General provisions and phases of satiation
The presence of food in the gastrointestinal tract is regarded in normal physiology as the second most important useful adaptive result of the functional nutrition system. It is the processes belonging to the external self-regulation link of this system that ultimately determine the development of food satiation.
Physiologically, this complex process is conventionally divided into two successive stages:
- Sensory satiation (also known as primary or emotional).
- Metabolic satiation (occurs later, as nutrients are digested and absorbed).
Sensory (primary) satiation
The sensory phase is an extremely rapid physiological process. In everyday life, this state is often described as «satiation right at the dinner table», when a person or animal stops eating even before the food has time to be digested and absorbed into the bloodstream.
The biological significance of this phenomenon is critically important for the survival of species in the wild. Rapid, anticipatory consumption of available food and the immediate relief of hunger allows the animal to quickly leave an open area and hide in a safe shelter before prolonged metabolic processes unfold. This is a reliable evolutionary mechanism for avoiding predators at a moment of vulnerability.
Neurophysiological mechanism
The cessation of food motivation is ensured by a strict sequence of reflex reactions:
- Activation of the receptor apparatus. Incoming food begins to exert mechanical and chemical effects on receptors located in the mucosa of the oral cavity and esophagus. However, the most significant impulses originate from receptors in the stomach when it is distended.
- Afferent pathway. The resulting nerve impulses travel along afferent pathways to the brain, reaching neurons of the ventromedial hypothalamus (traditionally referred to as the «satiety center»).
- Reciprocal inhibition. The excited cells of the ventromedial hypothalamus begin to exert a powerful inhibitory effect on neurons of the lateral hypothalamus (the so-called «hunger center», which was previously highly active due to food deprivation).
- Loss of motivation. As a result of the inhibition of the lateral hypothalamus, its activating ascending influences on other brain structures sharply decrease. Food motivation completely disappears, and the act of eating ceases.
Humoral mechanisms and satiation factors
In addition to nerve impulses, humoral factors are involved in the process. Excitation of the ventromedial hypothalamus triggers a cascade of reactions (via hormonal pathways and pathways not yet fully understood) that leads to the emergency emptying of nutrient depots in the body.
The released nutrients rapidly enter the systemic circulation. This deprives the blood of its previous «hungry» properties, due to which it previously exerted an irritating effect on hypothalamic structures. Concurrently, the blood is actively saturated with specific factors—oligopeptides—that consolidate the feeling of fullness.
The main satiation factors include:
- Cholecystokinin;
- Somatostatin;
- Bombesin;
- Glucagon;
- Leptin.
A special place in this series belongs to enterins. These are specific substances secreted by the duodenal mucosa that effectively reduce appetite. This group of compounds was described in detail by the prominent physiologist A.M. Ugolev.
Briefly: answers to topical questions
Through which afferent nerves do impulses from gastrointestinal receptors travel to the hypothalamus?
In the cited sources, the specific afferent pathway is named for the stomach: impulses are transmitted via afferent fibers of the vagus nerve (n. vagus) departing from the stomach; when food or a probe is introduced, this impulse generation is suppressed.
During sensory satiation, food irritates the receptors of the oral cavity, esophagus, and especially the stomach, and impulses are directed to neurons of the ventromedial hypothalamus. Excitation of the ventromedial hypothalamus inhibits the activity of lateral hypothalamic cells, reduces its ascending influences on other parts of the brain, which leads to the disappearance of food motivation and the cessation of eating.
For the oral cavity and esophagus, specific afferent nerves are not named in these sources.
What are the specific physiological mechanisms of metabolic satiation?
Metabolic, or true, satiation occurs after nutrients are absorbed into the blood. This process is based on the following mechanisms of internal homeostasis regulation:
- Changes in the intensity of tissue metabolism.
- Entry of nutrients into depots and their release from depots.
- Redistribution of nutrients in the body.
- Hormonal regulation.
The main regulated parameter is blood nutrients. Vascular chemoreceptors perceive the concentration of nutrients and, via a feedback link, transmit signals to the central nervous system—to the lateral and ventromedial hypothalamus.
Class and exam questions
What is the biological significance of sensory satiation?
It allows the animal to quickly become satiated and retreat to a safe shelter from predators even before the prolonged processes of digestion and assimilation (metabolism) of food begin.
Which brain region is responsible for inhibiting food motivation?
Ventromedial hypothalamus. Upon receiving signals from gastrointestinal receptors, it becomes excited and inhibits the lateral hypothalamus (the hunger center).
What happens to nutrient depots during primary satiation?
Excitation of the ventromedial hypothalamus causes the emptying of depots; nutrients enter the blood, eliminating its «hungry» properties and removing irritation from the hypothalamus.
Who described enterins and where are they produced?
Enterins were described by Academician A.M. Ugolev. They are secreted by the duodenal mucosa and contribute to appetite reduction.
What else to study on this topic
- Metabolic phase of satiation: mechanisms and timelines
- Functional nutrition system: internal and external links of self-regulation
- The role of the lateral hypothalamus in the formation of food motivation
- Hormonal regulation: pathways of hypothalamic influence on nutrient depots
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