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Endocrine System Flashcards

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1
Q

What does endocrine control provide?

A

Endocrine control provides a chemically mediated control system in the body alongside the nervous system.

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2
Q

How is the control exerted by the endocrine system different to that of the nervous system?

A
  • Nature of the control system - Endocrine exerts control by releasing chemicals where as the nervous system uses electrical control.
  • Rapidity of the control system- Endocrine control responses may last for weeks, months or years. (e.g. growth) compared to the nervous system which has very short lived responses. (e.g. reflexes)
  • Duration of the controlling signal - endocrine signals may take hours or days to initiate responses (e.g. steroid hormone action) in contrast with the rapid duration of neural signals e.g. action potentials.
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3
Q

How is endocrine control characterised?

A

Endocrine control is characterized by the release of chemical substances - hormones - which interact with target cells to produce biological responses.

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4
Q

Endocrine control can be considered to consist of 3 categories.
What are they?

A
  • Autocrine control
  • Paracrine control
  • Classical endocrine control
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5
Q

What is autocrine control?

A

Autocrine control is characterized by the release from a cell of a chemical which interacts with and influences the activity of the cell from which it was released.

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6
Q

What is paracrine control?

A

Paracrine control is characterized by the release of a hormone from a cell into the interstitial fluid surrounding the cell.
The hormone remains in the interstitial fluid and diffuses a short distance where it influences the activity of a target cell.

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7
Q

What is classical endocrine control?

A

Classical endocrine control is the most common form of control.
In this case, a hormone is released from a cell, enters the circulatory system, and travels some distance to influence biological activity in a target cell.

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8
Q

Endocrine organs are composed of what?

A

Endocrine organs may be composed of epithelial tissue and form classic endocrine organs (e.g. pancreas, thyroid)
Alternatively endocrine organs may be composed of neural tissue (e.g. posterior pituitary gland.) In this case the hormone is released from the axon terminals of neurons. This is termed neuroendocrine control.

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9
Q

Why is the chemical nature of a hormone important?

A

The chemical nature of a hormone is important because this determines how the hormone exerts its effects.
It determines whether it can enter the target cell.

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10
Q

Hormones fall into one of 2 chemical groups.
What are they?

A
  • Proteins, modified amino acids and peptides e.g. insulin, oxytocin and adrenaline
  • Steroids e.g. cortisol which are formed from modification of a cholesterol molecule.
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11
Q

Why are protein, peptides and modified amino acids unable to cross the lipid rich membrane of target cells?

A

Protein, peptides and modified amino acids are unable to cross the lipid rich membrane of target cells because they are water - soluble compounds.
Because of this their receptors are located in the membranes of target cells.

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12
Q

Why are steroids able to cross the membrane of target cells?

A

Steroids are lipid - soluble compounds. Therefore they are able to cross the membrane of target cells.
Steroid receptors are located in the cytoplasm of target cells.

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13
Q

What happens once the hormone binds to the receptor?

A

One bound, the hormone - receptor complex interacts with, and activates a G - protein.

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14
Q

What are G - proteins?

A

G - proteins are proteins found in the cell membrane s which act as intermediaries between hormone binding and the initiation of responses within the target cell.

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15
Q

What does the activated G - protein activate?

A

Subsequently, the activated G - protein activates an enzyme called adenylate cyclase.

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16
Q

What does adenylate cyclase do?

A

Adenylate cyclase takes a molecule of ATP and converts it to a molecule of cyclic AMP (cAMP).
cAMP is known as a second - messenger molecule - the first messenger is the hormone itself.

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17
Q

What does the rise in cAMP result in?

A

The rise in cAMP levels converts inactive intracellular protein kinases to active protein kinases.
Protein kinases are enzymes which phosphorylate other (target) proteins.
When a protein is phosphorylated its shape changes.

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18
Q

What does the now activated protein kinases do?

A

The now activated protein kinases phosphorylate other intracellular proteins.
These proteins may be for example enzymes or membrane bound transport proteins.
Since the shape of these proteins changes, so does their function therefore an inactivated enzyme may be changed to its active form and the target cell may begin producing a chemical.
This is the biological response of the target cell.

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19
Q

What does the activation of target cells need to be regulated mean?

A

The activation of target cells needs to be regulated - having been switched on by a hormone they need to be switched off.

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20
Q

How is activity in the target cell achieved?

A

Activity in the target cell is achieved by the conversion of cAMP to AMP.
This is done by the enzyme phosphodiesterase.
The process described here produce responses quickly e.g. immediate effects seen when adrenaline is released into the blood which leads to a increase in heart rate dilation of pupils.

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21
Q

What are steroids (hormones) ?

A

Steroids are lipid soluble therefore they do not easily dissolve in aqueous solutions (e.g. plasma in the body)
This means there is a potential difficulty in transporting them around the body from their site of production to their site of action.

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22
Q

How do steroids overcome the potential difficult of transporting them around?

A

Steroids are transported in blood plasma bound to transport proteins, known as steroid - binding proteins, which are found in the plasma.

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23
Q

What happens at the target cell (in regards to steroid)?

A

At the target cell, the steroid dissociates from the transport protein and as it is lipid soluble, passes through the cell membrane and enters the cytoplasm of the target cell.

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24
Q

Where does the target cell bind to its receptor?

A

In the cytoplasm.

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25
How does the hormone receptor complex bind to specific regions of DNA?
Passes through the nuclear membrane of the target cell
26
What is the outcome of the hormone receptor complex binding to DNA?
The outcomes of binding to DNA is that genes are activated - either on or switched off. This results in the production of proteins (e.g. enzymes) which alter the biological activity of the target cell. This represents the response of the target cell to the hormone.
27
What are the several mechanism which limits the activity of steroids within the target cell?
These include a finite number of receptors to which the steroid can bind, and in some cases the steroid may actually inhibit the synthesis of new receptors.
28
Compared with hormones, what is the response of of cells to steroids?
Compared with hormones, which have membrane bound receptors the response of cells to steroids is much slower (hours to days to produce effects) as it involves the activation of genes, production of proteins and post translation modification of proteins.
29
Why do plasma levels of hormones need to be regulated closely?
To ensure homeostasis is maintained.
30
Why is it essential that appropriate levels of hormones are maintained within the plasma?
Too little or too much of an individual hormone will have deleterious effects on normal physiological function. For example, diabetes is a consequence of too little of the pancreatic hormone insulin.
31
What do overall levels of an individual hormone represent?
The balance between its secretion and degradation.
32
Why does the secretion of hormones occur?
Secretions of hormones occurs in response to a stimulus. For example, the appearance of glucose in plasma following a meal results in the production of insulin.
33
Some hormones are circadian changes in secretions what does this mean?
For example, release of growth hormone is at a maximum during early stages of sleep. Circadian changes are the natural variations in physical, mental, and behavioural changes that occur over a 24-hour cycle.
34
Why may degradation of hormones occur?
Degradation of hormones may occur by several mechanisms: - Metabolism in the liver - Renal loss - Catabolic destruction in tissues This may result in some hormones having extremely short half - lives. On the other hand since steroids are bound to proteins they tend to have much longer half - lives (days.)
35
For the most part, how are levels of hormones controlled?
For the most part, levels of individual hormones are controlled by negative feedback mechanisms A rise in plasma levels of a hormone inhibits further secretion of it thus allowing the plasma level to return to normal.
36
What is the hypothalamo - pituitary axis?
The hypothalamo - pituitary axis is a dominant control system in endocrine control, controlling a significant number of other endocrine organs.
37
Why is the pituitary gland sometimes referred to as the 'conductor of the endocrine orchestra' ?
This is because it oversees the activity of a number of endocrine glands.
38
Where is the pituitary gland located?
The pituitary gland is located at the base of the brain in a bony structure called the Sella turcica. It is connected to the hypothalamus by the hypophyseal stalk.
39
The pituitary gland is divided into 2 distinct regions What are they?
The posterior pituitary gland is composed of neural tissue (formed from an outgrowth of the hypothalamus) where as the anterior pituitary gland is formed from pharyngeal epithelial tissue. This produces clear distinctions in how the regions function.
40
What does the posterior pituitary gland represent?
The ending of neurons which have their cell bodies in the hypothalamus.
41
What is neurosecretion?
Stimulation of neurons result in the release of substances directly into the plasma. In this case, neurotransmitters in these axons are actually hormones. Unlike normal they are released into the plasma instead of the synapse.
42
What are the 2 hormones secreted from the posterior pituitary gland?
- Oxytocin - Vasopressin
43
What is oxytocin?
Oxytocin is released from neurons that have their cell bodies in a region of the hypothalamus called the supra - optic nucleus.
44
What is oxytocin function?
Oxytocin stimulates milk ejection from the mammalian glands and also causes contraction of uterine smooth muscle during childbirth.
45
In the case of milk ejection how do action potentials work?
They travel via spinal cord to the hypothalamus. This activates neurons in the supra - optic region resulting in release of oxytocin into the blood and ultimately milk ejection.
46
What is vasopressin?
Vasopressin is secreted from neurons whose cell bodies lie in the paraventricular nucleus of the hypothalamus. The principal role of vasopressin is regulation of water balance.
47
What is the primary stimulus for the release of vasopressin?
The primary stimulus for the release of vasopressin is an increase in plasma osmolarity. This change is detected by osmoreceptors in the hypothalamus which in turn activates neurons which release vasopressin.
48
Where does vasopressin primarily act?
Vasopressin primarily acts in the collecting duct of the nephron to increase water reabsorption and consequently the osmolarity of the plasma is reduced.
49
What controls the secretions of the anterior pituitary gland?
The hypothalamus controls the secretions of the anterior pituitary gland itself by the release of hormones - so called release hormones (RH) and release - inhibiting hormones (RIH.) These hormones are released by cells in the hypothalamus into the plasma where they travel to the anterior pituitary gland to exert their effects.
50
What are the blood vessels which connect the hypothalamus to the anterior pituitary gland?
The blood vessels which connect the hypothalamus to the anterior pituitary gland are known as portal blood vessels. These are vessels which having passed through one organ pass through another before returning to the heart.
51
What do the release and release - inhibiting hormone do?
Leave the circulation and enter the anterior pituitary gland where they stimulate or inhibit the release of anterior pituitary hormones.
52
What are the release and release - inhibiting factors secreted from the hypothalamus? ( Hypothalamic hormones) Look over close to end
- Luteinizing hormone releasing hormone (LHRH) - Thyrotrophin releasing hormone (TRH) - Growth hormone releasing hormone (GHRH) - Corticotrophin releasing hormone (CRH) - Prolactin inhibiting factor (PIF) (also known as dopamine) - Growth hormone inhibiting hormone (GHIH) (also known as somatostatin)
53
In turn, these hypothalamic hormones stimulate or inhibit the release of the following anterior pituitary hormones: Look over close to end
- LHRH -stimulation of FSH and LH release - TRH - stimulation of thyroid stimulating hormone (TSH) - GHRH - stimulation of growth hormone release (GH) - CRH - stimulation of adrenocorticotrophin hormone (ACTH) release - PIF - inhibition of prolactin release - GHIH - inhibition of GH release
54
Each of the anterior pituitary hormones then influences activity in other endocrine organs: Look over close to end
- FSH and LH are collectively termed gonadotrophins. They are involved in regulation of reproduction. - TSH (thyrotrophin) stimulates thyroid gland to stimulate release of T3 and T4 - GH influences both metabolism and growth - ACTH regulates function of adrenal glands - PIF inhibits release of prolactin.
55
What is the close working relationship of the hypothalamus and the anterior pituitary gland also sometimes known as?
Hypothalamo - pituitary axis
56
What are thyroid hormones?
They are released from the thyroid gland and influence a number of physiological processes including control of metabolic rate.
57
Where is the thyroid gland?
It is located in the neck. It is small but has a dense blood supply.
58
What is the functional unit of the thyroid gland?
The functional unit of the thyroid gland is the thyroid follicle. The complete gland is made up of several thousand follicles. The follicle is characterized by its ability to absorb iodine from dietary intake in the form of iodide ion.
59
What does the thyroid follicle do?
The follicle concentrates iodide ions and oxidizes them to convert them to iodine. The iodine is then linked to tyrosine amino acid which are themselves linked to a protein called thyroglobulin which is found within the cytoplasm of the follicle this forms Mono iodo tyrosine (MIT) Subsequently, a second iodine molecule is attached this forms di - iodo tyrosine (DIT)
60
How are T3 and T4 formed?
The coming together of MIT and DIT result in the formation of active substances tri - iodothyronine (T3) and tetra-iodothyronine (T4)
61
How is T3 and T4 released?
Under the influence of TSH thyroglobulin is hydrolysed, releasing T3 and T4 into the circulatory system.
62
How are T3 and T4 transported around the body?
T3 and T4 are transported around the body bound to plasma proteins.
63
How do T3 and T4 enter target cells?
Both hormones enter target cells by active transport and once inside T4 is converted to T3
64
What are T3 and T4 involved in?
T3 and T4 influence virtually all cells in the body. They are involved in the following processes. - Production of heat ( helps control body temp) - Low levels of the hormone appear to stimulate the absorption of amino acids and protein anabolism, whilst high levels do the opposite. - They promote the breakdown of fat stores resulting in a rise in the plasma concentrations of free fatty acids. - Low levels stimulate uptake of glucose into muscle cells with an insulin like effect. High levels promote breakdown of glucose stores and synthesis of new glucose.
65
T4 is converted to T3 why?
T4 is converted to T3 because T3 is the active form of thyroid hormone, and cells in the body can use it more effectively than T4
66
What are the parathyroid glands?
The parathyroid glands are important regulators of plasma calcium levels.
67
Where are the parathyroid glands located?
The parathyroid glands are embedded in the thyroid gland.
68
What do the parathyroid gland do?
The parathyroid gland release a hormone called parathyroid hormone (PTH), which regulates in part, plasma calcium levels. PTH increases plasma calcium levels and decreases plasma phosphate levels. It targets are the skeleton, kidneys and gastrointestinal tract.
69
In healthy individuals with a normal plasma calcium concentration, what does PTH promote?
In healthy individuals with a normal plasma calcium concentration, PTH promotes activity in osteoblasts (bone - building cells) and the calcification of the bone.
70
What happens when plasma calcium levels drop?
When plasma calcium levels drop, PTH stimulates the release of calcium , initially from the surface of the bone but in the long term also activates osteoclasts (bone destroying cells) This results in a rise in plasma calcium concentration level.
71
How does PTH affect the gastrointestinal tract?
In gastrointestinal tract, PTH indirectly stimulates the absorption of calcium. It does this by promoting the production of a compound called 1,25 dihydroxy - cholecalciferol . This is synthesized from vitamin D and promotes the intestinal absorption of calcium.
72
How does PTH affect the kidney?
In kidneys, PTH simulates calcium reabsorption in the distal convoluted tubule. At the same time, it decreased the reabsorption of phosphate. The significance of the reduction in plasma phosphate levels is that calcium binds to phosphate. Therefore reducing phosphate concentrations resulting in further rise in calcium levels.
73
How is PTH secretion controlled?
PTH secretion is controlled by plasma levels of calcium. Low plasma calcium levels promote its release, whilst high levels inhibit its release.
74
Where is the growth hormone released from?
Growth hormone is released from the anterior pituitary gland and influences virtually all cells in the body.
75
What is the function of growth hormone?
Control of growth, whilst its activated throughout all life stages there are periods where its role is increased e.g. childhood and adolescence.
76
The actions of growth hormone can be divided into direct and indirect actions. What is the primary direct action of the growth hormone?
The primary direct action of growth hormone is its stimulatory effect on protein synthesis. These effects are of particular significance during childhood growth - especially in bone. It also stimulates the growth of cartilage In promoting growth it also produces a glucose - sparing effect promoting ,ATP production from fatty acid oxidation whilst conserving glucose.
77
What is the primary indirect action of growth hormone?
The primary indirect action of growth hormone is the maintenance of tissue.
78
What are the indirect effects of growth hormone?
The indirect effects of growth hormone are, in part, mediated via the production of substances called insulin - like growth factors (IGFs) in the liver. The principal IGFs are IGF - 1and IGF - 2
79
What does IGF - 1 and IGF - 2 do?
IGF - 1 plays a significant role in growth and amongst other roles stimulates mitotic cell division. IGF - 2 has an insulin like activity.
80
How is the release of growth hormone influenced?
The release of growth hormone from the anterior pituitary gland is influenced by 2 hormones from the hypothalamus. Growth hormone releasing hormone and somatostatin. GHRH is the most important factor. The release of GHRH is influenced by a number of stimuli to the hypothalamus. (e.g. pain, exercise and reduced blood sugar levels.)
81
What are the adrenal glands?
The adrenal glands are a pair of glands - one lying above each of the kidneys. Each gland consists of 2 regions the adrenal medulla and surrounding this the adrenal cortex.
82
What is the function of adrenal glands?
Involved in many physiological processes such as metabolism, fluid and electrolyte balance.
83
Where is the adrenal medulla located?
The adrenal medulla is the inner part of the adrenal gland.
84
What is the principal cells of the adrenal medulla?
Chromaffin cells.
85
When the sympathetic nervous system is simulated what are the 2 compounds the adrenal medulla releases?
When the sympathetic nervous system is simulated the 2 compounds the adrenal medulla releases is adrenaline and noradrenaline.
86
What are the effects of these 2 compounds released by the adrenal medulla?
These 2 compounds have common cardiovascular effects (e.g. increased heart rate and force of contraction) Adrenaline also has significant effects on metabolism.
87
When is activity in the adrenal gland increased?
Sympathetic activity and therefore activity in the gland increases during variety of states such as exercise and pain. (It increases HR)
88
Where is the adrenal cortex located?
The adrenal cortex surrounds the medulla and is itself divided into 3 distinct regions. Together these regions secrete a variety of hormones all of which are derivatives of cholesterol i.e. they are steroids.
89
What is the outermost region of the adrenal cortex?
The outermost region is the zona glomerulosa, it secretes mineralocorticoids. The principal mineralocorticoids is the hormone aldosterone, it acts to increase reabsorption of sodium in kidney.
90
What is the middle region of the adrenal cortex?
The middle region is the zona fasciculata which secretes glucocorticoid hormones. The principal glucocorticoid is cortisol. Together the hormones contribute to the ability to cope with a variety of stresses e.g. trauma and infection Cortisol also plays a role in the regulation of carbohydrates and protein metabolism.
91
What is the inner region of the adrenal cortex?
The inner region is the zona reticularis which secretes sex hormones. Androgens are produced in males and progesterone in females.
92
What are the pancreatic hormones responsible for?
Regulating plasma glucose levels.
93
What are the 2 hormones secreted by the pancreas?
The 2 hormones secreted by the pancreas are insulin and glucagon which are important in the maintenance of the appropriate plasma glucose concentration.
94
Where are the pancreatic hormones secreted from?
These hormones are secreted from endocrine regions of the pancreas known as islets of Langerhans
95
Where is insulin produced and stored?
Insulin is produced and stored in the beta cells of the islets of Langerhans in the pancreas.
96
What is the principal stimulus to insulin release?
The principal stimulus to insulin release is an increase in plasma glucose concentration. As concentration rises, glucose depolarises the beta cells and promotes a calcium mediated exocytosis of the hormone.
97
What happens in response to an increased plasma glucose concentration?
In response to an increased plasma glucose concentration (e.g. after food consumption) there is a biphasic release of insulin. The initial response corresponds to the release of stored preformed insulin whilst the later response corresponds to the release of newly synthesized hormone.
98
What are the main sites of action for insulin?
The skeletal muscle, liver and adipose tissue. In all tissues, the response is increased active uptake of glucose from plasma into these structures. Having entered, the majority of the glucose is converted to glycogen.
99
What other stimuli also control insulin secretion?
Activation of the sympathetic nervous system which results in a decrease in insulin release. Activity in the parasympathetic nervous system has opposite effect.
100
What are the actions of glucagon?
The actions of glucagon effectively oppose those of insulin. i.e. its release increases the plasma concentration of glucose. Therefore its release is stimulated when plasma glucose levels drop.
101
What is the principal target for glucagon?
The liver where it promotes breakdown f glycogen whilst at the same time inhibiting the production of new glycogen.
102
What does glucagon also promote?
The synthesis of glucose from amino acids and the release of free fatty acids from fat sources. The significance of the latter response is it ensures the brain continues to receive a supply of glucose whilst other sources are also able to meet energy requirements.

Physiological Systems (12 decks)

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