Week 3 The endocrine system I Flashcards

1
Q

What is the Endocrine System?

A

Complex organisms (both invert. & vertebrate spp.) require chemicals to communicate between cells

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2
Q

What are the chemicals which allow this communication & their properties?

A

neurotransmitters - signal between neurones

  • very fast (milliseconds to seconds)
  • transient (stop acting quickly)
  • over relatively short distances (μm to mm)

hormones – signal between endocrine glands and target cells

  • relatively slow (minutes to days)
  • sustained
  • over relatively long distances (cm to m)
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3
Q

Duration vs Speed in the Endocrine system:

A

Long distance regulatory system for “slow and sustained responses” (duration vs speed)

  • Nervous system = speed
  • Hormones = sustained action
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4
Q

Multiple ____ interact in the endocrine system

A

Multiple endocrine glands* interact in the endocrine system

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5
Q

What are the several distinct endocrine axes within the endocrine system?

A
  • Hypothalamo-pituitary-adrenal (HPA) axis
  • Hypothalamo-pituitary-gonadal (HPG) axis
  • Hypothalamo-pituitary-thyroid (HPT) axis
  • Growth hormone axis
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6
Q

What are the Endocrine Glands?

A

hormones - soley endocrine function

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7
Q

How are Endocrine Glands Classified?

A
  • Central (primary) endocrine glands
    • Hypothalamus (secretes tropic hormones; regulates endocrine function of anterior pituitary gland)
    • Pituitary gland (anterior and posterior lobes)
    • Pineal gland (melatonin)
  • Peripheral (secondary) endocrine glands e.g. gonads, adrenal, thyroid, pancreas etc.
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8
Q

What are the Functions of the Endocrine System?

A
  1. Regulate homeostasis (nutrient metabolism, electrolyte and water balance)
  2. Adaptation to stress
  3. Supports smooth, sequential growth & development
  4. Controls reproduction
  5. Regulates erythrocyte production
  6. Controls and integrates activities of circulatory, digestive and nervous systems
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9
Q

What is a Hormone?

A

“Long distance chemical mediator secreted by an endocrine gland into the blood, which transports it to its target cell” (Sherwood, 2016)

May be secreted from neurones - neurohormones / neuropeptides e.g. oxytocin & vasopressin

Distance of action is variable:

  • Autocrine actions (ECF)
  • Paracrine actions (ECF, blood or lymph)
  • Endocrine actions (blood)
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10
Q

How are Hormones Classified?

A

Hydrophilic

  • Peptide hormones (e.g. insulin or oxytocin)
  • Biogenic amines
    • Catecholamines (derived from Tyr e.g. adrenaline [epinephrine])
    • Indoleamines (derived from Trp e.g. melatonin)

Hydrophobic

  • Biogenic amines
    • Thyroid hormones (iodinated tyr derivatives)
  • Steroid hormones derived from cholesterol (e.g. cortisol, estradiol, progesterone and testosterone)
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11
Q

What are Peptide Hormones?

A
  • Encoded by genes – transcribed and translated in endocrine cells
  • Peptides can undergo post-translational modifications – form complex protein and glycoprotein hormones
  • Packaged in vesicles – secreted by exocytosis
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12
Q

What are Catecholamines?

A
  • Comprised of catechol group (C6H4(OH)2) (derived from Tyr) plus a side chain a.a.
  • Adrenaline (epinephrine) secreted from adrenal medulla – mediates acute stress
  • Neurotransmitters = noradrenaline (norepinephrine) and dopamine
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13
Q

What are Indoleamines?

A
  • Comprised of indole group (C8H7N) (derived from Trp) plus a side chain a.a.
  • Only hormone = melatonin (secreted by pineal gland – mediates circadian rhythm)
  • Neurotransmitter = serotonin (secreted by neurones – maintain mood)
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14
Q

How do Hydrophilic Hormones Act?

A

Hydrophilic hormones (peptide hormones, catecholamines and indoleamines)

  • Transmembrane receptor
  • Commonly leads to generation of a second messenger
  • Activates (protein) kinase enzymes
  • Alters activity of existing proteins (conformational change)
  • Rapid actions (minutes)
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15
Q

What are Thyroid Hormones?

A
  • Biogenic amines – iodinated derivatives of Tyrosyl residues in thyroglobulin
  • Thyroxine (tetraiodothyronine = T4) and triiodothyronine (= T3) secreted from thyroid gland
  • Act via intracellular / nuclear receptor – increase BMR
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16
Q

What are Steroid Hormones & their families?

A

Derived from cholesterol (C27)

Two families:

  • Corticosteroids – synthesised in adrenal cortex
    • Glucocorticoids (e.g. cortisol) (C21)
    • Mineralocorticoids (e.g. aldosterone) (C21)
    • Adrenal Androgens (e.g. DHEA) (C19)
  • Gonadal steroids – synthesised in ovary, testis and placenta
    • Progestogens (e.g. progesterone) (C21)
    • Androgens (e.g. testosterone) (C19)
    • Estrogens (e.g. estradiol) (C18)
17
Q

How do Hydrophobic Hormones Act?

A

Hydrophobic hormones (thyroid & steroid hormones)

  • Intracellular (nuclear) receptors
  • Ligand-dependent transcription factors
  • Regulate gene expression (activation / upregulation or repression by modifying histone acetylation)
  • New proteins transcribed and translated in target cell
  • Can be structural proteins or functional proteins (e.g. metabolic enzymes)
  • Slow actions (hours/days)
18
Q

Homeostasis relies on what negative feedback loops?

A

Homeostasis relies on negative feedback loops:

  • Direct feedback loops (only true example = ANP)
  • First order feedback loops (neural or neurohormonal)
  • Second order feedback loops (neural & endocrine)

Third order feedback loops (neural & endocrine)

In vertebrate taxa, most endocrine axes operate on second or third order feedback loops

19
Q

How is Hormone Secretion Regulated (Direct and First-order feedback loops)?

A

Direct feedback loops–> can only be regulated at 1 control point

First-order feedback loops–> can only be regulated at 1 control point

20
Q

How is Hormone Secretion Regulated (Second-order and Thrid-order feedback loops)?

A

Second-order feedback loops –> can only be regulated at 2 control point

Third-order feedback loops –> can only be regulated at 3 control point

21
Q

What is the role of Role for Neuroendocrine Reflexes?

A

Rapid increase in hormone secretion due to neural reflexaccelerates endocrine loop

  • E.g.1 ANS (sympathetic) control of catecholamine secretion from adrenal medulla for “fight or flight” response (norepinephrine and epinephrine increase heart rate, metabolic rate etc.)
  • E.g.2 Fergusson reflex from vagina to hypothalamus to accelerate OT secretion at parturition (stimulates myometrial contractions)
  • E.g.3 Suckling reflex from mammary gland to hypothalamus to accelerate OT secretion
22
Q

What is the Role for Diurnal / Circadian Rhythms?

A
  • Rhythmic variation in hormone secretion at different times of day / year
  • Entrained by external cues (e.g. photoperiod via melatonin)
    • E.g. Circadian rhythym in plasma concentration of cortisol
23
Q

What are Tropic Hormones?

A
  • Affect synthesis and/or secretion of another (downstream) hormone
  • Stimulate and maintain endocrine target tissue (can cause gland growth: “tropic” vs “trophic”)
  • Most (not all) hormones secreted by hypothalamus are tropic;
    • e.g. TRH stimulates TSH synthesis and secretion
  • All hormones secreted by anterior pituitary gland exert tropic actions on downstream glands; e.g. TSH stimulates thyroid gland; ACTH stimulates adrenal cortex; gonadotropins (LH & FSH) stimulate gonads
24
Q

Negative feedback loops of Tropic Hormones:

A
25
Q
A