Meninges Ventricles Cerebrospinal Fluid and Blood Supply

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Objectives

When you have completed this section, you should be able to

• describe the meninges of the brain;

• describe the system of fluid-filled chambers within the brain;

• discuss the production, circulation, and function of the cerebrospinal fluid that fills these chambers; and

• explain the significance of the brain barrier system.

Meninges

The meninges of the brain are basically the same as those of the spinal cord: dura mater, arachnoid mater, and pia mater (fig. 14.5). The dura mater, however, shows some significant differences. In the cranial cavity, it consists of two layers—an outer periosteal layer, equivalent to the periosteum of the cranial bone, and an inner meningeal layer. Only the meningeal layer continues into the vertebral

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Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition

520 Part Three Integration and Control

Primary Brain Vesicles
Figure 14.4 Primary and Secondary Vesicles of the Embryonic Brain. (a) The primary vesicles at 4 weeks; (b) the secondary vesicles at 5 weeks; (c) the fully developed brain, color-coded to relate its structures to the secondary embryonic vesicles.

Dura mater

Periosteal layer

Meningeal Jayer

Subdural space

Arachnoid -mater

Subarachnoid space

Pia mater

Blood vessel

Falx cerebri (in longitudinal fissure only)

Periosteal layer

Meningeal Jayer

Subdural space

Arachnoid -mater

Subarachnoid space

Pia mater

Falx cerebri (in longitudinal fissure only)

Arachnoid Villi Malformation

Skull

Arachnoid villus

Superior sagittal sinus

Gray matter White matter

Skull

Arachnoid villus

Superior sagittal sinus

Gray matter White matter

Brain

Figure 14.5 The Meninges of the Brain. Frontal section of the head.

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition canal. The cranial dura mater lies closely against the cranial bone, with no intervening epidural space like that of the spinal cord. In some places, the two layers are separated by dural sinuses, spaces that collect blood that has circulated through the brain. Two major dural sinuses are the superior sagittal sinus, found just under the cranium along the mid-sagittal line, and the transverse sinus, which runs horizontally from the rear of the head toward each ear. These sinuses meet like an inverted T at the back of the brain and ultimately empty into the internal jugular veins of the neck.

In certain places, the meningeal layer of the dura mater folds inward to separate major parts of the brain: the falx14 cerebri (falks SER-eh-bry) extends into the longitudinal fissure between the right and left cerebral hemispheres; the tentorium15 (ten-TOE-ree-um) cerebelli stretches like a roof over the posterior cranial fossa and separates the cerebellum from the overlying cerebrum; and the falx cerebelli partially separates the right and left halves of the cerebellum on the inferior side.

The arachnoid mater and pia mater are similar to those of the spinal cord. A subarachnoid space separates the arachnoid from the pia, and in some places, a subdural space separates the dura from the arachnoid.

14falx = sickle 15tentorium = tent

Insight 14.1 Clinical Application

Meningitis

Meningitis—inflammation of the meninges—is one of the most serious diseases of infancy and childhood. It occurs especially between 3 months and 2 years of age. Meningitis is caused by a variety of bacteria and viruses that invade the CNS by way of the nose and throat, often following respiratory, throat, or ear infections. The pia mater and arachnoid are most likely to be affected, and from here the infection can spread to the adjacent nervous tissue. In bacterial meningitis, the brain swells, the ventricles enlarge, and the brainstem may have hemorrhages. Signs include a high fever, stiff neck, drowsiness, and intense headache and may progress to vomiting, loss of sensory and motor functions, and coma. Death can occur within hours of the onset. Infants and toddlers with a high fever should therefore receive immediate medical attention.

Meningitis is diagnosed partly by examining the cerebrospinal fluid (CSF) for bacteria and white blood cells. The CSF is obtained by making a lumbar puncture (spinal tap) between two lumbar vertebrae and drawing fluid from the subarachnoid space. This site is chosen because it has an abundance of CSF and there is no risk of injury to the spinal cord, which does not extend into the lower lumbar vertebrae.

Ventricles and Cerebrospinal Fluid

The brain has four internal chambers called ventricles (fig. 14.6). The most rostral ones are the lateral ventricles,

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which form a large arc in each cerebral hemisphere. Through a tiny passage called the interventricular foramen, each lateral ventricle is connected to the third ventricle, a narrow medial space inferior to the corpus callo-sum. From here, a canal called the cerebral aqueduct passes down the core of the midbrain and leads to the fourth ventricle, a small chamber between the pons and cerebellum. Caudally, this space narrows and forms a central canal that extends through the medulla oblongata into the spinal cord.

These ventricles and canals are lined with ependymal cells, a type of neuroglia that resembles a simple cuboidal epithelium. Each ventricle contains a choroid (CO-royd) plexus, named for its histological resemblance to a fetal membrane called the chorion. The choroid plexus is a network of blood capillaries anchored to the floor or wall of the ventricle and covered by ependymal cells.

A clear, colorless liquid called cerebrospinal fluid (CSF) fills the ventricles and canals of the CNS and bathes its external surface. The brain produces about 500 mL of CSF per day, but the fluid is constantly reabsorbed at the same rate and only 100 to 160 mL are present at one time. About 40% of it is formed in the subarachnoid space external to the brain, 30% by the general ependymal lining of the brain ventricles, and 30% by the choroid plexuses. CSF forms partly by the filtration of blood plasma through the choroid plexuses and other capillaries of the brain. The ependymal cells modify this filtrate, however, so the CSF has more sodium and chloride than the blood plasma, but less potassium, calcium, and glucose and very little protein. Cerebrospinal fluid serves three purposes:

1. Buoyancy. Because the brain and CSF are very similar in density, the brain neither sinks nor floats in the CSF but remains suspended in it—that is, the brain has neutral buoyancy. A human brain removed from the body weighs about 1,500 g, but when suspended in CSF its effective weight is only about 50 g. By analogy, consider how much easier it is to lift another person when you are standing in a lake than it is on land. Neutral buoyancy allows the brain to attain considerable size without being impaired by its own weight. If the brain rested heavily on the floor of the cranium, the pressure would kill the nervous tissue.

2. Protection. CSF also protects the brain from striking the cranium when the head is jolted. If the jolt is severe, however, the brain still may strike the inside of the cranium or suffer shearing injury from contact with the angular surfaces of the cranial floor. This is one of the common findings in child abuse (shaken child syndrome) and in head injuries (concussions) from auto accidents, boxing, and the like.

3. Chemical stability. CSF is secreted into each ventricle of the brain and is ultimately absorbed into the bloodstream. It provides a means of rinsing metabolic

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition

522 Part Three Integration and Control

Anterior Cerebrospinal FluidCircumventricular Organs Midsagittal
Figure 14.6 Ventricles of the Brain. (a) Right lateral aspect; (b) anterior aspect; (c) superior aspect of a horizontal section of the brain, showing the lateral ventricles and other features of the cerebrum.

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition wastes from the CNS and homeostatically regulating its chemical environment. Slight changes in its composition can cause malfunctions of the nervous system. For example, a high glycine concentration disrupts temperature and blood pressure control, and a high pH causes dizziness and fainting.

The CSF is not a stationary fluid but continually flows through and around the CNS, driven partly by its own pressure and partly by rhythmic pulsations of the brain produced by each heartbeat. The CSF secreted in the lateral ventricles flows through the interventricular foramina into the third ventricle (fig. 14.7) and then down the cerebral aqueduct to the fourth ventricle. The third and fourth ventricles and their choroid plexuses add more CSF

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along the way. A small amount of CSF fills the central canal of the spinal cord, but ultimately, all of it escapes through three pores in the walls of the fourth ventricle—a median aperture and two lateral apertures. These lead into the sub-arachnoid space on the brain surface. From this space, the CSF is absorbed by arachnoid villi, cauliflower-like extensions of the arachnoid meninx that protrude through the dura mater into the superior sagittal sinus. CSF penetrates the walls of the arachnoid villi and mixes with the blood in the sinus.

Hydrocephalus16 is the abnormal accumulation of CSF in the brain, usually resulting from a blockage in its

16hydro = water + cephal = head

Cerebellum Csf Tap Site

Figure 14.7 The Flow of Cerebrospinal Fluid.

Locate the sites of the obstructions that cause hydrocephalus.

Figure 14.7 The Flow of Cerebrospinal Fluid.

Locate the sites of the obstructions that cause hydrocephalus.

Saladin: Anatomy & I 14. The Brain and Cranial I Text I © The McGraw-Hill

Physiology: The Unity of Nerves Companies, 2003 Form and Function, Third Edition

524 Part Three Integration and Control route of flow and reabsorption. Such obstructions occur most often in the interventricular foramen, cerebral aqueduct, and the apertures of the fourth ventricle. The accumulated CSF makes the ventricles expand and compress the nervous tissue, with potentially fatal consequences. In a fetus or infant, it can cause the entire head to enlarge because the cranial bones are not yet fused. Good recovery can be achieved if a tube (shunt) is inserted to drain fluid from the ventricles into a vein of the neck.

Blood Supply and the Brain Barrier System

Although the brain constitutes only 2% of the adult body weight, it receives 15% of the blood (about 750 mL/min) and consumes 20% of the body's oxygen and glucose. Because neurons have such a high demand for ATP, and therefore glucose and oxygen, the constancy of blood supply is especially critical to the nervous system. A mere 10-second interruption in blood flow can cause loss of consciousness; an interruption of 1 to 2 minutes can significantly impair neural function; and 4 minutes without blood causes irreversible brain damage.

Despite its critical importance to the brain, the blood is also a source of antibodies, macrophages, and other potentially harmful agents. Consequently, there is a brain barrier system that strictly regulates what substances can get from the bloodstream into the tissue fluid of the brain. There are two potential points of entry that must be guarded: the blood capillaries throughout the brain tissue and the capillaries of the choroid plexuses.

At the former site, the brain is well protected by the blood-brain barrier (BBB), which consists mainly of the tightly joined endothelial cells that form the blood capillaries and partly of the basement membrane surrounding them. In the developing brain, astrocytes reach out and contact the capillaries with their perivascular feet. They stimulate the endothelial cells to form tight junctions, which seal off the capillaries and ensure that anything leaving the blood must pass through the cells and not between them. At the choroid plexuses, the brain is protected by a similar blood-CSF barrier, composed of ependymal cells joined by tight junctions. Tight junctions are absent from ependymal cells elsewhere, because it is important to allow exchanges between the brain tissue and CSF. That is, there is no brain-CSF barrier.

The brain barrier system (BBS) is highly permeable to water, glucose, and lipid-soluble substances such as oxygen, carbon dioxide, alcohol, caffeine, nicotine, and anesthetics. It is slightly permeable to sodium, potassium, chloride, and the waste products urea and creatinine. While the BBS is an important protective device, it is an obstacle to the delivery of drugs to treat brain diseases. Trauma and inflammation sometimes damage the BBS and allow pathogens to enter the brain tissue. Furthermore, there are places called circumventricular organs (CVOs) in the third and fourth ventricles where the barrier system is absent and the blood does have direct access to the brain. These enable the brain to monitor and respond to fluctuations in blood glucose, pH, osmolarity, and other variables. Unfortunately, the CVOs also afford a route for the human immunodeficiency virus (HIV, the AIDS virus) to invade the brain.

Before You Go On

Answer the following questions to test your understanding of the preceding section:

4. Name the three meninges from superficial to deep.

5. Describe three functions of the cerebrospinal fluid.

6. Where does the CSF originate and what route does it take through and around the CNS?

7. Name the two components of the brain barrier system and explain the importance of this system.

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Responses

  • Negisti
    Is the dura mater fully protected by the bloodbrain barrier?
    8 years ago
  • Timba Hornblower
    What type of tissue in the ventricles that makes cerebrospinal fluid?
    8 years ago
  • Michelle
    What do the meninges and the cerebrospinal fluid have in common?
    6 years ago
  • arthur
    Where does the Csf originate and what route does it take through and around the CNS?
    5 years ago
  • essi
    How is cerebrospinal fluid supplied to the tissue of the nervous system?
    4 years ago
  • jean
    What do the meninges and the cerebrospinal fluid have in common quizlet?
    4 years ago

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