Hypothalamic Pitutary Axis & growth Hormone

Question 1

What is the HPA axis ?

Answer 1

The hypothalamus and pituitary gland form a complex functional unit that serves as a the major link between the endocrine and nervous system.

Question 2

Where is the location of the Pituitary gland ?

Answer 2

Sits beneath the hypothalamus in a socket of bone called the SELLA TURCICA

Question 3

What processes does the hypothalamus and P gland modulate ?

Answer 3

Body growth
Milk secretion

Lactation

Reproduction

Adrenal gland function

Water homeostasis

Thyroid gland function

Puberty

Question 4

What are the two components of the pituitary gland ?

Answer 4

1. Anterior Pituitary gland ( adenohypophysis)

2. Posterior pituitary gland ( neurohypophysis )

Question 5

Is the posterior pituitary section a gland or not ?

Answer 5

No , because it doesn’t actually synthesis the hormones it secretes. It secretes hormones that are synthesised by the hypothalamus.

Question 6

What is the embryological origin of the anterior pituitary gland ?

Answer 6

Arises from Evaginnation of oral ectoderm ( primitive gut tissue )

Question 7

What is the embryological origin of posterior pituitary gland ?

Answer 7

Originated from neuroecroderm ( primitive brain tissue )

Question 8

What structure connects the posterior Pituitary to the hypothalamus ?

Answer 8

Infundibulum

Question 9

What is the neurocrine function of the posterior pituitary gland ?

Answer 9

1. Oxytocin and ADH produced by neurosecretory cells in the supraoptic and paraventricular nuclei of the hypothalamus. More oxytocin is produced by the supraoptic nuclei than the paraventricular nuclei.
2. These two hormones are transported down the nerve cell axons to the posterior pituitary.
3. These hormones are then stored and released from posterior pituitary into general circulation to act on distant targets.

THIS IS AN EXAMPLE OF NEUROCRINE SIGNALLING

Question 10

Outline the process by which hormones are stimulated and secreted by the anterior pituitary gland ?

Answer 10

1. Hormones synthesised in the hypothalamus are transported down axons and stored in median eminence before released into HYPOPHYSEAL PORTSL SYSTEM ( connects one capillary bed to another )
2. These hormones then stimulate or inhibit target endocrine cells in the ANterior pituitary gland. ( neurocrine function )
3. Endocrine cells of anterior pituitary secrete variety of hormones into bloodstream to act on distant target cells ( endocrine function )

Question 11

What is the function of oxytocin ( OT ) ?

Answer 11

Regulates the positive feedback loop during childbirth - causes uterus contractions during childbirth. And involved in the milk let down reflex when the baby begins to suckle at the breast.

Question 12

What is the function of ADH ( vasopressin ) ?

Answer 12

Regulation of body water volume - promotes reabsorption of water through deposition of aqua porin channels in the collecting ducts of the kidney.

Question 13

What are tropic hormones ?

Answer 13

Hormones that are released by the hypothalamus and affect the synthesis and release of hormones from thr anterior pituitary gland.

Question 14

Name the 6 tropic hormones that are synthesised and secreted from the hypothalamus .

Answer 14

TRH : thyrotropin releasing hormone

PIH : prolactin release inhibiting hormone ( dopamine )

CRH : corticotropin releasing hormone

GNRH : gonadotropin releasing hormone

GHRH : growth hormone releasing hormone

GHIH : growth hormone-inhibiting hormone ( somatostatin )

Question 15

What are the 6 hormones produced by the anterior pituitary gland ?

Answer 15

1. TSH : thyroid stimulating hormone which is involved in the secretion of thyroid from thyroid gland.

ACTH : Adrenocorticotropic hormone : secretion of hormones from adrenal cortex

LH : Luteinising hormone : controls ovulation and secretion of sex hormones.

FSH : follicle stimulating hormone : development of eggs and sperm

PRL : prolactin : mammary gland development and milk secretion

GH : growth hormone : growth and energy metabolism and stimulates IGFS.

Question 16

Outline the mechanism of HPA when the hypothalamus detects stress such as pain , fever , hypoglycaemia or low blood pressure ?

Answer 16

1. Hypothalamus detects these signals
2. Hypothalamus responds by secreting Corticotropin releasing hormone ( CRH )
3. This acts on endocrine cells in the Anterior pituitary gland which results in the release of Adrenocorticotropic hormone ( ACTH) which acts on the adrenal cortex.
4. Adrenal cortex responds by releasing cortisol.
5. This has an effect on many target tissues.

Question 17

What are the 4 main factors that influence growth ?

Answer 17

1. Growth hormones
2. Environment
3. Nutrition
4. Genetics

Question 18

What are the two mechanisms by which cells can grow ?

Answer 18

Hypertrophy ( cell growth )

Hyperplasia ( increasing cell number )

Question 19

Where is growth hormone produced and what two hormones regulate its secretion ?

Answer 19

Released from the anterior pituitary gland

It is stimulated by hypothalamic GHRH
It is inhibited by hypothalamic somatostatin

Question 20

What type of hormone is growth hormone ?

Answer 20

Peptide hormone (191 aa) . It has a signal peptide that must be cleaved before folding.

Question 21

What does GH stimulate ?

Answer 21

Stimulates the production of insulin-like growth factor from the liver ( indirect effects )

Or it could have direct effects - where it acts directly on growth hormone receptors

Question 22

What is the function of growth hormone in childhood / teenage / adults ?

Answer 22

Growth hormone stimulates long bone growth - it lengthens and widens the bone BEFORE epiphyseal closure. Once those epiphyseal plates have been ossified , it can no longer grow in length , only by width.

In adults , GH and IGFS help maintain muscle and bone mass. And promotes tissue repair and healing.

Question 23

Apart from GHRH and Somatostatin, what other factors promote GH secretion / inhibition ?

Answer 23

After onset of deep sleep , there is a surge in GH secretion. That is why it is important for babies to have uninterrupted sleep because it allows them to grow.

2. Stress increases GH secretion.
3. Exercise Increases GH secretion
4. A decrease in glucose or fatty acids leads to an increase in GH secretion
5. Fasting leads to an increase in GH secretion
6. Obesity decreases GH secretion

Question 24

What is the long loop negative feedback mediated by ?

Answer 24

Mediated by IGFS , which act on the hypothalamus and inhibit release of GHRH. But stimulate release of somatostatin from hypothalamus. They also act on the anterior pituitary gland which inhibits the release of GH.

Question 25

What is the short loop mediated by ?

Answer 25

Mediated by GH itself by acting on the hypothalamus to stimulate somatostatin release.

Question 26

Outline the mechanism of action of growth hormone on its receptor ?

Answer 26

1. GH binds to a tyrosine kinase receptor .
2. Binding of GH to its receptor results in dimerisation of the GHR . This leads to a conformational change of the receptor
3. This leads to autophosphorylation of the Janus Kinases on the dimers.
4. This activates signalling pathways.
5. These signalling pathways leads to transcription factor activation and IGF production.

Question 27

What are the two types of IGFS ?

Answer 27

IGF1 ( major growth factor in adults ) produced by the liver

IGF2 ( mainly involved in fetal growth )

They act on IGF receptors to module cell growth , cell number increase , increase in rate of protein synthesis and increase rate of lipolysis in adipose tissue.

Question 28

Give an example of. Another hormone which influences growth

Answer 28

Androgens

These accelerate pubertal growth spurt. Increase muscle mass. Promote closure of epiphyseal growth plates.