26) Control of food intake Flashcards
How is the storage of food in the stomach accommodated?
- The ANS enables the storage of food in the stomach.
- All we have to do is eat and the stomach accommodates the food
What happens in the stomach during fasting?
- During the fasting state the stomach is contracted
What happens in the stomach during consumption?
- When the food is in the mouth it is known as receptive relaxation. This receptive relaxation is communicated to the vagus centre in the higher centres of the brain.
- As a result messages are sent along the inhibitory vagal fibre to the stomach to allow adaptive relaxation to occur
- Vasoactive Intestinal Peptide (VIP) and Nitric oxide (NO) allow accommodation to occur and are known as satiety factors
- PYY is also a satiety factor which decreases gut motility which means the stomach is not emptied so satiety/fullness is achieved
- As the food makes its way into the small intestines there is a feedback reaction occurring depending on the contents of the food
What happens in the stomach during emptying?
- During emptying a hunger fact (called ghrelin) is released causing emptying
- Emptying of the stomach causes us to feel hunger
What is adaptive relaxation?
- The presence of food in the stomach that allows accommodation to occur causing a sense of fullness (satiety)
What happens when lipids enter the small intestine?
- A feedback mechanism is mounted which informs the higher centres of the brain to release bile for the emulsification of the lipids.
- There is a release of CCK which causes the secretion of bile
- This allows lipase to works on emulsified lipids
- During this time no more food is allowed to pass through and hence the stomach relaxes
- This relaxed state of the stomach causes a state of fullness (satiety)
Why can the stomach respond to stimuli?
- The stomach is encircled by a lot of nerves which allow the stomach to respond to stimuli within the lumen of the gut
- The greater curvature of the stomach contains pacemaker cells which allow contractions to occur by initiating electrical impulses
- When the vagus nerve in the stomach is stimulated we end up with a vagovagal reflex which allow the food to be moved along the GI tract
What is vagotomy?
- A surgical operation in which one or more branches of the vagus nerve are cut
What is the effect of vagotomy on the stomach?
- Vagotomy impairs accommodation and emptying as we loose the pacemaker cells found on the greater curvature of the stomach.
- As a result it interferes with the vagovagal reflex which means there are less contractions pushing the food through.
- Hence the food is unable to be moved out of the stomach and causes early satiety in some patients
What is the effect of denervation of intestines and stomach?
- There may be no effect on food intake as there may be other tissues and organs which release other factors that can mediate other effects (i.e. hunger)
- These signals/factors can come from the pancreas, adipocytes, GIT and CNS
What is hunger?
- A state of discomfort caused by lack of food and desire to eat. It can be a strong physiological craving or drive for food
What is appetite?
- A physiological desire/drive to staisfy the body’s need for food
- It is stimulated by hunger
What is satiety?
- The state of being “full” after eating food. There is no longer the urge to continue eating
What is aphagia?
- The inability or refusal to swallow
What is hyperphagia/polyphagia?
- An abnormal desire for food (an extreme unsatisfied drive to eat)
What factors influence appetite?
- Family gatherings (e.g. during Christmas times)
- Emotional
- Food palatability (the way it is presented)
- Circadian (body clock)
- Habitual
- Cardiac rhythm
- Individual-based requirement (e.g. neural, metabolic and hormonal)
How do we know when to start/stop eating?
- Hunger, satiation and satiety are cues which tell our body when to start eating and when to stop
- Hunger tells us to start eating until we get full
- Satiation tells us to stop eating when we are full
- Satiety is the satisfaction between meals where we start to get hungry again
- This forms a cycle of:
Hunger –> Satiation –> Satiety —> Hunger
How is food intake controlled?
- Signals from the CNS and periphery control nutrient uptake
- There is a balance of stimulating and inhibiting forces in the hypothalamus which regulates feeding
- There are several nuclei located at the base of the hypothalamus which regulate energy homeostasis and control appetite
- There is also a diurnal variation in food metabolism. Carbohydrate metabolism occurs during the day while fats are metabolised in the night. The hypothalamus has to respond to switch between carbohydrate and fat metabolism
What are the reasons for differences in BMI?
- Genes
- How much we eat and its composition
What are the different nuclei of the hypothalamus that control food intake?
- Lateral hypothalamic nuclei
- Ventromedial nuclei
- Dorsomedial nuclei (DSN)
- Paraventricular nucleus (PVN)
- Arcuate nuclei
- Suprachiasmatic nucleus
- Medial amygdaloid nucleus
What is the function of the ventromedial nuclei?
- The satiety centre found in the hypothalamus and is important to satisfy our desire for food
- Stimulation of ventromedial nuclei leads to aphagia (swallowing difficulties) as we feel a sense of fullness
- Lesions/destruction of the ventromedial nuclei leads to hyperphagia (increased appetite or excessive hunger) causing weight gain
What is the function of the lateral hypothalamic nuclei?
- The feeding/hunger/thirst centre
- Stimulation of the lateral hypothalamic nuclei leads to increased feeding
- Lesions/destruction of the lateral hypothalamic nuclei can lead to aphagia
What are the experimental data of the effect of lesions on food consumption in cats?
- When a ventromedial hypothalamic lesion is carried out the cat starts to gain weight as it suffers from hyperphagia
- When the lateral hypothalamic lesion is carried out the cat starts to loose weight as it suffers from loss of appetite
What neurotransmitters control feeding behaviour?
- Orexigenic and anorexigenic neurotransmitters are found in the hypothalamus
- They modulate feeding behaviour by binding to hypothalamic nuclei
- Orexigenic neurotransmitters increase appetite
- Anorexigenic neurotransmitters decrease appetite