Module 5 - Feedback Mechanisms

• Homeostasis - condition in which a system, such as the human body, is maintained in a more-or-less steady state. It means to maintain dynamic equilibrium in the body
• Set Point and Normal Range


  This specific value is called the set point. For any given variable (e.g. body temperature) there is a particular setpoint that is the physiological optimum value. For example, in human body temperature, the setpoint is about 37 ºC (98.6 ºF). Fluctuations from the set point are normal as the body works to maintain homeostasis for a variable. The spread of values within which such fluctuations are considered acceptable is called the normal range. In the case of body temperature, for example, the normal range for an adult is about 36.5 to 37.5 ºC (97.7 to 99.5 ºF)
• Homeostatic Process


  Homeostatic process is an extremely complex act. However, regardless of the variable being kept within its normal range, maintaining homeostasis requires at least four interacting components: stimulus, sensor, control center, and effector.
• Four Interacting components

  • Stimulus
  • Sensor
  • Control Center
  • Effector
• Stimulus


  The change in the internal or external environment. It is the variable that is being regulated. It indicates that the value of the variable has moved away from the set point or has left the normal range.
• Sensor


  Monitors the values of the variable and sends the data to the control center.
• Control Center


  Matches the data from the sensor or receptor with the normal values. If the value is not at the set point or is outside the normal range, it sends a signal to the effector.
• Effector


  An organ, gland, muscle, or other structure that acts on the signal from the control center to move the variable back toward the set point.
• Biological feedback systems divided into two:

  • Negative feedback
  • Positive feedback
• Negative feedback - allows the system to self-stabilize. It's either increase or decrease the stimulus, but the stimulus is not allowed to continue as it did before the receptor sensed it. In other words, it opposes the direction of the stimulus.
• Thermoregulation - mechanism by which mammals maintain body temperature with tightly controlled self-regulation independent of external temperatures. The body maintains a relatively constant internal temperature to optimize chemical processes. Neural impulses from heat-sensitive thermoreceptors in the body signal the hypothalamus. The hypothalamus compares the body temperature to a set point value. The hypothalamus maintains the set point for body temperature through reflexes that cause vasodilation or vasoconstriction, as needed for shivering or sweating.
• Two examples of negative feedback:

  • Thermoregulation
  • Osmoregulation
• When body temperature drops, the hypothalamus initiates several physiological responses to increase heat production and conserve heat:

  Vasoconstriction - narrowing of surface blood vessels, decreases the flow of heat to the skin. This reduces heat loss from the surface
• When body temperature drops, the hypothalamus initiates several physiological responses to increase heat production and conserve heat:

  Shivering - due to muscle contractions commences, increasing production of heat by the muscles.
• When body temperature drops, the hypothalamus initiates several physiological responses to increase heat production and conserve heat:

  Adrenal glands - secrete stimulatory hormones such as norepinephrine (noradrenaline) and epinephrine (adrenaline) to increase metabolic rates. Adrenaline causes the breakdown of glycogen (the carbohydrate used for energy storage in animals) to glucose, which can be used as an energy source. This catabolic chemical process is exothermic, or heat producing.
• When body temperature drops, the hypothalamus initiates several physiological responses to increase heat production and conserve heat:

  The thyroid gland may be stimulated by the brain (via the pituitary gland) to secrete more thyroid hormones. This hormone increases metabolic activity and heat production in cells throughout the body.
• Whereas when body temperature rises, the hypothalamus initiates several physiological responses to decrease heat production and lose heat:

  Vasodilation - dilation of blood vessels in the skin, occurs to allow more blood from the warm body core to flow close to the surface of the body, so heat can be radiated into the environment.
• Whereas when body temperature rises, the hypothalamus initiates several physiological responses to decrease heat production and lose heat:

  • As blood flow in the skin increases, diaphoresis occurs. Diaphoresis is when sweat glands release water (sweat). When the sweat evaporates from the skin surface into the surrounding air, it takes the heat with it.
  • Breathing becomes deeper, and the person may breathe through the mouth instead of the nasal passages. This increases heat loss from the lungs
• Osmoregulation

  • the process of maintenance of salt and water balance (osmotic balance) across membranes within the body’s fluids.
  • terrestrial animals regulate their body fluid levels by controlling the amount of water released from the body as urine.
  • The medullary region of the kidneys is hypertonic and will draw water out of the collecting ducts and back into the circulating blood.
• Osmoregulation


  Osmoreceptors in the hypothalamus detect water levels in the blood and coordinate the release of the neurohormone, anti-diuretic hormone (ADH). Neurohormones are hormones released from nerve cells that target distant cells (as opposed to neurotransmitters which target nearby neurons). Diuresis means urination. Thus, anti-diuretic (“anti-diuresis”) hormone prevents diuresis or urination.
• Osmoregulation

  When blood water levels are low (e.g. dehydration), more ADH is released from the posterior pituitary. ADH stimulates the production of aquaporins in the collecting ducts of the kidneys, making them more permeable to water. Increasing the permeability allows more water to be reabsorbed from the urine back to the bloodstream. In summary, more ADH means less urination and more water retention. Thus, increasing blood water levels
• Osmoregulation

  Whereas when blood water levels are high, less ADH is released from the posterior pituitary. Also, less aquaporins are produced in the collecting ducts of the kidneys, making them less permeable to water. Opposite to the previous condition, less permeability allows less water to be reabsorbed from the urine back to the bloodstream. Meaning, less ADH means more urination and less water retention. Thus, decreasing blood water levels
• Positive Feedback Mechanism - maintains the direction of the stimulus, possibly accelerating it. It may either increase or decrease the stimulus, but the stimulus continues as it did before the receptor sensed it. The term positive feedback is typically used as long as a variable has an ability to amplify itself, even if the components of a loop (receptor, control center and effector) are not easily identifiable. Common terms that could describe positive feedback loops or cycles include “snowballing” sand “chain reaction".
• Childbirth


  1. Head of infant pushes against cervix

  2. Nerve impulses travel from cervix to hypothalamus in brain

  3. Hypothalamus sends hormone oxytocin to pituitary gland

  4. Pituitary gland secretes oxytocin into bloodstream

  5. Oxytocin stimulates uterine contractions

  6. Uterine contractions push baby harder against cervix

  7. Cervix starts to dilate

  8. Increasing oxytocin, stronger contractions, wider cervix dilation

  9. Baby pushed through birth canal and out of body

  10. Cervix no longer stimulated to send nerve impulses to brain
• Homeostatic Imbalance - the mechanisms fail. When they do, homeostatic imbalance may result, in which cells may not get everything they need or toxic wastes may accumulate in the body. If homeostasis is not restored, the imbalance may lead to disease or even death. Diabetes is an example of a disease caused by homeostatic imbalance.
• Homeostatic Imbalance

  In the maintenance of blood glucose levels, blood glucose levels rise when an animal has eaten. This is sensed by the nervous system. Specialized cells in the pancreas sense this, and the hormone insulin is released by the endocrine system. Insulin causes blood glucose levels to decrease, as would be expected in a negative feedback system. In the case of diabetes, blood glucose levels are no longer regulated and may be dangerously high. Medical intervention can help restore homeostasis and possibly prevent permanent damage to the organism
• Homeostatic Imbalance

  Normal aging may bring about a reduction in the efficiency of the body’s control systems. This makes the body more susceptible to disease. For example, older people may have a harder time regulating their body temperature. This is one reason they are more likely than younger people to develop serious heat-induced illnesses such as heat stroke.