REB 3. Hypothalamus-Pituitary-Target Organ Axes: Feedback Control 1 Flashcards
The hypothalamus and pituitary gland form a complex and are a dominant component of the endocrine system.
They regulate the function of… [3]
and are responsible for…[4]
Regulate the Function of:
[1] Thyroid Gland
[2] Adrenal Gland
[3] Reproductive Gland
They are responsible for... [1] Somatic Growth [2] Lactation [3] Milk Secretion [4] Maintaining Body Fluid Homeostasis
Describe the physical traits and location of the hypothalamus.
- represents less than 1% of brain mass
- around the size of a peanut
- below the hypothalamus
- above the pituitary gland
- on either side of 3rd ventricle
What are the 2 main sources that the hypothalamus receives signals from? Which nervous system branches are involved with each.
[1] External
- sensory pathways conveying info on the external environment
- e.g. emotional, instinctual, appetite (emotions are coordinated through connections with the CNS)
[2] Internal
- receptors of body temperature, osmolarity, pressure, volume, glucose concentration, hormone concentration
- the internal homeostatic responses are produced through ANS + endocrine system
- endocrine link is through the hypothalamic-pituitary axis
What is another name for the pituitary gland?
Hypophysis
What are the physical characteristics of the pituitary gland?
- small gland
- 1 cm in diameter
- 0.5 to 1 g in weight
- lies in Sella Turcica (bony cavity in base of brain)
Where is the pituitary gland located?
lies in the Sella Turcica - a bony cavity in the base of the brain
What is another name of the anterior pituitary gland?
Adenohypophysis
What is another name for the posterior pituitary gland?
Neurohypophysis
Do the anterior and posterior pituitary glands originate from the same place?
No, the 2 portions originate from different sources embryologically
Describe how the anterior pituitary gland is formed and the nature of its cells.
- arises from an upward + outward pouching from the epithelium of the pharynx (Rathke’s pouch)
- explains epithelioid nature of its cells (epithelial cells)
Describe how the posterior pituitary gland is formed and the nature of its cells.
- arises from neural tissue outgrowth from hypothalamus (direct connection to hypothalamus)
- large # of glial-type cells
How does the hypothalamus control the anterior and posterior pituitary glands?
How are both the posterior and anterior pituitary glands connected to the hypothalamus?
The hypothalamus controls the pituitary glands by nervous and hormonal signals.
Posterior pituitary is connected to hypothalamus by a neural pathway. - this forms a neuroendocrine system
Anterior pituitary is connected to hypothalamus by unique vascular link
Explain the whole process of the neuroendocrine system and how hormones are released from the posterior pituitary gland.
- neurosecretory neurons have cells bodies in the hypothalamus in 2 distinct clusters (supraoptic and paraventricular)
- the dendrities are located in the posterior pituitary gland
- the hormones are synthesized in the cell bodies (as prohormones)
- hormones are transported down to nerve endings over several days with carrier proteins (neurophysins)
- vasopressin (adh) is formed in supraoptic nuclei
- oxytocin is formed in the paraventricular nuclei
Can the nuclei synthesize any other hormone?
each nuclei can synthesize 1/6th as much of the second hormone as of its primary hormone
What are the hormones that are synthesized by the anterior pituitary gland?
- Growth Hormone - GH (Somatotropin)
- Adrenocorticotropin Hormone - ACTH (Adrenocorticotropin)
- Thyroid-Stimulating Hormone - TSH (Thyrotropin)
- Follicle-Stimulating Hormone - FSH (Gonadotropin)
- Leutinizing Hormone - LH, ICSH (Gonadotropin)
- Prolactin - PRL**
What are tropic hormones?
Tropic hormones are hormones that stimulate another hormone after going to the target gland.
List the tropic hormones.
- Growth Hormone
- Adrenocorticotropin
- Thyroid-Stimulating Hormone
- Follicle-Stimulating Hormone
- Leutinizing Hormone
What is the “outlier” from all the anterior pituitary gland hormones?
Prolactin - it is the only hormone that doesn’t stimulate the secretion of another hormone
What cell type creates growth hormone?
Somatotropes
What cell type creates adrenocorticotropin?
Corticotropes
What cell type creates thyroid-stimulating hormone?
Thyrotropes
What cell type creates follicle-stimulating hormone and leutinizing hormone?
Gonadotropes
What cell type creates prolactin?
Lactotropes
What is the most abundant cell type in the anterior pituitary gland?
Somatotropes at 30 to 40% of all the anterior pituitary cells
What is the second most abundant cell type in the anterior pituitary gland?
Corticotropes at 20% of all the anterior pituitary cells
What is the name of the vascular link that connects the anterior pituitary gland to the hypothalamus? What types of hormones are passed through this system?
Hypothalamic-Hypophyseal Portal System are minute blood vessels that connect the hypothalamus and anterior pituitary gland. In this, the general circulation is bypassed and almost all blood supplied to ant. pituitary has to pass through hypothalamus first
Hypothalamic Releasing + Hypothalamic Inhibitory hormones (factors) are secreted from hypothalamus that act on the glandular cells of the anterior pituitary.
Explain the process of how the hypothalamus can stimulate release of hormones in the anterior pituitary gland through the Hypothalamic-Hypophyseal Portal System.
- hypophysiotropic hormones (releasing hormones + inhibiting hormones) are produced by neurosecretory neurons in hypothalamus and enter hypothalamic capillaries
- hypothalamic capillaries rejoin to form hypothalamic-hypopyseal portal system
- portal system branches into capillaries of the ant. pituitary
- hypophysiotropic hormones, leave the blood across anterior pituitary capillaries, conrol the release of ant. pituitary hormones
- when stimulated by appropriate hypothalmic releasing hormone, the ant. pituitary secretes a given hormone into capillaries
- the ant. pituitary rejoins to form a vein - the hormones leave for ultimate distribution through the body by systemic circulation
What are hypophysiotropic hormones?
hormones that are produced by the hypothalamus and act to stimulate the pituitary gland
What are the 2 factors that regulate the anterior pituitary hormone secretion? Give some examples and explain.
[1] Hypothalamic Hypophysiotropic Hormones
- a single anterior pituitary hormone may be regulated by 2 or more hypothalamic hypophysiotropic hormones
- e.g. growth hormone regulated by growth hormone releasing hormone (GHRH) abd growth hormone inhibiting hormone (GHIH)
[2] Feedback by Target Gland Hormones
List the hypothalamic hypophysiotropic hormones. [7]
[1] Thyrotropin-Releasing Hormone (TRH) stimulates release of Thyrotropin (TSH) and PRL
[2] Corticotropin-Releasing Hormone (CRH) stimulates release of Corticotropin (ACTH)
[3] Gonadotropin-Releasing Hormone (GnRH) stimulates release of FSH and LH (gonadotropins)
[4] Growth Hormone Releasing Hormone (GHRH) stimulates release of GH
[5] Growth Hormone Inhibiting Hormone inhibits release of GH and TSH
[6] Prolactin-Inhibiting Hormone (PIH)/Dopamine inhibits release of PRL
[7] Prolactin-Releasing Hormone (PRH) stimulates release of PRL
What 2 hormones are identical to one another?
Prolactin-Inhibiting Hormone (PIH) and Dopamine
What are the 3 levels of feedback that the hypothalamus-pituitary axis is under?
[1] Long Loop Feedback
- e.g. coming from peripheral tissues and endocrine glands
[2] Short Loop Feedback - most common
- e.g. coming from the pituitary gland itself
[3] Ultra Short Loop Feedback
- e.g. hypophysiotropic hormones that regulate the hypothalamus
What is the most common level of feedback?
Short Loop Feedback
What is the CRH-ACTH-CORTISOL System?
Steps:
stress -> stimulates hypothalamus -> increase in corticotropin-releasing hormone (CRH) -> stimulates ant. pituitary -> increase in adrenocorticotropic hormone (ACTH) -> stimulates adrenal cortex -> increase in cortisol -> stimulates cells that leads to metabolic changes that help resist stress
Cortisol: inhibits the hypothalamus and anterior pituitary gland
Overall: levels of CRH increase in response to stress
