Circumventricular organ

Circumventricular Organ

The circumventricular organs (CVOs) are unique structures in the brain that are characterized by their extensive vasculature and lack of a typical blood-brain barrier. These organs are strategically located around the ventricular system of the brain and play crucial roles in maintaining homeostasis by facilitating communication between the central nervous system and peripheral blood.

Structure

Circumventricular organs are distinct from other brain regions due to their permeable capillaries, which allow for the exchange of molecules between the blood and the brain. This permeability is essential for their function as they detect changes in blood composition and relay this information to other parts of the brain.

The main circumventricular organs include:

• The Area postrema, located in the medulla oblongata, which is involved in the vomiting reflex.
• The Subfornical organ, which plays a role in osmoregulation and cardiovascular regulation.
• The Organum vasculosum of the lamina terminalis (OVLT), which is involved in the regulation of body fluids and temperature.
• The Median eminence, which is part of the hypothalamus and involved in the release of hypothalamic hormones.
• The Pineal gland, which secretes melatonin and regulates circadian rhythms.
• The Subcommissural organ, whose function is less well understood but is thought to be involved in the secretion of glycoproteins into the cerebrospinal fluid.

Function

Circumventricular organs serve as critical interfaces between the nervous system and the endocrine system. They detect changes in blood-borne signals such as hormones, electrolytes, and other solutes, and initiate appropriate physiological responses.

For example, the subfornical organ and the OVLT are sensitive to changes in blood osmolarity and play a role in the regulation of thirst and salt appetite. The area postrema detects toxins in the blood and triggers the vomiting reflex to prevent poisoning.

Clinical Significance

Due to their unique properties, circumventricular organs are involved in various physiological and pathological processes. They are targets for certain drugs that need to access the brain but are typically restricted by the blood-brain barrier. Additionally, dysfunction in these organs can contribute to disorders such as hypertension, heart failure, and neuroendocrine disorders.

Also see

• Blood-brain barrier
• Hypothalamus
• Endocrine system
• Neuroendocrinology