MEH 12 - Introduction to Endocrinology + Appetite Control

Question 1

What are the key features of a homeostatic control system?

Answer 1

1) A receptor - which detects the stimulus/change in environment.
2) Afferent pathway - which communicates signals from the receptor to control centre, either via nervous system (via AP’s) or by endocrine system (via hormones)
3) Control centre - determines the set point
4) Efferent pathway - communicates signals from control centre to effectors
5) Effectors - cause the change to bring value back toward set point.

Question 2

How are circadian rhythms of body temperature, cortisol and melatonin controlled?

Answer 2

• Via a small cluster of neurones in the suprachiasmatic nucleus (SCN), which receive environmental cues (Zeitgebers) to keep body on a 24 hour cycle.

Question 3

What is the difference between negative + positive feedback?

Answer 3

Negative = response to reverse the direction of change (the most common).

Positive = response to change the variable even more in the direction of change, e.g.: blood clotting + ovulation

Question 4

Describe how body fluid homeostasis is maintained in response to an increase or decrease in blood osmolality?

Answer 4

• High osmolality detected by osmoreceptors in hypothalamus, causes thirst + ADH release from posterior pituitary. ADH increases water reabsorption to pass small amounts of concentrated urine.
• Low osmolality detected by osmoreceptors in hypothalamus, posterior pituitary secretes less ADH, decreased water reabsorption in collecting ducts to pass large amounts of dilute urine.

Question 5

What are the 4 mechanisms of communication that hormones can use?

Answer 5

1) Autocrine - hormone signal acts back on cell of origin
2) Paracrine - hormone signal carried to adjacent cells over short distance via interstitial fluid
3) Endocrine - hormones released into blood stream to act on distant target cells
4) Neurocrine - hormone originates in neurone and released into blood stream to distant target cells after transport down neurone.

Question 6

What are the 4 main categories hormones can be classified into?
Which of these are water soluble?

Answer 6

1) Peptide hormones - all water soluble, short chains of amino acids, e.g.: insulin/glucagon/GH
2) AA derivatives - synthesised from aromatic AA’s, e.g.: AD/NA/Thyroid hormones. Adrenal medulla hormones water soluble, thyroid hormones lipid soluble.
3) Glycoproteins - all water soluble, include carbohydrate side chains, e.g.: LH, FSH, TSH
4) Steroids - all derived from cholesterol (need to know this), all lipid soluble - e.g.: cortisol, aldosterone, testosterone.

Question 7

How are hormones usually transported within the blood?

What is the role of this transport mechanism?

Answer 7

• Bound to carrier proteins, usually specific (e.g.: thyroxine-binding globulin/TBG).
• Increase solubility of hormone in plasma, increase half-life, create a readily accessible reserve
• Only the unbound form is biologically active

Question 8

What 3 factors determine the level of a hormone in the blood?

Answer 8

1) Rate of production - which is highly regulated
2) Rate of delivery - higher blood flow to a particular organ will deliver more hormone
3) Rate of degradation - hormones metabolised + excreted from the body

Question 9

What kind of receptors do water soluble hormones bind to, to exert their effects?

Answer 9

• Cell surface receptors (as they can’t cross plasma membrane) - e.g.: GPCR’s or tyrosine kinase receptors

Question 10

What kind of receptors do lipid soluble hormone bind to, to exert their effects?

Answer 10

• Intracellular receptors, typically cytoplasmic receptors to form a complex and move into nucleus
• Binding of complex to DNA (specifically the hormone response element sequence)
• Alteration of transcription to produce new proteins and different cellular response.

Question 11

What is the control centre (satiety centre) for appetite called + where is it located?
Describe the structure of this control centre.

Answer 11

• The arcuate nucleus, located within the hypothalamus.
• Neuronal, nutrient + hormonal signals are processed by 2 primary neurones within the arcuate nucleus

1) Stimulatory - containing neuropeptide Y (NPY) and Agouti-related peptide (AgRP) which promote hunger
2) Inhibitory - containing pro-opiomelanocortin (POMC) which yield several NT’s (a-MSH + B-endorphin) to promote satiety..

• Primary neurones synapse with secondary neurones and the signals integrated to alter feeding behaviour.

Question 12

What 2 hormone signals arising from the gut stimulate + suppress appetite?

Answer 12

• Ghrelin = peptide hormone release from stomach wall when empty. Stimulate excitatory neurones in arcuate nucleus to stimulate appetite.
• Peptide tyrosine tyrosine (PYY) = short peptide hormone released by cells of ileum + colon in response to feeding, inhibits excitatory and stimulate inhibitory neurone in ACN to suppress appetite.

Question 13

What 3 hormone signals from the body stimulate + suppress appetite?

Answer 13

1) Leptin = peptide hormone released from adipocytes to stimulate inhibitory + inhibit excitatory primary neurones in ACN to suppress appetite.
2) Insulin = Suppresses appetite, similar mechanism as leptin.
3) Amylin = peptide hormone secreted by B-cells of pancreas, known to suppress appetite. Mechanism not fully known.