Circumventricular organs

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Scheme showing relations of the ventricles to the surface of the brain. Circumventricular organs primarily surround the third ventricle and fourth ventricle.

Circumventricular organs are so named because they are positioned at distinct sites around the margin of the ventricular system of the brain. They are among the few sites in the brain which have an incomplete blood-brain barrier. As a result, neurons located in circumventricular organs can directly sense the concentrations of various compounds, particularly peptide hormones, in the bloodstream, without the need for specialized transport systems which move those compounds across the blood-brain barrier. A useful mnemonic device for remembering this aspect of their function, though not the source of the name, is that they allow factors to 'circumvent' the blood-brain barrier. These organs secrete or are sites of action of a variety of different hormones, neurotransmitters and cytokines.