Osmoreceptor

Osmoreceptors are specialized receptor cells that measure the concentration of dissolved molecules (the osmotic concentration ) of the fluid outside the cell and are involved in osmoregulation .

functionality

The cell interior ( cytoplasm ) in mammals has a precisely regulated osmotic concentration of 290 mosmol / l. In the equilibrium state, the extracellular osmotic concentration of mammals including marine mammals is also 290 mosmol / l. If the extracellular osmotic concentration is the same as the intracellular one, the diffusion currents into and out of the cell are equalized ; overall there is no change in the concentration.

Extracellular hypoosmolarity

However, if the osmotic concentration outside the cell drops - for example due to increased salt loss or due to water poisoning - liquid flows into the cell from the outside along the osmotic gradient. Cell swelling follows due to the hypotonic environment. This swelling opens mechanosensitive ion channels . The consequence is a depolarization of the membrane potential of the cell membrane and thus the release of transmitter substances or the generation of an action potential . The formation of the hormone vasopressin is curbed, as a result of which the kidneys absorb less water from the primary urine and more urine is formed ( diuresis ).

Extracellular hyperosmolarity

Correspondingly, when the osmolarity increases, the cells hyperpolarize outside the cell. In a hypertonic environment, the cells shrink. This gives the feeling of thirst . When thirsty, the hormone vasopressin is released, which increases the absorption of water in the kidneys. In humans, increased osmolarity also leads to a reduction in sweat production during physical activity. The question of how to end the feeling of thirst when drinking has not yet been clarified . It takes a few minutes for the amount of fluid absorbed to reach the blood and the osmoreceptors in the brain. The feeling of thirst disappears after a few seconds when drinking, presumably through osmoreceptors in the throat and digestive tract. The thirst is probably quenched without the stomach having to participate , as an infusion of hypotonic solutions also reduces thirst.

The body has a number of hormones that are involved in the homeostasis of osmotic concentration. These essentially include vasopressin or antidiuretic hormone (ADH), aldosterone and atrial natriuretic peptides (ANP or ANF).

Occurrence

Osmoreceptors are located in the hypothalamus and in the juxtaglomerular apparatus of the kidney . The osmoreceptors in the central nervous system are located in areas without a blood-brain barrier . Other osmoreceptors (peripheral osmoreceptors) are located in the throat, digestive tract, mesentery , portal vein and liver . The cell type of the peripheral osmoreceptors is unknown.

Individual evidence

1. ↑ ^{a b c d e f g h i j} Charles W. Bourque: Central mechanisms of osmosensation and systemic osmoregulation. In: Nature Reviews Neuroscience . 9, 2008, pp. 519-531, doi : 10.1038 / nrn2400 .