lecture 25- What is homeostasis and why is it important? What are hormones and how do they work?

Question 1

Homeostasis

Answer 1

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.

Question 2

Receptors

Answer 2

Sensitive to a particular environmental change or stimulus (detect change)

Question 3

Control center

Answer 3

Receives and processes the information supplied by the receptor that sends out commands

Compares stimulus to set point and signals

Question 4

Effectors

Answer 4

Responds to the commands of the control centre by opposing the stimulus

Question 5

Controlled variable

Answer 5

A variable that needs to be kept in a very small range in order for our body to function properly

Question 6

Set point

Answer 6

A set point is the physiological value around which the normal range fluctuates.

Question 7

Normal range

Answer 7

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

Question 8

Population variation in set point

Answer 8

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

Question 9

Each individuals ‘normal range’ is more…..than the population range

Answer 9

Narrow

Question 10

What does moving outside the individuals ‘normal range’ lead to sometimes?

Answer 10

May lead to symptoms of a disorder (even when hormone levels are within the population reference range)

Question 11

Neural versus endocrine control systems

Answer 11

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

Question 12

The endocrine system simple

Answer 12

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.

Question 13

The major endocrine glands in this course

Answer 13

Hypothalamus 
Pituitary gland 
Thyroid gland 
Adrenal gland 
Pancreas (pancreatic islets) 
Parathyroid glands

Question 14

Hypothalamus does…

Answer 14

The hypothalamus links the nervous system to the endocrine system and controls the secretion of many endocrine glands

Question 15

Which physiological variables in this course are maintained through hormones?

Answer 15

Blood sugar concentration
Growth and repair
Basal metabolic rate
Blood calcium concentration

Question 16

What is a hormone?

Answer 16

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

Question 17

Specificity of hormone action

Answer 17

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.

Question 18

Water soluble hormones

Answer 18

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

Question 19

Lipid soluble hormones

Answer 19

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

Question 20

Where are water soluble hormone receptors located?

Answer 20

Water soluble hormones (peptides and catecholamines) cannot cross the cell membrane therefore the receptors are located in the target cell/plasma membrane

Question 21

Where are lipid soluble hormone receptors located?

Answer 21

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

Question 22

Water soluble hormones - activation of receptor and cellular response

Answer 22

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

Question 23

Lipid-soluble hormones (steriod and thyroid hormones) - activation of receptor and cellular response

Answer 23

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)

Question 24

Receptors of water soluble vs lipid soluble hormones

Answer 24

Water soluble = cell surface

Lipid soluble = intracellular in cytoplasm or nucleus

Question 25

Mechanism of action water soluble vs lipid-soluble hormones

Answer 25

Water soluble = through chemical messengers

Lipid soluble = by altering gene function

Question 26

Speed of response water soluble vs lipid-soluble hormones

Answer 26

Water soluble = milliseconds to minutes

Lipid soluble = hours to days

Question 27

Negative feedback

Answer 27

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

Question 28

Positive feedback

Answer 28

In positive feedback, an initial stimulus produces a response that exaggerates or enhances the change in the original conditions rather than opposing it.

Question 29

Control of hormone secretion

Answer 29

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.

Question 30

Stimulus

Answer 30

a change in the environment