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Sem 2 - MEH > Introduction And Apetite Control > Flashcards

Introduction And Apetite Control Flashcards

1
Q

What is homeostasis?

A

Homeostatic mechanisms act to counteract changes in the internal environment.

Variables are regulated so that internal conditions remain stable and relatively constant.

Homeostasis is NOT a steady state but a dynamic equilibrium. Failure in homeostasis leads to disease.

Examples of homeostatic mechanisms exist at every level.

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2
Q

What are the characteristics of a control system?

A

1. Stimulus

2. Receptor - detects stimulus e.g. chemoreceptors, Thermoreceptors, Proprioceptors (sense of self awareness e.g where limbs are), Nociceptors (painful stimuli)

Afferent pathway (from receptor to control pathway)

3. Control centre - (e.g. hypothalamus) determines set point, analyses afferent input and determine response.

Efferent pathway

4. Effector - Causes changes e.g. sweat glands, muscles and kidneys. Negative feedback

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3
Q

Biological rhythms

A

Set point of control centre can vary because of Circadian (diurnal) rhythm.

This “biological clock” in brain in small group of neurones in suprachiasmatic nucleus.

Melatonin is produced from the pineal gland which is involved in setting the biological clock (higher at night).

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4
Q

What are Zeitgebers?

A

These are cues from the environment that keep the body on a 24hr cycle.

  • Light
  • Temperature
  • Social Interactions
  • Exercise
  • Eating / Drinking pattern

Long haul flights crossing time zones can result in mismatch between environmental cues and body clock causing Jet Lag.

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5
Q

Negative feedback?

A

Response in a way to reverse the direction of change. Most common form of feedback in physiological systems. E.g the HPA axis.

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6
Q

What is positive feedback?

A

Response in a way so as to change the variable even more in the direction of the change (rare, few examples as negative feedback is ore common)

Used when rapid change is desirable.

Examples of positive feedback:

  • Blood clotting
  • Ovulation
  • Ferguson reflex - oxytocin causes contraction which causes more oxytocin ect..
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7
Q

Regulation of body water?

A

Osmolality sensed in the hypothalamus. This is sensed by the amount of salt in the blood.

The osmotic pressue of blood plasma is monitored by osmoreceptors in the hypothalamus.

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8
Q

What is the difference between osmolarity and osmolality

A

Osmolarity = The number of osmoles per litre of solution VOLUME

Osmolality = The number of osmoles per Kg of solution MASS

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9
Q

What is an an osmole and when is serum osmolality useful?

A

Osmole = The amount of substance that dissociates in solution to form one mole of osmotically active particles e.g. 1mM solution of NaCl corresponds to an osmolarity of 2mOsmol/L

Serum osmolality is useful when investigating hyponatraemia (Low Na in blood). Reference range: 275-295mOsmol/Kg

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10
Q

What happens if blood osmolality is high? Low?

A
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11
Q

Plasma glucose homeostasis?

A

Glucose should stay at about 5mM.

In a fed state:

  • Plancreas B cells releases insulin.
  • This then stimulates gycogenesis in the liver and it stimulates glucose uptake into tissues (GLUT4).
  • This results in a plasma glucose decline.

Fasted state:

  • a Cells in the pancreas releases glucagon.
  • Glucagon stimulates glycgenolysis in the liver. Glucose in the released into the blood and plasma glucose increases.
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12
Q

What are the major endocrine glands?

A
  • Hypothalamus
  • Pituitary gland
  • Pineal gland
  • Parathyroid gland
  • Thymus
  • Adrenal glands
  • Pancreas
  • Ovary
  • Testis
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13
Q

What is the endocrine system?

A

The endocrine system is a collection of glands located throughout he body.

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14
Q

What are hormones?

A

Hormones are chemical signals produced in endocrine glands or tissue that travel in the bloodstream to cause an effect on other tissues.

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15
Q

What other organs and tissues release important hormones?

A
  • Heart (ANP and BNP)
  • Liver (IGF-1)
  • Stomach (Gastrin, Ghrelin)
  • Placenta (inhibin, Placental lactogen)
  • Adipose (Leptin)
  • Kidney (Erythropoietin, Renin, Calcitriol)
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16
Q

Autocrine communication

A

Hormone signal acts back on the cell of origin

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17
Q

Paracrine?

A

Hormone signal carried to adjacent cells over a short distance via interstitial fluid

18
Q

Endocrine?

A

Hormone signal released into bloodstream and carried to distant target cells.

19
Q

Neurocrine?

A

Hormone origionates in neurone and after transport down axon released into bloosstream and carried to distant target cells.

20
Q

What are the similarities of the endocrine and nervous system?

A

Both neurones and endocrine cells are capable of secreting.

Both neurones and endocrine cells can be depolarised.

Some molecules act as both neurotransmitter and hormone

The mechanism of action requires interaction with speciifc receptors in the target cells

Both systems work in parallel to control homeostasis.

21
Q

What are the four classifications of hormones?

A
  • Peptide
  • Amino acid devivatives
  • Glycoproteins
  • Steroids
22
Q

Peptide / polypeptide hormones?

A
  • Largest group
  • Short chain of amino acids
    • Insulin
    • Glucagon
    • Growth Hormone
  • All water soluble
23
Q

Amino acid derivative hormones?

A
  • Synthesised from aromatic amino acids.
    • Adrenaline (tyrosine)
    • Noradrenaline (tyrosine)
    • Thyroid hormones (tyrosine)
    • Melatonin (tryptophan)
  • Adrenal medulla hormones are water soluble
  • Thyroid hormones are lipid soluble
24
Q

Glycoproteins?

A
  • Large protein molecules
  • Often made up of subunits
  • Carbohydrate side chain
    • LH
    • FSH
    • TSH
  • All water soluble
25
Q

Steroid hormones?

A
  • All derived from cholesterol
  • Steroidogenic tissues convert cholestrol to different hormones
    • Cortisol
    • Aldosterone
    • testosterone
  • All Lipid Soluble
26
Q

How do hormones get transported?

A

Some hormones travel in blood in simple solution - Peptides / Adrenaline.

Most hormones MUST bind to (usually) proteins.

Often specific proteins -Thyroid hormones (Thyroid-binding globulin, TBG)

Dynamic equilibrium between bound and free forms of hormone in plasma. Only the free form is biologically active.

27
Q

What are the roles of carrier proteins?

A
  • Increase solubility of hormone in plasma
  • Increase half-life
  • Readily accessible reserve
28
Q

What are the three main factors that determine the hormone levels in the blood?

A

1. Rate of production - Syntheiss and secretion is the most highly regulated aspect of hormonal control.

2. Rate of delivery - Higher bloodflow to a particular organ will deliver more hormone

3. Rate of degradation - Hormones are metabolised and excreted from the body.

29
Q

What conc. are hormones in the blood?

A

Hormones usually circulate in blood at very low concentrations. -pmol/L which is 10-12M

30
Q

How are hormones specific?

A

Hormones only work on cells that have the specific receptor for them so although they are released into the bloodstream, they only exert an effect on their target cells.

31
Q

How do GPCRs work?

A
  • Dissociation of G protein a subunit
  • Activation of effector protein e.g. adenylyl cyclase
  • Formation of a second messgenger e.g. cAMP
  • Activation of protein kinase e.g. PKA
  • Phosphorylation of target proteins
  • Cellular response
32
Q

How do tyrosine kinase domains work?

A
  • Dimerisation (except insulin receptor which is already dimersied
  • Autophosphorylation of specific tyrosines
  • Recruitment of adaptter proteins and signalling complex
  • Activation of protein kinase (e.g. PKB)
  • Phosphorylation of target proteins
  • Cellular response.
33
Q

How do lipid soluble hormones work?

A

Lipid soluble hormones bind to intracellular receptors.

  • Lipid soluble hormones diffuse across the plasma membrane,
  • Type I -cytoplasmic receptor binds hormone and receptor hormone complex enters nucleus and binds to DNA
  • Type II- Hormones enters nucleus and binds to pre-bound receptor on DNA e.g. thyroid hormone. Bidning relieves repression of gene transcription.
  • The receptor then binds to a specifc DNA sequence called a hormone response element (HRE) in the promoter region of the specifc gene.
  • The expression of new protein mediates effect of hormone.
  • Effect of lipid soluble is often slower than effect of water soluble becauseit involves making a new protein.
34
Q

How do you control appetite?

A

The appetite control centre (satiety centre) is located in the hypothalamus.

Hypothalamus contains several clusters of neurones referred to as nuclei.

The acurate nucleus plays a central role in controlling appetite.

Other brain areas are also involved

Complex and emerging area.

Only a SIMPLE OVERVIEW given here.

35
Q

Neurones of the arcuate nucleus

A
  • Neuronal, nutrient and hormonal signals are processed by primary neurones in the arcuate nucleus.
  • Two types of primary neurones:
    • Stimulatory neurones contain neuropeptide Y (NPY) and Agouti-elated peptide (AgRP). These promote hunger.
    • Inhibitory neurones contain pro-opiomelanocortin (POMC) which yields several neurotransmtters including a-MSH and B-endorphin. These promote satiety.
  • Primary neurones synapse with secondary neurones in other regions of hypothalamus and the signals integrated to alter feeding behavior.
36
Q

What is ghrelin?

A
  • Peptide hormone released from stomach wall when empty
  • Stimulates the excitatory primary neurones in the arcuate nucleus and therefore stimulate appetite.
  • Filling of stomach inhbits ghrelin release.
37
Q

What is PYY?

A

Peptide tyrosine tyrosine

  • A Short peptide hormone released by cells in the ileum and colon in response to feeding.
  • Inhibits excitatory primary neurones of the arcuate nucleus and stimulates the inhibitory neurones. It threfore suppresses appetite.
  • Injections of PYY into mouse brains makes them anorexic
  • Blunted PYY response folling food intake in obese humans.
38
Q

What is leptin?

A
  • Peptide hormone released into blood by fat cells (adipocytes)
  • Has two effects in arcuate nucleus:
  1. Stimulates the inhibitory (POMC) neurones
  2. Inhibits the excitatory AgRP / NPY neurones in arcuate nucleus
  • Overall effect is therefore to supress appetite
  • Leptin induces the expression of uncoupling proteins in mitochondria. Energy is therefore dissipated as heat.
39
Q

How does insulin contribute to the contorl of appetite?

A
  • Suppress appetite by a similar mechanisn as leptin (Stimuate POMC and inhibit AgRP)
  • Seems less important than leptin in this respect
40
Q

How does amylin contribute to the control of apetite?

A
  • Peptide hormone also secreted by B cells in pancreas.
  • Role not fully understood but known to suppress appetite, decrease glucagin secretion and slow gastric emptying.
  • Pramlintide is an amylin analogue approved for the treatment of type 2 diabetes.
41
Q

Summarise the control of appetite

A

Sem 2 - MEH (22 decks)

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