Homeostatic control systems & Hormone introduction

Question 1

Balance (homeostasis) vs Imbalance (loss of homeostasis)

Answer 1

Homeostasis;:
- A relatively constant internal environment that supports cell function and life processes.
- Human body systems keep each regulated variable within an optimal range

Loss of homeostasis:
- As cells function they use up nutrients and produce byproducts
- Examples: during growth, reproductions, exercise, stress and illness

Question 2

What are regulated variables?

Answer 2

A variable for which there is a sensor and a system to ensure the variable is kept within a normal range, around a set point.

Examples of regulated physiological variables:
- Core temperature
- Plasma glucose concentration
- Plasma calcium concentration
- Basal metabolic rate
- Etc…

Question 3

Regulated variables: Set point vs normal range

Answer 3

Normal range:
- The restricted set of values for a regulated variable, that permits optimal cell functioning

Set point:
- The physiological value of any given variable, around which its normal range fluctuates
- Each person has a unique set point for each variable based on genetics and other factors

Question 4

What is a physiological example of set point vs normal range?

Answer 4

Core temperature control:
- Set point: 37C
- Normal range: a bit higher and a bit lower than 37C
- Control system: physiological negative feedback control

Question 5

What is the population reference range?

Answer 5

The set of values (high and low ends) for a regulated variable that are considered normal and /or healthy.
The population reference range tends to be wider than any given individual’s normal range.
- based on test results from large groups of healthy people
- based on the breadth of individual normal ranges within a population sample

Question 6

Population reference range vs Individual normal range

Answer 6

• Most individuals (~95%) will have a set point that is within the population reference range
• Remember: the normal range fluctuates around the set-point for any given variable
• If the variable moves outside an individual’s ‘normal’ range it may lead to illness, disease or symptoms of a disorder
• The may occur even if the variable remains within the population reference range

Question 7

Homeostasis of regulated variables: What is a negative feedback loop?

Answer 7

Negative feedback for balance.
Negative feedback control loops keep a regulated variable within its normal range, around its set-point.
- is the variable goes up, negative feedback brings it down
- if the variable goes down, the negative feedback brings it up

Question 8

What are the characteristics of a negative feedback loop?

Answer 8

Sensors: monitor the variable and detect changes (deviation from set-point)
Control centre: compares variable;e’s changed value to its set-point. Sends signals to effectors if correction is required.
Effectors: act to oppose the effect of the stimulus, thereby correcting the change and restoring the variable to its set-point

Question 9

Describe how negative feedback loops correct a rise in core temperature

Answer 9

Sensors: thermoreceptors (nerve cells) detect increased temperature
Control centre: brain cells compare 40C to set-point of 37C; then send signals to effectors
Effectors: produce sweat for convective heat loss; dilate blood vessels for heat loss via radiation

Question 10

Amplification of regulated variables: What is a positive feedback loop?

Answer 10

Positive feedback control drives a process to completion by amplifying the change to a regulated variable.
- if the variable goes up, positive feedback makes it go up more
- if the variable does down, positive feedback makes it go down more

Question 11

What are the characteristics of a positive feedback loop?

Answer 11

Sensors: monitor the variable and detect changes (deviation from set point)
Control centre: sends signals to effectors to amplify change
Effectors: action amplifies the change, to drive a process to completion

Question 12

Describe how a positive feedback loop will amplify breast milk release

Answer 12

Sensors: nerve cells detect suckling around nipple areola.
Control centre: pituitary glad triggers release of hormone to stimulate milk producing cells
Effectors: mild producing cells send milk to ducts for release from nipple, causing child to suckle more.

Question 13

Nervous and endocrine systems work together to control the body and maintain homeostasis. What are their differences?

Answer 13

Nervous system:
- Homeostatic control (eg breathing, core temp, etc)
- Sensory detection and interpretation
- Control of muscle and movement
- Higher functions (eg. thinking, speaking, etc)

Endocrine system:
- Homeostatic control (eg. plasma, pH, or glucose etc)
- Cellular metabolism
- Reproduction
- Growth and development
- Immunity and stress management

Question 14

Describe hows signals are sent/changes are made: Nervous system vs Endocrine system

Answer 14

Nervous system:
- Neurons produce action potentials and release neurotransmitter at synapses (with neurons, muscle, or glands)
- Neurotransmitter binds to chemically -gated ion channels on post synaptic cell
- Extremely fast signalling

Endocrine system:
- Endocrine cells release hormones into the bloodstream to travel to target cells
- Hormones bind to membrane or intracellular receptors of target cells
- Relatively slower, but longer lasting action compared to the nervous system

Question 15

What are the six endocrine glands we are focusing on in this unit?

Answer 15

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

Question 16

What are hormones?

Answer 16

• chemicals released by endocrine gland cells
• travel through blood
• bind to receptors either on the target cell membrane or within the target cell
• are made from amino acids or cholesterol

(Memory hook: eggs look kinda like endocrine cells. Eggs have macronutrients needed to make hormones: amino acids (in the form of protein) and cholesterol)

Question 17

How do hormones work/What is their process in the body?

Answer 17

1. Hormones travel through the blood towards target organs
   - Lipid soluble hormones require carrier proteins to travel in the blood
   - Water soluble hormones do NOT require carrier proteins
2. Hormones bind to target cell receptors
   - Lipid soluble hormones easily diffuse across plasma membranes to bind to intracellular receptors inside target cells’ cytosol or nucleus
   - Water soluble hormones cannot cross cell membranes so they bind to receptors on the membrane of the target cell
   - Each hormone only binds to a receptor designed specifically to receive it
3. Hormones stimulate target cells to produce a response
   - when lipid soluble hormones bind to intracellular receptors they stimulate the creation of new proteins - often enzymes - to produce a response
   - when water soluble hormones bind to membrane receptors they activate a 2nd membrane system to produce a response
4. Hormone effects stop when hormones are removed
   - effects of hormone stop when it unbinds from the receptor
   - used or excess hormone: gets broken down and/or recycled or secreted in sweat, urine or feces

Question 18

Lipid soluble vs Water soluble hormones

Answer 18

Lipid soluble:
- Chemical classification: Amines (thyroid hormones) and Steroids
- Synthesis and storage:
- Thyroid hormones: pre-made and stored
- Steroids: made as required (not stored)
- Form of transport: bound to carrier protein
- Receptor location: Intracellular receptors
- Action mechanism: stimulates gene transcription to make new proteins
- Response and duration: slower response; lasts hours to days
Examples: Thyroid hormone and Cortisol

Water-soluble:
- Chemical classification: Amines (catcholamines), Peptides and Proteins
- Synthesis and storage: All are pre-made and stored until needed
- Form of transport: Unbound (no carrier protein)
- Receptor location: Plasma membrane receptors
- Action mechanism: Activates 2nd messengers for an amplified response (ie. small amount of hormone = big response)
- Response and duration: faster response; lasts milliseconds to minutes
Examples: Adrenaline and most other hormones

Question 19

Describe lipid soluble hormone - action mechanism

Answer 19

• Lipid soluble hormone diffuses across the plasma membrane
• binds to receptor in cytosol OR in nucleus
• causes gene activation, transcription and mRNA production and translation
• for protein synthesis
• The protein creates a cellular response
• This is a relatively slow hormone response because protein synthesis takes time

Question 20

Describe water soluble hormone - action mechanism

Answer 20

• Water soluble hormone binds to a membrane receptor on a target cell
• activating the attached G-protein
• G-protein activates or inhibits 2nd messengers (eg. cAMP or Ca2+)
• 2nd messenger effect actions of other proteins (eg. enzymes or ion channels)
• because 2nd messengers are already pre-made, this is a relatively fast response
  Note: a small amount of hormone can cause a large response by activating 2nd messengers

Question 21

What are the six hormones and glands that we are looking at involved in?

Answer 21

• homeostasis of plasma calcium and glucose concentration
• growth and cell metabolism
• immunity and stress management
• endocrine disorders and disease