Empty Stomach Theory (Peripheral)
This concept, proposed by Anton Carlson, explains the feeling of hunger through signals from gastric mechanoreceptors. It was hypothesized that when food is evacuated, periodic motor contractions begin ("hunger motility"), which trigger the need for food.
However, this theory was proven untenable for several reasons:
- S.V. Anichkov's self-experiments showed that the sensation of hunger does not strictly coincide with bouts of motility and can occur even when the stomach is at rest.
- Surgical practice confirms that individuals with a completely resected stomach still experience hunger during food deprivation.
- Experiments on dogs with deafferentation (transection of the vagus nerves and spinal cord) demonstrated that even with a complete loss of gastrointestinal sensation, animals maintained the drive to eat.
Hunger Blood Theory (Humoral)
Developed by Ivan Pavlov after the empty stomach theory was heavily criticized. The core idea is that hunger arises from changes in the chemical composition of the blood, specifically a decrease in nutrient levels.
Experimental evidence:
- Administration of glucose to starved animals stopped feeding behavior if the dose met their carbohydrate requirement.
- Administration of insulin, which lowers blood sugar, conversely provoked feeding even in satiated animals.
Later, it was established that the decisive factor is not the absolute glucose level itself, but its arteriovenous concentration difference. When this difference approaches zero, the sensation of hunger forms, which, incidentally, coincides with the onset of gastric "hunger" motility.
Critique of the Humoral Theory and Twin Experiments
Despite its logical appeal, the humoral theory also proved incomplete. Initially, it was believed that the blood contained specific hunger and satiety factors.
The refutation came from observations of conjoined twins who shared a common circulatory system but had separate nervous systems. When one twin was fed, her blood (and her sister's blood) became enriched with nutrients. However, the second twin did not experience satiety—she continued to feel hungry. This proved that blood nutrients alone are insufficient to produce satiety; a neural component is strictly required.
Role of Afferentation and Anastomosis Experiments
To prove the importance of GI tract impulses reaching the nervous system, P.K. Anokhin conducted experiments involving heterogeneous nerve anastomoses.
- Anastomosis with a somatic nerve. The vagus nerve (n. vagus) was sutured to the median nerve. In hungry animals, the electrical activity of the forelimb muscles increased during contractions of the empty stomach, dropped at rest, and became minimal after eating.
- Anastomosis with an autonomic nerve. When the vagus nerve was sutured to the chorda tympani, salivatory secretion of the submandibular gland began to reflect the activity of the vagal nucleus: it increased during empty stomach contractions and dropped sharply after a meal.
Direct electrophysiological recording also demonstrated that an empty stomach generates high-amplitude impulses in the afferent fibers of the vagus nerve, which are suppressed upon food intake. This confirmed that the empty stomach, due to tonic contraction of its walls, continuously sends signals to the vagus nerve nuclei, playing a crucial role in shaping hunger.