Nervous Systems, Types of

(also called types of higher nervous activity), a complex of the main congenital and acquired individual characteristics of the nervous system that in man and animals determine differences in behavior and in reactions to the same external factors. The concept of types of nervous systems, introduced into physiology and medicine by I. P. Pavlov, the founder of the theory of higher nervous activity, is related to the concepts of temperament and of human and animal constitution, areas of interest since the dawn of human culture and civilization.

Higher nervous activity in animals exhibits common features and patterns, as well as characteristics that are peculiar to the nervous system of a given organism and to the organism’s main neural processes—excitation and inhibition—which characterize the activity of the cerebral cortex. According to Pavlov, the classification of types of nervous systems is based on the force, balance, and mobility of excitation and inhibition. The force of a neural process is determined by the ability of nerve cells to function normally during marked excitation or inhibition. The neural processes may be strong or weak, depending on the competence of the cortical cells. By balance is meant the equal force of the neural processes. Excitation and inhibition may be balanced or unbalanced, depending on the correlation of their force. That is, both processes may be equally strong, or one may dominate.

The degree of mobility of the neural processes is determined by the rapidity with which they arise and cease and by their ability to shift from one process to the other. Accordingly, the neural processes may be mobile (labile) or inert. The combination of these characteristics in the nervous system of each organism constitutes the individual differences of higher nervous activity in an animal and the animal’s characteristic behavioral traits. Pavlov distinguished four main types of nervous activity, three strong and one weak, among the many possible combinations of the main properties of excitation and inhibition—their force, balance, and mobility.

The neural processes of all animals are divided according to their force into strong, with highly pronounced excitation and inhibition, and weak, with limited excitation and inhibition. Animals with a strong type of nervous system are subdivided according to the balance of the neural processes. That is, they are subdivided according to the correlation of excitation and inhibition into an unbalanced type, in which excitation is dominant, and a balanced type, in which excitation and inhibition are equally strong. Animals of the strong balanced type are divided according to the mobility of the neural processes into an inert, or slow, type and a mobile, or rapid, type. In the strong unbalanced, or unrestrained (excitable), type, both neural processes have great force, but excitation distinctly prevails over inhibition. Positive conditioned reflexes develop with relative ease in animals (for example, dogs) that have this type of nervous system, but inhibitory conditioned reflexes develop with great difficulty and require long conditioning. The differentiating reflex is not always complete and is easily disinhibited. Higher nervous activity caused by pronounced inhibition is difficult and often impossible for such animals, since a conflict arises between the weakened process of inhibition and the greatly intensified process of excitation. Hence the absence of restraint and the use of the term “unrestrained.”

Animals belonging to the strong balanced inert, or slow (calm), type possess equally strong and fairly well-balanced processes of excitation and inhibition, but these processes are inert and almost entirely lacking in mobility. Consequently, they have a negative influence when a rapid change in the neural processes is required. Positive and inhibitory conditioned reflexes develop slowly, but once stabilized in a particular stereotype, they are difficult to disrupt or alter. When experimental dogs with this type of nervous system are set free, they appear bold but sluggish.

Animals of the strong balanced mobile, or quick (lively), type have equally pronounced strong and mobile neural processes. Positive and inhibitory conditioned reflexes develop quickly in animals with this type of nervous system and are easily altered when the meaning of the signal used as a stimulus is changed. It is relatively easy to shift such animals from one conditioned reflex to another and from one kind of experimental problem to another. Outwardly, the animals (experimental dogs) are quick, lively, energetic, and always active. According to Pavlov, their nervous system is the ideal, perfect type. They are calm, but when freed from physical restraint they are frisky, energetic, easily establish rapport with man, are indifferent to other dogs when in calm surroundings, and control themselves quickly and easily. Animals with this type of nervous system adapt with the greatest possible ease to changing environmental conditions. This is the most stable type of all.

Animals with the weak type of nervous system have weak excitatory and inhibitory neural processes. These animals develop positive and inhibitory conditioned reflexes with great difficulty; those reflexes that do develop are readily inhibited. Protective inhibition develops in the brains of the animals in response to very strong stimulation. Dogs with this type of nervous system are fearful and cowardly and shun contacts. They tire easily, and consequently the level of their performance becomes lowered; it cannot be improved by training.

Dogs with strong neural processes and highly competent nerve cells can also tolerate strong stimulation, react to it with vigorous activity, and perform difficult tasks. Strenuous nervous activity is beyond the capacity of dogs with weak neural processes and a low level of performance.

Different types of nervous systems vary in their autonomic functions. In animals of the strong, balanced, mobile type, the autonomic functions adapt readily to changes in the environment and recover rapidly and completely after disturbing factors are eliminated. Adaptation and recovery of the autonomic functions take place less rapidly in animals with a strong, balanced, inert type of nervous system. In animals of the strong unbalanced type, the autonomic functions recover slowly and unevenly after drastic changes. In animals of the weak type, the autonomic processes are sluggish, are readily disturbed by various factors, and recover incompletely and with difficulty. Neuroses and psychosomatic diseases are more common in animals with the strong unrestrained and weak types of nervous systems.

There are also intermediate types of nervous systems, since the correlation of the main characteristics of the neural processes varies from animal to animal. For example, in animals with a strong type of nervous system, both neural processes may be strong but with a slight predominance of the force of inhibition; or excitation may be strong but somewhat inert, and inhibition very weak. Some animals occupy an intermediate position between strong and weak with respect to the force of the neural processes: examples are a strong variant of the weak type and a weak variant of the strong type of nervous system. Variations are particularly numerous among animals of the weak type, including variations in lack of balance between the neural processes, and the processes’ greater or lesser inertness.

Pavlov believed that as a result of possible variations in the main properties of the nervous system, combinations thereof, and simple arithmetic calculation, 24 types of nervous systems could be distinguished. New classifications of types now make it possible to define the characteristics of the nervous system of animals with more precision. This is undoubtedly of great value in theoretical and practical medicine and in stock raising, particularly from the standpoint of genetics and selective breeding. However, these classifications are far from ideal in their present form. They are cumbersome and require numerous research techniques and functional tests to determine the types of nervous systems.

Pavlov extended the types of nervous systems he found in dogs to apply to man. He believed that the four main types common to man and animals coincided with Hippocrates’ classification of temperaments in human beings. The choleric temperament corresponded to the strong unbalanced type, the phlegmatic to the strong, balanced, inert type, the sanguine to the strong, balanced, mobile type, and the melancholic to the weak type. Pavlov proposed that in addition to the types common to man and animals, specifically human types be distinguished that would characterize the interaction of the first and second signal systems and the correlation between them. In the reflective type, the second-signal system predominated, and in the creative type, the first-signal system. In the intermediate type, both signal systems were equally represented. Pavlov’s concept of two signal systems has been confirmed by recent electrophysiological studies on brain function.

The Soviet psychologists B. M. Teplov and V. D. Nebylitsyn applied to man the theory of the characteristics and types of nervous systems that Pavlov had developed for animals, but they took into account man’s specific nature as a social being. Pavlov and most of his students had generally viewed the main characteristics of the nervous system as either positive or negative with respect to the organism. The theoretical and experimental studies of the Teplov-Nebylitsyn school proved that Pavlov’s approach, based on value judgments, was far from exhaustive. Teplov and Nebylitsyn formulated a constructive approach according to which each pole of any of the opposing characteristics of the nervous system, for example, strength-weakness, was a synthesis of positive and negative aspects: examples were strong-insensitive and weak-sensitive. They developed new and original techniques for evaluating the characteristics of the human nervous system that proved to be of great value in increasing existing knowledge of those characteristics.

Modern research techniques have indicated that in the human nervous system there are more than the three main characteristics of force, mobility, and balance. New characteristics that have been identified include lability—the result of splitting mobility into lability and mobility—and dynamism. According to Teplov and Nebylitsyn, lability is a property of the nervous system, which reflects the speed at which the neural processes arise and cease. By dynamism is meant another property reflecting the generating of an excitatory or inhibitory neural process by the brain during experiments—that is, the capacity to learn. Nebylitsyn believed that the characteristics of the human nervous system should be studied separately with respect to excitation and inhibition. Such characteristics as force, lability, dynamism, and mobility were termed primary characteristics. Balance within each of these was termed a secondary characteristic. Consequently, according to Nebylitsyn, the human nervous system has 12 aspects: eight primary characteristics (force, lability, dynamism, and mobility with respect to excitation and to inhibition) and four secondary characteristics (balance within each of the four basic characteristics).

The main characteristics of the nervous system may vary considerably in the different analyzers within man, and are not always in correlation with the individual’s personality traits, for example, with the emotions. This phenomenon is called partiality. Each analyzer has both its own and a relatively independent means of manifesting the characteristics of the nervous system. Nebylitsyn called these characteristics particular, or partial. Although they are largely hereditary, their influence on human behavior is limited. The particular characteristics determine only specific aspects of behavior, such as memory. Nebylitsyn conjectured that nervous systems have general characteristics as well as particular ones. The general characteristics determine man’s individual behavior in some of its most general manifestations and traits—activity, emotionality, and self-regulation—and can elucidate individual differences in personalities. The general characteristics of the human nervous system are manifestations of the brain’s integration of neural processes; these processes may be observed in the frequencies of distance synchronization on an electroencephalogram. The particular characteristics are manifestations of local integration; examples are the characteristics of the analyzers, hemispheres, and anterior structures of the brain.

Thus, there are functional and psychological distinctions between the general and particular characteristics of the human nervous system. The general typological characteristics determine temperament, and the particular characteristics are probably more important in determining specialized abilities.

The achievements of the Teplov-Nebylitsyn school have contributed greatly to present-day knowledge of the characteristics of the human nervous system. However, the complexity of the problems relating to the main types of human nervous systems demands more intensive study of the systems’ varied structures, functions, and manifestations. Solutions to these problems will elucidate the causes of individual differences and will ultimately aid in developing a scientific and multidimensional classification of types of human nervous systems as the most stable combinations of general and particular characteristics of the nervous system.

The theory of types of nervous systems is of great importance not only for physiology and medicine but for genetics, selective breeding, veterinary science, livestock raising, psychology, and pedagogy.