Week 10B

• Homeostasis


  Homeostatic control of internal states
• Homeostatic control systems

  1. Sensor detects changes in internal environment

  2. Integrator receives input from sensor and determines appropriate response

  3. Effector carries out response
• Homeostatic control systems


  • A set of processes that work together to maintain the internal environment of an organism within a narrow range of values known as the set point
  • Operate through feedback mechanisms that can detect changes in the internal environment and then trigger responses to counteract those changes
• Main components of homeostatic control systems

  • Sensor
  • Integrator
  • Effector
• The sensor detects changes in the internal environment and sends signals to the integrator
• The integrator is typically a brain region or other regulatory center that can receive input from the sensor and determines the appropriate response
• The effector is the organ or tissue that carries out this response
• Thermoregulation in mammals

  1. Decrease in body temperature activates temperature sensitive neurons in skin

  2. Integrator site in brain receives input

  3. Effector carries out response to counteract decrease in body temperature
• Integrator


  Compares the input temperature with the setpoint temperature and responds accordingly
• Integrator response

  1. Activating an effector such as skeletal muscle

  2. Inducing a response such as shivering

  3. Shivering generates heat

  4. Warms up the body

  5. Tries to return the organism back to its thermal set point
• Homeostasis


  The ability of an organism or system to maintain a stable internal environment despite changes in the external environment
• Negative feedback


  A process that plays a crucial role in maintaining homeostasis by counteracting any deviations from the desired set point
• Negative feedback mechanism

  1. Detecting changes in the internal environment

  2. Triggering responses that oppose those changes

  3. Bringing the system back to its desired set point
• Negative feedback mechanisms

  • Shivering
  • Sweating
  • Increased blood flow to the skin
  • Increased respiration
• Body temperature rises or falls above a certain set point

  Negative feedback mechanisms are activated to raise or lower the body temperature
• Negative feedback (in homeostasis)


  A regulatory mechanism that maintains stability and prevents deviations from the desired set point
• Negative feedback occurs at the organism level, organ level, and cellular level
• Negative feedback

  A critical component of homeostasis that helps maintain stability and prevent excessive fluctuations in the internal environment of an organism or of a system
• Negative feedback example

  1. Sensor activated (blood vessels)

  2. Integrator activated (brain)

  3. Hormones and nerves from brain travel to effector (kidney, heart, blood vessels)

  4. Response raises blood pressure back to normal
• Negative feedback


  Removes the stimulus to the sensor, turning off any additional signals
• Positive feedback


  Moves the system away from a homeostatic set point, in contrast to negative feedback
• Positive feedback


  A mechanism in which a deviation from the normal range of a physiological variable is reinforced, amplifying the deviation rather than counteracting it
• Negative feedback

  Restores homeostasis by counteracting deviations from the set point
• Positive feedback


  Increases the deviation and moves the system further away from its set point
• Positive feedback is less common than negative feedback in homeostatic control systems
• Positive feedback


  Used to amplify processes that need to proceed rapidly and reach a threshold before stopping abruptly
• Positive feedback in homeostatic control systems

  • Blood clotting when a blood vessel is damaged
• Blood clotting process with positive feedback

  1. Platelets adhere to the site of injury and release chemicals

  2. Chemicals attract more platelets

  3. Platelets aggregate and form a plug

  4. Plug releases more chemicals

  5. Amplifies clotting process until injury is sealed off
• Negative feedback is the predominant mechanism for maintaining homeostasis
• Feed forward regulation


  Mechanism by which homeostatic control systems can anticipate changes in internal or external environments and adjust its responses before these changes actually occur
• Feed forward regulation allows the system to prepare itself for an impending change and minimize the impact of that change on the internal environment
• Feed forward regulation

  1. Control system receives information from environment or other parts of body

  2. System generates response that prepares body for change

  3. Response occurs even before change has actually occurred

  4. Anticipatory response can reduce magnitude of change to help maintain homeostasis
• Examples of feed forward regulation

  • Anticipatory salivary production when animal sees or smells food
  • Regulation of body temperature during exercise - brain initiates sweat production and increased blood flow to skin to lower temperature
• Feed forward regulation


  Can help predict or anticipate changes in advance to prevent gross level deviations
• Feed forward regulation can be learned
• How homeostatic processes are realized


  Changes at local level allow cells to communicate using chemical signals such as Perrine signaling or neurotransmitter release
• Hormones

  Produced in the endocrine system and secreted into the blood for longer distance communication
• Long-distance communication


  • Can lose some specificity but is able to reach much more distant targets
• Homeostasis


  The process by which living organisms maintain a stable internal environment despite changes in the external conditions
• Solute movement is an important aspect of homeostasis