Gastric component and reception
The initial stage of nutritional need is associated with the physical state of the gastrointestinal tract. Following the evacuation of food, the stomach contracts, occupying a minimal volume while its walls thicken.
Such anatomical changes lead to irritation of mechanoreceptors in the organ wall. In parallel, chemoreceptors are activated, stimulated by the accumulation of gastric mucus and the so-called "emptiness" factor. The resulting afferent impulses trigger a complex cascade of neurohumoral regulation.
Neurohumoral chain (according to I.G. Akmaev)
The signal from the empty stomach receptors is transmitted to the brain via the following chain:
- The nuclei of the vagus nerve (n. vagus) are excited.
- The impulse reaches specialized neurons of the hypothalamus.
- Cells of the anterior pituitary gland (adenohypophysis) are activated via neurosecretory mechanisms (involving nerve growth factors, including insulin-like oligopeptides).
- The pituitary gland increases the production of tropic hormones.
The main effect of tropic hormones at this stage is the stimulation of nutrient deposition. Carbohydrates and fats from the bloodstream are actively stored in the liver, skeletal muscles, and adipose tissue. Clinical observation: in overweight individuals, these processes occur significantly more intensively, exacerbating the accumulation of reserves.
Theory of functional systems: two phases of hunger
According to the theory of functional systems, neural centers receive information about nutritional needs through two successive factors:
First phase (gastric afferentation): Develops as food is evacuated. At the level of the medulla oblongata, it triggers "hunger" periodic motility. At the level of the hypothalamo-pituitary system, a crucial process occurs: the enhanced transition of substances from the blood into storage depots. As a result, the level of nutrients in the plasma decreases, creating conditions for the transition to the second phase, even if actual reserves in the body are still plentiful.
Second phase ("Hungry blood"): Blood depleted of nutrients becomes saturated with signaling molecules — oligopeptides (ghrelin, motilin, pentagastrin, oxytocin, neuropeptide Y). If such substances are experimentally administered to a fed animal, it will begin to search for food.
Conscious awareness of need and clinical evidence
Mechanical impulsation from the empty stomach is not subjectively felt by a person on its own. The feeling of hunger is consciously perceived only when "hungry blood" factors reach the limbic structures of the brain and activate them. Only then does the subject switch to searching for food.
The dual nature of hunger (afferentation + humoral factors) is confirmed by classic clinical examples:
- Removal of the stomach (gastrectomy): Patients without a stomach continue to feel hungry. There is no mechanical component, but metabolic demand persists, so signaling molecules are formed in the bloodstream, creating the very "hungry blood".
- Conjoined twins: When sharing a common circulatory system, feeding one twin does not always satisfy the hunger of the other. The signaling from the empty stomach of the hungry child is so strong that it locally blocks the satiety-inducing effect of "sated blood" factors coming from the fed twin.
Briefly: answers to topical questions
Which specific hypothalamic nuclei form the hunger center and the satiety center?
The hunger center and satiety center are formed by specific nuclei belonging to various groups within the hypothalamus.
- Ventromedial nuclei form the satiety center and belong to the middle group of hypothalamic nuclei.
- Lateral nuclei form the "hunger center" (lateral hypothalamus) and belong to the outer group of nuclei.
Bilateral destruction of the ventromedial nuclei leads to a loss of the feeling of satiety and obesity, whereas destruction of the lateral regions causes refusal of food.
Which types of hypothalamic chemoreceptors respond to changes in blood nutrient concentrations?
Sources indicate that glucoreceptors of the hypothalamus respond to changes in nutrient concentrations associated with blood glucose levels.
- Glucoreceptors are found in the ventrobasal and lateral nuclei of the hypothalamus.
- Direct contact between hypothalamic neurons and the blood is enabled by the absence of the blood-brain barrier.
Which theories of feeding behavior regulation, besides the theory of functional systems, explain the formation of the feeling of hunger?
The formation of the feeling of hunger is also explained by the humoral and glucostatic theories.
- Humoral theory ("hungry blood" theory) postulates that the cause of hunger is a change in the chemical composition of the blood, manifested by a reduced nutrient content and the presence of specific hunger factors.
- Glucostatic theory states that the feeling of hunger arises when the arteriovenous difference in glucose content tends to zero, while the absolute glucose level is of secondary importance.
The rule of the two "G"s: The gastric phase (empty stomach) shifts substances into depots and prepares the humoral phase ("hungry blood"), whose peptides turn on the limbic system and the feeling of hunger.
Class and exam questions
Why is a person still able to experience hunger even after total gastrectomy?
Because the body's metabolic demand persists. In response to it, specific signaling molecules (oligopeptides) are released, forming "hungry blood" that directly stimulates brain centers.
How does an empty stomach affect nutrient concentrations in the systemic circulation?
Impulses from the empty stomach via the hypothalamus activate pituitary hormones. They stimulate the active transfer of nutrients from the blood into tissue depots, thereby lowering their levels in the plasma.
Can conjoined twins with shared circulation experience hunger independently?
Yes. If only one is fed, the other will retain impulsation from their own contracted stomach. This local neural impulsation blocks the action of satiety factors coming from the shared blood.
What else to study on this topic
- Receptor apparatus of the stomach: chemoreceptors and the "emptiness" factor
- Mechanisms of neurosecretion in the hypothalamo-pituitary system during hunger
- Features of nutrient deposition in obese individuals
- Characteristics of "hungry blood" oligopeptides (ghrelin, neuropeptide Y, motilin)
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