lecture 25- What is homeostasis and why is it important? What are hormones and how do they work? Flashcards
Homeostasis
Homeostasis is the presence of a stable internal environment. Maintaining homeostasis is absolutely vital to an organism’s survival
The system acts so that any variation outside the normal limits triggers a response that restores normal conditions.
Receptors
Sensitive to a particular environmental change or stimulus (detect change)
Control center
Receives and processes the information supplied by the receptor that sends out commands
Compares stimulus to set point and signals
Effectors
Responds to the commands of the control centre by opposing the stimulus
Controlled variable
A variable that needs to be kept in a very small range in order for our body to function properly
Set point
A set point is the physiological value around which the normal range fluctuates.
Normal range
Homeostatic control is not precise - it maintains a normal range rather than an absolute value.
Everyone has a normal range around their set point
Each individual will have a normal range within which their levels of a given variable fluctuate
Population variation in set point
The population has a reference range based on the breadth of individual normal ranges within population. The population reference range tends to be wider than normal fluctuations within an individual
Each individuals ‘normal range’ is more narrow than the population range
Moving outside the individuals ‘normal range’ may lead to symptoms of a disorder, even when hormone levels are within the population reference range
Most individuals will have a set point that is within the population reference range
Each individuals ‘normal range’ is more…..than the population range
Narrow
What does moving outside the individuals ‘normal range’ lead to sometimes?
May lead to symptoms of a disorder (even when hormone levels are within the population reference range)
Neural versus endocrine control systems
Synaptic (neural) - action potentials in axons and neurotransmitter release at synapse. Targeting achieved by specific ‘wiring’. Fastest transmission speed to minimise response delays. Good for brief responses rather than sustained ones.
Endocrine (hormonal) - hormones released into the blood. Targeting by presence of specific receptors on target cells. Relatively slow, but long-lasting action. Good for widespread and sustained responses
The endocrine system simple
Consists of endocrine gland cells, that secrete hormones (chemical messengers), which are carried in the bloodstream to the target cells which they act upon (usually these target cells are far away from the original endocrine gland.
The major endocrine glands in this course
Hypothalamus Pituitary gland Thyroid gland Adrenal gland Pancreas (pancreatic islets) Parathyroid glands
Hypothalamus does…
The hypothalamus links the nervous system to the endocrine system and controls the secretion of many endocrine glands
Which physiological variables in this course are maintained through hormones?
Blood sugar concentration
Growth and repair
Basal metabolic rate
Blood calcium concentration
What is a hormone?
True hormones are chemical messengers produced in one location and transported via the bloodstream to a second location (target cells) where they cause a response in those cells
Specificity of hormone action
Hormones can only affect cells with specific receptors for that hormone. Each receptor is a protein. It can be in the target cell membrane or inside of the cell.
Water soluble hormones
Mostly peptides (75% of hormones in the whole body) and some are catecholamines which includes adrenaline and noradrenaline
Made and stored until required (released by exocytosis when needed)
Travel dissolved in the blood until they find a cell with the appropriate receptor, bind to it and cause a response in that target cell
Lipid soluble hormones
Steroids and thyroid hormone (including both T3 and T4)
Steroids are made from cholesterol as required (not stored). Thyroid hormones are made in the thyroid cells and stored until required (this storage is unusual for lipid-soluble hormones)
Travel in the blood bound to a carrier protein
Where are water soluble hormone receptors located?
Water soluble hormones (peptides and catecholamines) cannot cross the cell membrane therefore the receptors are located in the target cell/plasma membrane
Where are lipid soluble hormone receptors located?
Lipid soluble hormones (steroids and thyroid hormones) can diffuse through the cell membrane into the target cell therefore receptors are located in the cytoplasm or nucleus
Water soluble hormones - activation of receptor and cellular response
1- Water soluble hormone binds to cell surface receptor
2- Changes that occur as a result of this within the cell
3- which leads to a secondary messenger pathway being activated
4- This second messenger pathway is going to cause a response within the cell
Lipid-soluble hormones (steriod and thyroid hormones) - activation of receptor and cellular response
0- Lipid soluble hormone dissociated from carrier protein
1- Hormone diffuses across the cell membrane
2- Hormone binds to intracellular receptor
3- Hormone receptor complex acts as a specific transcription factor
4- target gene is activated
5- New mRNA is generated
6-New protein is generated by translation of mRNA
7- New protein mediates cell specific response (slow process- not only fo you have to make the steroid hormone but you all have to form a whole new protein from DNA)
Receptors of water soluble vs lipid soluble hormones
Water soluble = cell surface
Lipid soluble = intracellular in cytoplasm or nucleus
Mechanism of action water soluble vs lipid-soluble hormones
Water soluble = through chemical messengers
Lipid soluble = by altering gene function
Speed of response water soluble vs lipid-soluble hormones
Water soluble = milliseconds to minutes
Lipid soluble = hours to days
Negative feedback
An effector is activated by the control centre opposes the original stimulus. Negative feedback therefore tends to minimise change, keeping variation in key body systems within limits compatible with our long term survival.
Reduce change until stimulus is removed or directly inhibit further release
Positive feedback
In positive feedback, an initial stimulus produces a response that exaggerates or enhances the change in the original conditions rather than opposing it.
Control of hormone secretion
Need to maintain hormone levels to maintain effects of hormones.
Amount of hormone in blood depends on rate of hormone secretion and rate of removal from blood.
Removal is controlled by enzymes in blood or in target cells. Secretion usually controlled by negative feedback loops.
The goal of hormones is to maintain homeostasis. Too much or too little hormone can lead to endocrine disorders.
Stimulus
a change in the environment
