ABSTRACT
The cerebral hemispheres occupy the interior of the skull, the cranial cavity� The brain in this photograph is seen mainly from above and from the side-one hemisphere has the meninges removed and the other is still covered with meninges�
The meninges, the connective tissue coverings of the brain, consist of three layers-the dura, the arachnoid, and the pia-with spaces or potential spaces between the layers (see Figure 7�2)�
The outermost layer of the meninges is the dura, a thick strong sheet of connective tissue� Within the cranial cavity, the dura and periosteum of the skull bones adhere to one another� The dural layer has additional folds within the skull that subdivide the cranial cavity and serve to keep the brain in place� The two major dural sheaths are:
• The falx cerebri in the sagittal plane, between the hemispheres (see Figure 7�4 and Figure 7�5; the falx has been removed from the interhemispheric fissure in Figure 2�2A)�
• The tentorium cerebelli in the transverse plane, between the occipital lobe and the cerebellum (see Figure 7�5 and Figure 7�6; the cut edge of the tentorium can be seen in the hemisected brain as in Figure 1�7)�
There is an opening in the tentorium for the brainstem, called the tentorial notch or incisura, located at the level of the upper midbrain (see Figure 7�5)� The falx and tentorium, because of their attachment to the skull
particularly at the sutures, do help to stabilize the brain hemispheres within the cranial cavity� These dural sheaths are further discussed with the venous sinuses (see Figure 7�4, Figure 7�5, and Figure 7�6)�
In this illustration, the falx cerebri is still in place within the interhemispheric fissure (see also Figure 2�2A)� At the attachment of the falx to the skull, there is a large venous sinus-the superior sagittal sinus (see Figure 7�2 and Figure 7�4)�
The subarachnoid space, between the arachnoid and pia, is filled with cerebrospinal fluid (CSF) (see Figure 7�2)� Therefore, the brain is actually “floating” inside the skull� Both the meninges and the CSF offer a certain measure of protection to the underlying brain tissue beyond that of the skull itself�
Any space-occupying lesion (e�g�, sudden hemorrhage, slow-growing tumor) sooner-or-later causes a displacement of brain tissue from one compartment to another within the skull� This is called a brain herniation syndrome, and it typically occurs:
• Under the falx cerebri itself-subfalcine herniation (see Figure 7�4)�
• Through the tentorial notch-uncal herniation (discussed with Figure 1�6)�
• Through the foramen magnum-tonsillar herniation (discussed with Figure 3�2 and in the introduction to this section)�
This pathological displacement itself causes damage to the brain�
These shifts are life-threatening and require emergency management�
Note to the Learner: This would be an opportune time to review the signs and symptoms associated with these clinical emergencies, such as testing of the pupillary light reflex and the pathways involved�
Dura
Superior sagittal sinus (opened)
Interhemispheric ssure
Parieto-occipital ssure
Central ssure
F = Frontal lobe P = Parietal lobe O = Occipital lobe
F P
O
The major layers of the scalp are shown in these illustrations (and labeled in the upper illustration), notably the skin (with hair), and the aponeurosis (the flattened tendon that connects the bellies of the frontalis and occipitalis muscles over the top of the cranium)� (This illustration has been modified from that found in the Integrated text�)
The skull itself has outer and inner layers, called tables, with the middle layer where there is bone marrow for the formation of the blood cells (see Figure 2�9B and Figure 3�2)�
The dura is composed of 2 layers-an outer periosteal layer which is firmly attached to the bony periosteum, and an inner meningeal layer� There is a potential space on the inner surface of the skull between the bone and the outer (periosteal) layer of the dura, the epidural space� This is where the artery that supplies most of the dura is located, the middle meningeal artery (from the external carotid artery, shown in the lower illustration)� This artery typically produces grooves in the temporal bone on the interior of the skull (see the video in the atlas Web site [www�atlasbrain�com])�
The two dural sheaths that separate the parts of the brain from each other and hold the cerebral hemispheres in place, the falx (shown in the lower illustration) and the tentorium (not seen in these illustrations), attach to the bone (see Figure 7�1; described with the venous sinuses in Figure 7�4, Figure 7�5, and Figure 7�6)� At the upper edge of the falx and at the lateral margins of the tentorium, where these dural sheaths are attached, the two dural layers split and form-within the dura-the cerebral venous sinuses� Therefore, we have the superior sagittal sinus at the upper edge of the falx (as shown in the lower illustration; also see Figure 7�1) and the transverse sinuses at the margins of the tentorium cerebelli (further discussed with the venous circulation; see Figure 7�4, Figure 7�5, and Figure 7�6)�
The next layer is the arachnoid� There is a potential space between the dura and arachnoid, and the cerebral veins pass through this “space�” These bridging veins, as they are called (shown in the lower illustration), course from the surface of the brain into the venous sinuses, particularly the superior sagittal sinus� There is a potential space between the dura and arachnoid, called the subdural space, where bleeding may occur, usually from venous sources (see later)�
The innermost layer, the pia, lies on the surface of the brain and follows all its folds� The subarachnoid space, between the arachnoid and the pia, contains the cerebrospinal fluid (CSF)� Large arteries and veins are also found in this space� The arachnoid granulations (shown in the lower illustration) convey the CSF from the subarachnoid space back to the venous circulation (discussed with Figure 7�8)�
Bleeding inside the skull occurs in somewhat typical situations and in predictable locations�
A forcible traumatic injury to the side of the head may lead to a fracture of the temporal bone and disruption of the middle meningeal artery� This causes the typical syndrome associated with an epidural hemorrhage� This life-threatening arterial bleeding often follows a typical course: trauma to the side of the head with or without a loss of consciousness, then a lucid period, and then rapid neurological deterioration that, if not recognized soon enough and managed appropriately, usually leads to brain herniation (discussed in the introduction to this section and with Figure 7�1) and possibly death�
The bridging veins may be disrupted by trauma to the head, in which case blood leaks into the potential space between the dura and arachnoid, to produce a subdural hemorrhage� Subdural (venous) hemorrhages usually occur slowly over time (subacute or chronic), but they may also manifest acutely� Typically, they occur more commonly in very young and very old people� Apparently, elderly people are more vulnerable to this type of bleeding because of the extra space available for the brain to “move about” as a result of age-related cerebral shrinkage� Any type of head trauma, even the most mild (e�g�, bumping your head getting into or out of a car), can cause these bridging veins to be disrupted (“sheared” or torn) as they pass through the potential subdural space� One should be aware of this possibility in an older person who has a sudden change of behavior (with or without the complaint of a headache)� Once recognized and assessed with neuroimaging, this condition may be easily treatable and, if caught soon enough, can resolve without sequelae� CT scanning can be very useful in determining the age of a bleed by virtue of the relative density of the hemorrhage� The more recent the hemorrhage, the higher the density of hemorrhage on CT scan (Hounsfield Units)�
Bleeding may occur into the CSF as a result of the bursting of a large blood vessel coursing within this space� This is called a subarachnoid hemorrhage� Typically, this occurs because of an aneurysm (known as a berry aneurysm), which is a weakening of the blood vessel wall, most often involving the arteries of the circle of Willis (discussed with Figure 8�2)� Meningeal irritation causes pain, and the person complains of an ultra-severe headache; because this is arterial bleeding, loss of consciousness may occur very quickly�
Bleeding also occurs within brain itself, a brain hemorrhage, destroying brain tissue� The clinical picture depends on the size and location of the bleed� It is estimated that about 15% of cerebrovascular “accidents” (CVA) are due to hemorrhage� CT scanning is mandatory for determining whether a bleed has occurred�
As discussed in the introduction to this section, any increase of volume inside the skull leads to an increase in intracranial pressure (ICP) and the possibility of a brain herniation syndrome� Note again the necessity of examining the optic discs for papilledema�
The meninges continue around the spinal cord within the vertebral canal� Three views of the spinal meninges are shown:
The vertebral canal and spinal cord are shown as in Figure 1�10 (also Figure 1�11)� The meningeal layers are illustrated with color coding (as in the previous illustration)—pia, arachnoid, and dura-with the cerebrospinal fluid (CSF colored blue), in the subarachnoid space� The CSF continues within the subarachnoid space around the spinal cord� The spinal cord with its pia ends at the vertebral level of L2-where the spinal cord ends in the adult, whereas the dura and arachnoid continue and end at the level of S2� This differential ending results in a large CSF space, known as a cistern, in the vertebral canal below the level of the spinal cord-called the lumbar cistern (see Figure 1�2, Figure 1�10, and Figure 1�11)� This site is used for sampling of CSF, called a spinal tap or lumbar puncture (see Figure 7�8 and discussed later)�
Note that the spinal cord dura is separated from the periosteum of the vertebra (and the intervertebral discs) by a space that is filled with fat in the lower vertebral region and that contains a plexus of veins� A strong ligament between the spinous processes, the ligamentum flavum, is also located in this region; this is of some clinical importance (see below)�
The schematic of the exiting nerves (also shown in Figure 1�12) shows the detail of the relationship of the dorsal root ganglion (DRG) and nerves with the vertebra and the intervertebral disc in the lumbar region�
This is a view of the three meningeal layers depicted in an axial view (this illustration has been slightly modified from The Integrated Text)� The top part of the illustration shows the pia surrounding the spinal cord, then the arachnoid is added with CSF in the subarachnoid space, and finally the dura�
Note particularly the exiting nerve roots ventrally (motor, see Figure 8�7) and the incoming (sensory) root with its DRG� A sleeve of arachnoid and dura, with CSF, accompanies the ventral and dorsal roots of the spinal nerves, until they come together within the intervertebral (neural) foraminal region to form the mixed spinal nerve�
Sampling of CSF for the diagnosis of meningitis, for inflammation of the meninges, or for other neurological diseases is done in the lumbar cistern� This procedure is called a lumbar puncture (also known as an LP, or spinal tap), and it must be performed using sterile technique�
The patient is positioned on her or his side, in the socalled fetal position, and the area of the lower back is cleansed� After appropriate local anesthesia, a trochar (which is a large needle with a smaller needle inside) is inserted below the termination of the spinal cord at the L2 vertebral level), in the space between the vertebra, usually between the vertebra L4 to L5� The trochar must pierce the very tough ligamentum flavum (shown in this illustration), then the dura-arachnoid, and then it “suddenly” enters the lumbar cistern� At this point the inner needle is withdrawn, and CSF drips out to be collected in sterile vials�
A pressure apparatus known as an open manometer is also used to measure the CSF pressure (also discussed in the Introduction to this section), normally 7 to 19 cm of water or 5 to 14 mm of mercury (Hg)� (The CSF is fully discussed with Figure 7�8�)
This is not a pleasant procedure for a patient and is especially unpleasant, if not frightening, for children� It is not uncommon to sedate children if it is necessary to perform a spinal tap on them�
At this point it is timely to consider the venous system inside the skull, as the venous sinuses are associated with the meninges� The venous return from the brain courses via a superficial system over the surface of the hemispheres and a deep system within the substance of the brain tissue� Both systems drain into the venous sinuses, which are formed within the dura at its various attachment points inside the skull�
The midline falx cerebri lies in the sagittal plane between the hemispheres (see Figure 2�2A)� It is attached anteriorly to a spike of bone, called the crista galli, that protrudes from the nose into the anterior cranial fossa; the bone is the ethmoid bone (shown in the video of the skull on the atlas Web site [www�atlasbrain�com])� After arching over the corpus callosum, the falx cerebri splays out posteriorly-at a 90-degreeangle, to form the tentorium cerebelli, above the cerebellum (see Figure 7�5 and Figure 7�6)�
Note to the Learner: In the discussion of brain herniation syndromes (with Figure 7�1), one of the syndromes is named subfalcine, which means under the falx cerebri�
The tentorium cerebelli lies between the occipital lobe and the superior surface of the cerebellum� The tentorium cerebelli attaches at the lateral margins of the posterior cranial fossa and forms below (inferior to) it the posterior cranial fossa, containing the cerebellum and brainstem (see also Figure 3�2)�
The venous sinuses are then the venous channels within the dura formed along the attachments of the dura to the bones of the skull; the dura splits to form these large venous spaces (review Figure 7�2)� A major venous sinus is the superior sagittal sinus, which is found along the upper (attached) border of the falx cerebri, in the midline (see Figure 7�1 and also Figure 7�8)� Most of the superficial veins of the hemispheres empty into the superior sagittal sinus� This sinus continues posteriorly, and at the back of the interior of the skull it divides to become the laterally placed transverse venous sinuses, one on each side, attached to the skull at the lateral edges of the tentorium
cerebelli� As will be explained, venous blood exits the skull via the sigmoid sinuses, which continue as the internal jugular veins on each side of the neck�
An exception to the rule of the location of the venous sinuses is the inferior sagittal sinus, located in the inferior margin of the falx cerebri, where there is no attachment to bone� This sinus drains blood from the medial surface of the brain and is joined by other veins that collect blood from the interior of the brain (see Figure 7�5 and Figure 7�6)�
The straight sinus is also not located at a bony margin; it is found within the dura itself as the falx “flattens out” to become the tentorium cerebelli (see Figure 7�5 and Figure 7�6)� This sinus drains the inferior sagittal sinus and joins with the superior sagittal sinus posteriorly�
This illustration also shows the pituitary fossa (the sella turcica) and the pituitary gland� The basilar artery (to be described subsequently in this section) is seen anterior to the brainstem; other cerebral arteries (not labeled) are also seen�
Note to the Learner: The Magnetic Resonance Venogram (MRV) of a similar view is shown in the upper illustration of Figure 7�7�
Blood flow within the cerebral venous sinuses is low flow, sluggish (as in most veins)� Under various clinical conditions, including hyper-coagulability of the blood or marked dehydration, there can be clotting of the blood in the venous sinuses-venous sinus thrombosis, most often in the superior sagittal sinus or transverse sinus� This leads to a back-up of blood and resulting back pressure in the area of drainage�
Should this occur in the deep venous system, the clinical consequence will be a venous infarction or hemorrhagic infarction, whereby the brain tissue is compromised because of the lack of venous drainage�
The space behind the thalamic area (the diencephalon) is a cistern of the subarachnoid space, outside the brain, not the 3rd ventricle� It is located posterior to the pineal gland and the colliculi and in front of the cerebellum (see also Figure 1�7), hence its name-the quadrigeminal cistern; the four colliculi are also called the quadrigeminal plate (see Figure 3�3 and Figure 7�8)� This space also contains some important cerebral veins that drain the interior of the brain including the great cerebral vein of Galen (these have been removed from this specimen)�
This is an illustration of the dural sheathes-the falx and tentorium-with the brain removed� Again, the superior and inferior sagittal sinuses can be seen�
The system of veins that drain the deep structures of the brain emerges medially as the internal cerebral veins, one from each hemisphere� These veins join in the midline in the region behind the diencephalon to form the great cerebral vein (of Galen)�
At this point there is another exception to the rule of the formation of venous sinuses� A sinus is located in the midline where the falx splays out laterally to form the tentorium� This is known as the straight sinus� The location of the straight sinus can also be appreciated in a mid-sagittal view of the brain (see Figure 1�7 and Figure 3�2; also Figure 7�8)� The great cerebral vein becomes continuous with the straight sinus, in the midline lying above the
cerebellum (see Figure 7�6; see also the video in the atlas Web site [www�atlasbrain�com])� At this point it is joined by the inferior sagittal sinus�
At the back of skull, the straight sinus joins with the superior sagittal sinus� The venous sinuses now divide, and the blood flows into the transverse sinuses, which are seen in Figure 7�6� The venous blood exits the skull via the sigmoid sinus into the internal jugular vein (on both sides, see Figure 7�7)�
Note to the Learner: The Magnetic Resonance Venogram (MRV) of a comparable view is shown in the middle illustration of Figure 7�7�
Additional Detail: This oblique view of the meninges, with the brain removed and the brainstem cut at the level of the midbrain, shows the free edge of the tentorium cerebelli (on one side)� The “space” created by the dural reflections of the tentorium is called the tentorial notch or “incisura,” and it exists for the passage of the brainstem (discussed as part of Brain Herniation syndromes, known as uncal herniation, with Figure 1�6 and Figure 7�1)�
This is an illustration of the tentorium cerebelli, seen from above in the horizontal (axial) plane� The hemispheres have been removed, and the cerebellum is still present, below the tentorium, in the posterior cranial fossa�
The straight sinus is shown in the midline, where the falx and tentorium are continuous�
The dura to the right side of the straight sinus is created by the falx cerebri (as labeled), which has been rolled up and folded down and held in place with forceps, to produce this specimen�
The sagittal and straight venous sinuses have united (see Figure 7�5) and now divide, sometimes unequally, to form the transverse sinuses; these are located where the
tentorium is attached to the bone along its lateral margins� The blood then flows (in a forward direction) until it reaches the anterior edge of the posterior cranial fossa (reminder to see the videos of Cerebrospinal Fluid and The Skull on the atlas Web site [www�atlasbrain�com])� At this point the sinuses leave the tentorium and enter the posterior cranial fossa as the sigmoid sinuses (because of their “S” shape), thus making a prominent groove in the skull interior� The venous blood then exits the skull via the jugular foramen (one on each side) to form the internal jugular vein�
Note to the Learner: The Magnetic Resonance Venogram (MRV) shown in the lower illustration of Figure 7�7 includes some of the venous sinuses seen in this illustration�
This dissection includes an unusual view of the hippocampal formation-to be further discussed with Figure 9�4�
A magnetic resonance venogram (MRV), like a magnetic resonance angiogram (MRA-see Figure 8�2) can be done both with and without contrast (gadolinium-based imaging)� In this case, no contrast was used� The sequences are called time-of-flight (TOF), either angiogram or venogram�
The three images correspond to the views shown in the illustrations of the meninges with the venous sinuses (see Figure 7�4, Figure 7�5, and Figure 7�6)�
This series depicts the cerebral veins as though the patient is turning their head-from lateral (upper illustration) to oblique (middle illustration) to antero-posterior (lower illustration)�
The upper figure, the lateral view, (see Figure 7�4), captures the superior and inferior sagittal sinuses in the upper and lower aspects of the falx cerebri� Some of the collector veins from the cerebral cortex are also seen� The straight sinus is also seen, as well as the transverse and
sigmoid sinuses, with the blood flowing into the internal jugular vein in the neck�
The middle image is an oblique view, capturing again the superior and inferior sagittal sinuses� Some of the veins draining the interior of the brain are also seen forming the great cerebral vein (of Galen)� Again, the transverse sinus is seen, as well as the sigmoid sinuses, and this time both internal jugular veins are visualized�
The lower image is an anteroposterior view� The superior sagittal sinus, now joined by the straight sinus, is seen dividing into the two transverse sinuses and both internal jugular veins can be seen exiting the skull�
Venograms sometimes reveal an unequal flow of blood in the transverse sinuses that is caused by a narrowing or stenosis of one of the transverse sinuses� If this is severe and thought to predispose the person to clotting in the superior sagittal sinus, procedures are now in place (done by interventional neuroradiologists) to “stent” the transverse sinus involved�
This is an illustration of the production, circulation, and reabsorption of cerebrospinal fluid (CSF) slightly modified from the illustration in The Integrated Nervous System� The ventricles of the brain are shown, as are the subarachnoid spaces around the brain, enlargements of which are called cisterns including the lumbar cistern, as well as the superior sagittal (and straight) venous sinus� The blood flow in the venous sinuses is also shown�
The cerebral ventricles of the brain (and the central canal of the spinal cord) are found within the brain tissue and are the remnants of the original neural tube from which the nervous system developed� There are four ventricles: one (ventricles I and II) in each of the cerebral hemispheres, also called the lateral ventricles (see Figure 2�1A and Figure 2�1B); the 3rd ventricle in the thalamic (diencephalic) region (see Figure 2�8); and the 4th ventricle in the brainstem region (see Figure 3�1, Figure 3�2, and Figure 3�3)�
Choroid plexus is found in the lateral ventricles (see Figure 2�1B and Figure 2�10A and also Figure 9�4; present in the atrium in Figure 6�5 but not labeled), the roof of the 3rd ventricle, and the lower half of the roof of the 4th ventricle� CSF produced in the lateral ventricles flows (as shown with black arrows, note the figure legend with the illustration) via the foramen of Monro (from each lateral ventricle-see Figure 2�9B and Figure 2�10A) into the 3rd ventricle, and then through the aqueduct of the midbrain into the 4th ventricle (review Figure 3�2)� CSF leaves the ventricular system from the 4th ventricle, as indicated schematically in the diagram� In the intact brain, this occurs via the medially placed foramen of Magendie (see lower illustration in Figure 1�9) and the two laterally placed foramina of Luschka, and CSF enters the enlargement of the subarachnoid space under the cerebellum, the cerebello-medullary cistern, the cisterna magna� This
cistern is located inside the skull, just above the foramen magnum of the skull (see Figure 3�2)�
CSF continues to flow through the subarachnoid space, between the pia and the arachnoid (darker blue arrows in the figure)� The CSF fills the enlargements of the subarachnoid spaces around the brainstem-the various cisterns (each of which has a separate name)� The CSF then flows upward around the hemispheres of the brain and is found in all the sulci and fissures� CSF also flows in the subarachnoid space downward around the spinal cord to fill the lumbar cistern (see Figure 1�10 and Figure 7�3)�
This slow circulation is completed by the return of CSF to the venous system� The return is through the arachnoid villi, protrusions of arachnoid into the venous sinuses of the brain, particularly along the superior sagittal sinus (see below and also Figure 7�2)� These can sometimes be seen on the specimens as collections of villi, called arachnoid granulations, on the surface of the brain lateral to the interhemispheric fissure�
Note to the Learner: These features are well shown in the video on the atlas Web site (www�atlasbrain�com)�
The CSF in the lumbar cistern is where lumbar puncture is performed for the sampling of CSF for clinical diagnostic purposes (discussed with Figure 7�3)�
The normal amount of CSF in the ventricles and cranio-spinal subarachnoid spaces is estimated to be around 150 ml and is replaced roughly every 6 to 8 hours; this indicates a continuous process of production and absorption of CSF, in effect a (slow) CSF circulation� CSF is returned to the venous circulation via the arachnoid granulations (see Figure 7�2) that protrude into the venous sinuses, particularly into the superior sagittal sinus (light blue arrows in the illustration)� A small pressure differential is thought to account for the transport of CSF across these villi and into the venous sinuses, thus completing the circulation of CSF�
Additional Note: The major arteries of the circle of Willis travel through the subarachnoid space (see Figure 7�2)� An aneurysm of these arteries, called a berry aneurysm, that “bursts” (discussed with Figure 8�2) will do so within the CSF space� This is called a subarachnoid hemorrhage (discussed with Figure 7�2)�
[TEXT CONTINUED]
Blockage of CSF flow, for example, at the level of the midbrain where the aqueduct connecting the 3rd with the 4th ventricle is very n arrow, would cause an increase in size, and of pressure, in the cerebral ventricles� This blockage, called obstructive or non-communicating hydrocephalus, would be visualized with computed tomography or magnetic resonance imaging as an enlargement of the lateral ventricles of the hemispheres� In an adult, because the sutures of the skull are fused, this process would be accompanied by raised intracranial pressure� In contrast, in a young child (e�g�, the first 2 years) with non-fused cranial sutures, the head itself would enlarge, and there would be a separation of the bones of the skull; at a very early age, the anterior fontanelle would bulge�
Blockage of the CSF flow can also occur at the level of the arachnoid granulations, and in fact this does occur following meningitis or other disorder which can cause increased protein in the CSF interfering with the function of the arachnoid villi (further discussed in The Integrated Nervous System)� If the villi are non-functional or blocked, or if there is a blockage of the venous sinuses (e�g�, venous sinus thrombosis) or stenosis of the sinuses, then CSF can no longer return to the venous circulation, and the result is an increase in CSF pressure� Hydrocephalus developing from CSF flow obstruction at a point outside the brain is called communicating hydrocephalus�
In young adults with brains of high tissue resistance or low compliance, the CSF pressure increases; in older adults with brains of lower tissue resistance or higher compliance, the CSF spaces such as the ventricles increase in volume� This latter condition can manifest as a disorder called normal pressure hydrocephalus�
The ventricles of the brain are lined with a layer of cells known as the ependyma� In certain loci within each of the ventricles, the ependymal cells and the pia meet, thus forming the choroid plexus, which invaginates into the ventricle� Functionally, the choroid plexus has a vascular layer (i�e�, the pia) on the inside and the ependymal layer on the ventricular side� The blood vessels of the choroid plexus are freely permeable, but there is a cellular barrier between the choroid plexus and the ventricular space-the blood-CSF barrier� The barrier consists of tight junctions between the ependymal cells that line the choroid plexus� CSF is actively secreted by the choroids plexus, and an enzyme is involved� The ionic and protein composition of CSF is different from that of serum�
A similar type of barrier exists between the brain capillaries and the extracellular space of the brain tissue, known as the blood-brain-barrier (BBB)� This barrier is also formed by tight junctions-between the endothelial cells lining the capillaries� This barrier allows for only small molecules to cross, including glucose and select amino acids�
A breakdown of the BBB occurs in certain diseases, under “toxic” conditions, and within and around tumors of the CNS� The use of a radio-opaque dye during imaging and its escape indicates a breakdown of the BBB�
