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Endocrinology > THE HYPOTHALAMO-HYPOPHYSIAL (PITUITARY) AXIS > Flashcards

THE HYPOTHALAMO-HYPOPHYSIAL (PITUITARY) AXIS Flashcards

1
Q

Describe the dimensions of the pituitary gland

A

Pituitary gland tiny – size of your thumbnail – attached to the base of the brain. Lies in a bony dip – shaped like a Turkish saddle – hence its name sella turcica

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2
Q

What is the pituitary gland also known as

A

The master gland. It is the source of a number of hormones, of which some have, as their major effect, the hormone production by other endocrine glands elsewhere in the body. The hypothalamus is therefore the overall conductor.

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3
Q

Describe the basic structure of the pituitary gland

A

Lies within a depression in the sphenoid depression beneath it (sella turcica)
In adults, the two lobes of the gland are quite distinct, with the anterior lobe being made up of various types of secretory cells; it is also called the adenohypophysis. The posterior is composed in large part of nerve axons from the hypothalamus, and is called the neurohypophysis.
Optic chiasm is near the anterior- tumour and visual disturbance
Mammillary body- nearer to the posterior

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4
Q

How does the pituitary gland appear on the MRI

A

We only see one entity- but functionally it has two distinct lobes.

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5
Q

Describe the differences in the embryological developments of the anterior and posterior lobes

A

The 2 components of the pituitary gland develop from different structures. There is a upward growth (ectodermal extension) from the buccal cavity called Rathke’s pouch towards a downward growth (ectodermal extension) from the base of the brain - these two growths combine to make one gland (5th week). So the pituitary gland has 2 components, 1 secretory part derived from the buccal cavity the adenohypophysis and 1 neural part, the neurohypophysis, made up of nerve axons derived from the base of the brain.

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6
Q

Describe the different regions of the anterior lobe of the pituitary gland

A

Pars distalis is the body of the anterior pituitary- contains the neurosecretory cells, the pars tuberalis is the part of the anterior pituitary which wraps around the pituitary stalk (infundibulum). We know that the regulation of anterior pituitary function is via the bloodstream - a special circulation.

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7
Q

What happens to the pituitary gland during pregnancy

A

It increases in size

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8
Q

Describe the hypothalamic nuclei

A

The hypothalamus is part of the brain, so is made up of neurones – neurones start off as cell bodies and then their axonal projections to various parts of the brain including the neurohypophysis. Groups of neuronal cell bodies are called nuclei. There are a number of hypothalamic nuclei – and some of these are very important as they send axons down the pituitary stalk to the posterior pituitary (neurohypophysis). A different population of neurons have shorter axons and these terminate in the part of the hypothalamus which abuts the pituitary gland – the median eminence.

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9
Q

Describe the median eminence

A

It is devoid of the blood-brain barrier and so can communicate with the systemic circulation

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10
Q

Describe the HYPOTHALAMIC – HYPOPHYSIAL PORTAL CIRCULATION

A

There is a very special blood system which flows to the anterior pituitary. It starts with blood arriving in the SUPERIOR hypophyseal artery and this enters a capillary network in the median eminence. From this capillary network (primary capillary plexus), blood drains to a second capillary network in the anterior pituitary via the portal vein – a portal network. A portal network is where blood from the systemic circulation drains from one capillary network to another before returning to the heart. From the secondary capillary system, blood would flow out into the venous system (jugular vein) via the cavernous sinus. This system is critical for control of anterior pituitary function.

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11
Q

What is the importance of the capillaries being fenestrated

A

Neurosecretions from hypothalamic nuclei can enter the capillaries and be transported to the pars distalis, and act on secretory cells.

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12
Q

What is a neurosecretion

A

Hypothalamic neurone

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13
Q

What does the adenohypophysis require

A

Blood Circulation

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14
Q

What is the portal system called

A

Hypothalamo-adenohypophysial portal system

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15
Q

Describe the steps that lead to the release of adenohypophysial hormone into the circulation

A

Axons from hypothalamic nuclei end at the primary capillary plexus in the median eminence. The neurosecretions from these nuclei enter the primary capillary plexus (granules released by exocytosis in nerve terminals upon depolarisation) and travel to the secondary capillary plexus via the portal system. Neurosecretions leak out of the fenestrations in the secondary capillary plexus, act on secretory cells- release of hormones into circulatory system

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16
Q

Describe the innervation of the adenohypophysis

A

There is no appreciable innervation of the adenohypophysis, although a few peptidergic nerve fibres do terminate within it and could have a controlling influence. There is also evidence for some innervation by sympathetic fibres, specifically to the vasculature. It is quite possible that regulation of the adenohypophysial blood flow could have some influence on hormone release into the circulation

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17
Q

What does trophin mean

A

means stimulates growth and development of’

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18
Q

List the different types of adenohypophysial cells and the hormones that they produce

A

SOMATOTROPHS produce Growth Hormone (Somatotrophin)

LACTOTROPHS produce PROLACTIN

THYROTROPHS produce Thyroid Stimulating Hormone (TSH, Thyrotrophin)

GONADOTROPHS produce Luteinising Hormone, Follicle Stimulating Hormone (LH and FSH)

CORTICOTROPHS produce   AdrenoCorticoTrophic Hormone 				     (ACTH, Corticotrophin)
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19
Q

What are two interesting characteristics of adenohypophysial hormones

A

They are greatly influenced by the CNS through the hypothalamus and its hormones.
Most of them have other endocrine glands as their targets

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20
Q

Describe the production and secretion of the hypophysial hormones

A

Precursor molecules called PROHORMONES
Enzymatic cleavage of prohormone yields
the bioactive HORMONE molecule
Adenohypophysial hormones stored
in secretory granules
Released by exocytosis into the bloodstream

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21
Q

Where are the enzymes that cleave the prohormones found

A

In the anterior pituitary cells.

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22
Q

Us the example of corticotrophin to describe how hormones are synthesised

A

POMC = ProOpioMelanoCorticotrophin) — CORTICOTROPHIN + Pro-gMSH + bLPH

23
Q

Describe what constitutes the different types of adenohypophysial hormones

A

All made of amino acids
PROTEINS:
GROWTH HORMONE (SOMATOTROPHIN ) 191 aa
PROLACTIN 199 aa

GLYCOPROTEINS: consisting of a and b sub-units (92 aa a
sub-unit common to all )
THYROID STIMULATING HORMONE (TSH, THYROTROPHIN) b-sub-unit 110 aa
2 GONADOTROPHINS both have 115 aa b-sub-unit
- LUTEINISING HORMONE (LH)
- FOLLICLE STIMULATING HORMONE (FSH)

POLYPEPTIDE:
ADRENOCORTICOTROPHIC HORMONE (ACTH, CORTICOTROPHIN) 39 aa
24
Q

What do glycoprotein hormones contain

A

Glycoprotein molecules – are protein molecules to which CHO molecules have been attached. These have 2 strands – alpha and beta subunits, the alpha subunit is common to all of them, the beta subunit distinguishes them

25
Q

Where are the adenohypophysial hormones stored

A

In secretory granules

26
Q

What are the two types of neurosecretions

A

Stimulatory or inhibitory

27
Q

How are action potentials generated in the hypothalamic nuclei

A

From the integration of incoming stimulatory and inhibitory signals

28
Q

Why is the pulsatile nature of hypothalamic hormone release into the median eminence important

A

it is essential in determining not only the nature, but also the amount of, anterior pituitary hormone to be released.

29
Q

What is corticotrophin also known as

A

Adrenocorticotrophic hormone

30
Q

What is special about prolactin

A

It is the only adenohypophysial hormone under dominant inhibitory control, exerted by dopamine from the hypothalamus.

31
Q

List the hypothalamic hormones that lead to the release of somatotrophin

A

Growth Hormone Releasing Hormone (GHRH, somatotrophin releasing hormone) - STIMULATORY
Somatostatin (SS)- INHIBITORY

32
Q

List the hypothalamic hormones that lead to the release of prolactin

A

Dopamine (DA)- INHIBITORY

Thyrotrophin releasing hormone (TRH)- STIMULATORY

33
Q

List the hypothalamic hormones that lead to the release of Thyroid stimulating hormone

A

TRH- Stimulatory

34
Q

List the hypothalamic hormones that lead to the release of LSH and FSH

A

Gonadotrophin releasing

hormone (GnRH)- Stimulatory

35
Q

List the hypothalamic hormones that lead to the release of ACTH

A

Corticotrophin releasing hormone (CRH)- STIMULATORY

Vasopressin (VP)- STIMULATORY

36
Q

When is the growth hormone very pulsatile

A

During puberty- has direct and indirect effects during development.

37
Q

What is a consequence of the release of hypothalamic hormones being pulsatile

A

The release of adenohypophysial hormones is also pulsatile

38
Q

What are the target cells of somatotropin

A

GENERAL BODY TISSUES,

PARTICULARLY THE LIVER

39
Q

What are the target cells of prolactin

A

BREASTS (LACTATING

WOMEN)

40
Q

What are the target cells of thyrotropin

A

THYROID

41
Q

What are the target cells of LSH and FSH

A

TESTES (MEN)

OVARIES (WOMEN

42
Q

What are the target cells of corticotrophin

A

Adrenal Cortex

43
Q

What happens after somatotropin binds to growth hormone receptors on hepatocytes

A

Lots of GH Rs on hepatocytes – binding of GH to these receptors causes production of IGF-1 (and 2) which mediates some of the effects of GH in the tissues.
IGF-2 is released- but more in foetus
IGF-1 and 2 are known as somatomedins

44
Q

What does growth involve

A

Growth involves muscular growth – GH stimulates protein synthesis. In lipolysis, triglycerides are broken down to form glycerol (which can then go on to contribute to glucose formation) and FFAs – a source of energy.

45
Q

List some of the metabolic effects of somatotropin

A

Stimulation of amino acid transport into cells (e.g. muscle) and protein synthesis
Increased gluconeogenesis
Stimulation of lipolysis leading to increased
fatty acid production
Increased cartilaginous growth and somatic cell growth

46
Q

What can stimulate the release of somatotropin

A 
Sleep (stages III and IV)
Stress
Oestrogens
Fasting (Hypoglycaemia)
Amino acids such as arginine
Ghrelin (from stomach).
47
Q

Describe the negative feedback loop in the production of somatotropin

A

Negative Feedback loops – IGF-1 has direct feedback to the ant pituitary and indirect effects on the hypothalamus to reduce GHRH. Short feedback = negative feedback of GH to the hypothalamus.
GH indirect effects on hypothalamus

48
Q

Describe the neuro-endocrine reflex arc that leads to the production of milk

A

Stimulation of post-partum Breast by suckling. Afferent neuronal pathway to hypothalamus, where Suppression of hypothalamic dopaminergic neurones occurs, leading to the secretion of prolactin and thus lactogenesis.

49
Q

What are some of the other effects of prolactin

A 
Inhibition of LH and FSH production.
Decreased libido
Mammary gland growth and development during pregnancy
Osmoregulation
Interactions with the immune system
50
Q

What inhibits the release of Prolactin

A

Oestrogen.

51
Q

What are the effects of LH and FSH in males

A

LH- Androgen production
FSH- Stimulates the Sertoli cells which are involved in various stages of spermatogenesis, as well as inhibin production.

52
Q

What are the effects of LH and FSH in females

A

LH- Androgens

FSH- Convert androgens into oestrogens.

53
Q

What are the effects of TSH

A

The overall effect of TSH is to stimulate the synthesis, storage and release of the iodothyronines. Stimulation of iodide pump, synthesis of enzyme peroxidase (located in the apical membrane facing the colloid), synthesis of intracellular thyroglobulin protein, iodination of thyroglobulin, the process of endocytosis to bring iodinated thyroglobulin back into the follicular cells from the colloid, migration of lysosomes towards the endocytosed colloid.

54
Q

What are the effects of ACTH

A

Increased synthesis of glucocorticoids (steroid hormones) and to a lesser extent the adrenal androgens.

Endocrinology (11 decks)

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