Endocrine System Flashcards
What does endocrine control provide?
Endocrine control provides a chemically mediated control system in the body alongside the nervous system.
How is the control exerted by the endocrine system different to that of the nervous system?
- 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.
How is endocrine control characterised?
Endocrine control is characterized by the release of chemical substances - hormones - which interact with target cells to produce biological responses.
Endocrine control can be considered to consist of 3 categories.
What are they?
- Autocrine control
- Paracrine control
- Classical endocrine control
What is autocrine control?
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.
What is paracrine control?
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.
What is classical endocrine control?
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.
Endocrine organs are composed of what?
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.
Why is the chemical nature of a hormone important?
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.
Hormones fall into one of 2 chemical groups.
What are they?
- 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.
Why are protein, peptides and modified amino acids unable to cross the lipid rich membrane of target cells?
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.
Why are steroids able to cross the membrane of target cells?
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.
What happens once the hormone binds to the receptor?
One bound, the hormone - receptor complex interacts with, and activates a G - protein.
What are G - proteins?
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.
What does the activated G - protein activate?
Subsequently, the activated G - protein activates an enzyme called adenylate cyclase.
What does adenylate cyclase do?
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.
What does the rise in cAMP result in?
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.
What does the now activated protein kinases do?
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.
What does the activation of target cells need to be regulated mean?
The activation of target cells needs to be regulated - having been switched on by a hormone they need to be switched off.
How is activity in the target cell achieved?
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.
What are steroids (hormones) ?
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.
How do steroids overcome the potential difficult of transporting them around?
Steroids are transported in blood plasma bound to transport proteins, known as steroid - binding proteins, which are found in the plasma.
What happens at the target cell (in regards to steroid)?
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.
Where does the target cell bind to its receptor?
In the cytoplasm.
How does the hormone receptor complex bind to specific regions of DNA?
Passes through the nuclear membrane of the target cell
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.
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.
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.
Why do plasma levels of hormones need to be regulated closely?
To ensure homeostasis is maintained.
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.
What do overall levels of an individual hormone represent?
The balance between its secretion and degradation.
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.
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.
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.)
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.
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.
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.
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.
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.
What does the posterior pituitary gland represent?
The ending of neurons which have their cell bodies in the hypothalamus.
