In this chapter, the student should learn:
- The anatomy and function of the coverings of the brain and their relationship to venous sinuses and subarachnoid fluid spaces (cisterns).
- The differences between the coverings of the spinal cord and the brain, including anatomical features of the lumbar cistern and its importance in lumbar puncture.
- The ventricular system of the brain; anatomical and functional characteristics of choroid plexuses; and the formation, composition, circulation, and function of the cerebrospinal fluid (CSF).
- Role of arachnoid villi in the absorption of CSF into the venous sinuses and alteration of CSF in pathological conditions.
- The nature of the blood-brain and blood-CSF barriers.
- Disorders associated with the circulation and absorption of CSF.
The tissues comprising the brain and spinal cord are very delicate and require special protection. This is provided by the bony cranial vault, the bony vertebral canal, and the meninges. The cranial cavity is generally divided into three regions known as the anterior, middle, and posterior fossae, which house the anterior frontal lobe, the temporal lobe, and the cerebellum and brain stem, respectively. Within the cranial cavity, the brain is surrounded by meninges.
The meninges consist of three layers of connective tissue membranes (dura, arachnoid, and pia mater). The arachnoid and pia are known as the leptomeninges (“lepto” means thin and fine in Greek). There are several differences in the meninges covering the brain and spinal cord, and, accordingly, they are discussed separately. The meninges consist of fibroblasts and collagen fibrils. The amount of collagen varies in different meningeal layers. For example, the dura mater contains copious amounts of collagen fibrils, whereas the arachnoid mater has no collagen.
Coverings Of The Brain
The cranial dura mater (Fig. 3-1) is a tough, fibrous membrane consisting of two connective tissue layers: an external periosteal layer and an inner meningeal layer. These two layers are fused together except where the dural venous sinuses are located (e.g., superior sagittal sinus). The periosteal layer of the dura mater adheres to the inner surface of the skull bone and is highly vascular and innervated. There is no space between the dura and the cranium (Fig. 3-1B).
Thus, the cranial epidural space is a potential space that becomes filled with a fluid only in pathological conditions. The cranial epidural space (when present) is located between the periosteal layer of the dura and the cranium. The meningeal layer of the dura is smooth and avascular and is lined by mesothelium (a single layer of squamous-like, flattened cells) on its inner surface. At the foramen magnum (a large opening at the base of the occipital bone through which the medulla is continuous with the spinal cord), the meningeal layer of the cranial dura joins the spinal dura.
Figure 3-1. The coverings of the brain and spinal cord.
(A) The brain and spinal cord are covered with three membranes: dura, arachnoid, and pia mater. The periosteal and meningeal layers of the dura are separate at the dural sinuses (e.g., superior sagittal sinus). At other places, the dura consists of fused periosteal and meningeal layers. The space between the arachnoid and pial membranes is called the subarachnoid space. The subarachnoid space is enlarged at some places (e.g., cisterna magna and chiasmatic cistern). Small tufts of arachnoidal tissue (arachnoid villi) project into the dural venous sinuses. Other structures are shown for orientation purposes. CSF = cerebrospinal fluid. (B) Magnified view of the dura, arachnoid, and pia maters.
Sheet-like processes, called septa, extend from the meningeal layer of the dura deep into the cranial cavity, forming freely communicating compartments. The function of these septa is to reduce or prevent displacement of the brain when the head moves. One of the septa, the falx cerebri, is vertically oriented, divides the cranium into two lateral compartments, and separates the two cerebral hemispheres. The tentorium cerebelli is attached dorsally to the falx cerebri in the midline and posteriorly to the ridges of the occipital bone.
Its rostral edge is free and forms the boundary of the tentorial notch through which the midbrain traverses. The tentorium cerebelli forms a tent-like roof over the posterior cranial fossa. The occipital lobes lie on the dorsal surface of the tentorium cerebelli, whereas the dorsal surface of the cerebellum lies inferior to it. The falx cerebelli consists of a vertically oriented triangular projection into the posterior fossa. It partially separates the cerebellar hemispheres located in the posterior fossa.
The anterior part of the dura is supplied by the anterior meningeal arteries (which arise from the anterior ethmoidal branches of the ophthalmic arteries), the posterior part is supplied by the branches of the vertebral and occipital arteries, and the lateral aspect is supplied by the middle meningeal artery and its branches. Some of the branches of these vessels supply bones of the scalp. As a result of severe head injury, these vessels may become damaged, leading to a hematoma that can cause a variety of neurological deficits. The dura is drained by meningeal veins that travel parallel to the meningeal arteries. Further discussion of the vascular supply of the meninges is presented in Chapter 4.
The location of arachnoid mater and the structures associated with it are shown in Figure 3-1. This membrane lies between the dura and pia mater. It is a delicate, avascular membrane and surrounds the brain loosely without projecting into sulci. The space between the arachnoid and pial membranes, called the subarachnoid space, is filled with cerebrospinal fluid (CSF). The formation and distribution of CSF are described later in this chapter.
Fine strands of connective tissue, called arachnoid trabeculae, arise from the arachnoid, span the subarachnoid space, and then connect with the pia. These trabeculae help to keep the brain suspended within the meninges. At several places in the cranial cavity, the subarachnoid space is enlarged; these enlargements are called subarachnoid cisterns. The cerebellomedullary cistern, located between the medulla and the cerebellum, is the largest cistern and is accordingly called the cisterna magna (Fig. 3-1).
To identify pathological processes, such as those caused by tumors in the brain, it is essential to use radiological procedures to visualize subarachnoid cisterns adjacent to the suspected site of the pathological process. For example, the chiasmatic cistern is located adjacent to the optic chiasm (Fig. 3-1), so to identify pathological processes adjacent to the optic chiasm, radiological visualization of the chiasmatic cistern may be necessary.
Small tufts of arachnoidal tissue, called arachnoid villi, project into the superior sagittal sinus (Fig. 3-1) and other dural sinuses. Large aggregations of arachnoid villi are called arachnoid granulations. The arachnoid villi consist of a spongy tissue with many interconnecting small tubules and function as one-way valves. The CSF flows from the subarachnoid space into the dural venous sinuses through arachnoid villi, but the blood from the dural venous sinuses cannot flow back into the subarachnoid space via these villi.
Normally, the pressure in the subarachnoid space is greater (about 200 mm H2O) than that in the dural venous sinuses (about 80 mm H2O); this pressure difference promotes the CSF flow into the dural venous sinuses through the fine tubules located in the arachnoid villi. However, even if the pressure in the dural venous sinuses exceeds that of the subarachnoid space, the blood from the dural sinuses does not flow back into the subarachnoid space because the tubules in the arachnoid villi collapse.
The location of pia mater is shown in Figure 3-1. This membrane is the innermost layer of the meninges. It is tightly attached to the surface of the brain and projects into the fissures as well as the sulci. Pia mater consists of small plexuses of blood vessels that are embedded in connective tissue and is externally covered with mesothelial cells (a single layer of flattened cells).
When small branches of blood vessels penetrate the brain tissue, they carry with them a cuff of pia and arachnoid into the brain for a short distance creating a small space, called the perivascular space, around the vessel. This space is continuous with the subarachnoid space. It has been suggested that the perivascular space may serve as a channel for movement of CSF into the brain tissue, but its exact function has not been established with certainty.
Coverings of the Spinal Cord
The conical-shaped caudal end of the spinal cord, known as the conus medullaris, is located at the caudal edge of the first or rostral edge of the second lumbar vertebra (Fig. 3-2). A thin filament enclosed in pia and consisting of ependymal cells and astrocytes (for the description of these cells, see Chapter 5) emerges from the conus medullaris. This filament is called the filum terminale internum. It extends from the conus medullaris and passes through the caudal end of the dural sac (which ends at the second sacral vertebra). At this level (S2), a caudal thin extension of the spinal dura, called the coccygeal ligament (filum terminale externum) surrounds the filum terminale. It emerges and anchors the dural sac to the vertebral canal.
Figure 3-2. The spinal cord.
(A) The lumbar cistern extends from the caudal end of the spinal cord (conus medullaris) to the second sacral vertebra (S2). The subarachnoid space (widest in this region) contains the filum terminale internum (a thin filament). L = lumbar. (B) The subarachnoid space in the lumbar cistern also contains the cauda equina (a bundle of nerve roots of all the spinal nerves caudal to the second lumbar vertebra). (C) The three membranes of the spinal cord: the dura, arachnoid, and pia mater. The dorsal and ventral sides of the spinal cord, spinal nerves, the dorsal and ventral roots of the spinal nerves, and dorsal root ganglion are shown for orientation purposes.
The spinal cord is also covered by three membranes: the spinal dura, arachnoid, and pia mater (Fig. 3-2C). These coverings are generally similar to those of the brain. However, there are some differences. First, the spinal dura is single-layered and lacks the periosteal layer of the cranial dura. Second, the spinal epidural space is an actual space in which venous plexuses are located and is used clinically for the administration of epidural anesthesia to produce a paravertebral nerve block.
(The cranial epidural space is a potential space that becomes filled with a fluid only in pathological conditions.) Third, the spinal epidural space is located between the meningeal layer of the dura (there is no periosteal layer) and the periosteum of the vertebra, whereas the cranial epidural space (when present) is located between the periosteal layer of the dura and the cranium.
Spinal Dura Mater
The spinal dura mater consists of only the meningeal layer and lacks the periosteal layer of the cranial dura. Rostrally, the spinal dura joins the meningeal layer of the cranial dura (Fig. 3-1) at the margins of the foramen magnum. The spinal epidural space separates the spinal dura from the periosteum of the vertebra and is filled with fatty connective tissue and plexuses of veins. Caudally, the spinal dura ends at the level of the second sacral vertebra (Fig. 3-2A). As mentioned earlier, at this level, it becomes a thin extension (the coccygeal ligament or filum terminale externum) and serves to anchor the fluid-filled spinal dural sac to the base of the vertebral canal.
Spinal Arachnoid Mater
The spinal arachnoid mater invests the spinal cord and is connected to the dura via connective tissue trabeculae (Fig. 3-2C). Rostrally, it passes through the foramen magnum to join the cranial arachnoid, and caudally it surrounds the cauda equina. The cauda equina consists of a bundle of nerve roots of all the spinal nerves caudal to the second lumbar vertebra (Fig. 3-2B).
Spinal Pia Mater
The spinal pia mater (Fig. 3-2C) is thicker than the cranial pia mater. It is a vascular membrane and projects into the ventral fissure of the spinal cord. At intervals, toothed ligaments of pial tissue, called dentate ligaments, extend from the lateral surfaces of the spinal cord; these ligaments serve to anchor the spinal cord to the arachnoid and the inner surface of the dura.
The lumbar cistern extends from the caudal end of the spinal cord to the second sacral vertebra. The subarachnoid space (Fig. 3-2) is widest in this region and contains the filum terminale internum and nerve roots of the cauda equina. Because of the large size of the subarachnoid space and relative absence of neural structures, this space is most suitable for the withdrawal of CSF by lumbar puncture.
This procedure is used to gain specific information about the cellular and chemical composition of the CSF in disorders such as meningitis. As noted earlier, the caudal end of the spinal cord in the normal adult is located at the caudal end of the first (L1) or rostral edge of the second (L2) lumbar vertebra. Therefore, a needle for lumbar puncture is usually inserted between the third and fourth lumbar vertebrae (L3–L4) in the adult patient.
In children, the caudal end of the spinal cord is usually located at the third lumbar vertebra (L3). Therefore, the needle for lumbar puncture is inserted at the L4–L5 level in children (Fig. 3-2A). Typically 5 to 15 mL of the CSF is removed during the lumbar puncture to perform the cell count, protein analysis, and microbiological studies. For the procedure of lumbar puncture, the patient is placed in a lateral recumbent position, and the CSF pressure is measured by a manometer.
Normally, the CSF pressure is between 100 and 150mm H2O (< 200mm H2O) in the adult person and between 60 and 150mm H2O (< 180mm H2O) in young children and infants. If the intracranial pressure (ICP) is high, withdrawal of CSF is contraindicated because brain tissue may get herniated through the foramen magnum.