THE SIGHT of breathtaking alpine vistas. The fragrance of homemade bread. The sound of that most-loved voice. A skilled musician performing a violin concert. The steady beat of the heart. Diverse experiences, indeed, but all require the performance of the nerve cells for the desired function. How do they accomplish this?
Because the nervous system is so in credibly complex and the integrated control and relay centers so numerous, modern science has only begun to understand its function. Yet, just as intricate man-made computers are constructed of wires and relatively simple electronic devices, so the human nervous system is made up of components that are relatively simple if viewed as single units. The great Spanish neuroanatomist Ramon y Cajal was the first to recognize that the nervous system was made up of numerous individual parts (neurons), each a functioning, living unit.
In early development inside the womb, all of our cells look very much the same. They all act similarly. Each has the complete blueprint necessary to produce a new individual. As development proceeds, however, various groups of cells begin to specialize. Some cells become manufacturers, and their sole purpose is to produce material that will promote the growth and function of other cells. Some specialize for movement, and we call them muscle cells. Still others are adapted for carrying things from one place to another.
The general rule is that a price must be paid by the cells as they specialize. The nerve cell is highly specialized, and it pays a very dear price. It can't go anywhere, it can't produce offspring or replace dead cells, but is uniquely specialized for carrying information from one place to another. Other cells can do this, but not as efficiently.
Every neuron, or nerve cell, has at least two long, thin projections extending away from its cell body. One of these carries impulses toward the cell body (dendrite); the other carries impulses away from the cell body (axon). Neurons have only one axon but may have up to several hundred dendrites branching out and resembling a tree without leaves. Therefore, the nerve cell is specialized for receiving input from numerous sources but has only one outlet for all of this information. The end of the axon may branch, but all the branches carry information from just the one axon. The tips of the many branches of a single axon contact other neurons. Often they communicate with a dendrite of another neuron. Thus a neuron is something like an octopus having a central body with long slender extensions reaching out and touching other nerve cells. Only one of these ex tensions carries information away from the cell body (Fig. 1).
Communication from a neuron to other neurons occurs at specialized con tact areas at the branching tips of its axon. These areas are called synapses (Fig. 2). Communication affects the receiving cell in one of two ways—either by promoting (excitation) or hindering (inhibition) its action.
When we realize that the brain is made up of at least 10 billion such neurons and that each of these cells makes contact with at least one hundred, and in some cases over a thou sand, other nerve cells we can see that the possibilities for interconnection and intercommunication are virtually unlimited! A single nerve cell, then, may receive literally hundreds and thou sands of impulses every second. Some of these are excitatory, that is, tell the nerve cell to answer Yes, and some are inhibitory and tell the nerve cell to answer No. The decision is made in the region of the cell body where the axon is attached, and it is the neuron's job to decide whether or not it should send off an impulse at that given moment. It does this on the basis of the total combined inputs it is receiving at that time.
The minimum number of positive messages required to send an impulse down the neuron's axon is called its threshold. For example, if the input at a given time is 75 excitatory charges and 15 inhibitory impulses, then the net sum (60 excitatory) of these will be such that the neuron will send an impulse out through its axon. However, if at a given moment the inhibitory impulses out number the excitatory impulses, this neuron will not send an impulse at that moment. In fact, it is prevented from doing this, and it would require more excitatory stimulations to allow the neuron to send an impulse than if the inhibition was not present.
Foundation of the Nervous System
This interplay between pluses and minuses, between positives and negatives, is the foundation of the nervous system and is the reason that certain functions can occur at one time and not at another. A nervous system would not function well if all its neurons were excitatory neurons. At the same time, however, the nervous system wouldn't be effective if there were only negative inputs.
Neurophysiologists are now able to listen in on the activities of individual nerve cells through the use of tiny pick up wires (microelectrodes), which can be placed inside these extremely small cells. This has yielded much information about the way in which these cells "talk" to one another. Research presently being conducted is helping us to better understand the nervous system.
As we have seen, the neuron is so constructed that the outflow of information is carried over the axon. Axons of most neurons are surrounded by a considerable amount of living insulation material called myelin. This tissue, made up of other cells that wrap them selves around the axon, closely resembles a length of carpet rolled up around a wooden stick (Fig. 3). The "wood" in the center would represent the nerve axon, and the layers of "carpet" would represent the layers of myelin. This myelin covering enables impulses to be sent over long distances, often several feet, with both rapid speed and virtually negligible loss of impulse magnitude.
Without myelin, axons do not conduct impulses nearly as fast, and would need greatly increased diameters to do so. The Swiss physiologist Von Muralt has calculated that if the rate of impulse conduction in man's brachial nerve, the one that supplies the arm, were achieved by the increased diameter of its axons, this nerve would be as large as the entire arm!
If, as a result of accident or disease, axons or dendrites are severed or other wise disrupted but the nerve cell body is not damaged, the nerve may regenerate these severed parts, but very slowly. If, however, the nerve-cell body is destroyed, the cell is lost forever, and no regeneration is possible.
This is one reason why we need to be careful of any practice that may directly or indirectly injure nerve cells—once they are lost they are gone forever. Obviously we should avoid this loss at all costs, for all of our communication with the world about us and our capacity for service—depends on nerve cells.
Messages to Young People sums it up this way: "The desire to honor God should be to us the most powerful of all motives. It should lead us to make every exertion to improve the privileges and opportunities provided for us, that we may understand how to use wisely the Lord's goods. It should lead us to keep brain, bone, muscle, and nerve in the most healthful condition, that our physical strength and mental clearness may make up faithful stewards." —Pages 149, 150.
