Chapter 9: 9.2: Homeostasis and Feedback Mechanisms

Negative Feedback

The response of a system that acts to maintain equilibrium by compensating for any changes made to the system

Negative Feedback Mechanisms

-primary mechanism of homeostasis: negative feedback (a stimulus resulting from a change in the external or internal environment triggers a response that compensates for the change)

Homeostatic mechanisms include three elements: 1. a sensor

2. an integrator

3. an effector

Sensor

The element of a feedback system that detects changes in the environment

-consists of tissues or organs that detect any change (or stimulus)—in external or internal factors

such as the pH, temperature and concentration of molecules (for example, hormone or glucose molecules)

-usually part of the nervous or endocrine system

Negative Feedback Mechanism Process

-sensor gathers the information

-information is transmitted to the integrator

- the integrator activates the effector-- This action is called the response

-negative feedback mechanisms use antagonistic effectors

-"antagonistic" means that they act to produce the opposite effect of the change recorded by the sensor.

Integrator

The element of a feedback system that compares existing conditions with ideal conditions

-acts as a processing or control centre

-compares the environmental conditions with the optimal functioning conditions, called set points

-usually part of the nervous or endocrine system

Set Point

The optimal value for a given variable of a system

- represents a range of values within which a condition controlled by the mechanism is to be main- tained. If the environmental condition is outside the set point, the integrator activates the effector

Effector

The element (or elements) of a feedback system that acts to return the system to its optimal state

-the system that returns the measured condition to the desired set point

-may include parts of any tissues and organs

The Thermostat as a Negative Feedback Mechanism

Most systems engineered by humans use antagonistic effectors.

-A sensor inside the thermostat measures the tem- perature

-A circuit (integrator) in the thermostat compares the measured temperature to the set point programmed by the user

If the temperature increases or decreases by any amount, the integrator circuit activates an electrical effector (either a furnace or an air conditioner), which returns the temperature to the set point.

-the temperature falls--> the furnace is activated and warmer air is added to the home until the temperature rises to the set point.

-the temperature rises--> the air conditioner is activated and colder air is added to the room until the temperature falls to the set point

Negative Feedback Mechanisms in Animals

-Mammals and birds have a homeostatic mechanism

-The integrator in this mechanism is located in a brain center called the hypothalamus (Portions of the hypothalamus act like a thermostat for the body)

-In the human body, groups of neurons in the preoptic region of the anterior hypothalamus receive information from thermoreceptors in various locations (including the skin, the spinal cord, and the hypothalamus itself)

This information is then compared to the set point

-For humans, the set point for body temperature has a small range, 35 to 37.8 °C, which is centred on 37 °C

-If the temperature deviates from the set point, the hypothalamus activates a set of physiological and behavioural responses to re-establish the normal body temperature.

If the temperature drops below the set point, the hypothalamus activates effectors that induce vasoconstriction in the skin

-As blood flow through the skin is reduced, less thermal energy is lost to the environment, which causes our body temperature to increase.

-Signals from the hypothalamus make us aware of our lowered body temperature

Another mechanism activated if body temperature rises above the set point

-The hypothalamus triggers effectors that induce vasodilation in the skin, which increases blood flow and loss of thermal energy to the environment

Signals from the hypothalamus make us aware of overheating

-Other effectors cause the body to sweat, which causes loss of thermal energy when the sweat evaporates

There are times when the ideal temperature set point changes and the feedback mechanisms work to readjust body temperature to the new set point

-For example, if you have an infection caused by a virus or bacteria, the homeostatic effectors increase your temperature, causing a fever. This increase helps the body fight off the infection. Once the infection is cleared, the set point is readjusted to its normal level.

-Birds and dogs pant to release thermal energy from their body

-Many terrestrial vertebrates use water to cool off. Mammals and birds regulate their internal temperature within a narrow range

reptiles alter their behaviour in response to changes in body temperature

-They absorb thermal energy by basking on sunny rocks during cooler parts of the day, and then move to cooler areas when the temperature becomes warmer

-Some large fish generate enough thermal energy by contraction of the swimming muscles to maintain a body temperature

Insects use feedback mechanisms to maintain their body temperature

-Flight muscles operate best at higher temperatures, so some insects bask in the sunlight to warm their muscles before taking off

-dragonflies, bees, and moths, contract their flight muscles in a process similar to shivering in mammals

-During the cold winter months, hives of honeybees form large masses to maintain body temperature, and contract their flight muscles to generate thermal energy

Even plants have thermal energy control mechanisms

-to aid in their development and to attract pollinators

-the lotus plant minimizes transpiration to stay cool and breaks down carbohydrates to increase thermal energy.

Positive Feedback Systems

the response of a system that acts to increase the effect of any changes made to the system

-the response of a system that acts to increase the effect of any changes made to the system

Positive feedback mechanisms usually (with some exceptions) do not result in homeostasis, since they cause the system to become unstable

Almost always operate when a continuous increase in some internal variable is required

-when an animal is attacked, the body releases adrenaline and hormones into the blood to prime the muscles and organ systems for the "fight or flight" reactions

-The release of these chemicals stimulates further release, in a positive feedback cycle, making the animal even more fit to survive the attack.

positive feedback occurs during reproduction and child care

-The initial uterine contractions during childbirth stimulate the release of the hormone oxytocin from the pituitary gland

-Oxytocin increases and intensifies the contractions, which results in the release of more oxytocin and stronger contractions

-In this positive feedback cycle, continued contractions, and thus increased pressure, eventu ally lead to the delivery of the baby as quickly as possible (thus minimizing the risk of harm to both the baby and the mother)

-Once the baby is delivered, the contractions stop, which, in turn, stops the release of oxytocin.

-During child rearing, mammalian young suckle milk from the mother. The sensation of suckling stimulates glands in the mother to produce milk. Of course, this milk production leads to more suckling from the young, again causing further milk production in a positive feedback cycle. When the baby is satiated, and ceases to suckle, the milk production is triggered to stop.

Positive feedback mechanisms often operate within a larger negative feedback mechanism