Physiological mechanisms
underlying the basic functions of the central nervous system

Study of Glia

Posted: December 4th, 2009 | Filed under: Memory | Tags: , , , , , , | No Comments »

The function of the brain, or the higher nervous activity, is linked with the activity of the nerve cells. This is universally recognized and beyond doubt. That is why an article published some years ago by the well-known American physiologist Robert Galambos caused a great deal of controversy. The scientist argued that perception of the outer world, the formation of conditioned reflexes, memory, all the main functions of the brain have nothing to do with the nerve cells, but are associated with the glia, the tiniest cells which surround the neuron bodies and fill the gaps between their processes.

Improbable ideas are not a rare occurrence in biology, but they are usually forgotten before they gain popularity. Galambos’ ideas became known even in the Soviet Union, which is traditionally concerned with research on the nervous system. At that time scientists were, however, not prepared to discuss the functions of the glia elements as they could not support their arguments with proven facts. Almost nothing was known about glia in spite of the fact that glia cells are much more numerous than nerve cells. It was previously thought that they only supported the neurons and provided them with all they need as the blood capillaries never come directly into contact with the nerve cells.

The idea advanced by Galambos seemed to be too groundless to last for long. It did, however, attract followers in different countries, including the Soviet Union. For example, certain Georgian physiologists have voiced the assumption that the role performed by the glia was much more important than that previously ascribed to it. Unlike Galambos, however, they do not ascribe to the glia the function of consciousness or memory, but claim that the glia elements ensure the function of closing the temporary connections when conditioned reflexes are set up.

For a long time now histologists have known that many endings of the nerve processes in the central nervous system are bare, that is they are not covered with a myelin sheath. Calculations show that the electric current coming from these bare nerve endings has to be dispersed and that they are bad transmitters of excitation to the adjacent fibres. The Georgian scientists surmised that the closure mechanism consists in (hat a previously bare nerve ending acquires a myelin sheath and becomes more active. The insulation is made up of giia cells whose processes entwine around the nerve fibre forming a multi-layer myelin sheath.

As yet, it is difficult to say whether these suggestions will prove correct as the study of glia is only just beginning. It is beyond doubt, however, that the investigations will result in a new approach to the physiological mechanisms underlying the basic functions of the central nervous system.


Conditioned-Reflex Theory of Memory

Posted: December 1st, 2009 | Filed under: Memory | Tags: , , , , , , , | No Comments »

According to the second theory of memory, the process of remembering involves the formation of a new system and the building of new links between the nerve cells. Can these potential nerve contacts last throughout man’s life? Can the fading of memory in old age (the ability to remember new events diminishes) be accounted for by the fact that the reserves of the nervous system are exhausted by that time? Mathematicians cannot throw any light on the subject. However, taking into consideration that any nerve cell receives several thousand nerve endings, it is very likely that the nerve network of human brain is capable of storing all the information required.

An argument in favour of this theory is that the nerve cells themselves have changed very little in the course of evolution. The biochemical processes occurring in the neurons of lower animals and man are very similar. Progress has mainly been made in the increase in the number of nerve cells and the improved organization of the nervous system.

Not everything that we know about memory at present corroborates this theory. If the larva of an insect, for example, of a flour beetle, is trained to turn only to the right when moving in a labyrinth, the adult beetle witl retain this habit. Hence, its memory has not been disturbed in spite of the fact that, when the larva changes into the pupa, its body structure changes, and all the nerve contacts and 90 per cent of nerve cells are destroyed. It remains a mystery how its memory is preserved.

At present it is difficult to say which of memory theories is correct. But, as to a conditioned-reflex memory, there is a unanimous opinion that temporary connections exist between the nerve centers, which retain recollections caused by a conditioned stimulus, and a command point governing the responses to it. However, this still leaves a lot to be explained. How this connection is formed is obscure. Some scientists claim that this bond is purely functional and merely transmits excitation more efficiently through certain synapses. Others consider that the formation of conditioned reflexes is accompanied by the appearance of new contacts between the neurons due either to the growth of their processes or of new synaptic formations on these processes.