A number of agents used in the analysis of axoplasmic transport were noted in the preceding chapters. In this chapter, they are set out in systematic fashion on the basis of what is known of its mechanism. In addition, some neuropathies that appear to be accounted for on the basis of an altered transport will be noted. More extensive accounts of the agents used and neuropathies referred to may be found in general works on these subjects and special volumes.

BLOCK OF SYNTHESIS IN CELL BODIES CAUSING FAILURE OF TRANSPORT

The paradigm of an interference of transport leading to pathological changes in the fiber is the Wallerian degeneration seen in the distal amputated nerve after transection (Chapter 9). It results from the loss of substances needed by the fiber that are continually being supplied to it by axonal transport. An interference with synthesis by the cell bodies similarly results in Wallerian degeneration. This is seen when protein synthesis is blocked by puromycin or cycloheximide. When either of these agents were injected into the dorsal root ganglia shortly before that of a labeled amino acid precursor, or even at the same time, the outflow of labeled proteins was almost completely blocked (Figure 13.1). Puromycin acts by blocking the translation of messenger RNA (mRNA) to protein. It does so because of the similarity of its molecular structure to transfer RNA.

Before the dawn of civilization, primitive man believed, as does primitive man today, in animism, magic, and supernatural forces to account for events in the world he experienced. The powers of nature are seen when, after the death of vegetation in winter, its rebirth occurs in spring. Storms with their lightning and thunder, wild animals, and the unpredictable and often turbulent behavior of man in relation to man were powers anthropomorphized through the action of spirits who were either beneficent or malevolent. The emotions felt within himself, man projected to other men, to other living beings, and even to inanimate objects moved by unseen forces.

With the rise of Greek philosophy and science, another view of nature and man arose: the belief that the cosmos and man are ruled by impersonal laws, that the gods do not take a providential interest in the affairs of man. As scientific knowledge evolved and the structures and functions of the various body organs became recognized, the nerves were singled out as having an integral relation to sensation and body movements. In some of the earliest accounts of nerve, they were thought of as channels carrying a spiritual influence to the brain in which consciousness and willed motor control over the body was located.

THE EARLY CONCEPTION OF NERVE CONFOUNDED WITH TENDONS

The artifacts and cave drawings left by prehistoric man attest to his powers of observation.

In the preceding chapters, various agents of nerve action were put forward to account for the rapidity with which nerves conduct sensations and produce motor responses as occurs in a reflex such as that in the example given by Descartes (Chapter 5), where a foot is burned and rapidly withdrawn even before the pain is sensed. The various new physical principles and chemical entities discovered in the Renaissance were advanced to serve this function, but failed to fit all the properties of nerve action. Electricity had properties that suggested that it might be the long sought-for agent of nerve action. It was invisible and imponderable; acting with lightning speed and having profound excitatory actions on the nerves and muscles. How electricity came to be accepted as the agent of nerve conduction is the theme of this chapter. Its history can be divided into three periods. The first period extended from ancient times to that of Galvani at the turn of the eighteenth century when electricity was generated as a static discharge and its potent effects on the body experienced. The second period extends from the introduction of the battery by Volta after the turn of the nineteenth century, when the flow of current in body tissues was investigated, though not differentiated from electrical conduction in metals.

Within the last half of the twentieth century, two fundamental properties of nerve were established: in midcentury, the ionic nature of the propagated action potential; and, later in the century, the process in the fibers known as axonal flow, axoplasmic transport, axonal transport, neuroplasmic transport, and so on. By means of the transport mechanism, essential components synthesized in the nerve cell bodies are carried out within the relatively long length of nerve fibers to maintain their viability and function. Components transported include the ion channels and ion pumps needed to maintain membrane potentials all along the length of the fibers, metabolic and structural components supporting the form and viability of the fibers, and substances providing for reception at sensory terminals and neurotransmitters at motor terminals. This is indeed a protean mechanism, fundamental for an understanding of modern neuroscience and a rational basis for interpretation of neuropathies and eventually their therapy.

Although the discovery of the properties and molecular nature of the transport mechanism and related topics is a major theme, this account is not restricted to the last half century. The concept can be traced back to its earliest beginnings in the sixth and fifth centuries b.c., respectively, when philosophy and science had their origins in ancient Greece. Nerves were then conceived of as channels carrying sensory impressions by animal spirits to the brain where consciousness awareness and reasoned judgment were located, and from it willed commands were carried by nerves to actuate the muscles.

The loss of sensation and muscle power after a nerve transection has been known from antiquity. In the nineteenth century, when the microscopic structure of the nerve fiber became known, the amputated stump of transected nerves was seen to undergo the characteristic breakdown called Wallerian degeneration. The phenomenon led to a major advance in understanding the different functions of the spinal cord roots; sensory fibers carried in the dorsal roots, motor fibers in the ventral roots, the Bell-Magendie law. On cutting a root, degeneration was seen only in that portion of its fibers separated from the cells in the ganglia. The inference of these results was that the cell bodies are required to maintain viability of their fibers. The pursuit of how this comes about led to the recognition of the neuron doctrine and the need for some means by which materials from the cells are carried out into their fibers, the mechanism of axoplasmic transport, which will be discussed in detail in Chapters 11 and 12. In this chapter, the analysis of Wallerian degeneration is presented and shown to be a two-stage process in which the earliest phase is a beading of the fibers.

THE BELL-MAGENDIE LAW

In a paper he had privately printed in 1811 and that was privately circulated, and only much later publically revealed, the famous English anatomist Charles Bell (1774–1842) reported that injury to the anterior (ventral) portion of the spinal cord marrow caused convulsive muscular movements in vivisected animals, more so than an injury to the posterior (dorsal) portions of the cord.

Just as broken bones can heal, so must it have seemed possible to the ancients that cut nerve could reunite and its function restored. Ancient authority is silent on this, but although not stating it explicitly, Galen's commentators in the Middle Ages suggested that he thought this to be so because of the prescriptions he gave for the treatment of nerve wounds that were aimed to bring about the “agglutination” of cut nerves. Paul of Aegina (seventh century) apparently followed Galen in using medications to promote agglutination, also mentioning suturing of divided nerves:

In the previous chapters, the concept of channels in nerve through which animal spirits are conveyed was an inference made from the empty blood vessels seen in optic nerves. When, starting in the seventeenth century, microscopes became available, they were eagerly taken up in the search for them. Despite the difficulties in handling the soft nerve tissue and imperfect lenses used in early microscopic studies, they did show nerve fibers that were cylindrical in form. Their internal composition, however, was a matter of dispute, whether fluid as the concept of moving spirits demanded, or solid as called for by vibratory theories. A resolution of this point was of major importance. When microscopes with achromatic lenses and with reduced spherical aberration became available in the nineteenth century, their greatly improved resolution showed the contents of the fibers to contain fluid and filamentous structures. The nerve fibers were seen to be extensions of the cell bodies, parts of the same entity, the concept expressed as the neuron doctrine. With the advent of electron microscopy, the filamentous structures within the fibers were resolved and shown to consist of several species of longitudinally organized protein polymers: neurofilaments, microtubules, and microfilaments that are collectively referred to as the cytoskeleton. The functions of these different protein structures were related to the shape of the fiber and the means by which materials are carried out into the fibers to maintain their structure and functions, by the mechanism known as axoplasmic or axonal transport, to be discussed in detail in Chapters 11 and 12.

After the death of Galen, little progress was made in studies of anatomy and experimental physiology. No successor approaching his level appeared until the Renaissance. The works he left remained the chief guide for the whole of the Middle Ages, with his teachings petrified into the dogmatism known as Galenism. Although he was lauded as representing the highest authority in medicine, on a par with Hippocrates, he was also derided for being opinionated and argumentative. He was even charged with being responsible for holding back progress in the Middle Ages. Modern historians in the twentieth century have given a more evenhanded account of his contributions. Some of his writings have only now been revealed. An important work on the brain, the later book of his anatomical work – On Anatomical Procedures – was translated and published as recently as 1962. Galen's concept of the vascular system lasted into the seventeenth century until it was overturned by Harvey's establishment of the circulation (Chapter 4). The question remains as to why anatomical and physiological investigations remained dormant for so long. The answer must lie in the political and social upheavals after the breakdown of the Roman Empire, during which – alongside the regrouping of the secular power of the nobility – the Church emerged as the dominant intellectual presence in the Middle Ages.

The ancient view that the spinal cord is a nerve-like prolongation of the brain received experimental support from Galen, who showed that by cutting the cord transversely, sensation from the body below the level of the cut was lost, as was motor power – effects mimicking those seen when cutting a peripheral nerve (Chapter 2). Even as late as the seventeenth century, Descartes looked on the cord as only a conduit for nerve tubules passing sensation to the brain, where reflexes are controlled (Figure 5.5). But, study of the decapitated animal indicated that reflexes remained present in the spinalized animal, with its purposive-like behavior leading to the hypothesis that some mind-like principle was present in the cord. As the anatomy and physiology of the nervous system became better understood, aided in large part by the discovery of the Bell-Magendie law in the early part of the nineteenth century, the question was then asked whether mind-like behavior could be accounted for by the complex interconnectivity of neurons in the spinal cord. This question was also raised with respect to the instinctive behavior seen in lower forms and the emergence of higher functions in the course of evolution. The development of the brain with centers for higher functions of learning and memory; in man ideation; caused the lower centers of the spinal cord to become more machine-like in its reflex behavior.

The characteristics of transport discussed in the preceding chapter were shown to depend on the energy supplied by oxidative metabolism. In this chapter, models advanced to account for how that energy is utilized for the movement of proteins, and the vesicles and other particles visualized by means of allen video-enhanced contrast differential interference contrast (AVEC-DIC) microscopy, are described. The view that has emerged is that all these materials are moved out along the microtubules by specific “motors.” The development of this model of fast transport is described in this chapter.

Slow transport on the other hand has remained a matter of contention. The old view that axoplasm moves down in bulk (Chapter 11) was replaced by the hypothesis that only the microtubules and neurofilaments are moving down at the slow rate. An opposing theory holds that these cytoskeletal organelles are stationary in the fibers with their protein subunits moving in the fluid axoplasm. The question raised is whether this requires the presence of a different mechanism of transport other than that serving for fast transport with the further complication that, in addition to fast and slow transport, a number of intermediary transport rates have been found. The hypothesis that a single mechanism termed the unitary hypothesis, can account for all the different transport rates will be taken up at the end of the chapter.

With the overturn of the old physiology underpinning Galenic medicine, new physiological foundations for medicine were searched for. Systems based on new physical principles proposed by Galileo and other physicists, and those on chemistry following its transformation from alchemy, were advanced: on physical principles by the iatrophysicists and on chemical principles by the iatrochemists. Both groups proposed agents to replace the ancient concept of animal spirits, but they essentially represented only a change of name. Alongside the nervous system by which sensations were perceived and motor nerves innervating muscles expressed the will, an involuntary nervous system was recognized – one by which the various bodily organs and its movements were carried out independently of the will (autonomously). Since Galen, the intercostal nerve chains were known, but were thought to be an offshoot of the vagus nerve originating from the brain, and it was so figured by Vesalius. These chains were then recognized as not connected directly to the brain, having its origin in neural connectives from the spinal cord. How the ganglia associated with this system, the intercostal chains and the ganglia found elsewhere with the involuntary nerves in the abdomen, their relation to the voluntary nervous system, and mode of action, became a matter of inquiry.

BOERHAAVE'S SYSTEM OF NERVE FIBERS

One of the most eminent clinicians of his day who incorporated the developments in physics and chemistry into a new system of physiology and medicine was Hermann Boerhaave (1668–1738), whose influential six-volume compendium, Institutiones medicae, was aimed at eliminating metaphysics from medicine and medical science.

The recovery of long-lost Greek literature and science manuscripts and their translation into Latin led not only to an admiration of the achievements of past masters, but also before long a desire to emulate and surpass them. Starting first in Florence, where the “Florentine Renaissance” began in the mid-fourteenth century with the enthusiasm of Petrarch in his search for ancient manuscripts, the restoration of learning spread elsewhere in Italy and then to all of Europe in the fifteenth and sixteenth centuries. The Renaissance gave rise to the remarkable literary and artistic achievements of such men as Botticelli, Donatello, Michelangelo, Leonardo da Vinci, Erasmus, among others. The enthusiasm for learning and the expression of individualism characterized those of the Renaissance, differentiating them from the conformity to church doctrine of the preceding centuries that came to be called the “Middle Ages” or the “Dark Ages.”

A contrast developed between the universities, with their need to provide training for clerics and where church matters took precedence, and the new humanistic circles in which laymen and clerics mingled freely under the protection of princes, popes, and such powerful families as the Medici, who gave support to the arts and sciences. Through them, “the modern age began with the acceptance of the autonomy of intellectual methods and problems” to fulfill the desire of knowledge for its own sake, as well as for worldly gain. New findings were investigated, and new things brought to light.

The machine-like reflex responses of spinal animals and the instinctive behavior seen in lower species contrast with the adaptability of the behavior controlled by the brain in the higher species, especially in man where higher cognitive functions and willed behavior predominate (Chapter 14). In some of the earliest speculations, the brain was held to be the site where a higher spiritual entity, the soul, was responsible for cognition and willed activities. As more was known of the complexities of the brain, the Alexandrians Herophilus and Erasistratus, and above all Galen, assigned the higher functions of imagination, reasoning, and memory to the passage of animal spirits in the ventricles. This localization of functions was enshrined in the “cell theory” which held sway throughout the Middle Ages (Chapter 2). As the anatomy of the brain became better known, higher functions were assigned to various brain structures. The cerebrum, with its complex gyrations and the greater expanse of the cortex over the surface of the cerebrum in man, became identified with the higher functions of reasoning, learning, and memory. When the neuron and its interactions were recognized as the basis of nervous integration, changes in neuronal structure, particularly of the dendrites where synaptic interactions on them were seen to occur, was held to account for learning and memory.

Great advances were made in physics after the studies of Galileo, as the findings of other scientists in the sixteenth century led to ever greater accretions of knowledge of physics and chemistry and awareness of the complexities of the nervous system. The latter part of the seventeenth and eighteenth centuries, the period known as the “Enlightenment” or the “Age of Reason,” was characterized by a new critical examination of received teachings. This was the age in which superstition and dogmatic religion were rejected by the Philosophes, the popular philosophers of France. The views on religion held by them and other educated intellectuals included those who believed in a providential God that continually acted on behalf of the affairs of man, the theists, and the deists – those who held that a God may have created the universe, but then left man and the world to run their course without Him. For the atheist, there was no reality at all to God and the soul of man was a material entity that operated by the inherent motion of its substance. This mechano-materialist view was forcibly presented by the English philosopher Thomas Hobbes (1588–1679). For him: “Thought is a form of motion of matter; my ‘ideas’ are vibrations in the matter of my brain and nerves.” In France, the atomism of the ancients was revived by the French philosopher Pierre Gassendi (1592–1655) and used by leading figures in science to advance their theories.

The ancient concept of animal spirits moving in hollow nerve fibers to account for sensation and motor control was replaced in the Renaissance with such surrogates as a gas, a thin vapor, a fiery fiuid, vibrating particles, and so on, until eventually the nerve impulse was recognized as being electrical in nature. However, this left still unaccounted for the slow onset of Wallerian degeneration appearing a day or so after nerve transection, along with the later slowly developing atrophy of muscles and sensory organs. Some other nerve principle was involved. With the establishment of the neuron doctrine, the question turned on the possibility of the loss of supply of some substance from the nerve cell to its fibers and the tissues innervated, a hormone, an enzyme, or whatever. And, another related question arose, the nature of the mechanism that transports that principle in the fibers.

EARLY HYPOTHESES OF TRANSPORT BASED ON CELL BODY CHANGES

The chromatolysis of cell bodies, loss of Nissl particles staining dark blue with aniline dyes that was seen to follow the transection of its nerve fibers (Chapter 9), drew the attention of Scott to this phenomenon. He found the particles to consist of a “nucleoproteid,” later identified as ribonucleic acid (RNA), having a remarkable resemblance to the granular material present in secretory gland cells, such as those in the fundus of the stomach and pancreas.

Our aim in this chapter is to consider how what we now know of the nervous system may account for understanding human behavior. Rather than the early view of animal spirits and surrogates for them, our view of the nervous system is that it is composed of neurons, with a mechanism of axoplasmic transport in them as schematized for a peripheral neuron in Figure 16.1. The same axonal mechanism is present as well in all neurons in the central nervous system. An example is that of cortical neurons crossing from one hemisphere to the other via the callosal tract. In addition to components required to maintain the viability of fibers and to provide the neurotransmitters acting at synaptic junctions. Other molecular signals are transported between neurons to establish and maintain the networks responsible for the integrated behavior of the organism. Networks formed in the brain under genetic control are responsible for perception, cognition and memory and are not permanently fixed. They are modified in the course of learning (Chapter 15). The broader theoretical issues relate to the degree of innateness of sensory and perceptual processes and how change is brought about.

Up through most of the latter century, the concept of reflexes was held as the key to understanding behavior. In his great book on reflexes of the nervous system, Sherrington wrote,