11.2: Support and Protection of the Brain

By the end of this section, you will be able to:

• Describe the meninges that protect the brain
• Describe the blood vessels that supply the brain
• Name the components of the ventricular system and the regions of the brain in which each is located
• Explain the production of cerebrospinal fluid and its flow through the ventricles

Protective Coverings of the Brain

The central nervous system (CNS) is crucial to the operation of the body and any compromise of function in the brain and spinal cord can lead to severe difficulties. The brain is protected by multiple structures. First, the bones of the skull enclose and house the brain. Underneath the skeletal structures, the brain is protected by membranes made of connective tissue, called meninges, that surround, support, stabilize and partition the nervous tissue (Figure \(\PageIndex{1}\)). In addition, the brain has a privileged blood supply, as suggested by the blood-brain barrier. The function of the tissue is crucial to the survival of the organism, so the contents of the blood cannot simply pass into the central nervous tissue. To protect this region from the toxins and pathogens that may be traveling through the blood stream, there is strict control over what can move out of the general systems and into the brain. Because of this privilege, the brain needs specialized structures for the maintenance of circulation. This begins with a unique arrangement of blood vessels carrying fresh blood into the brain and venous sinuses carrying deoxygenated blood out of the brain. Beyond the supply of blood, the brain filters the blood into cerebrospinal fluid (CSF), which is then circulated through the cavities of the brain, such as the subarachnoid space and the ventricles.

 

Figure \(\PageIndex{1}\): Protective structures of the brain. Superficially, the skin of the scalp and bones of the skull create the first layer of protection. Deep to these layers, the meninges (represented by the dura mater, arachnoid mater and pia mater) cover and partition the brain. Within the meninges, blood inside blood vessels and sinuses, and cerebrospinal fluid within the subarachnoid space and ventricles circulate to support the brain's function. (Image credit: "Cranial Meninges" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, modification of image by NIH Medical Arts.)

Cranial Meninges

The outer surface of the CNS is covered by a series of membranes, composed of connective tissue and mesothelial cells, called the meninges that protect, stabilize and partition the brain. From superficial to deep, the meningeal layers are the dura mater, arachnoid mater and pia mater. The dura mater is a thick fibrous layer and a strong protective sheath over the entire brain. It is anchored to the inner surface of the cranium and to the vertebral cavity. The arachnoid mater is a membrane of thin fibrous tissue that forms a loose sac around the brain. Beneath the arachnoid mater, there is a space called the subarachnoid space where a thin, filamentous mesh form the arachnoid trabeculae, that looking like a spider web give this layer its name. Directly adjacent to the surface of the brain is the pia mater, a thin fibrous membrane that follows the superficial convolutions of the brain and fits into other grooves and indentations (Figure \(\PageIndex{2}\)).

Figure \(\PageIndex{2}\): Meningeal Layers of the Brain. The layers of the meninges are shown under neath the bones of the skull. The dura mater is adjacent to the inner surface of the cranium. Within it, veins and the superior sagittal sinus lie. The pia mater is adjacent to the surface of the brain. The arachnoid and subarachnoid space are in-between. An arachnoid villus is shown emerging into the dural sinus to allow CSF to filter back into the blood for drainage. (Image credit: "Meningeal Layers" by OpenStax is licensed under CC BY 4.0, modifications by Jennifer Lange.)

Dura Mater

Like a thick cap covering the brain, the dura mater is a tough outer covering. The name comes from the Latin for “tough mother” to represent its physically protective role. It encloses the entire CNS and the major blood vessels that enter the cranium and vertebral cavity. It is directly attached to the inner surface of the bones of the cranium and to the walls of the foramen magnum and the C2-C3 vertebral canal. The dura mater of the brain has two layers, the periosteal layer (similar in structure to the periosteum of bones) is more superficial and attached to the skull, and the meningeal layer that lies deep to the periosteal layer. These two layers are usually fused together. However, in some regions of the brain they separate to form large space filled with venous blood called dural sinuses. The dura mater of the spinal cord is composed of only the meningeal layer. The region between the bones and the dura mater is called epidural space. In the cranium, the epidural space contains arteries and veins that supply and drain blood from the brain. In normal conditions, the cranial epidural space is a potential space and not a real space. However, trauma can cause the leakage of fluid (hematomas) which accumulates in the epidural space enlarging it and transforming it into a real space. In the vertebral column, the epidural space is a real space that contains areolar and adipose connective tissue for an extra layer of protective padding, as well as blood vessels. The spinal epidural space at the lumbar level is where epidural injections are administered. Deep to the dura mater, there is another potential space called the subdural space. As the epidural space, this space can be filled with fluid causing a subdural hematoma.

The meningeal layer of the dura mater extends into the cranial cavity at four locations. These flat partitions are called cranial dural septa and separate specific parts of the brain (Figure \(\PageIndex{3}\)). The falx cerebri goes through the midline separation of the brain while the falx cerebelli separates the two cerebellar hemispheres. The tentorium cerebelli separates the cerebellum from the superior part of the brain (cerebrum), forming a shelf-like tent. Another partition called diaphragma sellae surrounds the pituitary gland.

Figure \(\PageIndex{3}\): Cranial Dural Septa. The dura mater creates partitions called cranial dural septa. Two of these septa are the falx cerebri located in the longitudinal fissure and between the right and left cerebral hemispheres and the tentorium cerebelli between the cerebral occipital lobes and the cerebellum. (Image credits: "Dural Septa" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0; "Cranial Cavity, Superolateral" by BlueLink is licensed underCC BY-NC 4.0 with notification of the original authors.)

Arachnoid Mater

Deep to the subdural space lies the middle layer of the meninges called the arachnoid mater (Figure \(\PageIndex{2}\)). This layer is named for the spider-web–like projections called arachnoid trabeculae between this layer and the pia mater. The trabeculae are found crossing the subarachnoid space, which is filled with circulating CSF, making it a real space. The lining of the subarachnoid space is a modified epithelial layer, the mesothelium, that forms a continuous layer of cells providing a fluid-impermeable barrier between the CSF and the nervous tissue.  One portion of the mesothelium is in the arachnoid mater, the second portion is found on the surface of the pia mater.

The arachnoid mater protrudes into the dural sinuses as the arachnoid granulations, where the CSF is filtered back into the blood for drainage from the nervous system.

Pia Mater

The outer surface of the CNS is covered in the thin, fibrous membrane of the pia mater (Figure \(\PageIndex{2}\)). The name pia mater comes from the Latin for “tender mother,” suggesting the thin membrane is a gentle covering for the brain. The pia extends into every convolution of the CNS and anchors to the glial membrane that is part of the blood-brain barrier.  Blood vessels that are nourishing the central nervous tissue are between the pia mater and the nervous tissue.

DISORDERS OF THE...

Meninges: Meningitis

Meningitis is an inflammation of the meninges, the three layers of fibrous membrane that surround the CNS (Figure \(\PageIndex{4}\)). Meningitis can be caused by infection by bacteria or viruses. The particular pathogens are not special to meningitis; it is just an inflammation of that specific set of tissues from what might be a broader infection. Bacterial meningitis can be caused by Streptococcus, Staphylococcus, or the tuberculosis pathogen, among many others. Viral meningitis is usually the result of common enteroviruses (such as those that cause intestinal disorders), but may be the result of the herpes virus or West Nile virus. Bacterial meningitis tends to be more severe.

Figure \(\PageIndex{4}\): Meningitis. Inflammation from meningitis shows in an MRI as bright enhancements of meninges at the tentorium and in  dilatation of the ventricles. White blood cells accumulate within the meningeal layers. (Image credits: "FLAIR MRI of Meningitis" by Aneel Kumar Vaswani, Waseem Mehmood Nizamani, Muhammad Ali, Geeta Aneel, Bhesham Kumar Shahani, and Sajjad Hussain Uploaded by Mikael Häggström is licensed under CC BY 3.0, via Wikimedia Commons; "Meningitis Histopathology" by Marvin 101 is licensed under CC BY-SA 3.0, via Wikimedia Commons.)

The symptoms associated with meningitis can be fever, chills, nausea, vomiting, light sensitivity, soreness of the neck, or severe headache. More important are the neurological symptoms, such as changes in mental state (confusion, memory deficits, and other dementia-type symptoms). A serious risk of meningitis can be damage to peripheral structures because of the nerves that pass through the meninges. Hearing loss is a common result of meningitis.

The primary test for meningitis is a lumbar puncture. A needle inserted into the lumbar region of the spinal column through the dura mater and arachnoid membrane into the subarachnoid space can be used to withdraw the fluid for chemical testing. Fatality occurs in 5 to 40 percent of children and 20 to 50 percent of adults with bacterial meningitis. Treatment of bacterial meningitis is through antibiotics, but viral meningitis cannot be treated with antibiotics because viruses do not respond to that type of drug. Fortunately, the viral forms are milder.

Blood Supply to the Brain

A lack of oxygen to the CNS can be devastating, and the cardiovascular system has specific regulatory reflexes to ensure that the blood supply is not interrupted. There are multiple routes for blood to get into the CNS, with specializations to protect that blood supply and to maximize the ability of the nervous tissue to get an uninterrupted perfusion.

Arterial Supply to the Brain

The major artery carrying recently oxygenated blood away from the heart is the aorta. The very first branches off the aorta supply the heart with nutrients and oxygen. The next branches give rise to the common carotid arteries, which further branch into the internal carotid arteries. The external carotid arteries supply blood to the tissues on the surface of the cranium. The internal carotid artery enters the cranium through the carotid canal in the temporal bone.

A second set of vessels that supply the CNS are the vertebral arteries, which are protected as they pass through the neck region by the transverse foramina of the cervical vertebrae. The vertebral arteries enter the cranium through the foramen magnum of the occipital bone. The two vertebral arteries then merge into the basilar artery, which gives rise to branches to the brainstem and cerebellum. The left and right internal carotid arteries and branches of the basilar artery all become the circle of Willis, a confluence of arteries that can maintain perfusion of the brain even if narrowing or a blockage limits flow through one part (Figure \(\PageIndex{5}\)).

Figure \(\PageIndex{5}\): Circle of Willis. This inferior view shows the network of arteries serving the brain. The structure is referred to as the circle of Willis and is formed by branches of the internal carotid artery, vertebral artery and basilar artery. (Image credit: "Cerebral Arteries" by Jennifer Lange using brain image by Justin Greene is licensed under CC-BY-NC-SA 4.0, Circle of Willis original image from OpenStax.)

Interactive Link

Circle of Willis

Query \(\PageIndex{1}\)

Venous Return from the Brain

After passing through the CNS, blood returns to the circulation through a series of dural sinuses and cerebral veins (Figure \(\PageIndex{6}\) and Figure \(\PageIndex{7}\)). The dural sinuses are housed between the two layers of the dura mater: 

• the superior sagittal sinus runs in the superficial aspect of the longitudinal fissure of the brain while the inferior sagittal sinus runs deep in the fissure, just superior to the corpus callosum; 
• at the posterior end of the corpus callosum the inferior sagittal sinus drains into the straight sinus, which then runs posteriorly across the superior midline surface of the cerebellum;
• the superior sagittal sinus and the straight sinus drain into the confluence of the sinuses, along with the occipital sinus coming from the inferior midline of the cerebellum, located at the point where the two cerebral hemispheres and the two cerebellar hemispheres meet;
• blood then flows into the transverse sinuses that travel laterally along the transverse cerebral fissure between the cerebrum and the cerebellum; 
• the transverse sinuses connect to the sigmoid sinuses, which bend in an "S" shape to reach the jugular foramen and become the jugular veins. From there, the blood continues toward the heart to be pumped to the lungs for reoxygenation.

Figure \(\PageIndex{6}\): Locations of the Dural Sinuses.  The larger dural sinuses run in the fissures of the brain - the longitudinal fissure between the right and left cerebral hemispheres and the transverse cerebral fissure between the cerebrum and the cerebellum.  (Image credit: "Dural Sinus Locations" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0.)

Figure \(\PageIndex{7}\): Flow Through Dural Sinuses. Blood drains from the brain through a series of sinuses and veins that connect to the jugular veins. (Image credit: "Brain Sinuses" by OpenStax is licensed under CC BY 4.0)

 

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Dural Venous Sinuses by Anatomy by Doctor Jana on Sketchfab

 

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Visualizing the Dural Sinuses

MR cerebral venography (MRV) is an MRI examination of the head with either contrast-enhanced or non-contrast sequences to assess flow of the dural venous sinuses and cerebral veins.  This test will be able to determine if an obstruction is reducing flow through one of the sinuses.

Figure \(\PageIndex{8}\): Normal MR Venogram - Sagittal and Posterior Views.  All of the major dural sinuses can be clearly visualized.  (Image credit: "Slide16" from BlueLink is licensed under CC BY-NC 4.0 with notification of the original authors.) 

 

 

Figure \(\PageIndex{9}\): MRV Case Study. (Case courtesy of Fadi Aidi, Radiopaedia.org, rID: 84329)

Query \(\PageIndex{2}\)

 

Ventricular System

Cerebrospinal fluid (CSF) is produced by a type of specialized membrane made of ependymal cells called a choroid plexus. Ependymal cells (one of the types of glial cells described in the introduction to the nervous system) surround blood capillaries and filter the blood to make CSF. The fluid is a clear solution with a limited amount of the constituents of blood. It is essentially water, small molecules, and electrolytes and is continuous with the interstitial fluid. Oxygen and carbon dioxide are dissolved into the CSF, as they are in blood, and can diffuse between the fluid and the nervous tissue. CSF circulates through the nervous tissue to remove metabolic wastes from the interstitial fluids of nervous tissues and return them to the blood stream. The choroid plexus lines open spaces within the brain called ventricles. The CSF circulates through all of the ventricles to eventually emerge into the subarachnoid space where it will be reabsorbed into the blood.

Ventricles

There are four ventricles within the brain, all of which developed from the original hollow space within the neural tube, the central canal. The first two are named the lateral ventricles and are deep within the cerebrum (Figure \(\PageIndex{10}\)). The two lateral ventricles are shaped as a C and are located in the left and right hemispheres, and were at one time referred to as the first and second ventricles. These ventricles are connected to the third ventricle by two openings called the interventricular foramina. The third ventricle opens into a canal called the cerebral aqueduct that passes through the midbrain and connects the third ventricle to the fourth ventricle. The fourth ventricle is the space between the cerebellum and the pons and upper medulla (Figure \(\PageIndex{11}\)). The ventricular system opens up to the subarachnoid space from the fourth ventricle. The single median aperture and the pair of lateral apertures connect to the subarachnoid space so that CSF can flow through the ventricles and around the outside of the CNS. From the fourth ventricle, CSF can continue down the central canal of the spinal cord.

Figure \(\PageIndex{10}\): Ventricles. Within the brain, open spaces called ventricles are lined with ependymal cells that produce cerebrospinal fluid (CSF). Deep within the cerebral lobes are the lateral ventricles, which are connected to the third ventricle located in the diencephalon. The third ventricle opens into a canal called the cerebral aqueduct which connects to the fourth ventricle. The fourth ventricle is connected to the central canal of the spinal cord. (Image credit: "Human Ventricular System" by BodyParts3D[1] by DBCLS. is licensed under CC BY-SA 2.1 JP, via Wikimedia Commons. Labeling by Jennifer Lange.)

Figure \(\PageIndex{11}\): Brain Ventricles.  The ventricles are open spaces within the brain that are partially seen within a single sagittal or coronal section.  (Image credits: "Brain Ventricles - Midsagittal View" by Justin Greene and Jennifer Lange is licensed under CC-BY-NC-SA 4.0, "Brain Ventricles - Coronal Section" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0.)

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Cerebrospinal Fluid Circulation

The choroid plexuses are found in all four ventricles (Figure \(\PageIndex{11}\)). Observed in dissection, they appear as soft, fuzzy structures that may still be pink, depending on how well the circulatory system is cleared in preparation of the tissue. The CSF is produced from components extracted from the blood, so its flow out of the ventricles is tied to the pulse of cardiovascular circulation.

From the lateral ventricles, the CSF flows into the third ventricle, where more CSF is produced, and then through the cerebral aqueduct into the fourth ventricle where even more CSF is produced (Figure \(\PageIndex{12}\)). A very small amount of CSF is filtered at any one of the plexuses, for a total of about 500 milliliters daily, but it is continuously made and pulses through the ventricular system, keeping the fluid moving. From the fourth ventricle, CSF can continue down the central canal of the spinal cord, but this is essentially a cul-de-sac, so more of the fluid leaves the ventricular system and moves into the subarachnoid space through the median and lateral apertures.

Within the subarachnoid space, the CSF flows around all of the CNS, providing two important functions. As with elsewhere in its circulation, the CSF picks up metabolic wastes from the nervous tissue and moves it out of the CNS. It also acts as a liquid cushion for the brain and spinal cord. By surrounding the entire system in the subarachnoid space, it provides a thin buffer around the organs within the strong, protective dura mater. The arachnoid granulations are outpocketings of the arachnoid membrane into the dural sinuses so that CSF can be reabsorbed into the blood, along with the metabolic wastes. From the dural sinuses, blood drains out of the head and neck through the jugular veins, along with the rest of the circulation for blood, to be reoxygenated by the lungs and wastes to be filtered out by the kidneys.

Figure \(\PageIndex{12}\): Cerebrospinal Fluid Circulation. The choroid plexus in all the ventricles produce CSF, which is circulated through the ventricular system and then enters the subarachnoid space through the median and lateral apertures. The CSF is then reabsorbed into the blood at the arachnoid granulations, where the arachnoid membrane emerges into the dural sinuses. (Image credit: "CSF Circulation" by OpenStax is licensed under CC BY 4.0.)

DISORDERS OF THE...

Central Nervous System: Strokes

The supply of blood to the brain is crucial to its ability to perform many functions. Without a steady supply of oxygen, and to a lesser extent glucose, the nervous tissue in the brain cannot keep up its extensive electrical activity. These nutrients get into the brain through the blood, and if blood flow is interrupted, neurological function is compromised.

The common name for a disruption of blood supply to the brain is a stroke. It is caused by a blockage to an artery in the brain or by blood leaking out of blood vessels (hemorrhagic stroke), see Figure \(\PageIndex{13}\). The blockage is caused by some type of embolus: a blood clot, a fat embolus, or an air bubble. When the blood cannot travel through the artery, the surrounding tissue that is deprived starves and dies. Strokes will often result in the loss of very specific functions. A stroke in the lateral medulla, for example, can cause a loss in the ability to swallow. Sometimes, seemingly unrelated functions will be lost because they are dependent on structures in the same region. Along with the swallowing in the previous example, a stroke in that region could affect sensory functions from the face or extremities because important white matter pathways also pass through the lateral medulla. Loss of blood flow to specific regions of the brain can lead to the loss of specific higher functions, from the ability to recognize faces to the ability to move a particular region of the body. Severe or limited memory loss can be the result of a temporal lobe stroke.

Figure \(\PageIndex{13}\): Types of Stroke. Blood clot stops blood flow to an area of the brain in Ischemic (left) whereas blood leaks into brain tissue in case of Hemorrhagic (right). (Image credit: "Types of Stroke" by https://www.scientificanimations.com/, is licensed under CC BY-SA 4.0, via Wikimedia Commons.)

Related to strokes are transient ischemic attacks (TIAs), which can also be called “mini-strokes.” These are events in which a physical blockage may be temporary, cutting off the blood supply and oxygen to a region, but not to the extent that it causes cell death in that region. While the neurons in that area are recovering from the event, neurological function may be lost. TIAs usually resolve spontaneously by the body, that is why it is called "transient".

Recovery from a stroke (or TIA) is strongly dependent on the speed of treatment. Often, the person who is present and notices something is wrong must then make a decision. The mnemonic FAST helps people remember what to look for when someone is dealing with sudden losses of neurological function. If someone complains of feeling “funny,” check these things quickly: Look at the person’s face. Does he or she have problems moving Face muscles and making regular facial expressions? Ask the person to raise his or her Arms above the head. Can the person lift one arm but not the other? Has the person’s Speech changed? Is he or she slurring words or having trouble saying things? If any of these things have happened, then it is Time to call for help.

Sometimes, treatment with blood-thinning drugs can alleviate the problem, and recovery is possible. If the tissue is damaged, the amazing thing about the nervous system is that it is adaptable. With physical, occupational, and speech therapy, victims of strokes can recover, or more accurately relearn, functions.

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Concept Review

The CNS has a privileged blood supply established by the blood-brain barrier. Establishing this barrier are anatomical structures that help to protect and isolate the CNS. The skull and vertebral column are the first mean of protection of the brain and spinal cord, respectively. Layers of connective tissue called meninges support and stabilize the brain and spinal cord, as well as partition the brain into specific regions. The outer layer is the dura mater, the middle layer is the arachnoid mater and the inner layer is the pia mater.

The arterial blood to the brain comes from the internal carotid and vertebral arteries, which both contribute to the unique circle of Willis that provides constant perfusion of the brain even if one of the blood vessels is blocked or narrowed. That blood is eventually filtered to make a separate medium, the CSF, that circulates within the spaces of the brain and then into the surrounding space defined by the meninges, the protective covering of the brain and spinal cord.

The blood that nourishes the brain and spinal cord is behind the glial-cell–enforced blood-brain barrier, which limits the exchange of material from blood vessels with the interstitial fluid of the nervous tissue. Thus, metabolic wastes are collected in cerebrospinal fluid that circulates through the CNS. This fluid is produced by filtering blood at the choroid plexuses in the four ventricles of the brain. It then circulates through the ventricles and into the subarachnoid space, between the pia mater and the arachnoid mater. From the arachnoid granulations, CSF is reabsorbed into the blood, removing the waste from the privileged central nervous tissue.

The blood, now with the reabsorbed CSF, drains out of the cranium through the dural sinuses. The dura mater is the tough outer covering of the CNS, which is anchored to the inner surface of the cranial and vertebral cavities. It surrounds the venous space known as the dural sinuses, which connect to the jugular veins, where blood drains from the head and neck.

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Review Questions

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Critical Thinking Questions

Query \(\PageIndex{4}\)

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Glossary

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Contributors and Attributions

OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology

Figure	Image Credit
Figure 20.1.1: Mountain Climbers.	Image credit: "Aiguille du Midi, Mont-Blanc" by Cristian Bortes is licensed under CC BY 2.0