The Neuron Doctrine and the Synapse
If there exists any surface or separation at the nexus between neurone and neurone, much of what is characteristic of the conduction exhibited by the reflex-arc might be more easily explicable…. At the nexus between efferent neurone and the muscle-cell, electric organ, which it innervates, it is generally admitted that there is not actual confluence of the two cells together, but that a surface separates them…. In view, therefore, of the probable importance of this mode of nexus between neurone and neurone it is convenient to have a term for it. The term introduced has been synapse.
It is now recognized that, with only a few exceptions, there is a physical gap between nerve cells and between them and the muscles and glands they innervate. Over much of the last half of the nineteenth century, however, eminent anatomists argued over whether a gap existed between nerve cells or whether instead nerve fibers formed a continuous network with no separation between them. The controversy over the existence and the nature of the gap had to be resolved before the question of whether nerves communicated chemically or electrically could come to the fore. By 1906, as the above quote by Charles Sherrington indicates, it was recognized that there was at least a functional, if not a physical, separation between nerves and the muscles they innervate. How this agreement came about is described briefly in this chapter.2
The discovery and gradual improvement of the compound microscope between 1800 and 1850 had made it possible to examine plant and animal tissue in much greater detail. This paved the way for Theodor Schwann (1810–1882) and Matthias Schleiden (1804–1881) in Berlin to propose the “cell theory,” which stated that all plant and animal tissue is made up of separate cells, each surrounded by a membrane.3 By the 1850s most biologists had accepted the theory that the cell was the basic unit of living tissue, but it was not agreed that the cell theory applied to the nervous system. Although nerve cells had been described, several anatomists maintained that the nervous system functioned through a network of very fine fibers that were not separated from each other. Moreover, some anatomists did not believe that the nerve fibers were actually connected to the nerve cells. Although not the first to advance the idea, Wilhelm von Waldeyer [Hartz] (1836–1921) in Berlin proposed the “neuron doctrine” in 1891; this stated that the nerve cell and its fibers constitute the basic unit of the nervous system. However, neither the techniques for staining nerve cells nor the power of the compound microscopes available at the time was sufficient to determine with certainty whether the fibers from different neurons were contiguous or separated by a gap.
The situation had begun to change after 1873, the year Camillo Golgi (1843–1926) at the University of Pavia introduced a method of staining nerve cells by impregnating them with silver nitrate.4 The silver made nerve fibers stand out clearly as black against a background of a different color (the “black reaction”). Moreover, because only a small percentage of nerve cells were stained in this process, individual neurons could be followed more easily in their entirety.5 Despite the advantages of the Golgi silver stain, it was still not possible to see whether the fine terminals of neurons were actually touching or whether a minute gap existed between them. (The gap between neurons was not actually seen until the electron microscope became available in the 1950s.)
This question of whether nerve fibers from different cells were connected was in dispute throughout much of the last decade of the nineteenth century. Ironically, although Golgi and the Spanish anatomist Santiago Ramón y Cajal (1852–1934) both used the Golgi stain, they reached different conclusions about the existence of a gap. Golgi became the leading protagonist for the “reticular theory,” which held that the nerve fibers formed a contiguous network, or reticulum.6 Cajal, on the other hand, took the lead in summarizing the evidence for the neuron doctrine, which maintained that a gap exists between the terminals of nerve fibers and both other nerve cells and the muscles they innervate.7
Cajal was persuaded by several lines of evidence that a gap existed. By studying neurons in embryonic and immature animals, Cajal was able to show that developing nerve fibers first grow out of the nerve cell bodies, growing next to muscles and to other nerve cells only at later stages. He also drew on the observation that when a nerve fiber is cut, interrupting its connection to the cell body, the fiber degenerates, but the degeneration stops at the boundary of the next cell.8
Cajal also concluded that transmission between nerve fibers is unidirectional. Nerve impulses, Cajal argued, do not spread diffusely in all directions, as would seem to be the case in a nerve net. He called this unidirectionality the “law of dynamic polarization,” and he described the interface between neurons as acting as a valve controlling one-way traffic. He argued that the generally short and heavily branched fibers called dendrites are receptive in that they receive information from other cells and convey it to the cell bodies. The fibers called axons, which are generally longer and with fewer branches than the dendrites, conduct away from the cell toward other nerve cells or muscles. This conclusion was supported by the observation that the dendrites in sensory nerves, like the optic and olfactory nerves, have their dendrites connected to the eyes and nose, respectively, while their axons extend into the brain. In contrast, the axons of motor neurons extend into the muscles they innervate. Cajal proposes that where the axons of one neuron meet the dendrites of another neuron, transmission is always from axon to dendrite and never the other way around.
By 1906 the majority of neuroanatomists supported the neuron doctrine. That year, Golgi and Cajal received word from Stockholm that they were to share the Nobel Prize “in recognition of their work on the anatomy of the nervous system.” The Nobel Committee acknowledged Golgi as a “pioneer of modern research into the nervous system.” Today Golgi is recognized as one of the founders of neuroscience. In addition to the histological stain that bears his name, Golgi discovered a basic structure within the nucleus of a cell that is now known as the “Golgi apparatus.” He also discovered the “Golgi tendon organ” and a type of cell in the cerebellum of the brain that now bears his name. Golgi made other contributions as well, including the first full description of the course of malaria in humans.9 In the citation for Cajal, the Nobel award described him as the person most responsible for “giving the study of the nervous system the form it has taken in the present day.”
Cajal and nearly everyone else were taken aback when Golgi used his Nobel lecture, “The Neuron Doctrine, Theory and Fact,” to launch an attack on the neuron doctrine. He stated that he was still “far from being willing to admit the idea of individuality and of functional independence of each nerve element,” and went on to assert that the neuron doctrine was “generally recognized as going out of favor.” Not only was the lecture inappropriate for the occasion, but Golgi ignored much of the evidence and arguments that Cajal and others had collected over the previous decade. It was a strange performance by a man who had deservedly received the highest recognition for his many contributions, but who apparently could not accept that he might not always have been right.10 Golgi’s Nobel lecture had no detectable impact on the growing acceptance of the neuron doctrine, although the reticular theory lingered on for several years, supported by a small group of defenders.11
It was the pioneering neurophysiologist Charles Scott Sherrington (1857–1952), later Sir Charles, who introduced the term “synapse” to describe the gap between neurons and between neurons and the muscles they innervate. Sherrington, who would later share the 1932 Nobel Prize in Physiology or Medicine with Edgar Adrian, had studied spinal reflexes and how antagonistic muscles are reciprocally inhibited to allow for smooth movement. Even though he could not see it, Sherrington recognized that there must be some kind of separation between neurons. He was quite familiar with Cajal’s work and had even persuaded his own mentor in physiology, Michael Foster (1836–1907), to invite Cajal to give the 1894 Croonian Lecture to the Royal Society.12 Not only had Cajal’s work persuaded Sherrington that there must be at least a functional gap at the end of axons, but from his own work he recognized that the delay in transmission at the junction between neurons or between a neuron and a muscle could only be explained by the existence of a gap. In his 1906 book The Integrative Action of the Nervous System, Sherrington noted that the existence of a physical gap between a nerve and the muscle cell that controls the electric organ used in some fish to shock prey had already been demonstrated.
Sherrington first used the term “synapse” in 1897.13 He been asked by Michael Foster to write a separate volume on the nervous system for a revision of Foster’s classic Textbook of Physiology. In working on the volume, Sherrington found that he needed a word to refer to the junction point between neurons. With the help of a friend, a Greek scholar, he adopted the term “synapse” from a Greek word meaning “to clasp.” Sherrington found this word preferable to other alternatives that seemed to imply a closer physical bonding than he thought existed between neurons.14
Prior to the pioneering work of the men discussed above, early work with curare had in fact suggested that the point at which neurons and muscles meet has special properties. The renowned French physiologist Claude Bernard (1813–1878) had started to experiment with curare around 1840 after receiving a poison arrow coated with this substance from a friend, who had obtained it from a South American native.15 When Bernard thrust the arrow into the thigh of a rabbit the animal became paralyzed, although its heart was still beating. Bernard was fascinated by the action of the drug and began to investigate. The work of others had demonstrated that an animal injected with curare could be kept alive, although only by putting the animal on an artificial respirator, because the drug disrupted breathing by blocking all movement of the chest muscles.
In a series of cleverly designed but simple experiments, Bernard was able to show that curare neither paralyzed the muscle nor blocked nerve conduction. He demonstrated, for example, that muscles were still able to respond to direct electrical stimulation even after they were injected with curare. In another experiment, Bernard isolated the sciatic nerve and its attachment to the leg muscle. He then immersed the sciatic nerve in a bath of curare solution, but left its point of attachment to the muscle outside the bath. When he stimulated the sciatic nerve the muscle responded normally, indicating that the curare did not interfere with nerve conduction. However, if the point of attachment between the nerve and the muscle was placed in the curare solution, stimulation of the nerve no longer made the muscle contract. Although Bernard never published his conclusions, comments in his notebook indicate that he realized that curare must act on the junction between the nerve and the muscle and that therefore there must be some kind of independence between them which would explain the action of curare.16
Sherrington knew about Claude Bernard’s work because John Langley, one of his mentors, had studied curare and had noted that it as well as nicotine prevented muscles from responding to nerve stimulation. In one experiment, Langley dipped a thread in either nicotine or curare and carefully applied it directly at the point where nerve and muscle meet; in this way he confirmed Bernard’s observation that this was the only place these drugs were effective in preventing the nerve from stimulating the muscle.
Although Sherrington did not discuss how communication might take place across the synapse, by giving the gap a name he had helped to focus attention on the special properties of this region. The emphasis on the synapse eventually led to the question of whether nerve impulses are transmitted across the synapse electrically or chemically.
It has become almost mandatory in historical accounts of ideas about chemical transmission to cite the German electrophysiologist Emil Du Bois–Reymond (1818–1896). In his 1877 textbook, Du Bois–Reymond commented that there were two possible processes by which a nerve might stimulate a muscle:
Of known natural processes that might pass on excitation, only two are, in my opinion, worth talking about: either there exists at the boundary of the contractile substance a stimulatory substance in the form of a thin layer of ammonia, lactic acid, or some other powerful stimulatory substance, or the phenomenon is electrical in nature.17
Du Bois–Reymond, professor of physiology at the University of Berlin, was a brilliant man.18 He designed ingenious recording instruments (galvanometers) for detecting electrical discharges of nerves and is generally considered the person responsible for creating the field of electrophysiology. Although Du Bois–Reymond may have considered the idea that a chemical change was a component of muscle contraction, he did not consider the possibility that transmission between the nerve and muscle is chemical.19
The possibility that chemical mediators might be involved in synaptic transmission would not be seriously entertained for another forty years. It emerged slowly, from the studies of the effect of various drugs on visceral organs and on the skeletal muscles innervated by spinal nerves. Skeletal muscles, which are also called “striated” (or in older literature, “striped”) muscles, are attached to the skeleton and move the body in space. In contrast, the muscles of most, but not all, visceral organs, such as the intestines, lungs, and blood vessels, are called “smooth” (or in early literature, “plain”) muscles.20 How the study of the effect of drugs on visceral organs and skeletal muscles contributed to the discovery that nerves secrete chemical substances is introduced in the next chapter.