Introduction
INTRODUCTION
The Central Nervous System (CNS) consists of the brain (encephalon) and the spinal cord.
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It is contained within bony structures, the cranium (or skull) and the spine and is protected by meningeal coverings which are more widely called "the meninges". The brain consists of two cerebral hemispheres (one right and one left), which meet in their lower midline part to form the diencephalon, which extends into the brain stem. The brain stem joins the spinal cord in its lower part. Each cerebral hemisphere contains a cavity called the lateral ventricle. The two lateral ventricles take the shape of a crescent which is open anteriorly and which consists of three horns (frontal, temporal and occipital). They are connected in the midline to the underlying third ventricle through the foramina of Monro.
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Anatomy of CNS
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The ventricle cavities
The diencephalon is the place where the two cerebral hemispheres meet, in which the 3rd ventricle lies.
The brainstem is located between the brain above and the spinal cord below. It consists of 3 parts, the mesencephalon (or midbrain), the pons and the spinal bulb (or medulla).
The 4th ventricle is delineated anteriorly by the posterior surface of the pons and posteriorly by the cerebellum. It communicates superiorly with the 3rd ventricle through the aqueduct of Sylvius and inferiorly with the ependymal canal and the sub-arachnoid spaces through the lateral foramina of Luschka and the median foramen of Magendie.
Anatomy of skull, brain & ventricles
Cerebral ventricles are filled with a transparent "gin clear" fluid, which is called Cerebro Spinal Fluid (CSF).
CSF is produced in the ventricles by the choroid plexus at a rate of approximately 20 ml/h in adults (8 ml/h in infants).
The CSF circulates within the ventricular cavities and leaves the 4th ventricle through the foramina, entering the sub-arachnoid spaces surrounding the brain and spinal cord.
CSF Pathways
The fluid returns to the venous circulation by being absorbed through small formations, the arachnoid villi (Pacchioni granules), a type of outgrowth from the arachnoid, through the dura mater and spaces in contact with the sagittal sinus at the midline of the brain.
The meninges
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Cerebro Spinal Fluid (CSF) |
Under normal conditions, a perfect equilibrium exists between secretion and absorption of CSF. |
The average CSF pressure in adults when lying down is 120-180 mm H2O (~10 mm Hg). The pressure is almost zero or even negative when standing up. The reference measurement of CSF pressure is measured at the level of the cerebral ventricles: this is intraventricular pressure. It is often called intra-cranial pressure (ICP).
CSF is a transparent "gin clear" fluid
which, in addition to its mechanical protective role, takes part in metabolic
exchange between the nervous system and the rest of the body.
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