CENTRAL NERVOUS SYSTEM METABOLISM

1. What is the principal energy source for the CNS? How does it get into brain cells?

2. What percent of the body’s oxygen requirement is used by the CNS?

3. What is meant by the blood-brain barrier?

4.

5. Is there a cerebrospinal fluid-brain barrier?

6. What are some maximum limits of oxygen deprivation to the CNS before injury occurs?

The CNS receives its energy principally from carbohydrates, of which glucose is an important source. Unlike many tissues of the body, which require insulin for facilitated diffusion of glucose across cell membranes, the CNS receives glucose by simple diffusion and insulin is not required. This is advantageous for the animal when insulin is lacking or in short supply because it enables the CNS function to continue when other systems fail.

The relatively high rate of metabolism of the CNS compared with that of other tissues can be shown by noting its oxygen consumption. Although the CNS constitutes only 2% of body mass, it consumes approximately 20% of the total oxygen supplied to the body. Also, the metabolic rate of gray matter is three to four times higher than that of white matter.

Blood-Brain Barrier

Many substances in the blood do not readily enter the cells of the CNS, a limitation referred to as the blood-brain barrier. The capillaries of the CNS have tight junctions between their endothelial cells rather than slit pores, which limit the diffusion,of substances from capillaries. Lipid-soluble substances, however, such as oxygen and carbon dioxide, readily diffuse. Transport for most substances is provided for by the CNS cells known as astrocytes (a glial cell), which are interposed between the capillaries and CNS cells. Astrocytes are selective regarding the materials they transport - hence, the blood-brain barrier.

A barrier also exists between the cells of the choroid plexus and the cerebrospinal fluid that is provided for by the choroid plexus cells. A barrier probably exists between the cerebrospinal fluid and pia mater for certain substances, but most substances usually diffuse readily between the cerebrospinal fluid and the brain. Drugs in the blood might have no effect on the brain, but when placed directly into the cerebrospinal fluid, they can have a profound effect.

Blood Requirement

The CNS must have a continuous supply of blood for normal functioning. Other tissues can be deprived of a blood supply for extended periods and recover to normal function when the blood supply resumes. Five to 10 minutes of little or no blood to the brain injures higher brain cells (in the cerebrum) so that no recovery occurs. Respiratory and cardiovascular centers (in the medulla oblongata) are more resistant to hypoxia (deficient oxygen) and revival has occurred after 10 minutes without blood. The tolerance of an adult brain to hypoxia is much lower than the tolerance of a newborn brain.

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