Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

13. Ventricular System and Cerebrospinal Fluid

13.1 Ventricular System, Overview

A Overview of the ventricular system and important neighboring structures

Left lateral view. The ventricular system is a greatly expanded and convoluted tube that represents an upward prolongation of the central spinal canal into the brain. There are four cerebral ventricles, or cavities, filled with cerebrospinal fluid and lined by a specialized epithelium, the ependyma (see D, p. 197). The four ventricles are as follows:

 The two lateral ventricles, each of which communicates through an interventricular foramen with the

 third ventricle, which in turn communicates through the cerebral aqueduct with the

 fourth ventricle. This ventricle communicates with the subarachnoid space (see B).

The largestventriclesarethe lateral ventricles, each of which consists of an anterior, inferior, and posterior horn and a central part. Certain portions of the ventricular system can be assigned to specific parts of the brain: the anterior (frontal) horn to the frontal lobe of the cerebrum, the inferior (temporal) horn to the temporal lobe, the posterior (occipital) horn to the occipital lobe, the third ventricle to the diencephalon, the aqueduct to the midbrain (mesencephalon), and the fourth ventricle to the hindbrain (rhombencephalon). The anatomical relationships of the ventricular system can also be appreciated in coronal and transverse sections (see pp. 292ff and 304ff).

Cerebrospinal fluid is formed mainly by the choroid plexus, a network of vessels that is present to some degree in each of the four ventricles (see p. 195). Another site of cerebrospinal fluid production is the ependyma. Certain diseases (e.g., atrophy of brain tissue in Alzheimer's disease and internal hydrocephalus) are characterized by abnormal enlargement of the ventricular system and are diagnosed from the size of the ventricles in sectional images of the brain.

This unit deals with the ventricular system and neighboring structures. The next unit will trace the path of the cerebrospinal fluid from its production to its reabsorption. The last unit on the cerebrospinal fluid spaces will deal with the specialized functions of the ependyma, the circumventricular organs, and the physiological tissue barriers in the brain.

В Cast of the ventricular system

Left lateral view (a) and superior view (b). Cast specimens are used to demonstrate the connections between the ventricular cavities. Each lateral ventricle communicates with the third ventricle through an interventricular foramen. The third ventricle communicates through the cerebral aqueduct with the fourth ventricle in the rhombencephalon. The ventricular system has a fluid capacity of approximately 30 ml, while the subarachnoid space has a capacity of approximately 120 ml.

Note the three apertures (paired lateral apertures [foramina of Luschka] and an unpaired median aperture [foramen of Magendie]), through which cerebrospinal fluid flows from the deeper ventricular system into the more superficial subarachnoid space.

C Important structures neighboring the lateral ventricles

a View of the brain from upper left, b View of the inferior horn of the left lateral ventricle in the opened temporal lobe.

a The following brain structures border on the lateral ventricles:

• The caudate nucleus (anterolateral wall of the anterior horn)

• The thalamus (posterolateral wall of the anterior horn)

• The putamen, which is lateral to the lateral ventricle and does not border it directly.

b The hippocampus (see p. 206) is visible in the anterior part of the floor of the inferior horn. Its anterior portions with the hippocampal digitations protrude into the ventricular cavity.

D Lateral wall of the third ventricle

Midsagittal section, left lateral view. The lateral wall of the third ventricle is formed by structures of the diencephalon (epithalamus, thalamus, hypothalamus). Protrusions of the thalami on both sides may touch each other (interthalamic adhesion) but are not functionally or anatomically connected and thus do not constitute a commissural tract.

13.2 Cerebrospinal Fluid, Circulation and Cisterns

A Cerebrospinal fluid circulation and the cisterns

Cerebrospinal fluid (CSF) is produced in the choroid plexus, which is present to some extent in each of the four cerebral ventricles. It flows through the median aperture and paired lateral apertures (not shown; see p. 192 for location) into the subarachnoid space, which contains expansions called cisterns. Most of the CSF drains from the subarachnoid space through the arachnoid granulations, and smaller amounts drain along the proximal portions of the spinal nerves into venous plexuses or lymphatic pathways (see F). The cerebral ventricles and subarachnoid space have a combined capacity of approximately 150 ml of CSF (20 % in the ventricles and 80% in the subarachnoid space). This volume is completely replaced two to four times daily, so that approximately 500 ml of CSF must be produced each day. Obstruction of CSF drainage will therefore cause a rise in intracranial pressure (see E, p. 197).

В Choroid plexus in the lateral ventricles

Rear view of the thalamus. Surrounding brain tissue has been removed down to the floor of the lateral ventricles, where the choroid plexus originates. The plexus is adherent to the ventricular wall at only one site (see D) and can thus float freely in the ventricular system.

C Choroid plexus in the fourth ventricle

Posterior view of the partially opened rhomboid fossa (with the cerebellum removed). Portions of the choroid plexus are attached to the roof of the fourth ventricle and run along the lateral aperture. Free ends of the choroid plexus may extend through the lateral apertures into the subarachnoid space on both sides (“Bochdalek’s flower basket”).

D Taeniae of the choroid plexus

Superior view of the ventricular system. The choroid plexus is formed by the ingrowth of vascular loops into the ependyma, which firmly attach it to the wall of the associated ventricle (see F). When the plexus tissue is removed with a forceps, its lines of attachment, called taeniae, can be seen.

E Histological section of the choroid plexus, with a detail showing the structure of the plexus epithelium (after Kahle)

The choroid plexus is a protrusion of the ventricular wall. It is often likened to a cauliflower because of its extensive surface folds. The epithelium of the choroid plexus consists of a single layer of cuboidal cells and has a brush border on its apical surface (to increase the surface area further).

F Schematic diagram of cerebrospinal fluid circulation

As noted earlier, the choroid plexus is present to some extent in each of the four cerebral ventricles. It produces CSF, which flows through the two lateral apertures (not shown) and median aperture into the subarachnoid space. From there, most of the CSF drains through the arachnoid granulations into the dural venous sinuses.

G Subarachnoid cisterns (after Rauber and Kopsch)

Basal view. The cisterns are CSF-filled expansions of the subarachnoid space. They contain the proximal portions of some cranial nerves and basal cerebral arteries (veins are not shown). When arterial bleeding occurs (as from a ruptured aneurysm), blood will slep into the subarachnoid space and enter the CSF. A ruptured intracranial aneurysm is a frequent cause of blood in the CSF (methods of sampling the CSF are described on p. 197).

13.3 Circumventricular Organs and Tissue Barriers in the Brain

A Location of the circumventricular organs

Midsagittal section, left lateral view. The circumventricular organs include the following:

 Posterior pituitary with the neurohemal region (see p.222)

• Choroid plexus (see p. 195)

 Pineal body (see p. 224)

• Vascular organ of the lamina terminalis, subfornical organ, subcommissural organ, and area postrema (see B).

The circumventricular or ependymal organs all have several features in common. They are composed of modified ependyma, they usually border on the ventricular and subarachnoid CSF spaces, and they are located in the median plane (except the choroid plexus, though it does develop from an unpaired primordium in the median plane). The blood- brain barrier is usually absent in these organs (see C and D; except the subcommissural organ).

В Summary of the smaller circumventricular organs

In addition to the four regions listed below, the circumventricular organs include the posterior pituitary, choroid plexus, and pineal body. The functional descriptions are based largely on experimental studies in animals.

Organ

Location

Function

Vascular organ of the lamina terminalis (VOLT)

Vascular loops in the rostral wall of the third ventricle (lamina terminalis); rudimentary in humans

Secretes the regulatory hormones somatostatin, luliberin, and motilin; contains cells sensitive to angiotensin II; is a neuroendocrine mediator

Subfornical organ(SFO)

Fenestrated capillaries between the interventricular foramina and below thefornices

Secretes somatostatin and luliberin from nerve endings; contains cells sensitive to angiotensin II; plays a central role in the regulation of fluid balance ("organ of thirst”)

Subcommissural organ (SCO)

Borders on the pineal body; overlies the epithalamic commissure at the junction of the third ventricle and cerebral aqueduct

Secretes glycoproteins into the aqueduct that condense to form the Reiss- ner fiber, which may extend into the central canal of the spinal cord; blood-brain barrier is intact; function is not completely understood

Area

postrema

(AP)

Paired organs in the floor of the caudal end of the rhomboid fossa, richly vascularized

Trigger zone for the emetic reflex (absence of the blood-brain barrier); atrophies in humans after middle age

C Demonstration of tissue barriers in the brain (after Kahle) a Blood-brain barrier, b blood-CSF barrier. The upper drawings show an inferior view of a transverse section through a rabbit brain, and the lower drawings show the brainstem from the basal aspect. The function of these barriers is to protect the brain from harmful substances in the bloodstream. These include macromolecular as well as small molecular pharmaceutical compounds.

a Demonstration of the blood-brain barrier: The intravenous injection of trypan blue dye (first Coldmann test) stains almost all organs blue except the brain and spinal cord. Even the dura and choroid plexus show heavy blue staining. Faint blue staining is noted in the tuber cinereum (neurohemal region of the posterior pituitary), area postrema, and spinal ganglia (absence of the blood-brain barrier in these regions). The same pattern of color distribution occurs naturally in jaundice, where bile pigment stains all organs but the brain and spinal cord, analogous to trypan blue in the first Coldmann test, b Demonstration of the blood-CSF barrier: When the dye is injected into the CSF (second Coldmann test), the brain and spinal cord (CNS) show diffuse superficial staining while the rest of the body remains unstained. This shows that a barrier exists between the CSF and blood, but not between the CSF and the CNS.

D Blood-brain barrier and blood-CSF barrier

a Normal brain tissue with an intact blood-brain barrier; b Blood-CSF barrier in the choroid plexus.

a The blood-brain barrier in normal brain tissue consists mainly of the tight junctions between capillary endothelial cells. It prevents the paracellular diffusion of hydrophilic substances from CNS capillaries into surrounding tissues and in the opposite direction as well. Essential hydrophilic substances that are needed by CNS must be channeled through the barrier with the aid of specific transport mechanisms (e.g., glucose by an insulin-dependent transporter), b The blood-brain barrier is absent at fenestrated capillary endothelial cells in the choroid plexus and other circumventricular organs (see A), which allow substances to pass freely from the bloodstream into the brain tissue and vice versa. Tight junctions in the overlying ependyma (choroid plexus epithelium) do, however, create a two-way barrier between the brain tissue and ventricular CSF in these regions. In other words, the diffusion barrier is shifted from the vascular endothelium to the cells of the ependyma and choroid plexus.

E Obtaining cerebrospinal fluid samples

a Lumbar puncture: This is the method of choice for sampling the CSF. A needle is inserted precisely in the midline between the spinous processes of L3 and L4 and is advanced into the dural sac (lumbar cistern). At this time a fluid sample can be drawn and the CSF pressure can be measured for diagnostic purposes by connecting a manometer to the needle. Lumbar puncture is contraindicated if the intracranial pressure is markedly increased, as it may cause a precipitous cranial to spinal pressure gradient, causing the brainstem to herniate through the foramen magnum. This would exert pressure on vitally important centers in the medulla oblongata, with a potentially fatal outcome. Thus, the physician should always check for signs of increased intracranial pressure (e.g., papilledema, see p.133) before performing a lumbar puncture.

b Suboccipital puncture: This technique should be used only in exceptional cases where a lumbar puncture is contraindicated (e.g., by a spinal cord tumor), because it may, rarely, produce a fatal complication. The mortality risk results from the need to passa needle through the cerebellomedullary cistern (cisterna magna), which may endangervital centers in the medulla oblongata.

F Comparison of cerebrospinal fluid and blood serum

Infection of the brain and its coverings (meningitis), subarachnoid hemorrhage, and tumor métastasés can all be diagnosed by CSF examination. As the table indicates, CSF is more than a simple ultrafiltrate of blood serum. Its primary function is to impart buoyancy of the brain (the brain has an effective weight of only about 50 g despite a mass of 1300 g). Decreased CNS production therefore increases pressure on the spine and also renders the brain more susceptible to injury (less cushioning).

 

CSF

Serum

Pressure

50-180mm H20

 

Volume

100-160mL

 

Osmolarity

292-297 mOsm/L

285-295 mOsm/L

Electrolytes

Sodium

137~145mM

136-145 mM

Potassium

2.7-3.9 mM

3.5-5.0mM

Calcium

1-1.5 mM

2.2~2.6mM

Chloride

116-122 mM

98-106mM

pH

7.31-7.34

7.38-7.44

Glucose

2.2-3.9 mM

4.2-6.4mM

CSF/serum glucose ratio

> 0.5-0.6

 

Lactate

1-2mM

0.6-1.7 mM

Total protein

0.2-0.5g/L

55-80 g/L

Albumin

56-75%

50-60%

IgC

0.01-0.014g/L

8—15 g/L

Leukocvtes

< 4 cells/ці

 

Lymphocytes

60-70%