9.3 Cerebrospinal fluid The cerebrospinal fluid (CSF, cerebrospinal liquor) is a clear, colourless fluid produced by an active secretion by the cells of the choroid plexus. Its composition differs from the composition of plasma by a lower content of proteins (15–40 mg%) and glucose (2.5–3.9 mmol/L, 45–60 mg%), higher content of chloride (113 mmol/L) and, under normal circumstances, it contains only a few lymphocytes (0–3/ml). The CSF circulates from the lateral ventricles through the interventricular foramina into the 3rd ventricle and thence via the mesencephalic aqueduct into the 4th ventricle. A small amount of CSF continues into the central canal of the spinal cord, but most of the CSF passes through the foramen of Magendie and foramina of Luschka into the subarachnoid space (see Chapter 10). The CSF is produced continuously at a rate of about 0.4 ml/min and is simultaneously reabsorbed partially into the superficial veins of the brain, but mainly into the sinuses of dura mater, especially into the superior sagittal sinus. The arachnoidea forms numerous arachnoid villi which invaginate through the sinus wall into the lumen of the sinus. At these sites reabsorption occurs because of the higher hydrostatic pressure in the subarachnoid space and greater osmotic pressure of the venous blood. The total production of CSF is about 500 ml/24 hours. The total volume of CSF in the CNS is about 150 ml. The ventricles contain about 25 ml of CSF, the intracranial subarachnoid space about 100 ml, some 25 ml are in the spinal subarachnoid space. The pressure of CSF, as measured at the lumbar puncture in a sitting patient, is normally 70–180 mm H20. The functions of CSF are multiple. It provides neutral buoyancy for the brain, which floats in it, and protects it from injury when the head is jolted or hit, serving as a cushion. It participates in chemical stability by rinsing the metabolic waste from the CNS through the blood-brain barrier and allows for homeostatic distribution of neuroendocrine factors. It also plays an important role in prevention of brain ischemia by decreasing the amount of CSF in the limited space inside the skull in case of intracranial hypertension. Removal of a larger volume of CSF by a lumbar puncture produces severe headaches because of the loss of buoyancy and the ensuing pull on the nerve roots. When the circulation of CSF is impaired by an obstruction of the interventricular foramina, mesencephalic aqueduct or 4th ventricle apertures, the ventricles dilate and the brain tissue is compressed and atrophies. This is internal hydrocephalus. In small children with unclosed sutures of the cranial bones the whole head is enlarged. Decompression of the dilated ventricles can be achieved by a shunt connecting the ventricles to the internal jugular vein or to the intraperitoneal cavity. Impairment of the reabsorption from the subarachnoid space (e.g. due to adhesions following meningitis) leads to external hydrocephalus.
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10/ Coverings of the CNS The coverings of the CNS, the meninges, are divided into the pachymeninx (thick membrane) or dura mater, which is the outermost membrane, and the leptomeninges (thin membranes), which form two layers, the arachnoid mater and the pia mater (Fig. 10.1). 3
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Fig. 10.1 Meninges, cerebral vessels and subarachnoideal space. Coronal section through the superior sagital sinus 1 – emissary vein, 2 – cranial dura mater, 3 – superior sagittal sinus, 4 – arachnoid granulations, 5 – subdural space, 6 – superior cerebral vein, 7 – pia mater, 8 – cortex, 9 – falx cerebri, 10 – subarachnoid space, 11 – arachnoid, 12 – cerebral artery
10.1 Dura mater The dura mater is a rather strong fibrous membrane which ensheathes both the brain and the spinal cord.
10.1.1 Cranial dura mater The cranial dura mater is at the base of the skull tightly adherent to the periosteum; in the frontoparietal area is a narrow extradural (epidural) space. The dura mater forms reflections and folds. 99 /
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A large reflection, the falx cerebri, extends from the crista galli to the internal occipital protuberance and reaches into the longitudinal median fissure, between the two hemispheres. It continues in the posterior cerebral fossa between the cerebellar hemispheres as the smaller falx cerebelli. Between the sulcus transversus and the upper margin of the petrous bone is spread the tentorium cerebelli separating the occipital cerebral lobes from the cerebellum. In the tentorium is an opening for passage of the brain stem, the incisura tentorii, tentorial notch. A horizontal sheet of dura mater above the hypohysis forms the diaphragma sellae. The semilunar ganglion of the trigeminal nerve is enclosed in a pocket of dura mater, the trigeminal cavity (of Meckel). The cranial dura mater extends into the orbit as the external sheath of the optic nerve. In some locations the layers of the dura mater separate to enclose the dural venous sinuses (Fig. 10.2). The walls of the dural sinuses are formed by the endothelium and the dura mater, they have no other layers and contain no valves. These sinuses are the main drainage system for collection of blood from the brain (see Chapter 11.1.2).
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Fig. 10.2 Dura mater duplications and dural sinuses. Lateral view 1 – superior sagittal sinus, 2 – falx cerebri, 3 – superior cerebral veins, 4 – inferior sagittal sinus, 5 – great cerebral vein, 6 – tentorium cerebelli (partially cut), 7 – transverse sinus, 8 – sphenoid sinus, 9 – internal acoustic meatus and CN VII, VIII, 10 – jugular foramen, CN IX, X, XI (internal jugular vein), 11 – hypoglossal nerve, 12 – radix spinalis of CN XI, 13 – sigmoid sinus, 14 – occipital sinus, 15 – falx cerebelli, 16 – confluens of sinuses, 17 – straight sinus
The vessels and nerves run on the surface of the dura mater. The arteries, accompanied by small veins, come from several sources and send branches both to the dura mater and cranial bones. The frontal region is supplied by the anterior meningeal branch of the anterior ethmoidal artery and the recurrent meningeal branch of the lacrimal artery. The largest area, the parietal, temporal, and partly frontal and 100 /
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