Hypothalamo-Adenohypophysial Axis

Question 1

where does the pituitary gland/hypophysis sit?

Answer 1

in a depression of the sphenoid bone called the sella turcica

Question 2

where does the adenohypophysis originate from?

Answer 2

it is the anterior pituitary and embryologically it grows up from the buccal cavity this is called Rathke’s Pouch (therefore has mainly secretory function)

Question 3

where does the neurohypophysis originate from?

Answer 3

from the brain tissue, neural tissue grows down from the base of the brain. this forms the posterior pituitary gland.

Question 4

what is the pars distalis?

Answer 4

body of the anterior pituitary

Question 5

what is the pars tuberalis?

Answer 5

the bit of the anterior pituitary that wraps around the pituitary stalk.

Question 6

why is the hypothalamus important?

Answer 6

it controls and regulates secretion of hormones from the hypophysis.

Question 7

what is significant about the median eminence in the hypothalamic-hypophyseal portal circulation?

Answer 7

it is devoid of the Blood Brain barrier

Question 8

what do hypothalamic nuclei do?

Answer 8

produce the release factors, they are a group of cell bodies with their axonal projections either to the median eminence which just touches the pituitary or to the base of the posterior pituitary

Question 9

describe the hypothalamic-hypophyseal portal circulation

Answer 9

blood enters via the SUPERIOR HYPOPHYSEAL ARTERY to the fenestrated primary capillary plexus known as the MEDIAN EMINENCE which sits just above the stalk of the pituitary. Fenestrations allow neurosecretions from the hypothalamus to enter the blood. LONG PORTAL VEINS take blood with neurosecretions to the secondary capillary plexus which is fenestrated to allow factors out of blood into the pituitary to release their hormones. Venous drainage is vis the CAVERNOUS SINUS which empties in the jugular veins.

Question 10

where do the short hypothalamic neurones terminate?

Answer 10

at the median eminence.

Question 11

what types of cells are found in the Adenohypophysis and what do they produce?

Answer 11

Somatotrophs- growth hormone 
Corticotrophs- ACTH
Thyrotrophs- TSH
Lactotrophs- prolactin
Gonadotrophs- LH and FSH

(note —trophin means stimulating growth and development of…)

Question 12

what type of hormones are produced by the adenohypophysis?

Answer 12

PROTEIN: somatotrophin and prolactin
GLYCOPROTEIN: 99 amino acid α subunit common to all + unique β chain: TSH, LH, FSH
POLYPEPTIDE: ACTH

Question 13

which hypothalamic hormones control the release of somatotrophin?

Answer 13

primarily, growth hormone releasing hormone (GHRH) promotes the release of somatotrophin

SOMATOSTATIN can inhibit release of somatotrophin.

Question 14

which hypothalamic hormones control the release of Prolactin?

Answer 14

Primarily, Dopamine INHIBITS the release of prolactin

Thyroid releasing hormone (TRH) PROMOTES secretion of prolactin

Question 15

which hypothalamic hormone promotes the release of TSH?

Answer 15

Thyroid Releasing Hormone, TRH

Question 16

which hypothalamic hormone promotes the release of LH and FSH?

Answer 16

GnRH, Gonadotrophin Releasing Hormone

prolactin Inhibits release

Question 17

which hypothalamic hormones control the release of ACTH?

Answer 17

Corticotrophin Releasing Hormone, CRH

Vasopressin

Question 18

how are these hormones (ACTH) released?

Answer 18

in a pulsatile pattern

Question 19

where do the adenohypohyseal hormones act (target cells)

Answer 19

Growth Hormone - all body tissues esp. Liver
Prolactin - breast in lactating women
Thyrotrophin- thyroid
Gonadotrophin- testes and ovaries
corticotrophin- adrenal cortex

Question 20

what does somatotrophin do?

Answer 20

it causes growth and development of all cells due to metabolic actions. In the liver it triggers production os somatomedins like IGF1 and IGF2 which mediates growth in all tissues to prevent overgrowth.

Question 21

metabolic actions of IGF and Somatotrophin

Answer 21

increased amino acid transport therefore increased protein synthesis
increases gluconeogenesis and lipolysis for energy
increased cartilaginous and somatic growth

Question 22

why is sleep important for growth?

Answer 22

Big Pulses of GH occur during sleep as sleep stimulates GHRH.

Question 23

describe the somatotrophin axis?

Answer 23

sleep, stress, oestrogens, exercise, fasting (hypoglycaemia), amino acids stimulates the release of GHRH which stimulates release of somatotrophin, gremlin also stimulates the release of somatotrophin directly. the somatotrophins and somatomedins also negatively feedback onto the hypothalamus and pituitary.

Question 24

how is prolactin secretion activated?

Answer 24

suckling of the nipple causes tactile receptors to get activated causing an afferent neural pathway to the hypothalamus. this causes suppression of hypothalamic dopaminergic neurones. less dopamine is released by the hypothalamic nuclei hence causes prolactin secretion to increase in an efferent endocrine pathway. This causes post part milk production.