06-11-23 - The hypothalamus-pituitary axis - Part 1 Flashcards
Learning outcomes
- Explain the general types of hormones and modes of secretion
- Identify the main ways hormone release is stimulated and the role of negative feedback
- Describe the different patterns of hormone secretion
- Summarise the structural and functional relationships between the hypothalamus and pituitary
- Explain the processing and function of the hypothalamic neurohormones released from the posterior pituitary
- Review the various hypothalamic neurohormones controlling the release of adenohypophyseal (anterior pituitary) hormones
What is the role of endocrine glands?
What structure do endocrine glands lack?
What are 3 examples of discrete endocrine organs?
What is an example of an endocrine organ associated with other tissues?
What are 3 complex organs that contain endocrine glands?
- Endocrine glands release hormones into the blood
- Endocrine glands are ductless (lack ducts)
- 3 examples of discrete (separate) endocrine organs:
1) Pituitary
2) Adrenals
3) Thyroid - The pancreas is an endocrine gland associated with other tissues
- 3 complex organs that contain endocrine gland:
1) Ovary
2) Testes
3) GI
How vascular are endocrine glands?
What type of capillaries do they have?
What are the 3 types of endocrine secretion?
- Endocrine glands are very vascular
- Endocrine glands have fenestrated capillaries
- 3 types of endocrine secretion:
1) Classic endocrine secretion
* Some hormones diffuse directly into capillaries to act on distant target organs
2) Paracrine secretion
* Hormones secreted and act more locally (paracrine secretion)
* Beta-cells can act on alpha-cells in the pancreas
3) Autocrine secretion
* Some hormones act directly on the cell that produces it
What are the embryological origins of endocrine glands?
What do Gland/cell characteristics reflect?
Describe the 5 chemical classes of hormones (in picture).
Give examples of each
- Many endocrine glands have complex embryological origins
- Gland/cell characteristics reflect types of hormone secretions
- 5 chemical classes of hormones (in picture).
Describe the 3 different methods of hormone release from endocrine glands (in picture)
What is the role of negative feedback mechanisms in hormone release?
What hormones exert negative feedback on the hypothalamic-pituitary-thyroid axis?
- Feedback mechanisms protect the body from hormone excess.
- Negative feedback is fundamental for hormone homeostasis
- T3 (and a lesser degree T4) in the hypothalamic-pituitary-thyroid axis exerts feedback inhibition on the other elements higher up in the axis
What are the 3 different patterns of hormone secretion?
What is an example of a hormone that uses each pattern?
- 3 different patterns of hormone secretion:
1) Diurnal (cortisol)
* The peak level secretion occurs in the morning (07:00–08:00 a.m.), which is considered the active phase, while its lowest secretion is around 02:00–04:00 a.m. at night
2) Set-point (T4)
* Levels held steady throughout the day
3) Episodic (insulin)
* Insulin is released in response to high glucose levels
What occurs when hormone patterns/feedback are disrupted?
How do we test for endocrine disease?
How does this vary among different hormones and secretion patterns?
How can we measure levels of GH?
- When hormone patterns/feedback are disrupted then disease occurs
- If wanting to test for an endocrine disease the regular pattern of hormone secretion needs to be known.
- Some hormones can be measured at any time (e.g., T4), while diurnal hormones want to measure if sufficient levels at peak
- For episodic hormones, a dynamic test is necessary. (e.g., GH):
- Normally, glucose induces negative feedback on GH secretion, so can challenge patient with glucose and determine whether GH is suppressed
- If GH is not supressed, this could suggest acromegaly
Where is the pituitary located (in picture)?
- The pituitary lies immediately posterior to the optic chiasma and sphenoid sinus (in picture)
What are the 3 major neuroendocrine organs?
What is the function of the nuclei in the hypothalamus?
Where are hormones transported to from the major neuroendocrine organs?
- 3 major neuroendocrine organs:
1) Hypothalamus
2) Posterior pituitary
3) Adrenal medulla - The hypothalamus contains several nuclei that regulate essential functions through release of various neurohormones
- From the neuroendocrine organs, hormones are delivered directly into the posterior pituitary, or via the portal blood to the anterior pituitary.
Where is AVP and oxytocin produced?
What is the role of the posterior pituitary?
Where are the cells that produce anterior pituitary hormones located?
What is the role of the hypothalamus in production of anterior pituitary hormones?
- Large-bodied neurons in the hypothalamus produce AVP and oxytocin (OT), which are then transported down axons to the posterior pituitary
- The posterior pituitary which basically stores hormones produced by the hypothalamus (AVP and Oxytocin)
- The clusters of cells that produce the six anterior pituitary hormones are under hypothalamic control.
- Small bodies neurons in the hypothalamus makes releasing hormones and transports them down their axons, and secretes them, into the primary capillary plexuses
- These releasing hormones regulate the anterior pituitary cells
What are the 7 Neurohormones controlling the release of anterior pituitary hormones?
- 7 Neurohormones controlling the release of anterior pituitary hormones:
1) Corticotropin-releasing hormone (CRH; 41 a.a. peptide)
2) Thyrotropin-releasing hormone (TRH; modified 3 a.a. peptide)
3) Growth hormone-releasing hormone (GHRH; 43 a.a. peptide)
4) Somatostatin (SS; also called Growth hormone-inhibiting hormone, GHIH; cyclic 14 a.a. peptide)
5) Gonadotropin-releasing hormone (GnRH; 10 a.a. peptide)
6) Prolactin releasing hormone (PRH; yet to be identified)
7) Prolactin-inhibiting hormone (PIH; dopamine)
What are the 2 Neurohormones released from the posterior pituitary?
- 2 Neurohormones released from the posterior pituitary:
1) Oxytocin (OXT; 9 a.a. peptide) –
2) Vasopressin (AVP) also called anti-diuretic hormone (ADH; 9 a.a. peptide)
Describe the following for each of the hypothalamic neurohormones:
1) Factor/hormone (9)
2) Structure
3) Hypothalamic nucleus they are synthesised in
4) Effect on the anterior pituitary (in picture)
Processing of hypothalamic neurohypophyseal hormones.
What is the role of AVP/ADP?
What 2 other proteins is AVP/ADH synthesised with?
What is the other protein Oxytocin is synthesised with?
- Processing of hypothalamic neurohypophyseal hormones
- AVP/ADP helps to retain water in order to control blood volume
- Vasoconstriction from ADP is only seen in severe trauma
- 2 other proteins is AVP/ADH synthesised with:
1) Neurophysin 2
* Binds to AVP in the circulation and protects it from degrading, then releases it at its target tissue
2) Copeptin
- Oxytocxin is also synthesised with Neurophysin 1, which has similar actions to neurophysin 2
What are 3 triggers for AVP/ADH release (in picture).
What does ADH/AVP release lead to?
What is Oxytocin (OXT) produced by?
What is it bound by?
What is Oxytocin release directly controlled by?
What process is oxytocin a key hormone in?
- Oxytocin is produced by hypothalamic neurons in the paraventricular and supraoptic nuclei
- It is bound by neurophysin I and carried in the axons to the posterior pituitary
- Oxytocin release is controlled directly by nervous impulses form the hypothalamus
- Oxytocin is a key hormone in childbirth by stimulating contraction of the uterus
Describe the Hypothalamic control of milk production and ejection – Roles of prolactin and oxytocin (in picture)
Hypothalamic- Anterior Pituitary Endocrine Axis (in picture)
