Metabolism and Protection of our Nervous System
The complicated and busy nervous system consumes energy at an astounding rate. Nerve cells, therefore, require far more fuel, oxygen and glucose, than other cells; thus they must have a continuous and rich supply of blood or they will die quickly. Nerve cells in the brain alone use up as much as 20 % of all the oxygen available in the adult body, for they are working at a frantic pace for 24 hours a day. That may seem curious, since we think of sleep as "giving the brain a rest." But the brain is indisputably active during sleep periods. Quite aside from maintaining breathing and the rate of heartbeat, the brain may use these periods to try to search out answers to particularly difficult problems and to reconsider experiences of the day that were flowing in too fast to be dealt with carefully at the time.Night or day, the brain's need for constant supplies of blood and the oxygen it carries never slows. Normally, during the course of every 24 hours, we lose some neurons simply because they wear out and die. Fortunately, great numbers would have to die to produce any noticeable effect, but the small losses inevitably accumulate. Neurologists today believe that the slow loss of mental efficiency that occurs in old age, the process we call senility, is a direct result of day-to-day destruction of a few neurons here and there, until the total loss begins to mount up and is accelerated by narrowing of blood vessels to the brain. Whatever the cause of the loss, once a neuron is destroyed, it ceases to exist. No new nerve cell will ever take its place; a year-old baby has all the neurons it will ever have. If a nerve fiber is only cut, and the cell itself is not killed, a new fiber may eventually grow out along the course of the cut fiber. But if the nucleus of the neuron is destroyed, the cell is gone forever. The only exception is the olfactory system, in which neurons are continuously stimulated to grow.
Because the body could not tolerate the loss of too many neurons, and because nervous tissue is very fragile, the nervous system operates behind an impressive wall of protection. The brain is encased in the tough armor plate of the skull. The spinal cord is enclosed in a strong but flexible bony sheath made up of the vertebrae. Both brain and spinal cord, furthermore, are surrounded by a clear cerebrospinal fluid which serves, among other purposes, as a shock absorber.
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The rest of the nervous system is also well protected. The nerve trunks throughout the body are deeply buried between layers of muscle, except in such rare places as the point where the ulnar nerve crosses the elbow just beneath the skin (Figure 8). This is the easily shockable "funny bone." The shock that vibrates in the fingers and courses up the spinal cord to the brain when the back of the elbow is struck is really an electric impulse, originating not in a bone but in the ulnar nerve. This nerve extends the entire length of the arm and is well protected everywhere except at that one tender spot. (The misleading term "funny bone" may have stemmed from a word play on "humerus," the large bone in the upper arm.)
Many of the delicate receptor endings of sensory nerves are also carefully shielded. The nerve endings in the retina lie at the back of the eye, which is buried in a deep bony socket; they are protected from too much light by the self-adjusting shutterlike covering of the iris. Sound receptors and balance receptors are encased deep within the ear in tiny bony caverns in the skull. Certain receptors, such as those sensitive to pain, touch, heat and cold, which lie at or near the surface of the skin, are relatively exposed. They are so widely distributed, however, that the loss of a few here and there is not usually important.
Under this massive cloak of protection, the nervous system conducts its complex activities with efficiency and experience. Nerve impulses are transmitted so rapidly that a whole sequence of actions, interpretations, and responses can take place before we are even aware that anything has happened. In fact, a single stimulus may fire off thousands upon thousands of responses within a fraction of a second. After being accustomed to operating in a gravitational environment, however, the brain and nervous system must be reprogrammed in space to allow for many strange new signals and new movement possibilities, such as floating. This results in new levels of disorientation for the first few hours or days of weightlessness. The problem is that the brain and nervous system have no experience in the microgravity environment of space. In fact, over two-thirds of all astronauts experience space motion sickness for up to the first three days in space. The symptoms of this malady are similar in many respects to the symptoms of motion sickness on Earth. Such space sickness appears to continue until the brain and nervous system gain some microgravity experience and begin to adapt. A great deal of work is being done to characterize, understand, and develop strategies to alleviate this problem. We are going to look at some of the things that are being done regarding space motion sickness, and we will examine some other very interesting effects of space flight on the sensory and balance system. We're coming closer to the end of the book, so let's move on!