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Endocrine System > The Hypothalamus-Hypophysis Axis > Flashcards

The Hypothalamus-Hypophysis Axis Flashcards

1
Q

Abnormal Hormone Secretion Types

A

Hyposecretion

Hypersecretion

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2
Q

Hyposecretion

A

Too little hormone is secreted.

Primary hyposecretion:

Too little hormone is secreted due to abnormality within the target gland.

Secondary hyposecretion:

Gland is normal but too little hormone is secreted due to deficiency of its trophic hormone.

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3
Q

Hypersecretion

A

Too much hormone is secreted.

Causes:

  1. Tumors that ignore normal regulatory input and continuously secrete excess hormone.
  2. Immunologic factors.

Primary hypersecretion:

Too much hormone is secreted due to abnormality within the target gland.

Secondary hypersecretion:

Excessive stimulation from outside the gland causes over-secretion.

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4
Q

Hormone Abbreviations

A

TSH: thyroid-stimulating hormone

ACTH: adrenocorticotropic hormone

FSH: follicle-stimulating hormone

LH: luteinizing hormone

GH: growth hormone

PRL: prolactin

MSH: melanocyte-stimulating hormone

ADH: antidiuretic hormone

Oxytocin/Vasopressin

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5
Q

Role of feedback in hormone secretion

A

The secretion of hormones is usually dependent upon feedback mechanisms.

Negative feedback:

A stimulus causes an endocrine response (hormone secretion) which will decrease the level of that stimulus.

Positive feedback:

A stimulus causes a response which will increase the level of that
stimulus.

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6
Q

Trophic/tropic hormones

A

Regulates hormone secretion by another endocrine
gland

Stimulates and maintains their endocrine target tissues

Example:
Thyroid-stimulating hormone (TSH):

-Secreted from anterior pituitary stimulates thyroid hormone
secretion by thyroid gland
-Also maintains structural integrity of thyroid gland

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7
Q

Hypothalamus-Pituitary Axis

A

Releasing hormones from hypothalamus instruct the anterior pituitary what to release into the blood.

Trophic hormones from the pituitary instruct specific endocrine glands to grow and produce their hormones.

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8
Q

What determines the size of hormone

effects?

A
  1. The amount of hormone in the circulation (reaching the target tissue).
    - the more hormone, the greater the effect.
  2. The presence and number of receptors for that hormone on the target tissue.
    - no receptor, no response
    - some receptors, some response
    - many receptors, higher response
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9
Q

Regulation of hormone secretions

A

Primarily controlled by negative feedback mechanism :increased
target organ hormone levels inhibits release of hormones

Neural Stimuli

Hormonal Stimuli

Humoral Stimuli

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10
Q

Neural Stimuli

A

Neurons synapse with cells producing hormone (i.e., norepinephrine release from the adrenal gland).

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11
Q

Hormonal Stimuli

A

Hormones bind to endocrine cells, regulating release of another hormone (i.e., TSH stimulates thyroid hormone release)

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12
Q

Humoral Stimuli

A

Endocrine cells respond to levels of other factors in the circulation (i.e., glucose causes increased insulin secretion fromSthe pancreas)

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13
Q

Link between Hypothalamus and

Adenohypophysis

A

Liberins

Statins

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14
Q

Liberins
14
Link between Hypophysis and
Adenohypophysis

A

Thyrotropin-releasing hormone (TRH) /Thyroliberin-TSH

Corticotropin-releasing hormone (CRH)/Corticoliberin-ACTH, and MSH

Gonadotropin-releasing hormone (GnRH)/Gonadoliberin-LH and FSH

Growth hormone-releasing hormone (GHRH)/somatostatin - GH

Prolactin- hormone-releasing hormone (PCLRH) -PCL

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15
Q

Statins

A

Growth hormone-inhibitory hormone (GHIH)-hormone inhibits release of GH

Somatostatin inhibits the secretion of GH and
TSH (to lesser extent)

Prolactin-inhibitory hormone (PCLIH)

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16
Q

Patterns of Hormone Secretion

A

There are three basic patterns of secretion:

Pulsatile:
-Relatively constant level of hormone, over a long period.

Acute:
-Rapid increase in hormone level for a short time in response to a stimulus.

Cyclic:
-Hormone increases and decreases in a constant pattern.

17
Q

What is Half-Life

A

Hormones are eventually broken down (metabolized) and/or excreted from the body.

The rate of removal from the circulation is fairly constant for a given hormone.

The length of time it takes to remove half of the amount of hormone from the circulation is the half-life of that hormone.

In general, water-soluble hormones have shorter half-lives than lipid soluble hormones (rapid degradation in kidney, liver, lungs)

Hormones with short half-lives exhibit rapid changes in hormone levels.

18
Q

Hypothalamic hormones

A

There are eight(8) hormones produced in hypothalamus of which six(6) regulate the anterior pituitary and the remaining two(2) are released into capillaries in the posterior pituitary when hypothalamic neurons are stimulated.

Releasing (liberins) and inhibiting (statins) hormones stimulate or inhibit hormone secretion by the adenohypophysis.

TRH, PCLRH, CRH, GnRH, and GHRH: affect the formation and secretion of TSH, PCL, ACTH, FSH, LH, and GH.

PCLIH inhibits PCL, and somatostatin
inhibits secretion of GH and TSH by the adenohypophysis

19
Q 
Growth Hormone (GH)
(somatotropic hormone or somatotropin)
A

Consists of 191 AA,

Synthesized, stored, and secreted by somatotropic
cells within anterior pituitary gland.

GH is the most abundant anterior pituitary
hormone.

“Somatotrophs are cells in the anterior pituitary.

These cells constitute 40-50% of anterior pituitary cells (the most abundant)

Release GH in response to GHRH.

Inhibited by GHIH (somatostatin), both received from the hypothalamus

GH is metabolized rapidly in liver.

A half-time of < 20 minutes.

20
Q

Functions of the Growth Hormone

A
  1. Stimulates growth

2. Stimulates cell reproduction

21
Q

Functions of the Growth Hormone:

Stimulates Growth

A

Hypertrophy: increase in size of cells.

Example: increase in bone thickness.

22
Q

Functions of the Growth Hormone:

Stimulates cell reproduction

A 
Hyperplasia: increase in number of cells and
proliferation rate (increases the rate of mitosis).

Example: increase in bone length

23
Q

Physiological Effects of the Growth Hormone

A

The physiological effects of the growth hormone can be classified as either Direct or Indirect Effects.

Direct:
Are the result of growth hormone binding to its receptor on target cells.

Indirect:
Imediated primarily by insulin-like growth
factor (IGF-1).

24
Q

Physiological Effects of the Growth Hormone:

Direct:

A

Are the result of growth hormone binding to its receptor on target cells.

-GH has direct actions on adipocyte cells, liver and muscles which are not growth promoting but are metabolic in function.

25
Q

Physiological Effects of the Growth Hormone:

Indirect:

A

Mediated primarily by insulin-like growth factor (IGF-1).

The majority of the growth promoting effects of GH is
due to IGF-I acting on its target cells.

The effect of GH on the liver is to produce a number of
Somatomedins or growth factors , i.e., IGFs.

The actions of GH are mediated mainly through IGF1→ stimulate growth in bone, protein synthesis in
muscle and lipolysis of fat.

26
Q

How Does GH exert its effects?

A

GH has important effects on protein, lipid and carbohydrate metabolism, as well as widespread effects on various body tissues.

Examples: cartilage, bone, muscle, and adipocytes

GH exerts much of its effect through intermediate
substances called “Somatomedins” (also called “Insulin Like Growth Factors” (IGF)) → similar to the effects of insulin on growth .

GH causes the liver to form several small proteins
called somatomedins that have the potent effect of
increasing all aspects of bone growth.

Somatomedin C is also called insulin-like growth factor 1 (IGF-1)

27
Q

Metabolic Effects of GH

A
  1. Increase Protein synthesis and Deposition
  2. Increase Lipolytic and Ketogenic effect
  3. Decrease utilization of Carbohydrates for energy
    production
28
Q

Metabolic Effects of GH

Increase Protein synthesis and Deposition

A

Adequate insulin and carbohydrates are necessary
for GH to be effective.

Increase amino acid transport inside cells →Increase protein accumulation in all cells.

Increase DNA transcription to form mRNA → Increase protein synthesis.

Increase RNA translation → Increase protein synthesis.

29
Q

Metabolic Effects of GH

Increase Lipolytic and Ketogenic effect

A

Increases mobilization and use of fatty acids for supplying energy.

Excess GH → excessive mobilization of fat from
the adipose tissue → ketosis (large quantities of
aceto-acetic acid are formed by the liver and
released into the body fluids - “Ketogenic Effect).

30
Q

Metabolic Effects of GH

Decreases utilization of carbohydrates for energy production

A

GH is one of the hormones that serves to maintain blood glucose within a normal range:

Increase uptake of glucose by cells → Increase blood
glucose (diabetogenic effect).

Decrease use of glucose for energy production due to
increased utilization of fatty acids for energy.

It inhibits hexokinase enzyme → inhibits glucose
uptake by muscles (opposite to insulin effect).

Increase hepatic glucose output (stimulates
glycogenolysis).

Increase insulin output → burn out of beta cells of
pancreas.

The above GH effects contribute to hyperglycaemia
→ INCREASE insulin secretion (hyperinsulinemia).

31
Q

Effects of GH on Cartilage

A

In adolescents: (before union of epiphysis) GH
stimulates chondrogenesis (proliferation of
epiphyseal cartilage and widening of epiphyseal
plates). Stimulation of osteoblasts → increase
bone matrix → increase length of long bones.

In adults: (after union of epiphysis) linear growth
is impossible.

32
Q

Effects of GH on Bone growth

A

NB: Growth requires the coordinated action of several
hormones.

The major role of GH in stimulating body growth is
to stimulate the liver and other tissues to secrete IGF-I.

IGF-I stimulates proliferation of chondrocytes
(cartilage cells), resulting in bone growth.

Insulin-like growth factor I (somatomedin C) → Increase’s growth promoting activities in many tissues and cartilage with a prolonged duration of action (20 hours).

GH stimulates the liver and other tissues to secrete IGF-I.

IGF-I stimulates differentiation and proliferation of
chondrocytes (cartilage cells), resulting in bone growth.

IGF-I is also a key player in muscle growth→ It stimulates both the differentiation and proliferation of myoblasts.

33
Q

Abnormalities in GH Secretion

A

Growth Hormone Deficiency

Growth Hormone Defect

34
Q

Abnormalities in GH Secretion:

Growth Hormone Deficiency

A

Due to pituitary defect or hypothalamic dysfunction:

  • Hyposecretion of GH in child is one cause of dwarfism
  • Deficiency in adults produces relatively few symptoms
35
Q

Abnormalities in GH Secretion;

Growth Hormone Defect

A

Most often caused by tumor of GH-producing cells of anterior pituitary

Symptoms depend on age of individual when abnormal secretion begins

Gigantism:
Caused by overproduction of GH in childhood before
epiphyseal plates close

Acromegaly:
Occurs when GH hypersecretion occurs after adolescence

36
Q

Growth Hormone Deficiency

A

Idiopathic GH deficienc:
-Lacks hypothalamic GHRH

Pituitary tumors
-Cannot produce GH

Dwarfism:
-Inadequate secretion of GH during childhood.

  • Laron-type dwarfism:
  • Hereditary defect in IGF production
37
Q

Effects of abnormal GH secretion on

body growth

A

Gigantism:
-Excess GH secretion in children-Maintain normal body proportions.

Acromegaly:
*Excess GH secretion in adults after the epiphyseal
discs are sealed-No increase in height.

*Growth of soft tissue-Elongation of jaw, deformities in hands, feet, and bones of face.

38
Q

Dwarfism Types

A 
• Achondroplasia
• Levi – Lorain dwarfism
• Psychogenic Dwarfism
• Spondyloepiphyseal dysplasia
congenita
• Diastrophic Dysplasia
• Campomelic Dysplasia
• dystrophia adiposogenitalis
• Hypophosphatemia
• Hallerman-Streiff Syndrome
• Cartilage Hair Hypoplasia (CHH)
• Robinow dwarfism/syndrome
• Turner Syndrome
• Panhypopituitarism

Endocrine System (5 decks)

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