Circumventricular organs
- Regions where the blood-brain barrier is absent.
- Function
- Anatomical adaptation allows areas of the brain to monitor homeostatic changes within the systemic circulation.
- Divided into
- Secretory organs
- Sensory organs
Secretory organs
- Structures that release their products directly into the bloodstream or cerebrospinal fluid. The products may either be neurohormonal or other proteins.
Neurohypophysis (Posterior pituitary gland)
- Originates from neuroectoderm
- Stores hypothalamic hormones (namely oxytocin and vasopressin).
- Hormones are delivered to the posterior pituitary gland by way of nerve fibres travelling from the paraventricular and supraoptic hypothalamic nuclei.
Pineal gland
- Situated in the posterior aspect of the 3rd Ventricle.
- The gland is encapsulated and lobulated (internally).
- Constituent cells – pinealocytes –
- Produce and cyclically release melatonin with the oscillating circadian rhythm (which is regulated by the suprachiasmatic nucleus).
- The absence of a blood-brain barrier here allows melatonin to be secreted into the rich blood supply coming from the posterior choroidal artery (branch of the posterior communicating artery) and internal cerebral veins (tributary to the great vein of Galen).
Subcommissural organ
- Situated near to the caudal limit of the pineal recess (in the third ventricle) at the opening of the cerebral aqueduct of Sylvius.
- Here, it is caudally and ventrally related to the posterior commissure.
- The ependymal cells here, unlike those covering the other circumventricular organs, are tall ciliated columnar cells.
- The capillaries at this level are not as abundant or significantly fenestrated as those in other circumventricular organs.
- The subcommissural organ releases SCO-spondin
- A glycoprotein that aggregates within the third ventricle and forms Reissner’s fibres.
- These fibres have been implicated in maintaining the patency of the aqueduct of Sylvius and their absence results in congenital hydrocephalus.
Median Eminence
- Has an intricate communication with the hypophyseal portal system that permits communication between systemic circulation and cerebrospinal fluid by way of the large number of fenestrated capillary beds it contains.
- Located at the floor of the hypothalamus, is anterior to the tuber cinereum (ventral extent of the third ventricle).
- Contains specialised cells known as tanycytes, which assist in modifying the permeability of the membrane to allow macromolecules to enter the peripheral circulation
Sensory organs
- Responsible for monitoring the peripheral circulation and responding appropriately to reverse these changes or eliminate toxins.
Subfornical organ
- Located in a small region at the interventricular foramen of Monro
- Comprised of glial cells, neurons and a densely packed tuft of fenestrated capillaries.
- Like other circumventricular organs, the subfornical organ is covered by flattened ependymal cells.
- The subfornical organ has multiple homeostatic functions,
- Including
- Cardiovascular regulation
- Osmoregulation
- Energy regulation
- This concept is supported by its efferent projections to the lateral hypothalamus, the median preoptic area and organum vasculosum.
Organum vasculosum
- AKA
- Organum vasculosum of the lamina terminalis (OVLT)
- Vascular organ
- Located
- Cranial to the optic chiasm
- Caudal to the anterior commissure.
- The vascular bed of this organ is highly fenestrated and the ependyma contains flattened cells with very sparsely distributed cilia.
- Function
- Regulate fluid balance. As such, it receives afferent fibres from the subfornical organ, several hypothalamic nuclei and the locus coeruleus. It then projects to the supraoptic and median preoptic nuclei.
Area postrema
- A paired structure
- Located at the caudal extent of the floor of the fourth ventricle.
- Vomiting centre
- The absence of a blood-brain barrier in this location allows the area postrema to identify chemical irritants and stimulate a vomiting response.
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