Physiology

• Motivation


  The set of factors that initiate, sustain, and direct behaviors
• Instinct


  • A complex behavior that is automatic, unlearned, and unmodifiable, and occurs in all the members of a species
• Drive theory


  Motivation in terms of needs arising from physical conditions such as hunger, thirst, and body temperature
• Incentive theory


  People are motivated by external stimuli, not just internal needs
• Arousal theory


  People behave in ways that keep them at their preferred level of stimulation
• Both drive theory and incentive theory share a common neural mechanism of increased release of dopamine in the brain, which can result in addiction
• Homeostasis


  The body's tendency to monitor and maintain internal states, such as temperature and blood sugar, at fairly constant and stable levels

  
• Homeostatic control system


  1. Stimulus

  2. Receptor

  3. Control unit

  4. Effector
• Allostasis


  Achieving stability through change; the process that maintains homeostasis, recognizing that 'set points' and other boundaries of control may change with environmental conditions

  
• Allostatic responses


  • Involve the sympathetic nervous system and HPA axis
  • Lead to release of catecholamines and secretion of adrenocorticotropin (ACTH) and glucocorticoids
  • Return to baseline concentrations of cortisol and catecholamines when the challenge has passed
• Exposure to elevated levels of stress hormones (an allostatic state) can result in allostatic load and overload, with resultant pathophysiological consequences
• Hunger motivation


  We eat to live. The food we take is digested and nutritional substances are absorbed. The biochemical processes get their energy from the food in order to sustain life. When these substances are exhausted, some imbalance exists.
• Hunger


  The biological drive of hunger, or desire to eat, relates to the end of this motivated behavior once we are full, called satiety. Basically, we eat because there is a deprivation in the amount of glucose in our blood to sustain activity; once we have restored that glucose level to its set point, hunger ends. In other words, we have achieved homeostasis again.
• Hunger signals


  Signals arise from both the stomach and the blood, but the stomach plays less of a role than we might realize.
• Hunger signals

  • Hunger pangs
  • Stomach growling
  • Feeling of being full
• Glucose levels


  When our glucose levels are low, we are motivated to eat and restore these levels.
• Hormones regulating hunger


  • Ghrelin (increases appetite)
  • Leptin (reduces appetite)
  • GLP-1 (reduces appetite, tells pancreas to release insulin)
  • CCK (improves digestion by reducing rate of food emptying from stomach)
  • PYY (causes satiety by being secreted into bloodstream and binding to brain receptors)
  • Neuropeptide Y (leads to increased consumption of carbohydrates)
• Hypothalamus


  The hypothalamus plays a pivotal role in the regulation of hunger. The lateral hypothalamus is involved in knowing when to start eating, the ventromedial hypothalamus tells an animal to stop eating, and the paraventricular nucleus is involved in satiety and regulating blood sugar levels.
• External cues for eating


  • Time of day
  • Social factors (eating with friends)
  • Sight/smell of food
  • Comfort eating
  • Cultural factors
  • Holidays
• Thirst motivation


  The body is about 70% water, so maintaining water balance is critical to life. Water is needed to maintain cells, keep blood flowing, transport nutrients, and dispose of waste.
• Types of thirst


  • Intracellular thirst (caused by excessive salt intake, drinking water can restore)
  • Extracellular thirst (caused by sweating, diarrhea, vomiting, bleeding, drinking saline solution can help)
• Thirst regulation


  Thirst is regulated by peripheral and central nervous system mechanisms, involving both intra- and extra-cellular water content. Central structures like the subfornical organ and lateral hypothalamic nucleus help regulate thirst.