The brain is suspended within the skull, protected by a specialized fluid environment that shields it from injury and supports its function. Within this delicate protective system are spaces called cisterns, which are essentially fluid-filled expansions at the base of the brain. These cisterns are reservoirs that hold the cerebrospinal fluid (CSF), a clear, colorless liquid that performs several mechanical and physiological roles for the central nervous system. Understanding these cisterns provides insight into the brain’s anatomy, the circulation of its protective fluid, and how medical conditions can affect this enclosed system.
Anatomical Definition and Location
A brain cistern is a natural enlargement of the subarachnoid space, the interval situated between two of the protective layers that cover the brain and spinal cord, known as the meninges. The meninges consist of three membranes: the tough outer dura mater, the web-like arachnoid mater, and the delicate inner pia mater. The pia mater closely adheres to the contours of the brain’s surface.
The arachnoid mater bridges over the deeper grooves and depressions of the brain, creating an open space between the two membranes. This separation results in wider areas where CSF can pool, forming the cisterns, which are most prominent at the base of the brain and around the brainstem. These interconnected pockets allow for the passage of blood vessels and cranial nerves as they enter and exit the brain tissue. They are separated by a porous wall of fine connective tissue known as arachnoid trabeculae.
Major Named Cisterns and Their Contents
The brain contains several named cisterns, each positioned near different brain structures and housing unique neurovascular elements. The Cisterna Magna, also known as the cerebellomedullary cistern, is the largest space, located between the cerebellum and the back of the medulla oblongata. It receives CSF directly from the fourth ventricle and contains the vertebral artery and the origins of the lower cranial nerves (IX through XII).
Moving anteriorly, the Pontine Cistern surrounds the front surface of the pons. This space is a major thoroughfare, housing the basilar artery, a major blood supply to the brain, and the abducens nerve (cranial nerve VI). The Pontine Cistern is continuous with the spinal subarachnoid space below and the interpeduncular cistern above, facilitating fluid and vascular continuity.
The Interpeduncular Cistern is situated at the base of the midbrain, nestled between the two cerebral peduncles. This cistern is shaped like a star or pentagon on cross-section and contains the bifurcation of the basilar artery and the oculomotor nerve (cranial nerve III).
The Role in Cerebrospinal Fluid Dynamics
The cisterns function as reservoirs and distribution points for the Cerebrospinal Fluid (CSF), which is constantly produced and circulated throughout the central nervous system. CSF provides buoyancy, reducing the brain’s weight from about 1,500 grams to approximately 50 grams. This minimizes mechanical stress on neural tissue and blood vessels, acting as a shock absorber to protect the brain from sudden impacts.
The cisterns also play a role in pressure regulation by buffering changes in intracranial pressure that occur with natural physiological events like the cardiac cycle. As the heart beats, blood is rapidly pumped into the cranial cavity, causing a momentary increase in pressure. The interconnected cisterns allow for the displacement and flow of CSF to accommodate these pulsations, helping to maintain a stable environment.
Furthermore, the continuous flow of CSF through the cisterns facilitates the clearance of metabolic waste products from the brain tissue, a process associated with the glymphatic system. The cisterns ensure the fluid travels from the ventricles and around the brainstem and spinal cord for eventual reabsorption into the bloodstream. The patency and health of these cisterns are tied to the overall metabolic maintenance and fluid balance of the brain.
Clinical Relevance in Brain Health
The cisterns serve as a roadmap for diagnostic imaging, particularly computed tomography (CT) and magnetic resonance imaging (MRI), because their appearance can signal serious underlying medical conditions. Radiologists assess the cisterns to look for signs of brain swelling, which can compress or “efface” these spaces. This compression indicates a potentially dangerous increase in intracranial pressure or a risk of brain herniation.
A common condition where cisterns are directly involved is subarachnoid hemorrhage (SAH), often caused by a ruptured aneurysm. Since the cisterns are expansions of the subarachnoid space, blood from a rupture pools here, appearing as a bright white density on a CT scan. The distribution of blood within a specific cistern, such as the interpeduncular cistern, can help a clinician determine the source of the bleeding.
The cisterns are also implicated in hydrocephalus, a condition where there is an abnormal accumulation of CSF. Blockage of the CSF flow within or between the cisterns, often due to inflammation or scarring, can impede the fluid’s circulation. This obstruction can lead to the enlargement of the brain’s ventricles, requiring medical or surgical intervention to restore proper fluid dynamics and prevent neurological damage.