|Venous drainage & CSF of Brain
Dural venous sinus
The cranial venous sinuses drain blood from the veins of the brain to the internal jugular vein. They are located between the inner and outer layers of the dura mater.
The major venous sinuses are :
(1) Superior Sagittal Sinus - between the falx cerebri and the midline of the skull. It drains into the confluence of the sinuses.
(2) Inferior Sagittal Sinus - located at the free edge of the falx cerebri, drains into the straight sinus.
(3) Straight Sinus - at the junction of the falx cerebri and the tentorium cerebelli, receives the great cerebral vein (of Galen) and the inferior sagittal sinus and drains into the confluence of the sinuses.
(4) Transverse Sinuses - between the tentorium cerebelli and its attachment to the occipital bone. Begins at the confluence of the sinuses and drains into the sigmoid sinus.
(5) Sigmoid Sinuses - in the posterior cranial fossa. Is a continuation of the transverse sinus, and drains into internal jugular vein.
(6) Cavernous Sinuses - on either side of the sella turcica. Receive blood from hypophysis, ophthalmic and facial veins and drain into petrosal sinuses. Internal carotid a. and abducens n. (CN VI) pass through this sinus and the oculomotor n. (CN III) and trochlear n. (CN IV), ophthalmic n. (CN V1) and maxillary n. (CN V2) pass along its lateral wall before exiting the cranial cavity.
(7) Superior Petrosal Sinuses - at the attachment of the tentorium cerebelli to the petrous temporal bone. Drain blood from the cavernous sinus into the commencement of the sigmoid sinus.
(8) Inferior Petrosal Sinuses - drain blood from cavernous sinus into the internal jugular vein.
Deep cerebral veins
The deep cerebral veins drain blood from the deep structures of the cerebral hemispheres.
The major deep cerebral veins include:
(i) the thalamostriate veins (paired), which run in the groove between the caudate nucleus and the thalamus and drain blood from both these structures,
(ii) the choroidal veins (paired), which drain blood from the choroid plexus of the lateral ventricles,
(iii) the septal veins (paired), which drain blood from the septum pellucidum,
(iv) the internal cerebral veins (paired), which are formed by the union of the thalamostriate and septal veins and also receives blood from the choroidal vein. They drain into the great cerebral vein (of Galen).
(v) the great cerebral vein (of Galen) which is formed by the union of the right and left internal cerebral veins. It also receives blood from the basal vein. It unites with the inferior sagittal sinus to form the straight sinus, which drains to the confluence of the sinuses.
(vi) the basal veins (paired) pass dorsally around the cerebral peduncles to enter the great cerebral vein. They drain the basal structures of the brain.
The choroid plexuses are the major sites of production of cerebrospinal fluid (CSF).
They are located within the ventricular system of the brain, in the inferior horn, trigone and body of the lateral ventricles and in the roofs of the third and fourth ventricles. In each case the choroid plexus lies adjacent to the subarachnoid space.
The choroid plexus are formed by invagination of vascular pia mater (tela choroidea) into the ventricles and consists of 3 layers:
(1) Outermost - the endothelium of a cerebral capillary, which is fenestrated (ie. contains small openings or pores).
(2) Middle - pia mater (connective tissue)
(Layers (1) and (2) together form the tela choroidea)
(3) Inner - ependymal cells which line the ventricle. These are columnar secretory cells which are joined by tight junctions.
Production of CSF by choroid plexus:
Cerebrospinal fluid is formed by selective filtration of substances from blood plasma through the fenestrated capillaries, followed by active transport of substances across the ependymal lining into the ventricle. Finally there is a passive movement of water to maintain the osmotic equilibrium between the blood plasma and cerebrospinal fluid.
Cerebrospinal Fluid (CSF) is a colourless fluid which is low in cells and proteins. It is similar in composition to blood plasma but has different concentrations of ions and glucose. CSF has higher concentrations of chloride and magnesium ions, lower concentrations of calcium and potassium ions and only about half the glucose level of blood.
CSF is located throughout the ventricles and subarachnoid space and has several functions:
(2) it may provide a sink for substances produced by the brain which would then be absorbed back into the venous system,
(3) it helps to maintain a constant extracellular environment.
The normal volume of CSF varies from 80 - 150 mls, of which 15 - 40 mls is located in the ventricles, the remainder is in the subarachnoid space.
Production and Circulation:
CSF is being constantly produced by the choroid plexus and ependymal lining of the ventricles at a rate of 400 - 500 mls per day, resulting in a complete turnover of CSF two to three times daily. The CSF is absorbed back into the venous system through specialisations known as arachnoid villi. It circulates from the lateral ventricles to the third and then the fourth ventricle. It escapes from the fourth ventricle into the subarachnoid space via the foramina of Magendie and Luschka (the median and lateral apertures of the fourth ventricle respectively). It circulates through the subarachnoid space (aided by rhythmic pulsations of arteries in the space) to the superior sagittal sinus (mainly), where the arachnoid villi are located.
The subarachnoid space is bounded by the pia mater (internally) and the arachnoid mater (externally) and surrounds the entire brain and spinal cord. It is filled with cerebrospinal fluid (CSF) and also contains the major arteries of the brain and spinal cord (eg. the circle of Willis).
The subarachnoid space communicates with the ventricular system through the foramen of Magendie (median aperture of the fourth ventricle) and the foramina of Luschka (lateral apertures of the fourth ventricle).
The pia and the arachnoid are connected across the subarachnoid space by strands known as arachnoid trabeculae. Areas where the subarachnoid space is enlarged are known as subarachnoid cisterns.
The third ventricle is a cavity which occupies the midline of the diencephalon. Like the rest of the ventricular system it is filled with cerebrospinal fluid.
The boundaries of the third ventricle are as follows:
Laterally - the thalamus and hypothalamus.
Roof - a sheet of choroid plexus which is suspended between the stria medullaris thalami of each side (stria medullaris thalami is a tract which runs along the medial surface of the thalamus).
Anteriorly - lamina terminalis (the adult remnant of the rostral end of the neural tube)
Floor - the hypothalamus
Posteriorly - it is continuous with the cerebral aqueduct.
The third ventricle communicates with each of the lateral ventricles via the interventricular foramen (also known as the foramen of Monro).
In approximately 70% of human brains the midline nuclei of the two thalami are fused in the midline, forming the massa intermedia, which crosses the third ventricle. The appears as a donut-like hole in the centre of the third ventricle in casts or diagrams of the ventricular system.