5.2 Stimulus response model

What is the definition of homeostasis?

The maintenance of a stable internal environment within narrow limits depsite changes to the external environment

Why is it important that homeostasis is maintained? How is it regulated and what is regulated?

Homeostasis:  The maintenance of a stable internal environment within narrow limits despite changes to the external environment

• It maintains optimal conditions for enzyme and cell function throughout the body

• Includes regulation of: Water balance, core body temperature and blood glucose regulation

• Homeostasis is regulated by the endocrine system and nervous system

What is the tolerance limit? What happens if an organism exceeds it?

The upper and lower values of an abiotic factor beyond which an organism cannot survive

EG Temperature, pH, salinity

What are the defintions for each component of the stimulus response model?

Stimulus: A change in the internal or external environment

Receptor: An organ or cell that detects the stimulus

Control centre: An organ that determines the appropriate response

Effector: A muscle or gland that produces a response

Response: A change that occurs in the organism resulting from the stimulus

What is the pathway of the stimulus responses mode?

1. A change in the internal or external environment occurs

2. The receptor detects the stimulus

3. The information is sent to the control centre where it decides the appropriate response

4. The information is sent to the effectors which carry out the response

5. A change occurs in the organism to either reinforce or reverse the stimulus

How can the stimulus response model (the homeostatic pathway) occur?

The homeostatic pathway can occur through nerves (NS) or hormones (ES)

• Nerves provide immediate, temporary effects

• Hormones take longer as they travel via the bloodstream but have longer lasting effects

What is the role of positive and negative feedback loops and how do they differ?

• The two feedback loops that maintain homeostasis and homeostatic mechanisms (EG Thermoregulation, BG regulation)

Negative feedback loops: When the response reverses the stimulus

Positive feedback loops: When the responses reinforces the stimulus

• Majority of homeostatic mechanisms in animals involve negative feedback to bring the body back to the tolerance limit

What are examples of negative feedback loops?

Negative feedback example	Stimulus	Response
Thermoregulation	Increased body temperature	Sweating and vasodilation to decrease body temp
Blood glucose regulation	Decreased blood glucose	Glycogen released as glucose to increase BG
Osmoregulation	Increased blood osmolarity	Water reabsorption from filtrate in the nephrons into the bloodstream to increase blood osmolarity

What are the few examples of positive feedback loops?

Positive feedback example	Stimulus	Response
Uterine contractions during childbirth	Baby pushing head on cervix	Releases oxytocin which strengthens contractions producing more oxytocin
Breast milk production	Baby sucking	More breast milk is produced
Platelets	Vessel damage	Platelets clot blot at the injury site causing more to travel to the site and clot blood to fully cover the injury