Introduction to the endocrine system

Question 1

The endocrine system and nervous system are the two major control systems. outline the role of the endocrine system.

Answer 1

1) Required for effective cell-cell communication
2) Monitor and coordinate internal environment and make appropriate adaptive changes (homeostasis!);
3) Regulate growth, development, reproduction, senescence
4) Enable you to respond and adapt to changes in external environment (eg: food intake, water availability, temperature change, “stressors”etc);

Question 2

what is a hormone?

Answer 2

A chemical messenger produced and secreted by a specialized endocrine gland that is transported in the bloodstream to a distant target organ/cell where it elicits a physiological response

Question 3

names the 3 classes of hormones and give examples.

Answer 3

1) Proteins / Peptides: Growth hormone (hydrophilic)
2) Cholesterol Derivatives: Steroids, Vitamin D (hydrophobic)
3) Modified Amino Acids: Adrenaline (hydrophilic) , Thyroid hormones (hydrophobic)

Question 4

explain how the class of hormone and the type of receptor that it has varies between peptide and steroid hormones.

Answer 4

1) protein/peptide hormones find their receptors decorating the plasma membrane of target cells
2) steroid hormones have intercellular receptors. Because steroids are hydrophobic they can easily pass through the cell membrane and get into the cell without a cell surface receptor. they bind to their receptor in the cytoplasm in the cell and then translocate to the nucleus where they act as transcription factors

Question 5

1) what can hormones affect?
2) what are the two types of effects they can have on the body?
3) where do hormones act?

Answer 5

1) Affect growth, development, metabolic activity and function of tissues
2) May be stimulatory or inhibitory
3) May act on several tissues or just one specific target tissue. Responsive tissues must have specific receptors for that hormone

Question 6

list the 3 main factors that influence the level of circulating hormone in the body.

Answer 6

1) Rate of secretion: by endocrine gland
2) Rate of metabolism: by target tissue, blood, liver, kidney
3) Serum binding proteins: transport in blood , solubility issue

Question 7

what are the 3 things that regulate hormone secretion?

Answer 7

1) physiological changes
2) endogenous rhythms
3) Feedback Mechanisms

Question 8

physiological changes regulate hormone secretion. give examples of this

Answer 8

1) Blood glucose regulates insulin & glucagon release
from pancreas
2) ‘Stress’ regulates adrenaline release from adrenal
medulla
3) Blood calcium regulates parathyroid hormone
release from parathyroids

Question 9

endogenous rhythms regulate hormone secretion. Give examples of this

Answer 9

1) Ultradian - cycles in minutes
- GnRH pulses (90-120 min.)
2) Circadian - daily cycles
- testosterone (24 hr.)
- cortisol: stress hormone (24h.)
3) Infradian cycles- monthly
- LH in females (~28 day menstrual cycle)

Question 10

Feedback Mechanisms regulate hormone secretion. Give examples of this

Answer 10

1) Mostly Negative (“closed-loop” feedback)
- maintains homeostasis
2) Some Positive feedback ( production stimulates even more production)
- milk-ejection reflex - involving oxytocin
- pre-ovulatory LH surge - involving oestrogen

Question 11

list the different types of Endocrine Dysfunction

Answer 11

1) Hyposecretion: production of a bodily secretion at an abnormally slow rate or in abnormally small quantities
2) Hypersecretion
3) Ectopic hormone secretion: hormone made outside normal endocrine gland (eg: Tumour)
4) End Organ Insensitivity/Resistance

Question 12

outline the main reason why Hyposecretion occurs

Answer 12

1) Genetic - e.g.Congenital Adrenal Hyperplasia; defect in cortisol production
2) Dietary - e.g.Iodine deficiency → thyroid malfunction
3) Immunological (auto-immune disease) -
-Destruction of thyroid tissue in Hashimoto’s Disease
- Destruction of adrenal cortex in Addison’s Disease
4) Cancer/Tuberculosis - e.g.Addison’s Disease, d
destruction of adrenal cortex
5) Iatrogenic (=doctor-caused) - e.g. removal of parathyroid glands during thyroidectomy.
6) idiopathic - unknown cause

Question 13

describe the treatment for Hyposecretion and discuss the problems.

Answer 13

1) Hormone Replacement
2) if Steroids or Thyroid Hormones are missing
- Straightforward
- Absorbed in GI. Tract
- High Success
3) Protein Hormones missing, harder to treat
- Difficult
- Must be injected
- Species specific: Human GH must only come from humans or it wont work

Question 14

discuss the main reason why Hypersecretion might occur

Answer 14

1) Functional tumour - e.g. Prolactin secreting pituitary adenoma
2) Ectopic hormone-secreting tumour - e.g. Oat cell carcinoma of lung secreting ACTH → cortisol excess (Cushing’s Disease)
3) Immunological - Grave’s Disease, antibodies that bind and activate TSH receptor on thyroid
4) Also, consider excess hormones due to :Substance abuse - Anabolic steroid use, exogenous source of high hormone levels

Question 15

describe the treatment for Hypersecretion and outline the problems.

Answer 15

1) Surgical removal of the gland that is hyper-secreting- but will then need replacement therapy
–e.g. Hyperthyroidism
2) Irradiation of Gland – reduce function (will then need replacement therapy)
–e.g. Hyperthyroidism
3) Antagonists of the hypersecreted hormone
–e.g. ß blocker for pheochromocytoma (NorAd/Adrenaline-secreting tumour of adrenal medulla)

Question 16

What are the causes of End Organ Insensitivity / Resistance

Answer 16

1) Most common problem is a genetic defect
- e.g. Laron Syndrome: GH receptor mutation → non-functional receptor cant bind to GH
- normal/raised GH levels
- Very low IGF
2) Acquired Defect: e.g. Type 2 Diabetes Mellitus
- impaired insulin receptor signalling
- Normal insulin levels, but compromised physiological response

Question 17

what is the role of the pituitary system?

Answer 17

1) Plays major role in coordinating entire endocrine system
2) A major interface between brain and periphery
3) Organises appropriate hormonal responses to stimuli from higher centres of brain in response to:
- changes in external environment (e.g. photoperiod, temperature, food availability, stress)
- changes in internal environment (e.g. hormonal feedback signals from target organs, nutritional status, water balance, salt balance, BP, heart rate)

Question 18

where is the pituitary gland located and what does it comprise of?

Answer 18

1) sits in bony cavity in base of brain case called the Sella Turcica
2) comprises of 2 distinct lobes with different embryological origins: Posterior lobe, Anterior lobe (bigger)

Question 19

how many hormones are made in the anterior pituitary?

name them

Answer 19

1) 6
2) single chain proteins:
- adrenocorticotrophin (ACTH)
- growth hormone
- prolactin
3) Two sub unit hormones:
- luteinizing hormone
- Follicile-stimulating hormone
- thyroid-stimulating hormone

Question 20

outline the role of the hypothalamus

Answer 20

1) Regulatory centre for many vital functions
2) Receives nerve fibres (directly or indirectly) from virtually all areas of brain
3) Some of these pathways synapse with hypothalamic “parvocellular” neurones that control hormone secretion from Anterior Pituitary
4) Other pathways synapse with “magnocellular” neurones that directly innervate the Posterior Pituitary

Question 21

what is the role of parvocellular neurons? (anterior system)

Answer 21

1) parvocellular neurons synapse with a tiny system of blood vessels which runs down to the cells which make up the anterior pituitary .
2) upon stimulation these will release a factor called “releasing factor” into the Hypophyseal Portal system which flows down to the anterior pituitary
3) the releasing factor binds to receptors on the anterior pituitary stimulating them to make a particular hormone

Question 22

what is the role of Magnocellular neurons? (posterior system)

Answer 22

1) magnocellular neurons have long axons which stretch down into the posterior pituitary where they synapse with capillaries in the pituitary itself
2) these Magnocellular neurons release their hormone directly into the circulation

Question 23

name the two hormones made in the Posterior Pituitary

Answer 23

vasopressin (ADH) and Oxytocin

Question 24

1) how many different types of Parvocellular hypothalamic neurons are there?
2) where exactly do the neurons terminate?
3) is their action stimulatory or inhibitory ?

Answer 24

1) 5 types each dedicated to controlling a different endocrine cell-type in Ant. Pit.
- Each secretes a specific neuropeptide (releasing factor)
2) Neurones terminate close to Hypophyseal Portal System. they release their hormones into this blood system which quickly travels down to the anterior pituitary stimulating production of the relevant hormone
- Hypothalamic hormones discharged in a pulsatile manner
3) Most have stimulatory action on Ant. Pit.

Question 25

Multiple hypothalamic neuropeptides regulate different endocrine cell-types in AP. list some of the releasing factors and state their function

Answer 25

1) Thyrotrophin-releasing hormone (TRH): stimulates the release of thyroid stimulating hormone
2) ganadotrophin-releasing hormone (GnRH): stimulates the release of (luteinizing hormone) LH and (follicle-stimulating hormone) FSH
3) Growth hormone-releasing hormone (GHRH): stimulates release of GH
4) Growth hormone-inhibiting (somatostatin) : inhibits release of GH
5) corticotrophin-releasing hormone (CRH):stimulates release of ACTH (adrenocorticotrophin)

Question 26

list the 6 hormones secreted from the Anterior pituitary

• posterior produces 2 : Anti-diuretic hormone (ADH) and
  Oxytocin

Answer 26

1) Adrenocorticotrophic hormone (ACTH)
2) Thyroid-stimulating hormone (TSH)
3) Luteinising hormone (LH)
4) Follicle-stimulating hormone (FSH)
5) Prolactin (PRL)
5) Growth hormone (GH)
- (Melanocyte-stimulating hormone (MSH))