Brain Protection

Brain Protection

The brain is surrounded by bones, fluids, and membranes. It lies inside the skull’s cranial cavity and is soft and delicate. Between the bony coverings and the soft brain tissues are layered membranes known as meninges­ that protect the brain and spinal cord (­FIGURE 12- 12). The singular term meninx describes just one of the meninges. Also, the CSF cushions the brain and the blood–brain barrier protects it from harmful substances carried in the blood. There is also a blood–CSF barrier, which is formed by specialized ependymal cells surrounding the capillaries of the choroid plexus.

Meninges

There are three layers of membranes in the menin-ges: the dura mater, arachnoid mater, and pia mater. The outermost layer (dura mater) is made up of fibrous, tough, white connective tissue. It has many blood vessels and nerves and attaches to the inside of the cranial cavity; it also extends inward between the brain lobes to form protective partitions. It con-tinues into the vertebral canal to surround the spinal cord, ending in a sac at its end. The dura mater has two layers of fibrous connective tissue. Its periosteal layer, which is more superficial, attaches to the peri-osteum (the inner surface of the skull). Around the spinal cord, there is no dural periosteal layer. The meningeal layer actually covers the brain, continu-ing caudally as the spinal dura mater in the vertebral canal. The two dural layers of the brain are fused in most areas. In certain places they separate, enclosing dural venous sinuses , which collect blood from the brain and channel it to the internal jugular veins in the neck.

Dural septa limit excessive brain movement and are formed from the meningeal dura mater. It con-tains three primary features. Its falx cerebri is a large fold that dips into the longitudinal fissure between the hemispheres of the cerebrum. It contains two large venous sinuses: the superior sagittal and the inferior sagittal. The falx cerebelli continues inferiorly from the more posterior falx cerebri along the vermis of the cerebellum. The tentorium cerebelli is a nearly hor-izontal dural fold extending into the transverse fissure and cerebellum. It contains the transverse sinus.

The membrane around the spinal cord has an ­epidural space separating it from the vertebrae. The epidural space contains loose adipose and con-nective tissues, protecting the spinal cord. The thin, web-like arachnoid mater lies between the dura and pia maters. The subdural space is a narrow serous cavity that contains a fluid film.

The thin pia mater has many blood vessels and nerves that nourish the brain and the spinal cord. The pia mater is closely aligned with the surfaces of these organs. It is comprised of many tiny blood vessels and delicate connective tissue. The pia mater is bound tightly to the brain and its convolutions. Small ragged bits of pia mater are briefly carried by small arteries entering the brain. There are also spinal meninges, discussed later.

Cerebrospinal Fluid

Between the arachnoid and pia maters is a sub-arachnoid space containing the watery and clear cerebrospinal­ fluid (CSF). This is similar to blood plasma but contains less protein. CSF contains more sodium, chloride, and hydrogen ions but less calcium and potassium than blood plasma. Inside the sub-arachnoid space are web -like extensions that func-tion partially to bind the arachnoid mater to the pia mater. This space also contains CSF and the primary brain blood vessels. However, these blood vessels are not protected well because the arachnoid mater is very fine and elastic. Arachnoid villi are knob-like ­projections that protrude superiorly through the dura mater into the superior sagittal sinus. They absorb CSF into the venous blood of the sinus. In adults, clusters of arachnoid villi form large arachnoid granulations, where CSF is actually absorbed into the venous circulation­.

Small red choroid plexuses secrete CSF and project into the brain ventricles. Most CSF is formed in the lateral ventricles. CSF also enters the menin-ges’ subarachnoid space via the two lateral apertures and the single median aperture and is reabsorbed into the blood. CSF surrounds the brain and spinal cord, maintaining a stable ionic concentration and protect-ing CNS structures. The brain floats in CSF, which cushions it and prevents the bottom of the brain from being crushed by its own weight. The CSF also helps to nourish the brain and may assist in carrying chemical signals concerning sleep and appetite. It also may carry hormones. The total CSF volume is about 150 mL, which is replaced every eight hours.

Blood Supply to the Brain

The human brain’s billions of neurons continuously demands oxygen and nutrients. The neurons do not have carbohydrates or lipids as energy reserves. They also do not have myoglobin as an oxygen reserve. Therefore, the blood supply to the brain is extensive. Arterial blood is supplied through the internal carotid arteries and vertebral arteries. The majority of the brain’s venous blood leaves the cranium through the internal jugular veins that drain the dural sinuses. If a head injury damages cerebral blood vessels, there may be bleeding into the dura mater. This can occur near the dural epithelium or between the outer dura mater and skull bones. Both are serious conditions, since blood entering these spaces will compress and distort the brain’s soft tissues.

Blood–Brain Barrier

The brain requires a constant internal environment to function normally. To maintain this, the blood– brain barrier acts to selectively allow certain mol-ecules to pass and to keep others from reaching the brain. The maintenance of a constant environment keeps the brain’s neurons from firing uncontrolla-bly. Before bloodborne substances can move from brain capillaries to reach neurons, three layers of the blood–brain barrier await them: the capillary wall endothelium, a thick basal lamina surround-ing every capillary’s external side, and bulb-like feet of the astrocytes that are bound to the capillaries.

These portions of astrocytes give signals to the endo-thelial cells and cause them to form tight junctions. The junctions form the actual barrier by causing a seamless pattern of endothelial cells. This capillary barrier is the least permeable of all capillary struc-tures in the body.

The selective blood–brain barrier allows certain electrolytes, nutrients, and essential amino acids to pass through. It does not allow passage of proteins, bloodborne metabolic wastes, most drugs, and specific toxins. Also, nonessential amino acids and potassium ions are actively pumped across the capillary endothe-lium away from the brain. The blood–brain barrier is not effective against fatty acids, fats, carbon dioxide, oxygen, and other fat-soluble molecules that can easily diffuse through the body’s plasma membranes. There-fore, bloodborne anesthetics, alcohol, and nicotine affect the brain.

The blood–brain barrier is not uniform through-out the brain and is even absent near the third and fourth ventricles. In the brain’s vomiting center, blood-borne molecules can easily cross to the neural tissue, but this is part of this center’s monitoring for poisons. The hypothalamus also does not have a barrier, allow-ing it to sample the blood’s chemical composition so it can regulate many metabolic activities. In newborns and premature infants, the blood–brain barrier is not fully formed. As a result, certain toxins can enter the CNS and cause certain conditions that do not gener-ally affect adults. Also, the blood–brain barrier can be broken down by brain injuries. When these occur, the capillary endothelial cells or their tight junctions are commonly affected.