Subfornical organ

The subfornical organ, situated on the ventral surface of the fornix, at the foramen of Monro, is one of the circumventricular organs of the brain.

Relations with other circumventricular organs
Other circumventricular organs are the area postrema in the brainstem and the organum vasculosum of the lamina terminalis (OVLT).

The OVLT and the SFO are both strongly interconnected with the nucleus medianus, and together these three structures comprise the so called "AV3V" region - the region anterior and ventral to the third ventricle. The AV3V region is very important in the regulation of fluid and electrolyte balance, by controlling thirst, sodium excretion, blood volume regulation, and vasopressin secretion.

The SFO is outside the blood-brain barrier, and so neurons in this region can respond to factors that are present in the systemic circulation.

Neurons
Some neurons in the SFO are osmoreceptors, being sensitive to the osmotic pressure of the blood. These neurons project to the supraoptic nucleus and paraventricular nucleus to regulate the activity of vasopressin-secreting neurons. These neurons also project to the nucleus medianus (also called the median preoptic nucleus) which is involved in controlling thirst.

Neurons in the SFO have receptors for many hormones that circulate in the blood but which do not cross the blood-brain barrier, including angiotensin, atrial natriuretic hormone, endothelin and relaxin.