Learning Intentions
"Homeostasis is the process by which the body maintains a relatively constant internal environment; it involves a stimulus-response model in which change in external or internal environmental conditions is detected and appropriate responses occur via negative feedback" (SU)
What is homeostasis? (Chpt 10.1)
- Refers to a state of constancy in a system.
- In its normal, or resting state, a system often is said to be in homeostasis.
- Physiologists use the term homeostasis to refer to maintaining a constant stable internal environment.
- A homeostatic system makes adjustments to lessen the internal impact of major external disturbances.
- Example: During exercise, sweating increases to maintain a constant internal temperature.
- Homeostasis in multicellular organisms is monitored and maintained by the nervous and endocrine systems.
- This is achieved by negative feedback
The Internal Environment
- The internal environment of an organism is the conditions that prevail within the body of an organism, particularly with respect to the composition of the tissue fluid (extracellular fluid).
- This fluid environment surrounds cells, exchanges nutrients and wastes, and acts as a buffer.
- Each cell contributes to the maintenance of homeostasis and each is cell is dependent on the overall maintenance of homeostasis.
- Homeostatic regulation occurs in two stages:
- Stimulus: Detection of a change from the stable state.
- Response: an organism counteracting the change.
Detecting the stimulus
Signals come from the external environment, other parts of the organism, or within the cell.
There are literally millions of receptors which detect stimuli, in turn allowing the response to occur.
Some receptors are internal and some are external.
Five main types of receptors are:
There are literally millions of receptors which detect stimuli, in turn allowing the response to occur.
Some receptors are internal and some are external.
Five main types of receptors are:
- Chemoreceptors: These are Internal Receptors, they detect specific chemicals e.g. oxygen and ion levels are detect by chemoreceptors located in Aorta and Carotid arteries.
- Osmoreceptors: detect changes to osmotic pressure of fluid surrounding cells
- Mechanoreceptors: These are responsible for detecting mechanical stilumli. Eg. touch receptors in the skin.
- Photoreceptors: these detect light stimuli. Eg. photoreceptors in eyes.
- Thermoreceptors: these detect temperature variations. eg. thermoreceptors on skin or in hypothalamus
- Pain receptors: these detect pain. eg. skin pain receptors, organ pain receptors, joint pain receptors.
Coordinating a Response (Chpt 10.2)
The response stage of homeostatic regulation can be completed by two body systems (or both)...
THE NERVOUS SYSTEM:
THE ENDOCRINE SYSTEM:
HOW DO PLANTS DETECT AND RESPOND?
THE NERVOUS SYSTEM:
- Comprised of the brain, spinal cord (Central Nervous System) and motor and sensory neurons (Peripheral Nervous System).
- Sensory neurons are the body's receptors,
- they detect stimuli and relay the information to the central nervous system's Interconnecting neurons (aka relay neurons and interneurons)
- Interneurons relay the information onto the motor neurons
- Motor neurons signal to the body's effectors, to respond to the stimuli
- Effector is a term for any organ or structure which can respond to the original stimuli and couteract it.
- Eg. muscles, skin, pancreas, etc.
THE ENDOCRINE SYSTEM:
- The endocrine system is a collection of organs and glands which secrete chemical signals to elicit a response to a stimulus.
- Chemical signals are referred to as hormones.
- Hormones are not one particular class of molecule, for example, hormones can be proteins, fatty acids, steroids, or even amino acids.
- Hormones are specific to particular tissues or organs which have receptors for them. This way, they can flood the entire bloodstream yet only induce a response in the targeted effector.
- Endocrine responses tend to be slower than nervous system responses.
HOW DO PLANTS DETECT AND RESPOND?
- Plants do not have a nervous system, so their responses are controlled by hormones.
- Tropism:
- Tropism refers to the turning away or toward a stimulus
- Example: Plant roots grow down because of specialised receptor cells in roots which can detect gravity.
- Example: A light sensitive hormone called auxin stimulates cells on the dark side of a plant to grow longer (causing leaves and branches to bend toward the light)
- Response to disease:
- Plants do not have an immune system, but they do detect and respond to infection.
- Example: plants will kill off its own cells in a wide border around an infection site to prevent the pathogen from spreading.
- Example: plants produce toxins to fight off pathogens, such as salicylic acid to kill bacteria
- Example: new evidence suggests plants even release signals to nearby plants that they are infected to "warn" them, allowing them to prepare for incoming threat.
The Stimulus Response Feedback Model
Stimulus – change in the internal or external environment which will cause the system to act.
Receptor – sensory cells, tissues or organs which detect the stimulus.
Modulator – is the control centre responsible for processing information received from the receptor and for sending information to the effector.
Effector – organs which respondsto a message regarding a stimulus and carry out the response.
Response - the action by the effectors to counteracts or reverses the stimulus
Feedback – is achieved because the original stimulus has been changed by the response.
Receptor – sensory cells, tissues or organs which detect the stimulus.
Modulator – is the control centre responsible for processing information received from the receptor and for sending information to the effector.
Effector – organs which respondsto a message regarding a stimulus and carry out the response.
Response - the action by the effectors to counteracts or reverses the stimulus
Feedback – is achieved because the original stimulus has been changed by the response.
The role of negative feedback in maintaining homeostasis
- Negative feedback systems are needed to maintain homeostasis within systems/organisms
- These negative feedback systems restore the internal environment to its original state
- The response produced reduces the effect of the original stimulus
- The response provides feedback so that there is a negative effect on the stimulus
- The operation of the negative feedback system involves a disturbance or stimulus which is detected by a receptor
- This sets off a chain of events that results in change that essentially reduces the impact of the original disturbance.
- Thus, homeostasis is maintained.
An example...
Follow the arrows in the diagram below and see if you can determine what would be the stimuli, receptor, modulator and effector(s) in this scenario:
Positive Feedback:
This is rare.
In a positive feedback scenario, the body reinforces the original stimulus, instead of counteracting it.
This enhances the deviation from stable, ideal homeostatic conditions.
While it is rare, it is at times necessary.
EXAMPLES:
In a positive feedback scenario, the body reinforces the original stimulus, instead of counteracting it.
This enhances the deviation from stable, ideal homeostatic conditions.
While it is rare, it is at times necessary.
EXAMPLES:
- Bloodclotting: platelets produce clotting factors which cause more platelets to congregate at the site of injury.
- Metamorphisis: Development of metamorphosing animals, like frogs, is control by hormones which follow a positive feedback model. (see thyroxin in tadpoles if you're interested)
- Lactation: the more breastmilk is released from the breast, the more the breast will produce. This enables the mother's milk production to account for the infant's growth and its higher nutritional needs.
- Labor: When baby's head presses on cervix it causes a release of oxytocin, this results in a contraction of the uterus which again forces the baby's head back onto the cervix.
Watch this:
Now you should:
- COMPULSORY:
- Read pages 332 to 341 (Chapters 10.1 and 10.2)
- 2022 Creelman Questions Chapter 4 Questions 10, 29, 33a, 35a, 36a, 37a, 41
- OPTIONAL EXTRA:
- Attempt Question sets 10.1 and 10.2