Keywords
ventricles, cerebral spinal fluid, choroid plexus, computerized tomography , magnetic resonance imaging, hydrocephalus
Chapter Outline
The Brain Contains Four Ventricles, 27
A Lateral Ventricle Curves Through Each Cerebral Hemisphere, 27
The Third Ventricle Is a Midline Cavity in the Diencephalon, 27
The Fourth Ventricle Communicates With Subarachnoid Cisterns, 27
The Ventricles Contain Only a Fraction of the CSF, 28
Choroid Plexus Is the Source of Most CSF, 28
CSF Is a Secretion of the Choroid Plexus, 28
CSF Circulates Through and Around the CNS, Eventually Reaching the Venous System, 28
CSF Has Multiple Functions, 29
Imaging Techniques Allow Both CNS and CSF to Be Visualized, 29
Tomography Produces Images of Two-Dimensional “Slices,” 29
CT Produces Maps of X-Ray Density, 29
Magnetic Resonance Imaging Produces Maps of Water Concentration, 29
Disruption of CSF Circulation Can Cause Hydrocephalus, 29
The ventricular system , the remnant of the space in the middle of the embryonic neural tube (see Fig. 2.5 ), is an interconnected series of cavities that extends through most of the central nervous system (CNS).
The Brain Contains Four Ventricles
There is a pair of lateral ventricles in the telencephalon (one for each cerebral hemisphere), a midline third ventricle in the diencephalon, and a fourth ventricle that straddles the midline in the pons and medulla. Cerebrospinal fluid ( CSF ) is secreted within the ventricles, fills them, and flows out of the fourth ventricle through three apertures to fill subarachnoid space.
A Lateral Ventricle Curves Through Each Cerebral Hemisphere
Each lateral ventricle is basically a C -shaped structure. This C shape curves from an inferior horn in the temporal lobe through a body in the parietal lobe and a bit of the frontal lobe, ending at the interventricular foramen where each lateral ventricle joins the third ventricle. Along this C -shaped course two extensions emerge—a posterior horn that extends backward into the occipital lobe and an anterior horn that extends farther into the frontal lobe ( Fig. 5.1 ). The expanded area where the body and the inferior and posterior horns meet is called the atrium . Each lateral ventricle represents the cavity of an embryonic telencephalic vesicle, so telencephalic structures like the caudate nucleus and the hippocampus border much of it; the thalamus, a diencephalic derivative, also forms part of its floor.
The Third Ventricle Is a Midline Cavity in the Diencephalon
The third ventricle is a midline slit in the diencephalon, with walls formed by the hypothalamus and much of the thalamus. The thalamus grows, pushing together in the middle of the third ventricle in most human brains, forming an interthalamic adhesion , so that the ventricle winds up looking like a misshapen doughnut with the doughnut hole being the area where the thalami have “bumped” together. The third ventricle connects with the fourth ventricle by a small cerebral aqueduct through the midbrain of the brainstem.
The Fourth Ventricle Communicates With Subarachnoid Cisterns
The fourth ventricle extends from the cerebral aqueduct to a mid-medullary level, where it narrows down into the vestigial (not actively utilized after birth) central canal of the caudal medulla and the spinal cord. It reaches its widest extent at the pontomedullary junction, where it extends into a lateral recess on each side. The tentlike roof of the fourth ventricle pokes up into the cerebellum.
The ventricular system communicates with subarachnoid space through three apertures of the fourth ventricle, two lateral apertures at the end of each lateral recess, and a median aperture above the point where the fourth ventricle narrows down into the central canal.
The Ventricles Contain Only a Fraction of the CSF
Although CSF is made in the ventricles, most of it is physically located in subarachnoid space (about 25 mL in the ventricles vs. 150 mL or so in subarachnoid space). CSF also distributes down around the outside of the spinal cord.
Choroid Plexus Is the Source of Most CSF
The ependymal lining of choroid plexus is specialized as a secretory epithelium.
A long strand of choroid plexus follows the C shape of each lateral ventricle, grows through the interventricular foramen, and joins the roof of the third ventricle. Separate strands of choroid plexus grow in the roof of the fourth ventricle, extending laterally through the lateral apertures and caudally to the median aperture.
CSF Is a Secretion of the Choroid Plexus
Capillaries in choroid plexus, unlike most other capillaries inside the arachnoid barrier, are permeable to plasma solutes. Plasma solutes therefore leak out, cross the pial layer, get stopped by the choroid epithelial diffusion barrier, and form the substrate for active secretion of CSF into the ventricles by the choroid epithelium. The resulting CSF is clear and colorless, low in protein, and similar (but not identical) to serum in its ionic composition.
CSF Circulates Through and Around the CNS, Eventually Reaching the Venous System
The CSF secreted by the choroid plexuses moves through the ventricular system ( Fig. 5.3 ), pushed along by newly formed CSF. It leaves the fourth ventricle through the lateral and median apertures, moves through subarachnoid space until it reaches the arachnoid villi (most of which protrude into the superior sagittal sinus), and finally joins the venous circulation ( Fig. 5.4 ).
