by Isaac Asimov
The spinal cord consists of gray matter (at the center) and white matter (on the periphery); to it are attached a series of nerves that are largely concerned with the internal organs—heart, lungs, digestive system, and so on—organs that are more or less under involuntary control.
In general, when the spinal cord is severed, through disease or through injury, that part of the body lying below the severed segment is disconnected, so to speak. It loses sensation and is paralyzed. If the cord is severed in the neck region, death follows, because the chest is paralyzed, and with it the action of the lungs. It is this that makes a broken neck fatal, and hanging a feasible form of quick execution. It is the severed cord, rather than a broken bone, that is fatal.
The entire structure of the central nervous system, consisting of cerebrum, cerebellum, brain stem, and spinal cord, is carefully coordinated. The white matter of the spinal cord is made up of bundles of nerve fibers that run up and down the cord, unifying the whole. Those that conduct impulses downward from the brain are the descending tracts, and those that conduct them upward to the brain are the ascending tracts.
In 1964, research specialists at Cleveland’s Metropolitan General Hospital reported the isolation from rhesus monkeys of brains that were then kept independently alive for as long as eighteen hours. This offers the possibility of detailed specific study of the brain’s metabolism through a comparison of the nutrient medium entering the blood vessels of the isolated brain and of the same medium leaving it.
The next year they were transplanting dogs’ heads to the necks of other dogs, hooking them up to the host’s blood supply, and keeping the brains in the transplanted heads alive and working for as long as two days. By 1966, dogs’ brains were lowered to temperatures near freezing for six hours and then revived to the point of showing clear indications of normal chemical and electrical activity. Brains are clearly tougher than they might seem to be.
Nerve Action
It is not only the various portions of the central nervous system that are hooked together by nerves, but, clearly, all the body that, in this fashion, is placed under the control of that system. The nerves interlace the muscles, the glands, the skin; they even invade the pulp of the teeth (as we learn to our cost at every toothache).
The nerves themselves were observed in ancient times, but their structure and function were consistently misunderstood. Until modern times, they were felt to be hollow and to function as carriers of a subtle fluid. Rather complicated theories developed by Galen involved three different fluids carried by the veins, the arteries, and the nerves, respectively. The fluid of the nerves, usually referred to as animal spirits, was the most rarefied of the three. Galvani’s discovery that muscles and nerves could be stimulated by an electric discharge laid the foundation for a series of studies that eventually showed nerve action to be associated with electricity—a subtle fluid, indeed, more subtle than Galen could have imagined.
Specific work on nerve action began in the early nineteenth century with the German physiologist Johannes Peter Muller, who, among other things, showed that sensory nerves always produce their own sensations regardless of the nature of the stimulus. Thus, the optic nerve registers a flash of light, whether stimulated by light itself or by the mechanical pressure of a punch in the eye. (In the latter case, you “see stars.”) This emphasizes that our contact with the world is not with reality at all but with specialized stimuli that the brain usually interprets in a useful manner, but can interpret in a non-useful manner.
Study of the nerves was advanced greatly in 1873, when an Italian physiologist, Camillo Golgi, developed a cellular stain involving silver salts that was well adapted to react with nerve cells, making clear their finest details. He was able to show, in this manner, that nerves are composed of separate and distinct cells, and that the processes of one cell might approach very closely to those of another, but that they do not fuse. There remained the tiny gap of the synapse. In this way, Golgi bore out, observationally, the contentions of a German anatomist, Wilhelm von Waldeyer, to the effect that the entire nervous system consists of individual nerve cells or neurons (this contention being termed the neuron theory).
Golgi did not, however, himself support the neuron theory. This proved to be the task of the Spanish neurologist Santiago Ramon y Cajal, who, by 1889, using an improved version of Colgi’s stain, worked out the connections of the cells in the gray matter of the brain and spinal cord and fully established the neuron theory. Golgi and Ramon y Cajal, although disputing the fine points of their findings, shared the Nobel Prize for medicine and physiology in 1906.
These nerves form two systems: the sympathetic and the parasympathetic. (The terms date back to semimystical notions of Galen.) Both systems act on almost every internal organ, exerting control by opposing effects. For instance, the sympathetic nerves act to accelerate the heartbeat, the parasympathetic nerves to slow it; the sympathetic nerves slow up secretion of digestive juices, the parasympathetic stimulate such secretions, and so on. Thus, the spinal cord, together with the subcerebral portions of the brain, regulates the workings of the organs in an automatic fashion. This set of involuntary controls was investigated in detail by the British physiologist John Newport Langley in the 1890s, and he named it the autonomic nervous system.
REFLEX ACTION
In the 1830s, the English physiologist Marshall Hall had studied another type of behavior which seemed to have voluntary aspects but proved to be really quite involuntary. When you accidentally touch a hot object with your hand, the hand draws away instantly. If the sensation of heat had to go to the brain, be considered and interpreted there, and evoke the appropriate message to the hand, your hand would be pretty badly scorched by the time it got the message. The unthinking spinal cord disposes of the whole business automatically and much faster. It was Hall who gave the process the name reflex.
The reflex is brought about by two or more nerves working in coordination, to form a reflex arc (figure 17.3). The simplest possible reflex arc is one consisting of two neurons, a sensory (bringing sensations to a reflex center in the central nervous system, usually at some point in the spinal cord) and a motor (carrying instructions for movement from the central nervous system).
Figure 17.3. The reflex arc.
The two neurons may be connected by one or more connector neurons. A particular study of such reflex arcs and of their function in the body was made by the English neurologist Charles Scott Sherrington, who won a share in the 1932 Nobel Prize for medicine and physiology in consequence. It was Sherrington who, in 1897, coined the word synapse.
Reflexes bring about so rapid and certain a response to a particular stimulus that they offer simple methods for checking the general integrity of the nervous system. A familiar example is the patellar reflex or, as it is commonly called, the knee jerk. When the legs are crossed, a sudden blow below the knee of the upper leg will cause it to make a quick, kicking motion—a fact first brought into medical prominence in 1875 by the German neurologist Carl Friedrich Otto Westphal. The patellar reflex is not important in itself, but its nonappearance can mean some serious disorder involving the portion of the nervous system in which that reflex arc is to be found.
Sometimes damage to a portion of the central nervous system brings about the appearance of an abnormal reflex. If the sole of the foot is scratched, the normal reflex brings the toes together and bent downward. Certain types of damage to the central nervous system will cause the big toe to bend upward in response to this stimulus, and the little toes to spread apart as they bend down. This is the Babinski reflex, named for a French neurologist, Joseph Francois Felix Babinski, who described it in 1896.
In human beings, reflexes are sometimes decidedly subordinate to the conscious will. Thus, you may up your rate of breathing when ordinary reflex action would keep it slow and so on. The lower phyla of animals are much more strictly controlled by their reflexes than human beings are and also have them far more highly developed.
One of the best examples is a spider spinning its web. Here the reflexes produce such an elaborate pattern of behavior that it is difficult to think of it as mere reflex action; instead, it is usually called instinctive behavior. (Because the word instinct is often misused, biologists prefer the term innate behavior.) The spider is born with a nerve-wiring system in which the switches have been preset, so to speak. A particular stimulus sets it off on weaving a web, and each act in the process in turn acts as a stimulus determining the next response.
Looking at the spider’s intricate web, built with beautiful precision and effectiveness for the function it will serve, it is almost impossible to believe that the thing has been done without purposeful intelligence. Yet the very fact that the complex task is carried through so perfectly and in exactly the same way every time is itself proof that intelligence has nothing to do with it. Conscious intelligence, with the hesitations and weighings of alternatives that are inherent in deliberate thought, will inevitably give rise to imperfections and variations from one construction to another.
With increasing intelligence, animals tend more and more to shed instincts and inborn skills. Thereby they doubtless lose something of value. A spider can build its amazingly complex web perfectly the first time, although it has never before seen web spinning or even a web. Human beings, on the other hand, are born almost completely unskilled and helpless. A newborn baby can automatically suck on a nipple, wail if hungry, and hold on for dear life if about to fall, but can do very little else. Every parent knows how painfully and with what travail a child comes to learn the simplest forms of suitable behavior. And yet, a spider or an insect, though born with perfection, cannot deviate from it. The spider builds a beautiful web, but if its preordained web should fail, it cannot learn to build another type of web. A child, on the other hand, reaps great benefits from being unfettered by inborn perfection. One may learn slowly and attain only imperfection at best, but one can attain a variety of imperfections of one’s own choosing. What human beings have lost in convenience and security, they have gained in an almost limitless flexibility.
Recent work, however, emphasizes the fact that there is not always a clear division between innate and learned behavior not only in the case of human feedback but among lower animals as well. It would seem, on casual observation, for instance, that chicks or ducklings, fresh out of the shell, follow their mothers out of instinct. Closer observation shows that they do not.
The instinct, however, is not to follow their mother but merely to follow something of a characteristic shape or color or faculty of movement. Whatever object provides this sensation at a certain period of early life is followed by the young creature and is thereafter treated as the mother. This may really be the mother; it almost invariably is, in fact, but it need not be! In other words, following is instinctive, but the “mother” that is followed is learned. (Much of the credit for this discovery goes to the remarkable Austrian naturalist Konrad Zacharias Lorenz. Lorenz, during the course of studies now some thirty years old, was followed hither and yon by a gaggle of goslings.)
The establishment of a fixed pattern of behavior in response to a particular stimulus encountered at a particular time of life is called imprinting. The specific time at which imprinting takes place is a critical period. For chicks, the critical period of mother imprinting lies between thirteen and sixteen hours after hatching. For a puppy there is a critical period between three and seven weeks, during which the stimulations it is usually likely to encounter imprint various aspects of what we consider normal doggish behavior.
Imprinting is the most primitive form of learned behavior, one that is so automatic, takes place inso limited a time, and under so general a set of conditions that it is easily mistaken for instinct.
A logical reason for imprinting is that it allows a certain desirable flexibility. If a chick were born with some instinctive ability of distinguishing its true mother so that it might follow only her, and if the true mother were for any reason absent in the chick’s first day of life, the little creature would be helpless. As it is, the question of motherhood is left open for just a few hours, and the chick may imprint itself to any hen in the vicinity and thus adopt a foster mother.
ELECTRICAL IMPULSES
As stated earlier, it had been Galvani’s experiments just before the opening of the nineteenth century that had first indicated some connection between electricity and the actions of muscle and nerve.
The electrical properties of muscle led to a startling medical application, thanks to the work of the Dutch physiologist Willem Einthoven. In 1903, he developed an extremely delicate galvanometer, one delicate enough to respond to the tiny fluctuations of the electric potential of the beating heart. By 1906, Einthoven was recording the peaks and troughs of this potential (the recording being an electrocardiogram) and correlating them with various types of heart disorder.
The more subtle electrical properties of nerve impulses were thought to have been initiated and propagated by chemical changes in the nerve. This was elevated from mere speculation to experimental demonstration by the nineteenth-century German physiologist Emil Du Bois-Reymond, who by means of a delicate galvanometer was able to detect tiny electric currents in stimulated nerves.
With modern electronic instruments, researches into the electrical properties of the nerve have been incredibly refined. By placing tiny electrodes at different spots on a nerve fiber and by detecting electrical changes through an oscilloscope, it is possible to measure a nerve impulse’s strength, duration, speed of propagation, and so on. For such work, the American physiologists Joseph Erlanger and Herbert Spencer Gasser were awarded the 1944 Nobel Prize for medicine and physiology.
If you apply small electric pulses of increasing strength to a single nerve cell, up to a certain point there is no response whatever. Then suddenly the cell fires: an impulse is initiated and travels along the fiber. The cell has a threshold: it will not react at all to a stimulus below the threshold; and to any stimulus above the threshold, it will respond only with an impulse of a certain fixed intensity. The response, in other words, is “all or nothing.” And the nature of the impulse elicited by the stimulus seems to be the same in all nerves.
How can such a simple yes-no affair, identical everywhere, lead to the complex sensations of sight, for instance, or to the complex finger responses involved in playing a violin? It seems that a nerve, such as the optic nerve, contains a large number of individual fibers, some of which may be firing and others not, and where the firing may be in rapid succession or slowly, forming a pattern, possibly a complex one, shifting continuously with changes in the over-all stimulus. (For work in this field, the English physiologist Edgar Doulas Adrian shared, with Sherrington, the 1932 Nobel Prize in medicine and physiology.) Such a changing pattern may be continually scanned by the brain and interpreted appropriately. But nothing is known about how the interpretation is made or how the pattern is translated into action such as the contraction of a muscle or secretion by a gland.
The firing of the nerve cell itself apparently depends on the movement of ions across the membrane of the cell. Ordinarily, the inside of the cell has a comparative excess of potassium ions, while outside the cell there is an excess of sodium ions. Somehow the cell holds potassium ions in and keeps sodium ions out so that the concentrations on the two sides of the cell membrane do not equalize. It is now believed that a sodium pump of some kind inside the cell keeps pumping out sodium ions as fast as they come in. In any case, there is an electric potential difference of about 1/10 volt across the cell membrane, with the inside negatively charged with respect to the outside. When the nerve cell is stimulated, the potential difference across the membrane collapses, and this represents the firing of the cell. It takes a couple of thousandths of a second for the potential difference to be re-established; and during that interval, the nerve will not react to another stimulus. This is the refractory period.
Once the cell fires, the nerve impulse travels down th
e fiber by a series of firings, each successive section of the fiber exciting the next in turn. The impulse can travel only in the forward direction, because the section that has just fired cannot fire again until after a resting pause.
Research that related, in the fashion just described, nerve action and ion permeability led to the award of the 1963 Nobel Prize for medicine and physiology to two British physiologists, Alan Lloyd Hodgkin and Andrew Fielding Huxley, and to an Australian physiologist, John Carew Eccles.
What happens, though, when the impulse traveling along the length of the nerve fiber comes to a synapse—a gap between one nerve cell and the next? Apparently, the nerve impulse also involves the production of a chemical that can drift across the gap and initiate a nerve impulse in the next nerve cell. In this way, the impulse can travel from cell to cell.
One of the chemicals definitely known to affect the nerves is the hormone adrenalin. It acts upon nerves of the sympathetic system, which slows the activity of the digestive system and accelerates the rate of respiration and the heartbeat. When anger or fear excites the adrenal glands to secrete the hormone, its stimulation of the sympathetic nerves sends a faster surge of blood through the body, carrying more oxygen to the tissues; and by slowing down digestion for the duration, it saves energy during the emergency.
The American psychologists and police officers John Augustus Larsen and Leonard Keeler took advantage of this finding in 1921 to devise a machine to detect the changes in blood pressure, pulse rate, breathing rate, and perspiration brought on by emotion. This device, the polygraph, detected the emotional effort involved in telling a lie, which always carries with it the fear of detection in any reasonably normal individual and therefore brings adrenalin into play. While far from infallible, the polygraph has gained great fame as a lie detector.