D-2 (page 2)

The axons of the neurons in the subfornical organ are sent to the median preoptic nucleus, which is a small nucleus that is wrapped around the front of the anterior commissure. It is the fiber bundle that connects the amygdala and anterior temporal lobes. In ENGLISH.... =) this means that the median preoptic nucleus is wrapped around the front (anterior) of the commissure of the ventral fornix. And it forms something similar to a "T" with the base originating in the SFO, and the bar of the "T" linking the front temporal lobe and the amygdala.

So it is the areas in front of the third ventricle that serve as the "osmometric and volumetric thirst hot spot" where all the stimuli congregate and are integrated.

Volumetric Thirst:

• Atrial baroreceptors
  • Then it goes to the nucleus of the solitary tract
  • Next the signal travels to the Median Preoptic Nucleus
  • Finally drinking is initiated
• Angiotensin
  • Then it goes to the Subfornical organ (SFO)
  • Next the signal travels to the Median Preoptic Nucleus
  • Finally drinking is initiated

Osmometric Thirst:

• Solute concentration of food is measured
• OVLT has osmoreceptors that detect the concentration change
• Next the signal travels to the Median Preoptic Nucleus
• Finally drinking is initiated

Looking at this outline of the path signals take to initiate drinking, you can trace both types easily. Osmometric thirst is the "shorter pathed" of the two. It causes a change inthe solute concentration in the blood, and this is detected by the OVLT. The osmoreceptors then send signals across the blood-brain barrier and these reach the median preoptic nucleus, which in turn stimulates drinking.

Volumetric thirst takes one of the two paths, depending on which one, the set of atrial baroreceptors or the production of angiotensin, or both, happens. If the atrial baroreceptors are activated, they send a signal to the nucleus of the solitary tract, and it goes from there to the median preoptic nucleus. If angiotensin is released, then this causes changes in the SFO, on the blood side of the blood-brain barrier, and finally the SFO sends a signal to the median preoptic nucleus.

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