Flashcards in MEH - Hypothalamic Pituitary Axis Deck (34):
What is the hypothalamic pituitary axis?
The hypothalamus and pituitary gland form a complex functional unit together which serves as the major link between the endocrine and nervous systems
Where is the pituitary gland found?
Beneath the hypothalamus in a socket of bone called the sella turcica
Give some examples of processes modulated by the hypothalamus and the pituitary gland
- body growth
- adrenal gland function
- water homeostasis
- milk secretion
- thyroid gland function
What are the two parts of the pituitary gland?
- anterior pituitary gland (adenohypophysis)
- posterior pituitary gland (neurohypophysis)
What is the embryological origin of the anterior pituitary?
It arises from evagination of the oral ectoderm (Rathke's pouch) - this is primitive gut tissue
What is the embryological origin of the posterior pituitary?
It originates from the neuroectoderm - primitive brain tissue
How is the posterior pituitary connected to the hypothalamus?
It is physically connected through the infundibulum (pituitary stalk), because the hypothalamus originally drops down to form the posterior pituitary gland
How are oxytocin and ADH produced and released?
- produced by neurosecretory cells in supraoptic and paraventricular nuclei of hypothalamus
- transported down nerve cell axons to the posterior pituitary
- stored/released from posterior pituitary into the general circulation to act on distant targets
Note: produced in HYPOTHALAMUS, released from POSTERIOR PITUITARY
How does the anterior pituitary function?
- hormones synthesised in hypothalamus are transported down axons and stored in median eminence before release into hypophyseal portal system
- hormones stimulate/inhibit target endocrine cells in the anterior pituitary gland
- endocrine cells secrete hormones into the bloodstream (endocrine) and also effect neighbouring cells (autocrine/paracrine function)
Summarise the difference in mechanism of the anterior and posterior pituitary
- POSTERIOR: direct effects on distant target tissues via oxytocin and antidiuretic hormone
- ANTERIOR: hormones only go into hypophyseal portal system, and affect endocrine cells within the anterior pituitary
What is the difference between antidiuretic hormone and vasopressin?
They're the same!
What does it mean if a hormone is tropic?
It affects the release of other hormones in the target tissue
What are the six tropic hormones produced by the hypothalamus?
- thyrotropin releasing hormone (TRH)
- prolactin release-inhibiting hormone (PIH/dopamine)
- corticotropin releasing hormone (CRH)
- gonadotropin releasing hormone (GnRH)
- growth hormone releasing hormone (GHRH)
- growth hormone inhibiting hormone (GHIH/somatostatin)
What are the six hormones produced by the anterior pituitary?
- thyroid stimulating hormone (TSH)
- adrenocorticotropic hormone (ACTH)
- luteinising hormone (LH)
- follicle stimulating hormone (FSH)
- prolactin (PRL)
- growth hormone (GH)
Give some factors which influence growth
Nutrition, genetics, environment, hormones (growth hormone is the most important)
What stimulates growth hormone?
GHRH from the hypothalamus
What inhibits growth hormone?
Somatostatin from hypothalamus
Why must growth hormone be injected rather than taken orally?
It is a protein and would be digested
True or false - the signal peptide remains on the growth hormone protein?
False - it must be cleaved before proper folding
What are IGFs?
Insulin-like growth factors (somatomedins) produced by the liver and skeletal muscle in response to GH
What is the role of GH and IGFs in adults?
Help maintain muscle and bone mass, promote healing and tissue repair, modulate metabolism and body composition
What is the role of GH and IGFs in children?
- GH stimulates long bone growth (length and width before epiphyseal closure, just width after)
- IGFs stimulate bone and cartilage growth
What events can cause GH secretion to rise?
- rise in secretion after onset of deep sleep
- stress (eg. Trauma, surgery or fever)
- decrease in glucose or fatty acids
What events can cause GH secretion to fall?
- rapid eye movement sleep
- increase in glucose or fatty acid levels
How does long loop negative feedback inhibit GH?
- IGFs inhibit release of GHRH from the hypothalamus
- IGFs stimulate release of somatostatin from the hypothalamus
- IGFs inhibit release of GH from anterior pituitary
How does short loop negative feedback regulate GH?
It is mediated by GH itself via stimulation of somatostatin release
What is pituitary dwarfism?
A growth hormone deficiency in childhood leading to proportionate dwarfism. The height is below the 3rd percentile on standard growth charts and there is delayed/no sexual development during teenage years. Responds to GH therapy
What disorder can growth hormone excess in childhood cause?
Gigantism - in adulthood it leads to acromegaly
How does GH exert its effect on cells?
GH receptors activate 'janus kinases' which cross phosphorylate and activate signalling pathways. Transcription factor activation and IGF production then occurs.
What are the two IGFs found in mammals and what are they responsible for?
- IGF1 is a major growth factor in adults
- IGF2 is mainly involved in foetal growth
What can IGFs modulate?
- cell growth (hypertrophy)
- cell number (hyperplasia)
- increase in rate of protein synthesis
- increase in rate of lipolysis in adipose tissue
What is the growth related action of insulin?
Enhances somatic growth, interacts with IGF receptors
What is the growth related action of thyroid hormones?
Promotes CNS development and enhances GH secretion