In this video I describe how hunger is turned on (the orexigenic signal) and off (the anorexigenic signal) and how hormones and other cues influence motivation to eat. The lateral hypothalamus is involved in turning on hunger, while the ventromedial hypothalamus is involved in signaling satiety. Other hormones such as ghrelin, leptin, and insulin also influence motivation to eat, in addition to other cues like visual stimuli.
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Video Transcript
Hi, I’m Michael Corayer and this is Psych Exam Review. In previous videos I’ve talked about drives and drive reduction theory and I frequently use the example of a hunger drive in order to illustrate these. So we had this idea that the hunger drive is turned on this motivates us to eat and then as we eat we have a reduction of this hunger drive until eventually we’re no longer motivated to eat.
So in this video I want to take a look at hunger signaling in more detail in order to see the complexities of this process. So we’ll start with a sort of a basic overview, which is that there’s two parts to this signal. We have an “on” signal telling us to eat and then an “off” signal telling us to stop eating. So this on signal, this motivation to eat, is referred to as the orexigenic signal and orexigenic means the source of appetite. And then we have the anorexigenic signal which will be without the source of appetite this would be the off signal that tells us to stop eating or indicates that we’re full.
So let’s look at the on signal in more detail. This hunger signal is triggered in part by the release of a hormone called orexin and this is released by the activity of neurons in the lateral hypothalamus. So this is an area of the hypothalamus and one way that we can see the role of the lateral hypothalamus on the release of orexin and the on hunger signaling is in rats who have lesions to their lateral hypothalamus. And so they have damage to this area, they can no longer turn on the hunger signal. What this means is that they can’t activate these neurons to release orexin, and then without the release of orexin, they no longer have the motivation to eat. And so they’ll starve to death unless they’re force-fed. They’re simply not getting the signal that they should be hungry, and so they don’t eat.
Now it’s not the case that orexin is the only source of hunger. We also have the release of other hormones in other areas of the body. So another hormone that’s involved in turning on the hunger signal is a hormone called ghrelin and this is released by the stomach. This also motivates eating behavior, and you might think of your stomach growling just before a meal to help you remember ghrelin. And we also have another hormone that you’ve probably heard of, which is insulin. Insulin is released by the pancreas and this is involved in regulating blood glucose levels, so as your blood glucose fluctuates then this influences the release of insulin and that can also influence your motivation to eat.
Now, let’s look at the satiety signal in some more detail and the satiety signal also comes from an area of the hypothalamus, in this case from the ventromedial hypothalamus. And one way we can see this is again in rats who have lesions to this area. So if you destroy the ventromedial hypothalamus in rats, what you find is they continue eating. They don’t get the off signal, so they get hungry and then they just don’t stop eating. They’ll eat, you know, continuously and so they become very, very large. Rats with lesions to the ventromedial hypothalamus can become 2 to 3 times the normal bodyweight of rats of their breed. And here’s a picture of one of these rats with a lesion to the ventromedial hypothalamus, you can see he’s become quite large.
And again, it’s not simply the case that the ventromedial hypothalamus is the only part to the off signal. We also have other hormones and one of these is leptin, and this comes from the Greek leptos for “thin” and this is actually released by fat cells. So fat cells send a signal of satiety when they’re full. So they store energy and then they send a signal to stop eating because they have already stored energy and this is kind of interesting because it means you can actually think of your fat cells as glands, right? They’re involved in the release of hormones. And another way we see the role of leptin is in people who are obese and one thing that we see in obesity is reduced sensitivity to this leptin signal. So, in other words, people who are obese, their fat cells are releasing leptin and trying to tell their body that they’re full, they should stop eating, but the signal is not getting there, right? It’s, there’s a lost sensitivity to leptin and this may be playing a role in obesity.
Now what I hope you get from this video is that even something as simple as a hunger drive, you think this is probably one of the most basic motivations we need to have in order to survive, we need to eat, we need to know when to stop eating, this seems very simple it’s actually very, very complex. There’s a number of different hormones involved, there’s other hormones that I didn’t get into in this video, and this is without even getting into psychological cues that are also involved in hunger signaling. So your memory of the last time that you ate might influence how hungry you feel. Or visual stimuli can influence you, you could be totally full and then you see this really great looking dessert and suddenly you find that you’re hungry again. And then you also have preferences, and these can be broad preferences in terms of culture, which foods are considered more or less appetizing in different cultures, and then even on an individual basis looking at the same image might make me very hungry and motivated to eat, whereas you don’t like that food and so you don’t find it particularly appetizing. And here we can see the incredible complexity of something that initially might appear fairly simple.
And so when we see something that might be going wrong with the hunger signal, we see it’s not easy to know what the solution is. This is something we’ll take a look at in the next video when we look at eating disorders. So I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!
