Conditions - Chiari and Syringomyelia 101

by John Oró, MD

A Brief Look at Neuroanatomy

The brain is enclosed and protected by a rounded skull made of rigid bone. The bottom of the skull contains multiple openings called foramen through which nerves and blood vessels pass. The inside of the skull, called the intracranial space, is partly separated into two compartments by a tent like structure called the tentorium. The large compartment above the tentorium is called the supratentorial compartment and the compartment below the tentorium is called the infratentorial compartment. (Supra means above, and infra means below.) Most doctors call the infratentorial compartment the posterior fossa.

The supratentorial compartment contains the two halves of the cerebrum known as cerebral hemispheres. The hemispheres come together in a deep central area called the diencephalon. Through an opening in front of the tentorium, the diencephalon connects to the brain stem. On the back of the brainstem sits the cerebellum. The brain stem continues through an opening at the bottom of the skull, called the foramen magnum, and connects with the spinal cord. The spinal cord runs in the spinal canal.

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There are four cavities filled with spinal fluid within the brain called ventricles. [see Figure 1] Two large C-shaped ventricles, called the lateral ventricles, are located within the cerebral hemispheres and are connect by two small tunnels (the foramen of Monro) to the third ventricle. From the third ventricle spinal fluid flows through a small tunnel known as the Aqueduct of Sylvius to the fourth ventricle which is located between the brain stem and the cerebellum. Spinal fluid flows out of the fourth ventricle through three openings: two openings at the side of the fourth ventricle called the foramen of Luschka and one at the bottom of the fourth ventricle called the foramen of Magendie. From these three foramina spinal fluid flows out of the ventricular system to the surface of the brain as well as down the spinal canal and back up. Spinal fluid is created by a tuft of vascular tissue called the choroid plexus that is present within each ventricle. With every heartbeat, blood passing in the choroid plexus is filtered to create a clear colorless fluid that looks like water called cerebral spinal fluid or CSF. The spinal fluid flows down through the ventricles, exits the through the three openings of the fourth ventricle, and flows around the brain and spinal cord. The spinal fluid is taken back up in the vascular system through a large vein located at the top of the brain called the sagittal sinus. By way of special connections called arachnoid granulations, the spinal fluid drains into the sagittal sinus. Here it becomes part of the blood that drains through the jugular veins to the heart. With each heartbeat, a small amount of spinal fluid is created inside the ventricles at the same time a small amount is taken up by the veins, keeping the system in balance.