Short memory of neurophysiology

Neurophysiology developed in parallel to all the other medical and physiological sciences (see Rose and Bynum, 1982; Finger, 1994). We could point at Thomas Willis’s anatomical description of the brain in 1664 as the first major step. His book was illustrated by Christopher Wren, the famous English artist and architect. Willis suggested the term “neurology” in 1681. It was only a few decades after another major event that was William Harvey’s explanation of the circulation of the blood in 1628. Most physicians, still using Galen’s text, believed that the blood ebbed and flowed like a tide through the whole body.

In fact, experimental study of neural mechanisms of learning has short memory. Since a theoretical progress in understanding the brain as the " organ of mind”, little progress had been made for a long time in understanding that ablation by successive slices was not a method well suited to the discovery of cortical localisation, even more so to the studying of intelligence.

It was Descartes' articulation of a mechanism for automatic, differentiated reaction that led to his generally being credited with the founding of reflex theory. Descartes included far more complex behaviour as reflexes than we imagine today. At the same time, he considered “reflex” a common basis for explanation both man and animals’ behaviours.

The term “reflex” in its modern sense, was introduced in 1736 by Jean Astruc. Reflexes were defined as cycles of actions of the senses led to rather immediate responses by the muscles. By that time, in 1717, Antony van Leeuwenhoek described nerve fibbers in cross section. In 1791 Luigi Galvani published work on electrical stimulation of frog nerves. Investigating the effects of electrostatic stimuli applied to the muscle fibre of frogs he discovered he could also make the muscle twitch by touching the nerve with various metals without a source of electrostatic charge, and greater reaction was obtained when two dissimilar metals were used. He attributed the effect to “animal electricity” and thus claimed to demonstrate that electrical energy was generated in the nervous systems of animals. This work initiated research into electrophysiology.

One of the first to advocate the use of experimental techniques in physiology was Johannes Peter Mü ller, German physiologist and comparative anatomist, one of the great natural philosophers of the 19th century and author of the most influential textbook of that times, “Handbuch der Physiologie des Menschen fü r Vorlesungen, 2 vol. (1834-40; Elements of Physiology)”. Mü ller ‘s most important achievement was the discovery that each of the sense organs responds to different kinds of stimuli in its own particular way or, as Mü ller wrote, with its own specific energy. The phenomena of the external world are perceived, therefore, only by the changes they produce in sensory systems. Mü ller's monograph “On Imaginary Apparitions” was published in 1826. According to this theory the eye as a sensory system not only reacts on external optical stimuli but can also be excited by internal stimuli generated by the imagination. Thus, persons who report seeing religious visions, ghosts, or phantoms may actually be experiencing optical sensations and believe them to be of external origin, even though they do not in fact have an adequate external stimulus. Mü ller examined many problems in physiology, evolution, and comparative anatomy. He studied the passage of impulses from afferent nerves (going to the brain and spinal cord) to efferent nerves (going away from the same centres), further elucidating the concept of reflex action. By careful experiments on live frogs, he confirmed the law named after Charles Bell and Franç ois Magendie, according to which the anterior roots of the nerves originating from the spinal cord are motor and the posterior roots are sensory.

One of the most talented Mü ller ‘s students, Hermann von Helmholtz, in 1849 measured the speed of frog nerve impulses and thus found that the nerve impulse was perfectly measurable. Helmholtz was equally distinguished in physiology, mathematics, and experimental and mathematical physics. His physiological works are principally connected with the eye, the ear, and the nervous system. His work on vision is regarded as fundamental to modern visual science.

At the same time with Mü ller, French physiologist Marie-Jean-Pierre Flourens was the first to demonstrate the general functions of the major portions of the vertebrate brain. Flourens conducted a series of experiments (1814-22) to determine physiological changes in pigeons after removal of certain portions of their brains. He found that removal of the cerebral hemispheres, at the front of the brain, destroys will, judgement, and all the senses of perception; that removal of the cerebellum, at the base of the brain, destroys the animal's muscular coordination and its sense of equilibrium; and that removal of the medulla oblongata, at the back of the brain, results in death. These experiments led him to conclude that the cerebral hemispheres are responsible for higher psychic and intellectual abilities, that the cerebellum regulates all movements, and that the medulla controls vital functions, especially respiration. Flourens was also the first to recognise the role of the semicircular canals of the inner ear in maintaining body equilibrium and coordination. Later work by Pavlov would prove that conditional responses could not be learned by dogs after removal of the cerebral cortex. Thus cerebral cortex was determined to be critical for the formation and storage of conditioned reflexes.

Flourens is generally credited with being the father of experimental brain research. He measured both behaviour and brain events. He contended, in opposition to the prior claims of phrenology, that behaviours were not localised in the cortex, and argued, as Carl Lashley later would, that behaviours were diffusely localised, and that lesions led to general losses (Lashley’s law of mass action). He additionally noted that if the lesions were not too severe, then recovery of function was possible and that restitution of a function was the result of compensation by the remaining intact brain (Lashley’s law of equipotentiality; see details in Chapter 9).

The first anatomical proof of the localisation of brain function was also attributed to Paul Broca, French surgeon whose study of brain lesions contributed significantly to understanding the origins of aphasia, the loss or impairment of the ability to form or articulate words. Much of Broca's research concerned the comparative study of the craniums of the races of mankind. He used original techniques and methods to study the form, structure, and topography of the brain and sections of prehistoric craniums. In 1861 he announced his discovery of the seats of articulate speech in the left frontal region of the brain, since widely known as the convolution of Broca.

Broca’s name is usually associated with names of Carl Wernicke and John Hughlings Jackson. German neurologist Carl Wernicke related nerve diseases to specific areas of the brain. He is best known for his descriptions of the aphasias, disorders interfering with the ability to communicate in speech or writing. In a small book published in 1874, Wernicke tried to relate the various aphasias to impaired mental processes in different regions of the brain; the book included the first accurate description of a sensory aphasia located in the temporal lobe. Wernicke also demonstrated the dominance of one hemisphere in brain functions in these studies. His " Textbook of Brain Disorders" (1881) was an attempt to comprehensively account for the cerebral localisation of all neurological disease. Jackson was British neurologist whose studies of epilepsy, speech defects, and nervous-system disorders arising from injury to the brain and spinal cord remain among the most useful and highly documented in the field. One of the first to state that abnormal mental states may result from structural brain damage, Jackson discovered epileptic convulsions, now known as Jacksonian epilepsy. Jackson's epilepsy studies initiated the development of modern methods of clinical localisation of brain lesions and the investigation of localised brain functions.

The concept of the neurone as a discrete structural and functional entity had been proposed by Deiters in 1865 and by Waldeyer in 1891 (who introduced the word neuron), on the basis of microscopic dissection and staining of cells in the nervous system.

Two major questions confronted neurologists at the end of the nineteenth and beginning of the twentieth centuries (Allen, 1998):

What was the basic anatomical element of the nervous system (individual cells, or a continuous nerve network)?

How were parts of the nervous system integrated to produce an overall functioning system?

By the early 1900s the first question was resolved ultimately in favour of the neurone theory: individual nerve cells serve as the basic structural and functional units of the nervous system. Central to that debate was the work of a Spanish cytologist Santiago Ramó n y Cajal whose study of the embryological development of the nervous system helped to demonstrate that the nervous system arises from many discrete individual cells. The second question concerning structural and functional organisation of the nervous system has been an area of great advancement during the twentieth century. As A.R. Damasio (1994) noted, even in the small world of brain science, since the 1860s, two camps were beginning to form. One held that psychological functions such as language or memory could never be traced to a particular region of the brain. If one had to accept, reluctantly, that the brain did produce the mind, it did so as a whole and not as a collection of parts with special functions. The other camp held that, on the contrary, the brain did have specialised parts and those parts generated separate mind functions. The rift between the two camps was not merely indicative of the infancy of brain research; the argument endured for another century and, to a certain extent, is still with us today.