Lecture 13

Regulation of Meal Intake and Weight Regulation, continued

 
 

• The Hypothalamus has a Feeding Center and a Satiety Center.
  
  • Lateral-Hypothalamic Nuclei – Feeding Center.
    
    • It is saying eat; it creates a hunger sensation.  It is constantly telling you to eat.
      
  • Ventro-medial Nuclei – Satiety Center.
    
    • It tells the Feeding Center to shut-up.
      
    • It works; the feeding center shuts up when it is told to.
      
    • The Satiety Center bases what it says to the Feeding center on many physical parameters that it is monitoring.
      
      • In the blood it monitors the uptake of glucose by cells.  It does this by monitoring the amount of glucose in the blood of the atria and compares it to the amount there is in the veins.
        
        • When glucose is being taken up, it puts a positive into the Satiety Center, which will then send its message to tell the Feeding center to shut up (putting negatives into the Feeding Center).
          
      • High Amino Acid (AA) and Lipid levels stimulate the Satiety center.
        
      • In the GI tract there are Stretch Receptors – they indicate the amount of food there is in the GI tract and if they are stretched they stimulate the Satiety Center.
        
      • Also in the GI tract, we are going to monitor the levels of glucose, protein, and fat.
        
      • Glucose is monitored directly with their own glucose receptors in the gut (the more glucose we have the more we stimulate the Satiety Center)
        
      • Protein and the fats are monitored indirectly and are going to stimulate a hormone called CCK.  CCK is monitored by the Satiety Center of the Hypothalamus (if you eat proteins or fats it increases CCK)  
        
    • We used to look for a set point for the Satiety center, but it is the Feeding Center that is the set point.
      
      • The Feeding Center is receiving all of this physiological input, but it ignores it, and the Feeding Center just sits there and says I will do whatever the Satiety Center tells me to do
        
      • If the Feeding Center is destroyed, you will become obese but up to a new level.
        
      • The feeding center is a slave of the satiety center.
        
      • The things being monitored here are all based upon metabolites and volume in the gut
        
    • This is a short term meal system
      

 
 

 
 

• Weight Regulation:
  
  • Meal intake is part of Weight Regulation System – (the Ventral Medial Nuclei and the Lateral-Hypothalamic Nuclei are functioning with this Weight Regulation System
    
  • These are all in the nuclei of the Hypothalamus
    
    • Paraventricular – sits on either side of the 3^rd ventricle, and there are 2 of them
      
    • Dorsal-medial nuclei – sits on either side of the 3^rd ventricle, and there are 2 of them
      
    • Arcuate nucleus – is the only single nuclei, b/c it sits below the 3^rd ventricle
      
  • The main regulator of body weight is a hormone that is produce by Adipocytes (fat cells) called Leptin.
    
    • The more adipose fat tissue you have the higher the level of Leptin you produce, and vice versa.
      
    • Leptin receptors are found in all of the Hypothalamic nuclei (they are regulating the weight)
      
    • The 3 nuclei above all have receptors for Leptin.
      
    • If we have high levels of Leptin causes:
      
      • A reduction in your Feed Intake
        
      • A stimulation of catabolism of fat (breaking something down)
        
      • An increase in the Basal Metabolic Rate (BMR) (this increases the amount of calories that you are burning)
        
    • Low levels of Leptin have exactly the opposite effect:
      
      • Anabolism of fat
        
      • Increases Food Intake
        
      • Decrease BMR.
        

Water Balance System in the Hypothalamus

• Balance means that intake has to equal output. This is done by the Hypothalamus.
  
  • Water balance levels are set to maintain:
    
    • Blood volume
      
    • Blood pressure
      
    • Osmotic gradient of the plasma
      
  • Intake of water is regulated by Thirst.
    
  • Output of water is regulated by ADH (Anti-Diuretic Hormone) – is a hormone that when stimulated causes the kidneys to reabsorbe more water thus lowering your urine output
    
• Thirst:
  
  • The osmotic receptors located in the Anterior Hypothalamus that monitor the osmotic gradient of the plasma.
    
  • The osmotic receptors themselves are just cells that shrink in response to a high osmotic gradient (they swell in a low osmotic gradient).
    
    • When they shrink they are lacking water 
      
    • When they swell they are full of water.
      
  • When the gradient is high, meaning there is lots of salt in your body (NaCl is the main one we look at) the receptors will stimulate the thirst center in the Lateral Hypothalamus (they say get thirsty)
    
    • Thirst Center  + or - (puts in) Osmotic gradient.
      
    • When positives are put in, it is a dryness of the mouth.
      
  • The stomach does not absorb water, which is why you get full when you drink water. 
    
  • The main site of water re-absorption is the Duodenum.
    
  • If there is food in the stomach the fluid can get into the small intestine rather quickly because it bypasses the bolus of food. This is because the stomach contracts and relaxes around the food (putting the food in the center), and allows spaces/a pathway for the fluid to get by, into the small intestine.
    
  • There are receptors in your Pharynx, Stomach, and Hepatic portal vein – when these are stimulated by water, to say you cannot drink anymore.
    
    • You drink water b/c water hit the osmotic receptors of the pharynx, stomach, and hepatic portal vein and then they will equalize and you will get another drink of water (you continue to do this until the osmotic receptors say that's enough)
      
  • None of that water has gotten to the osmotic gradient receptor in the hypothalamus.
    
  • GI Tract Osmotic Receptors – put in a negative and send it to the thirst center – you are no longer thirsty. They can only put in a positive.
    
  • You keep getting thirsty and then stopping, until the osmotic gradient drops to where you don't drink for a while.
    
    • This prevents you from drinking too much water.
      
  • These receptors can be fooled with Gatorade, which changes the osmotic gradient, so you can drink a lot of it quickly. Gatorade can easily bypass those receptors b/c that is what they are set to do (you can drink a lot more Gatorade than water)
    
• When you give a pint of blood and you get thirsty, this is called Hypovolemia – (means low volume blood) – drop in blood volume.
  
  • 2 things are working here and are monitored by the hypothalamus
    
  • 1.) Angiotensin II – this is a powerful blood pressure raiser monitored by the hypothalamus, and it comes from the kidney and secrets Renin.  The Renin will change to Angiotensin I, which will change to Angiotensin II.  If you raise the Angiotensin II levels it will increase the levels of the thirst b/c you are trying to raise your blood pressure (you do this by drinking more).
    
  • 2.) Baroreceptors – this is a pressure receptor.  The Baroreceptors in the atria of the heart monitor the blood volume that is sitting in the veins.  Baroreceptors found in your carotid sinusand that monitor the blood pressure.
    
    • Increasing the firing rates of the Baroreceptors (that is increasing the blood pressure and volume of blood) will reduce the thirst
      
      • This means that a high angiotensin II levels and high osmolatiy of plasma increase ADH. This increases absorption of water by the kidney and lessens your urine output.
        
    • If we have a drop in volume or blood pressure, this will increase the thirst
      
• There a lots of social reasons for drinking.
  
  • When you drink beer it blocks ADH, which tells you kidneys to stop producing ADH (ADH tells kidneys to reabsorb water) – this increases your urine output.
    

 
 

Regulation of Body Temperature:

• There are Thermoreceptors in the pre-optic area of the Hypothalamus.
  
  • These receptors monitor the temperature of the blood.
    
• Normal body temperature is 37C– this is 98F
  
• There are Heat Sensitive Neurons in the Hypothalamus.
  
  • They increase their firing rate when the temperature goes above 37C.
    
• There are Cold Sensitive Neurons in the Hypothalamus
  
  • They increase their firing rate when the temperature goes below 37C.
    
• The hypothalamus gets input from the skin from its hot and cold receptors.
  
• Input also comes from the blood.
  
• When the hypothalamus and skin receptors contradict each other, the hypothalamus will override the peripheral receptors and make the appropriate response.
  
• Response to the cold is mainly due to peripheral receptor stimulation while response to heat stimulation is due to hypothalamic stimulation, this is why you have more cold receptors in the skin, and more heat receptors in the hypothalamus.
  
• Response to a cold environment:
  
  • Increase your chemical heat production by:
    
    • Shivering
      
    • Increase your voluntary activity.
      
    • Eat – eating not only increases the amount of metabolites that you can burn to keep warm, but eating also has a Specific Heat of Digestion (there is a certain amount of heat that is produced by your body digesting your food).  
      
      • In the winter, the specific heat of digestion is conserved and used for heat.
        
      • In the summer, the specific heat of digestion is lost as radiant heat (you don't use this heat). 
        
    • Increase the Circulating Catecholamines, which have a calorigenic effect on the body.
      
    • Increase our metabolism of our body – this occurs every winter.
      
    • Stimulate your Brown Adipose Tissue (BAT) – Brown Fat.
      
      • A brown adipocyte is a small globule (brown thing) that the burns a lot of fat quickly.
        
      • A normal adipocyte is huge and filled with a lot of fat (white).
        
      • Babies have a large amount of BAT b/c they lose a lot of heat.  They have a large surface area compared to their volume of weight
        
      • When born, there is BAT in the back of your neck and a small part of back
        
      • When you get to be older, all of this disappears, it is still brown adipose tissue but it looks like white adipose tissue.
        
      • If you are exposed over and over to the cold environment the BAT changes back to a type that will function to produce heat.
        
      • If you are exposed to the cold over and over again, it will cycle to the functional form much quicker.
        
      • The more frequent it cycles, the quicker it will cycle back to a functional form.  Once you stimulate it, it will keep on being stimulated.
        
      • There is a protein in the mitochondrial membrane of these cells called Thermogenin
        
      • Electron transport chain – generally burning Hydrogen to create ATP.
        
      • Thermogenin uncouples Oxidative Phosphorylation from ATP production and all the energy is used for heat.
        
  • Decrease heat loss in the cold by:
    
    • Increasing Cutaneous Vasoconstriction – blood vessels beneath skin constrict (don't want to lose the heat in your blood).
      
    • Reduce your surface area – you do this by curling up.
      
    • Haripilation – it stands the hair up – it increase the R value of the hair insulation purposes (in humans this is called Goosebumps, but it is more useful for animals (ex. Dogs) by increasing their insulation ability and decreases their heat loss)
      
      • R-value is how much heat loss is occurring as a function of the hair standing up.
        
• Response to a hot environment:
  
  • Decrease heat production by:
    
    • Anorexia – eat less
      
    • Become inert and docile –less activity
      
    • Lower BMR
      
    • Inhibit BAT – a non-functional form.
      
  • Increase Heat Loss by:
    
    • Cutaneous Vasodilation – loss of heat
      
    • Sweat – as it evaporates, it takes heat from our bodies.
      
    • Increase respirations – the air that you breath is usually cooler than what comes out so you can increase respiration – that increases heat loss.