The Ventricular System

The brain and spinal cord float within a protective bath of cerebrospinal fluid (CSF), which is produced continuously by the choroid plexus within the ventricles of the brain.

Each part of the CNS contains a component of the ventricular system. There are four interconnected ventricles in the brain: two lateral ventricles, a third ventricle, and a fourth ventricle. A lateral ventricle is Located deep within each cerebral hemisphere. Each lateral ventricle communicates with the third ventricle via an interventricular foramen (foramen of Monro). The third ventricle is found in the midline within the diencephalon and communicates with the fourth ventricle via the cerebral aqueduct (of Sylvius), which passes through the midbrain. The fourth ventricle is located between the dorsal surfaces of the pons and upper medulla and the ventral surface of the cerebellum. The fourth ventricle is continuous with the central canal of the lower medulla and spinal cord (Figure IV-3-1).

Arachnoid granulation

Superior sagittal sinus

Lateral ventricle

Interventricular foramen of Monro

Foramen of Magendie (median aperture)

Third ventricle Cerebral aqueduct

Foramen of Luschka (lateral aperture)

Fourth ventricle Subarachnoid space

Arachnoid granulation

Superior sagittal sinus

Lateral ventricle

Foramen of Magendie (median aperture)

Interventricular foramen of Monro

Third ventricle Cerebral aqueduct

Foramen of Luschka (lateral aperture)

Fourth ventricle Subarachnoid space

Figure IV-3-1. Sagittal Section of the Brain

Clinical Correlate

CSF Abnormalities

Hydrocephalus is caused by an excess volume or pressure of CSF, producing ventricular dilatation.

Communicating hydrocephalus is caused by oversecretion of CSF without obstruction in the ventricles or by CSF circulation or absorption problems from the subarachnoid space. Choroid plexus papilloma is a possible cause of oversecretion, a tumor in the subarachnoid space limits circulation, or meningitis may limit absorption into the superior sagittal sinus.

Noncommunicating hydrocephalus is caused by obstruction to the CSF flow inside the ventricular system at a foramen of Monro, in the cerebral aqueduct, or in the fourth ventnde. CSF is prevented from exiting through the foramina of Magendie or Luschka in the fourth ventricle into the subarachnoid space.

Normal pressure hydrocephalus results when GF is not absorbed by arachnoid villi and the ventricles are enlarged, pressing the cortex against the skull. Patients present with confusion, ataxia, and urinary incontinence.

CSF DISTRIBUTION, SECRETION, AND CIRCULATION

CSF fills the subarachnoid space and the ventricles of the brain. The average adult has 90 to 150 mL of total CSF, although 400 to 500 mL is produced daily. Only 25 mL of CSF is found in the ventricles themselves.

Approximately 70% of the CSF is secreted by the choroid plexus, which consists of glomerular tufts of capillaries covered by ependymal cells that project into the ventricles (the remaining 30% represents metabolic water production). The choroid plexus is located in parts of each lateral ventricle, the third ventricle, and the fourth ventricle.

CSF from the lateral ventricles passes through the interventricular foramina of Monro into the third ventricle. From there, CSF flows through the aqueduct of Sylvius into the fourth ventricle. The only sites where CSF can leave the ventricles and enter the subarachnoid space outside the CNS are through three openings in the fourth ventricle, two lateral foramina of Luschka and the median foramen of Magendie.

Within the subarachnoid space, CSF also flows up over the convexity of the brain and around the spinal cord. Almost all CSF returns to the venous system by draining through arachnoid granulations into the superior sagittal dural venous sinus.

Normal CSF is a clear fluid, isotonic with serum (290-295 mOsm/L).

The pH of CSF is 7.33 (arterial blood pH, 7.40; venous blood pH, 7.36).

Sodium ion (Na+) concentration is greater in serum and CSF (=138 mEq/L).

CSF has a higher concentration of chloride (Cl~) and magnesium (Mg2"1") ions than does serum.

CSF has a lower concentration of potassium (K+), calcium (Ca2+), and bicarbonate (HCCTJ) ions, as well as glucose, than does serum.

The concentration of protein (including all immunoglobulins) is much lower in the CSF as compared with serum.

Normal CSF contains 0 to 4 lymphocytes or mononuclear cells per cubic millimeter. Although the presence of a few monocytes or lymphocytes is normal, the presence of polymorphonuclear leukocytes is always abnormal, as in bacterial meningitis.

Red blood cells (RBCs) are not normally found in the CSF but may be present after traumatic spinal tap or subarachnoid hemorrhage.

Increased protein levels may indicate a CNS tumor.

Tumor cells may be present in the CSF in cases with meningeal involvement.

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Essentials of Human Physiology

Essentials of Human Physiology

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