2. Introduction to the hypothalamic-pituitary axis Flashcards
Where is the pituitary gland located?
Below the brain, encased in skull
What is the are of brain directly above the pituitary gland
hypothalamus
where is the hypothalamus
directly above the pituitary gland
What does the infundibulum do?
Connects the pituitary gland to the brain
How many separate parts is the pituitary gland divided into and what are they?
Two separate parts:
anterior pituitary
posterior pituitary (neurohypophysis)
These function independently
Describe the posterior pituitary gland
The posterior pituitary gland is a down growth from the brain
sometimes called the neurohypophysis
What does the posterior pituitary gland secrete
Antidiuretic hormone (ADH also called vasopressin) Oxytocin
Describe ADH and oxytocin
- peptide hormones, nine amino acids (7 identical ones)
- hormones not synthesised within the gland but are synthesised by hypothalamic neurons (supraoptic nuclei and paraventricular nucleus)
- the hormones are synthesised as inactive precursors
- converted to active hormone within the posterior pituitary
How are inactive hormones transported from the hypothalamus to the posterior pituitary gland?
- From supraoptic and paraventricular nuclei along hypothalamico-neurohypophyseal tract
- stored within the posterior pituitary
What influences anti-diuretic hormone
- plasma osmolarity
- stimulation of V2 receptors causes translocation of aquaporins to the membrane of the kidney tubule, thus allowing water reabsorption
What does stimulation of V1 receptors cause?
Vasoconstriction but not at physiological concentrations
What do disorders of ADH secretion lead to?
Disorders of fluid balance
How does oxytocin act?
- Oxytocin acts via Ip3 to cause contraction of smooth muscle
- secretion of oxytocin is stimulated by stimulation of the genitals and nipples
When is oxytocin most important?
oxytocin is most important at parturition and during lactation
When does oxytocin synthesis increase?
- during pregnancy
- but parallel increase in oxytocinase activity
what does oxytocin cause during parturition?
During parturition, oxytocin causes contraction of the uterine smooth muscle to help expel the infant
Continued stimulation of the cervix causes continues secretion
How does oxytocin affect lactation?
- After parturition, oxytocin is responsible to milk ‘let down’ (expulsion)
- suckling is stimulus
- delay between start of suckling and milk let down
What is the role of oxytocin in labour?
- Oxytocin isn’t necessary for initiation of normal labour
- oxytocin can be administered to induce labour
Describe the anterior pituitary gland
-The anterior pituitary gland is an up growth from the palate
What hormones are secreted by the anterior pituitary gland?
Growth hormone prolactin thyroid stimulating hormone adrenocorticotrophic hormone follicle stimulating hormone luteinising hormone
How are the hormones synthesised within the anterior pituitary gland?
Separate cells within the anterior pituitary gland by separate cells.
E.g. thyrotrophe cells - thyroid stimulating hormones
E.g. corticotrophe cells - adrenocorticotrophic hormone
E.g. gonadotrophe cells - follicle stimulating hormone
TRH
- thyrotrophin releasing hormone
- from hypothalamus
- stimulates secretion of TSH (thyroid stimulating hormone)
CRH
-corticotrophin releasing hormone
-from hypothalamus
stimulates secretion of adrenocorticotrophic hormone (ACTH)
GnRH
- Gonadotrophin releasing hormone
- from hypothalamus
- stimulates secretion of follicle stimulating hormone (FSH)
GHRH
- growth hormone releasing hormone
- from hypothalamus
- stimulates secretion of growth hormone
Describe the route of releasing hormones to the anterior pituitary
The releasing hormones are secreted by hypothalamus
- into the hypothalamico-adenohypophyseal portal vessel
- this carries them into the anterior pituitary
Where do the releasing hormones act?
- On the pituitary gland
- do not enter the systemic circulation
How do you test pituitary function?
-measurement of releasing hormones and the effects of hormone administration (negative feedback)
What do you need to consider when testing pituitary function?
Timing of the tests (measuring hormones and their effects)
because of diurnal rhythms
Clinical investigations
- start with signs and symptoms
- follow up with biochemical tests (basal levels)
What are some biochemical tests?
TSH & FT4 Cortisol LH & FSH Prl (prolactone) Testosterone/"periods"
What must you do after biochemical tests?
Imaging e.g. MRI
Then
Visual Field Tests
What is a common visual field finding related to the hypothalamic-pituitary axis?
- Bilateral hemianopsia
- due to compression of optic chiasm
- overgrowth of pituitary gland
Describe prolactin
- 199 amino acid peptide
- synthesised in lactotrophe cells in the anterior pituitary
How is prolactin secretion regulated?
- secretion of prolactin is under hypothalamic control
- prolactin release inhibiting factor aka DOPAMINE inhibits prolactin release
- TRH stimulates prolactin synthesis
Prolactin secretion is stimulated by:
- mild stress
- nipple stimulation
- coitus
What’s the primary role of prolactin?
- promote lactation
- develop breast tissue
- most important in females after puberty and during/after pregnancy
Who secretes prolactin
prolactin is secreted in both males and females
Where is growth hormone synthesised?
In the somatotrophe cells of the anterior pituitary gland
How is growth hormone secretion controlled?
-by hypothalamus
-via growth hormone releasing hormone (GHRH)
AND
-growth hormone releasing inhibiting hormone (GHRIH, somatostatin)
How is growth hormone secretion regulated?
stimulated by:
- decreased carbohydrates
- decreased fatty acids
- increased amino acids
- in some tissues, GH acts via second messengers which are produced in the liver
How does growth hormone act in some tissues?
Via second messengers which are produced in the liver:
- insulin-like growth factor 1
- insulin-like growth factor 2
What are the effects of growth hormone?
- promotes linear growth of adolescence
- by increasing protein synthesis and collagen deposition
- mediated via the nucleus
- foetal growth is relatively independent of growth hormone
- growth hormone is also involved in tissue turnover and repair
