MEH - Hypothalamic Pituitary Axis

Question 1

What is the hypothalamic pituitary axis?

Answer 1

The hypothalamus and pituitary gland form a complex functional unit together which serves as the major link between the endocrine and nervous systems

Question 2

Where is the pituitary gland found?

Answer 2

Beneath the hypothalamus in a socket of bone called the sella turcica

Question 3

Give some examples of processes modulated by the hypothalamus and the pituitary gland

Answer 3

• body growth
• reproduction
• adrenal gland function
• water homeostasis
• milk secretion
• lactation
• thyroid gland function
• puberty

Question 4

What are the two parts of the pituitary gland?

Answer 4

• anterior pituitary gland (adenohypophysis)

- posterior pituitary gland (neurohypophysis)

Question 5

What is the embryological origin of the anterior pituitary?

Answer 5

It arises from evagination of the oral ectoderm (Rathke’s pouch) - this is primitive gut tissue

Question 6

What is the embryological origin of the posterior pituitary?

Answer 6

It originates from the neuroectoderm - primitive brain tissue

Question 7

How is the posterior pituitary connected to the hypothalamus?

Answer 7

It is physically connected through the infundibulum (pituitary stalk), because the hypothalamus originally drops down to form the posterior pituitary gland

Question 8

How are oxytocin and ADH produced and released?

Answer 8

• 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

Question 9

How does the anterior pituitary function?

Answer 9

• 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)

Question 10

Summarise the difference in mechanism of the anterior and posterior pituitary

Answer 10

• 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

Question 11

What is the difference between antidiuretic hormone and vasopressin?

Answer 11

They’re the same!

Question 12

What does it mean if a hormone is tropic?

Answer 12

It affects the release of other hormones in the target tissue

Question 13

What are the six tropic hormones produced by the hypothalamus?

Answer 13

• 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)

Question 14

What are the six hormones produced by the anterior pituitary?

Answer 14

• thyroid stimulating hormone (TSH)
• adrenocorticotropic hormone (ACTH)
• luteinising hormone (LH)
• follicle stimulating hormone (FSH)
• prolactin (PRL)
• growth hormone (GH)

Question 15

Give some factors which influence growth

Answer 15

Nutrition, genetics, environment, hormones (growth hormone is the most important)

Question 16

What stimulates growth hormone?

Answer 16

GHRH from the hypothalamus

Question 17

What inhibits growth hormone?

Answer 17

Somatostatin from hypothalamus

Question 18

Why must growth hormone be injected rather than taken orally?

Answer 18

It is a protein and would be digested

Question 19

True or false - the signal peptide remains on the growth hormone protein?

Answer 19

False - it must be cleaved before proper folding

Question 20

What are IGFs?

Answer 20

Insulin-like growth factors (somatomedins) produced by the liver and skeletal muscle in response to GH

Question 21

What is the role of GH and IGFs in adults?

Answer 21

Help maintain muscle and bone mass, promote healing and tissue repair, modulate metabolism and body composition

Question 22

What is the role of GH and IGFs in children?

Answer 22

• GH stimulates long bone growth (length and width before epiphyseal closure, just width after)
• IGFs stimulate bone and cartilage growth

Question 23

What events can cause GH secretion to rise?

Answer 23

• rise in secretion after onset of deep sleep
• stress (eg. Trauma, surgery or fever)
• exercise
• decrease in glucose or fatty acids
• fasting

Question 24

What events can cause GH secretion to fall?

Answer 24

• rapid eye movement sleep
• increase in glucose or fatty acid levels
• obesity

Question 25

How does long loop negative feedback inhibit GH?

Answer 25

- IGFs inhibit release of GHRH from the hypothalamus - IGFs stimulate release of somatostatin from the hypothalamus - IGFs inhibit release of GH from anterior pituitary

Question 26

How does short loop negative feedback regulate GH?

Answer 26

It is mediated by GH itself via stimulation of somatostatin release

Question 27

What is pituitary dwarfism?

Answer 27

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

Question 28

What disorder can growth hormone excess in childhood cause?

Answer 28

Gigantism - in adulthood it leads to acromegaly

Question 29

How does GH exert its effect on cells?

Answer 29

GH receptors activate 'janus kinases' which cross phosphorylate and activate signalling pathways. Transcription factor activation and IGF production then occurs.

Question 30

What are the two IGFs found in mammals and what are they responsible for?

Answer 30

- IGF1 is a major growth factor in adults | - IGF2 is mainly involved in foetal growth

Question 31

What can IGFs modulate?

Answer 31

- cell growth (hypertrophy) - cell number (hyperplasia) - increase in rate of protein synthesis - increase in rate of lipolysis in adipose tissue

Question 32

What is the growth related action of insulin?

Answer 32

Enhances somatic growth, interacts with IGF receptors

Question 33

What is the growth related action of thyroid hormones?

Answer 33

Promotes CNS development and enhances GH secretion

Question 34

What are the growth related actions of androgens?

Answer 34

Accelerate pubertal growth spurt, increase muscle mass, promote closure of epiphyseal plates, inhibit somatic growth