A SECOND neuronal area of importance in CONTROLLING OSMOLARITY and ADH SECRETION
***located along the anteroventral region of the third ventricle, called the AV3V region. At the upper part of this region is a structure called the SUBFORNICAL ORGAN, and at the inferior part is another structure called the ORGANUM VASCULOSUM of the lamina terminalis. Between these two organs is the MEDIAN PREOPTIC NUCLEUS, which has multiple nerve connections with the two organs, as well as with the SUPRAOPTIC NUCLEI and the blood pressure control centers in the medulla of the brain.
**Lesions of the AV3V region cause multiple deficits in the control of ADH secretion, thirst, sodium appetite, and blood pressure. Electrical stimulation of this region or stimulation by angiotensin II can increase ADH secretion, thirst, and sodium appetite.
In the vicinity of the AV3V region and the supraoptic nuclei are neuronal cells that are excited by small increases in extracellular fluid osmolarity; hence, the term OSMORECEPTORS has been used to describe these neurons. These cells send nerve signals to the supraoptic nuclei to control their firing and secretion of ADH. It is also likely that they induce thirst in response to increased extracellular fluid osmolarity.
Both the subfornical organ and the organum vasculosum of the lamina terminalis have vascular supplies that LACK the typical blood-brain barrier that impedes the diffusion of most ions from the blood into the brain tissue. This makes it possible for ions and other solutes to cross between the blood and the local interstitial fluid in this region