Hypothalamo pituitary axis

Question 1

Endocrine secretion

Answer 1

Hormones diffuse directly into capillaries to act on distant target organs

Question 2

Paracrine secretion

Answer 2

Secreted and act more locally

Question 3

Autocrine secretion

Answer 3

Act on themselves

Question 4

What are the major endocrine glands?

Answer 4

• Pineal gland
• Hypothalamus
• Pituitary
• Thyroid
• Parathyroid
• Thymus
• Adrenal gland
• Pancreas
• Gonads - ovary and testes

Question 5

What are the 3 categories of factors controlling hormone release from endocrine glands?

Answer 5

• Humoral
• Neural
• Hormonal

Question 6

Describe the humoral factors affect hormone release

Answer 6

Response to changes in extracellular fluid e.g.
• Capillary blood has a low concentration of Ca2+ which stimulates the secretion of parathyroid hormone by parathyroid glands
• Release of insulin in response to blood glucose

Question 7

Describe the neural factors affecting hormone release

Answer 7

Preganglionic sympathetic nervous system fibres stimulating the adrenal medulla cells to secrete catecholamines

Question 8

What are the hormonal factors affecting hormone release?

Answer 8

• Hypothalamus secretes hormones/neurohormones that stimulate the anterior pituitary to secrete hormones that stimulate other glands to release hormones
• Amplifiable response

Question 9

What is the pituitary gland enclosed by?

Answer 9

• Sella tunica of the sphenoid

* enclosed superiorly by diaphragma sellae

Question 10

Where does the pituitary gland lie?

Answer 10

Immediately posterior to the optic chiasma and sphenoid sinus, on the floor of the midbrain

Question 11

What are the parts of the anterior pituitary?

Answer 11

• Pars anterior (distalis)
• Pars tubercles (PT) surrounds the neural stalk
• Pars intermedia - exists between anterior and posterior pituitary

Question 12

What is the anterior pituitary derived from?

Answer 12

• Rathke’s pouch

Question 13

Describe the embryological development of the pituitary

Answer 13

• Anterior pituitary: upgrowth of epithelium from the oral cavity (Rathke’s pouch) - hypophysial diverticulum
• Posteiror pituitary: down growth from the brain (infundibulum) - neurohypophyseal

Question 14

Craniopharyngiomas

Answer 14

Slow growing tumours that can develop along the track of Rathke’s pouch

Question 15

Describe chromaphils

Answer 15

• Take up stain, there are two types
• Acidophils
• Basophils

Question 16

What is the name given to cells that do not take up stian

Answer 16

Chromophobes

Question 17

Describe the nuclei in the hypothalamus

Answer 17

• Large cell bodies: 
- paraventricular 
- supraoptic 
- infundibular 
• Small cell body nuclei

Question 18

Describe the course of the axons of the large cell bodied nuclei

Answer 18

• Extend down through the median eminence
• Come together to form a nerve tract: hypothalmo-hypophyseal tract
• Fibres terminate in the posterior pituitary on a capillary network supplied by the inferior hypophyseal artery
• Herring bodies

Question 19

Describe the course of the axons of the small cell bodied nuclei

Answer 19

• Axons terminate higher up on a capillary network in the median eminence or pituitary stalk
• Capillary network is supplied by the superior hypophyseal artery: called the primary capillary plexus
• Neurohormones released into the network drain into the hypothalmo-hypophyseal portal system which goes on to form a secondary capillary network within the anterior lobe of the pituitary
• The neurohormones diffuse out of the fenestrated capillaries and act on receptors (trophic cells) in the anterior pituitary to regulate secretions from adenohypophyseal cells

Question 20

What ar the neurohormones released from the posterior pituitary?

Answer 20

• Vasopressin/ADH

* Oxytocin

Question 21

What are the trophic hormones of the anterior pituitary

Answer 21

• Thyrotropin/ thyroid stimulating hormone
• Corticotropin/adrenocorticotropic hormone
• Gonadotropins: LH and FSH

Question 22

What hormones released from he anterior pituitary act on peripheral targets?

Answer 22

• Somatotropin/ Growth hormone
• Prolactin
• Melanocyte stimulaitng hormone

Question 23

Describe growth hormone release control

Answer 23

• Neurosecretory cells in the arcuate nucleus of the hypothalamus secrete growth hormone releasing hormone
• Reaches somatotrophs via the hypophyseal portal blood supply
• GHRH causes somatotrophs to synthesise and release growth hormone (in the anterior pituitary) by activating a stimulating g protein coupled receptor

Question 24

Describe the inhibition of growth hormone release

Answer 24

• Somatotropin is released from the periventricular region
• Inhibits the release of GH by the somatotrophs
• Activates an inhibiting G protein coupled receptor

Question 25

Describe feedback of growth hormone

Answer 25

• IGF-1 directly inhibits GH release by suppressing the somatotrophs • IGF-1 indirectly inhibits GH release by: - suppressing GHRH release - increasing the secretion of somatostatin • GH inhibits its own secretion via short loop feedback on somatotrophs

Question 26

Describe the circadian rhythms for growth hormone secretion

Answer 26

* Pulsatile release of GH throughout the day | * Spikes during sleep

Question 27

What are the physiological action fo growth hormone?

Answer 27

• Direct anti insulin effect: - increased lipolysis in adipose tissue - increased blood glucose • Indirect actions (IGF-1 release from the liver) - Increased cartilage formation and bone growth - increased general protein synthesis and cell growth/division

Question 28

What are the physiological consequences of growth hormone?

Answer 28

• Increased linear growth and lean body mass • Vital importance for normal post natal development and rapid growth throughout puberty • Maintenance of protein synthesis and tissue function in adults - maintains protein turnover rate

Question 29

GH deficiency

Answer 29

* Dwarfism in children | * Decreased availability of lipids and glucose for energy: treat with growth hormone

Question 30

GH excess- before puberty

Answer 30

• Gigantism due to excess stimulation of the epiphyseal plates

Question 31

GH excess after puberty

Answer 31

* Periosteal bone growth causing enlarged hands, jaw and foot size * Soft tissue growth leading to enlargement of the tongue and coarsening of facial features * Insulin resistance and gluten intolerance * Treat with synthetic long acting somatostatins

Question 32

Where is ADH synthesised?

Answer 32

In the neurosecretory cells in the supra-optic nucleus and paraventricular nucleus

Question 33

What increases ADH release?

Answer 33

* Increased blood osmolarity detected by osmoreceptors in the brain * Decreased blood volume detected by volume receptors (carotid, aorta, left atrium) * RAAS - angiotensin II

Question 34

What is the effect of ADH release?

Answer 34

Recruitment of aquaporin water channels -> water retention

Question 35

ADH deficiency

Answer 35

* Diabetes insipidus | * Neural or nephrogenic

Question 36

What are causes of cranial diabetes insipidus?

Answer 36

* Tumours - 30% * Disease induced/trauma -> damage to the hypothalamus * Familial disorders of the neurosecretory cells

Question 37

What are the causes of nephrogenic diabetes insipidus?

Answer 37

* Sex linked genetic defect in the collecting tubule (receptor doesn't recognise ADH) * Non sex linked genetic defect in the aquaporin water channel

Question 38

Describe the control of milk production

Answer 38

* Suckling stimulus travels through the spinal cord to the hypothalamus * Neurones from the hypothalamus inhibit dopamine release from the arcuate nucleus leading to prolactin release -> milk production * Neurones from the spinal cord stimulate the production and release of oxytocin from the paraventricular and supraoptic nuclei in the posterior pituitary -> systemic circulation -> breast and my-epithelial cells * neurones in the spinal cord inhibit neurones in the arcuate nucleus and preoptic area of the hypothalamus causing a fall in GnRH, inhibiting the ovarian cycle

Question 39

Where is the pineal gland?

Answer 39

• In the midline in the posterior part of the 3rd ventricle

Question 40

What are the functions of the pineal gland?

Answer 40

* In darkness the pineal gland secretes melatonin * Regulates circadian rhythm * Regulates reproductive processes including the onset of puberty in humans * Effects on ageing and regulation fo the immune system * Accumulate calcium phosphate