Blood Supply Blood Brain Barrier Phenomena and Cerebrospinal Fluid

As mentioned earlier, the brain lies within the skull, and the spinal cord within the vertebral column. Between the soft neural tissues and the bones that house them are three types of membranous coverings called meninges: the dura mater next to the bone, the arachnoid in the middle, and the pia mater next to the nervous tissue. A space, the subarachnoid space, between the arachnoid and pia is filled with cerebrospinal fluid (CSF). The meninges and their specialized parts protect and support the central nervous system, and they produce, circulate, and absorb the cerebrospinal fluid. (As described later, a portion of the cerebrospinal fluid is also formed in the cerebral ventricles.)

The cerebrospinal fluid circulates through the interconnected ventricular system to the brainstem, where it passes through small openings out to a space between the meninges on the surface of the brain and spinal cord (Figure 8-47). Aided by circulatory, respiratory, and postural pressure changes, the fluid ultimately flows to the top of the outer surface of the brain, where most of it enters the bloodstream through one way valves in large veins. Thus, the central nervous system literally floats in a cushion of cerebrospinal fluid. Since the brain and spinal cord are soft, delicate tissues with a consistency similar to Jello, they are somewhat protected by the shock-absorbing fluid from sudden and jarring movements. If the flow is obstructed, cerebrospinal fluid accumulates, causing hydrocephalus ("water on the brain"). In severe untreated cases, the resulting elevation of pressure in the ventricles leads to compression of the brain's blood vessels, which may lead to inadequate blood flow to the neurons, neuronal damage, and mental retardation.

Under normal conditions, glucose is the only substrate metabolized by the brain to supply its energy requirements, and most of the energy from the oxidative breakdown of glucose is transferred to ATP. The brain's glycogen stores being negligible, it is completely dependent upon a continuous blood supply of glucose and oxygen. In fact, the most common form of brain damage is caused by a stoppage of the blood supply to a region of the brain. When neurons in the region are without a blood supply and deprived of nutrients and oxygen for even a few minutes, they cease to function and die. This neuronal death results in a stroke.

Although the adult brain makes up only 2 percent of the body weight, it receives 12 to 15 percent of the

Vander et al.: Human Physiology: The Mechanism of Body Function, Eighth Edition

II. Biological Control Systems

8. Neural Control Mechanisms

© The McGraw-Hill Companies, 2001

Neural Control Mechanisms CHAPTER EIGHT

Neural Control Mechanisms CHAPTER EIGHT

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Essentials of Human Physiology

Essentials of Human Physiology

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