Osmosensing neurons (ONs) swell to detect a decrease in salt concentration.

Bourque/Macmillan

Cells that passively swell and contract with changes in osmolarity are the key to detecting those changes, say Zizhen Zhang and Charles Bourque (McGill University, Montreal, Canada).

Most cells have at least two mechanisms for maintaining homeostasis in response to changes in salt concentrations. Without these mechanisms, osmotic forces take over: more water and thus less salt outside cells leads water to flood into a cell. But this swelling doesn't occur in most cells because they pump ions out. Even if there is some swelling, the cells prevent membrane stretching by adding extra membrane to compensate.

The osmosensory neurons studied by Zhang and Bourque lack both of these responses, so they swell and contract with changes in salt concentration. “It's a behavior you expect from passive diffusion across a membrane,” says Bourque.

The neurons have membrane reserves so that they do not pop as they swell. But channels in the membrane are affected: the ironing out of membrane folds, tension on underlying cytoskeleton, or both act to turn off the channels. The resultant hyperpolarization and lack of firing leads to less vasopressin release so that the kidney directs more water to be excreted. ▪

Reference:

Zhang, Z., and C.W. Bourque. 2003. Nat. Neurosci. 10.1038/nn1124.