ingestive behavior

• The constancy of the internal milieu is a necessary component for a free life.” – Claude Bernard
• animals have evolved from single-cell organisms that live in the ocean. In order to “carry” this environment with us (i.e. water and solutes), our body and its cells must regulate their fluid balance
• homeostasis – process by which the body’s substances and characteristics (such as temp and glucose level) are maintained at their optimal level
• Mammals maintain homeostatic control of our body’s fluid and energy through our ingestive behavior – intake of food, water, and minerals such as sodium
• Ingestive Behavior Eating or Drinking
• HOMEOSTASIS Process by which body’s substances and characteristics are maintained at their optimal level
• Physiological regulatory mechanisms one that maintains the constancy of some internal characteristic of the organism in the face of external variability e.g. keeping body temp constant despite changes in ambient temp
• System variable – the characteristic to be regulated; e.g. temp
• Set point – the optimal value of the system variable; e.g. 78°
• Detector – monitors the value of the system variable; e.g. thermostat
• Correctional mechanism – restores the system variable to its set point; e.g. AC or heater
• Negative feedback – a process whereby the effect produced by an action serves to diminish or terminate that action; when AC or heater is turned on and successfully changes temp back to set point, the detector senses this and turns AC/heater off
• Satiation mechanism – a brain mechanism that causes cessation of hunger or thirst, produced by adequate and available supplies of nutrients or water
• Fluid balance -Body contains 4 major fluid compartments:

  Intracellular Fluid

  Extracellular Fluid

  Intravascular Fluid

  Interstitial Fluid
• Drinking Most times, we ingest more water or solutes than needed; these are then excreted by kidney
• Drinking

  When levels of either water or solutes are too low, corrective mechanisms are activated: thirst, or salt appetite (rare for modern humans)

  Our bodies lose water continuously, through sweating, breathing, urination, defecation, and in some circumstances through vomiting
• Osmometric (osmotic) thirst – occurs when the tonicity (solute conc.) of the ISF increases; e.g. when eat salty meal with no water
• Osmoreceptors – neuron that detects changes in the solute conc. of the ISF that surrounds it
• Hypertonicity of blood plasma (where salt is absorbed into) draws water from ISF, which then causes water to leave cells; when blood volume increases, the kidneys begin to excrete both water and solutes, allowing the blood plasma volume to remain constant
• Osmotic thirst (con’t)

  Osmoreceptors located in the anterior hypothalamus, one of the circumventricular organs (CVO’s)

  OVLT (organum vasculosum of the lamina terminalis) – CVO located on the blood side of the BBB, and thus substances dissolved in the blood are able to pass through to the ISF of this organ
• Volumetric thirst Produced when blood plasma volume is low Leads to both thirst and salt appetite
• 2 types of receptor systems

  Renin-angiotensin system – hypovolemia activates kidneys to release an enzyme called renin, which then catalyzes the conversion of a blood protein called angiotensinogen into a hormone called angiotensin (AngII)
• 2 types of receptor systems

  atrial baroreceptors stimulates secretion of hormones by posterior pituitary and adrenal gland to conserve water and solutes, and stimulates drinking and salt appetite
• Our bodies need food for “building blocks” (i.e. to construct and maintain our organs and muscles) and “fuel”

  Fuel comes from food we have consumed that travels through the digestive tract, but those nutrients must be able to be stored for when the gut is empty

  2 types of reservoirs: short-term (carbs) and long-term (fats)
• Nutrient reservoirs

  Short-term Located in cells of liver and muscles

  Cells filled with complex, insoluble carb called glycogen

  Cells in the liver convert glucose (obtained from diet) into glycogen and store it; this storage is stimulated by the presence of insulin, a peptide hormone produced by the pancreas
• Nutrient reservoirs

  When glucose enters the body, some is stored as glycogen and some is used as fuel

  When there are low levels of glucose in the blood, the pancreas begins to secrete glucagon, which stimulates the conversion of glycogen back into glucose

  This reservoir primarily serves to fuel the CNS
• Nutrient reservoirs

  Long-term

  Adipose tissue – filed with triglycerides (complex molecules that contain glycerol, a soluble carb, combined with 3 fatty acids)

  Found beneath the skin and in various locations in the abdominal cavity

  Cells can expand in size

  Reservoir for rest of body besides brain
• Nutrient reservoirs

  Long-term

  What keeps us alive when we are fasting; when body starts to use carb reservoir, fat cells start converting triglycerides into fuel that cells can use

  Fatty acids can be metabolized by cells in all of the body except the brain; glycerol can be converted to glucose in the liver for use in the brain

  So, why does brain get all the glucose? Insulin must be present at a cell in order for it to take up glucose into it However, neurons and glia do not require insulin to take up glucose
• Fasting phase – the phase of metabolism during which nutrients are not available to from the digestive system; glucose, amino acids, and fatty acids are derived from glycogen, protein, and adipose tissue during this phase
• Absorptive phase – the phase of metabolism during which nutrients are absorbed from the digestive system; glucose, and amino acids constitute the principle source of energy for cells during this phase, and excess nutrients are stored
• What starts a meal?

  Social and environmental factors

  Often we eat out of habit or because of some stimuli present in our env’t (e.g. clock, smell food)

  Meal schedule very important: rarely adjust times of meals, but can adjust size of meals
• What starts a meal?

  Social and environmental factors

  If we have eaten recently or if a previous meal was large, we tend to eat a smaller meal

  However, due to other social factors, such as parental cues (“finish your plate”) or peer influence, satiety signals can be ignored

  DeCastro and DeCastro (1989) found that the amount of food eaten was directly proportional to the amount of other people who were present during a meal
• Glucoprivation – a dramatic fall in the level of glucose available to cells
• Hunger can also be caused by lipoprivation ( fall in level of fatty acids available to cells)
• 2 sets of detectors for these metabolic fuels: one set located in the brain (sensitive to glucoprivation) and the other in the liver (sensitive to both glucoprivation and lipoprivation)
• What stops a meal?

  2 types of satiety signals

  Long term - from adipose tissue OB MOUSE LEPTIN
• What stops a meal?

  short-term info from gastrointestinal tract
• WHAT Stops a Meal?

  Live Factors

  Last Stage of satiety occurs in liver

  First organ to learn that foods is finally being received from intestines

  Insulin

  Permits organ other than brain to metabolize glucose

  Promotes entry of nutrients into fat cells where they are converted into triglycerides
• What stops a meal?

  Gastric factors

  Stomach not necessary for feelings of hunger?

  Not completely true: when stomach is empty, a peptide called ghrelin is secreted which activates a hunger signal

  The stomach also contains receptors that can detect the presence of nutrients
• What stops a meal?

  Intestinal factors

  Afferent axons from the duodenum (first portion of small intestine) are sensitive to the presence of glucose, amino acids, and fatty acids