to do is give exogenous vasopressin and that can be in tablet form or injection
nephrogenic diabetes insipidus
means that The kidney itself is insensitive to the effects of vasopressin and this can be caused by various renal disease, but more often through mutations in the vasopressin receptor gene or actually in the genes which encode that aquaporin which is inserted into the membrane after vasopressin has bound to its receptor
- the only way to treat nephrogenic diabetes insipidus is by increasing water consumption.
