Endocrine system Flashcards
What are some key features common to all control systems, including those in the body involved in homeostasis?
- Communication - NS and endocrine system
- Control centre
- Receptor
- Effectors
- Feedback
What is paracrine control?
Endocrine control caused by the local release of hormones (rather than into the blood) which act locally
What is autocrine control?
Endocrine control caused by a variety of agents which effect the releasing cell itself, not others around it or large distances away from it
What is the afferent branch of the peripheral nervous system?
The peripheral nerves that carry signals from the sensory input towards the brain
What is the efferent branch of the peripheral nervous system?
The peripheral nerves that carry signals from the brain towards the motor output
What are the two important control centres in the brain for homeostasis?
- Hypothalamus in the diencephalon
2. Medulla oblongata in the brain stem
What is the role of the control centre in a control system?
To determine the reference set point, to analyse the afferent input and determine the appropriate response
The hypothalamus is involved in the control of what system?
Endocrine system
Regions of the medulla are involved in the control of what systems?
Ventilation -movement of air between lungs&environment
Cardiovascular system
What do body sensors usually consist of?
Specialised nerve endings
What is the role of sensors in the body?
To detect stimuli such as changes in the environment
What do chemoreceptors respond to?
chemical stimuli
What do thermoreceptors respond to?
absolute and relative changes to temperature
What do proprioreceptors respond to?
Movement and position of body
What do nocireceptors respond to?
potentially damaging stimuli and sends signals which are usually detected as pain
What are effectors?
Agents that receive outputs from the control centres down efferent pathways and cause a change e.g. sweat glands responding to efferent signals from the control centre in response to high temperature afferent signals form thermoreceptors
What is feedback?
When the output (effect) has an effect on the control centre
What is negative feedback?
When the output inhibits the function of the control centre and the effector acts to oppose the stimulus
What is the purpose of negative feedback in control systems?
It gives them stability by allowing the set point to be controlled within fine limits
What is positive feedback?
The stimulus produces a response which tends to increase its effect rather than counteract it. Therefore positive feedback creates a rapid catastrophic change in state
There are not many examples of positive feedback in the human body, give two
- Blood clotting cascade - change in state of blood from liquid to solid
- Ovulation - build up of FSH causes release of an oocyte form a follicle in the ovary
What is meant by biological rhythms?
When the set point of a control system varies throughout the day instead of being a fixed steady value
Explain the biological rhythm that cortisol shows and the effect of this on cortisol measurments
The levels of cortisol in blood vary throughout the day from a peak at 7am to a trough at 7pm.
Therefore you should always note the time of day that a blood sample was taken for cortisol measurement and repeated measurement should be taken at the same time of day
The menstrual cycles is an example of a biological rhythm. What varies throughout the cycle?
A woman’s core body temperature. A sudden increase in body temperature can be used as a marker for ovulation
Where is the “biological clock” of the brain found?
Suprachiasmatic nucleus in the hypothalamus
List 4 nucleases in the hypothalamus and their functions
- Suprachiasmatic nucleus - “biological clock”
- Supraoptic nucleus - oxytocin
- Paraventricular nucleus - ADH (vasopressin)
- Arcuate nucleus - satiety centre
What is a meant by a CNS nucleus?
A cluster of neurones in the brain
What is meant by human’s “natural diurnal cycle”?
We keep on a 24 hr 11min cycle maintained by keys in the environment (Zeitgebers)
What is the cause of jet-lag?
A mismatch between environmental keys and our “body clocks”
What hormone is involved in setting the biological clock?
Melatonin released from the pineal gland in the brain
What are the three main body compartments that hold body water?
- Intracellular fluid
- Extracellular fluid
- Blood plasma (70kg male - approx. 4.6l)
Define osmolality
The number of osmole per Kg of solution
Define osmolarity
The number of osmoles per Litre of solution
Define an osmole
The amount of substance that dissociates in solution to form one mole of OSMOTICALLY ACTIVE PARTICLES
What detects the osmotic pressure and sodium ion concentration of blood plasma?
Osmoreceptors in the hypothalamus (supraoptic and paraventricular nuclei)
What effects do the osmoreceptors in the paraventricular and supraoptic nuclei have?
The influence feelings of thirst and release the hormone ADH from the posterior pituitary
What effect does ADH have on the body?
ADH increases the permeability of collecting duct to water and therefore increases the reabsorption of water in the kidneys from urine into blood. This makes urine more concentrated and decreases the loss of water into urine.
What is the effect of high blood osmality on the osmoreceptors in the hypothalamus?
Hypertonic blood - therefore increase ADH secretion and create feeling of thirst
What is the effect of low blood osmality on the osmoreceptors in the hypothalamus?
Hypotonic blood - therefore decrease ADH secretion and do not generate feeling of thirst
A target cell for a hormone will produce a response when what occurs?
Normally when a CHANGE in concentration of the hormone occurs
List the four categories of chemical type of hormones
- Peptide/polypeptide (largest group)
- Glycoprotein
- Amino acid derivatives (amines)
- Steroid
What type of hormones are insulin, glucagon, growth hormone and placental lactogen?
Peptide/ polypeptide hormones (largest group)
What type of hormones are LH, FSH and TSH?
Glycoprotein hormones
What type of hormones are adrenaline (a catecholamine) and thyroid hormones (thyroxine and triiodothyronine)
Amino acid derivatives (amines)
What type of hormones are these cholesterol-derived hormones: cortisol, aldosterone, testosterone and oestrogen?
Steroid
What is the difference in storage between polypeptide & catecholamines and steroid hormone endocrine cells?
Polypeptide and catecholamine producing cells normally store their hormones in vesicles within their cells, prior to secretion.
Steroid hormone producing cells normally store the precursor, cholesterol, as cholesterol esters in the form of lipid droplets.
The thyroid gland is an exception to polypeptide, catecholamine and steroid hormone storage. Where does the thyroid gland store its main hormone?
Extracellulary in the form of protein colloid
Which categories of hormones are relatively hydrophilic and are transported in the bloodstream dissolved in the blood plasma?
Polypeptide hormones, glycoproteins and adrenaline
Which categories of hormones are relatively hydrophobic and are transported in the bloodstream bound to specialised transport proteins?
Steroid hormones and thyroid hormones
Generally, the effect that a hormone has on its target cell depends on its concentration in the bloodstream. How does this differ when considering steroid hormones and thyroid hormones?
These hormones bind specifically or non-specifically to proteins in the blood and in their case it is the concentration of unbound or “free hormone” that matters
How are thyroid hormones transported in the blood?
~75% bound to thyronine binding globulin (TBG)
~25% non-specifically to other proteins , such as albumin
Excess secretion of which hormones produces acromegaly (characteristic changes in the shape of the face and body and other metabolic effects)?
growth hormone
How is the rate of secretion of a hormone normally controlled?
negative feedback
Pancreatic beta-cells secrete insulin in direct response to what?
Blood glucose levels rising above 5mM
Define tropic hormones
Hormones that have other endocrine glands as their targets
Define trophic hormones
Hormones that stimulate growth in the target tissue
Where are the majority of tropic hormones secreted from?
Anterior pituitary
Name the six main hormones that the anterior pituitary secretes?
FSH - follicle stimulating hormone (as below)
LH - lutenising hormone (affects ovary&testis function)
TSH
ACTH - adrenocorticotropic hormone
Growth hormone - affects metabolism
Prolactin - affects breast development and milk production
List the tropic hormones of the anterior pituitary
TSH
ACTH
LH
FSH
In addition to the action of negative feedback, how can the secretion of the anterior pituitary glands be controlled?
By releasing or inhibiting hormones
Where do releasing or inhibiting hormones come from and how do they affect the secretion of anterior pituitary hormones?
They originate from nerve cells in the hypothalamus and travel to the gland via specialised blood vessels known as the hypophyseal portal vessels. This allows the brain to control hormones secretion and explains how hormone secretion can change during stress.
List 4 examples of releasing or inhibiting hormones
Thyrotropin releasing hormone (TRH) - releasing
Corcticotropin releasing hormone (CRH) - releasing
Somatotropin releasing hormone (SRH) - GH release
Somatostatin - inhibits GH release
Where do lipophilic hormones bind to their receptors?
Cytoplasm and/ or nucleus of target cells
Where do hydrophilic hormones bind to their receptors?
Receptors on the plasma membrane of target cells
What can binding of hormones to receptors trigger in the target cell?
- Activity of enzymes or proteins
2. Expression of genes
When hormones bind to receptors on the cell surface, what is often released inside the cell?
A second messenger e.g. cAMP, cGMP, Ca2+, IP3 and DAG
Hormones that create a quick response (seconds-minutes) often work by…
Altering the activity of functional proteins (enzymes, membrane transport proteins) involved in the response mechanism
Hormones that create a response over a longer time period (minutes-hours, and may even work after the hormone concentration has returned to normal) work by…
changing the rate of gene expression in target cells
Where does hormone inactivation occur?
Kidneys, liver and sometimes target tissues
How are steroid hormones inactivated?
By a small change in their chemical structure which makes them more hydrophilic so they can be excreted in urine or bile
How are protein hormones inactivated?
They undergo more extensive chemical changes and are degraded to amino acids that are reused for protein synthesis
What can cause disorder of the endocrine system that can cause clinical consequences?
- Changes in the endocrine tissues -> under/over-secretion of hormones or structurally abnormal and less effective hormones
- The responsiveness of endocrine tissue may be altered by the presence, in the circulation, of abnormal proteins such as antibodies
- The physiologically effective concentration of hormone in circulation may be reduced by binding to circulating proteins
- There may be changes in the responsiveness of target tissue to hormones resulting from changes to receptors and/or post-receptor events
Describe the two ways that negative feedback occurs on the hypothalamus-pituitary-adrenal (HPA) axis
- ACTH inhibits release of CRH
2. Cortisol inhibits release of CRH from hypothalamus and ACTH from anterior pituitary
What triggers the release of CRH?
Stress (e.g. pain, fever, hypoglycaemia, low blood pressure)
