Osmoreceptors

Classification and Localization

Osmoreceptors belong to interoceptors, which inform the nervous system about processes in the body's internal environment. These are individual receptor structures that do not form complex anatomical sensory organs. Information from receptors that do not form sensory organs does not reach the cerebral cortex, so their stimulation is unconscious.

Based on their localization, two main groups are distinguished:

• Central osmoreceptors — located in the anterior hypothalamus (in the supraoptic and paraventricular nuclei). They are sensitive to the osmotic pressure of blood in the cerebral capillaries.
• Peripheral osmoreceptors — localized in the liver, spleen, skin, tongue, and the sinocarotid region (carotid sinus).

Note: during pregnancy, the number of various receptors increases, including baroreceptors, osmoreceptors, and chemoreceptors. This is important for the perception and transmission of nerve impulses from the fetus to the mother.

Mechanism of Action of Central Osmoreceptors

In the cytoplasm of hypothalamic neurons that function as osmoreceptors, there is a vacuole whose size changes depending on osmolarity.

• Isoosmolar state: vacuoles are of medium size, which is accompanied by the generation of action potentials (APs) at an optimal frequency.
• Hyperosmolar state: water leaves the vacuoles, their size decreases, which leads to a change in neuronal activity and stimulates the secretion of antidiuretic hormone (ADH).

Stimulation of ADH secretion via osmoreceptors occurs during intracellular dehydration accompanied by an increase in intracellular sodium concentration, or during extracellular dehydration accompanied by an increased concentration of angiotensin II in the hypothalamic capillary network.

Role in Blood Volume Regulation

Osmoreceptors participate in the regulation of circulating blood volume (CBV), as CBV is closely linked to water-electrolyte balance. In acute blood loss and a drop in systemic arterial pressure, the following compensatory cascade is triggered:

1. A decrease in systemic pressure stimulates baroreceptors in the walls of afferent arterioles, leading to the release of renin. Renin triggers a cascade of transformations leading to angiotensin II, which stimulates aldosterone production.
2. Aldosterone enhances the reabsorption of $Na^+$ ions, causing the osmotic pressure of the blood to rise.
3. High osmotic pressure stimulates the hypothalamic osmoreceptors.
4. ADH (vasopressin) is released, which enhances water reabsorption.
5. The increase in CBV, combined with the vasoconstrictor effect of angiotensin II, restores arterial pressure.

Clinical Significance in Pathologies

Disruption of the hypothalamic centers housing central osmoreceptors leads to disorders of water and electrolyte balance:

• Central diabetes insipidus: occurs when 90% or more of the neurons in the supraoptic and paraventricular nuclei are destroyed or dysfunctional. ADH secretion ceases. It manifests with agonizing thirst, polydipsia, and hypoosmolar polyuria — the excretion of at least 3 liters of urine per day with an osmolarity of 50–150 mOsm/L. Hypothalamic damage can impair the sensation of thirst, leading to severe hyponatremia. For differential diagnosis with the nephrogenic form, 5 IU of ADH is administered: if urine osmolarity increases by no more than 50%, the nephrogenic form of diabetes insipidus is diagnosed; replacement therapy in this case is ineffective.
• Syndrome of inappropriate antidiuretic hormone secretion (SIADH / Schwartz-Bartter syndrome): associated with abnormal ectopic ADH secretion, most commonly in bronchogenic carcinoma or other malignant tumors. It is characterized by hypervolemia, hyponatremia below 130 mmol/L, low plasma osmolarity below 275 mOsm/kg, and the excretion of highly concentrated urine. Clinically, it manifests with weight gain, weakness, nausea, altered mental status, and epileptic seizures.