Neurophysiology

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neurophysiology

[¦nu̇r·ō‚fiz·ē′äl·ə·jē]
(neuroscience)
The study of the functions of the nervous system.

Neurophysiology

 

the branch of physiology that studies the functions of the nervous system. Together with the morphological disciplines, neurophysiology constitutes the theoretical basis of neurology.

Although ideas regarding the reflex principle of nervous system functioning were advanced in the 17th century by R. Descartes and in the 18th century by J. Prochaska, neurophysiology did not begin developing as a science until the first half of the 19th century, when experimental methods were first used to study the nervous system. The development of neurophysiology was aided by the accumulation of anatomical and histological data on the structure of the nervous system, for example, the discovery of the nervous system’s structural unit—the nerve cell, or neuron. Also of great significance was the development of a method for tracing nerve pathways that was based on the degeneration of nerve fibers as they are cut from the neuron’s cell body. Early in the 19th century, C. Bell (1811) and F. Magendie (1822) independently discovered that transection of posterior spinal roots results in a loss of sensitivity, while transection of the anterior roots results in a loss of motor ability; this is evidence that the posterior roots transmit nerve impulses to the brain and the anterior roots transmit them from the brain. Extensive use was made thereafter of transection, destruction, and artificial stimulation of various brain structures to locate the different bodily functions in the nervous system.

An important stage in the development of neurophysiology was reached with I. M. Sechenov’s discovery of central inhibition in 1863; this is the phenomenon that arises when stimulation of certain centers of the nervous system (Sechenov’s nuclei) does not result in excitation but in suppression of activity in those centers. The interaction of excitation and inhibition was subsequently found to underlie all types of nervous activity. Detailed information was obtained in the second half of the 19th century and in the early 20th on the functional role of the various divisions of the nervous system and on the main patterns of reflex activity in each division.

N. E. Vvedenskii, V. M. Bekhterev, and C. Sherrington made major contributions to the study of function in the central nervous system (CNS). The role of the brainstem, especially in the regulation of the cardiovascular and respiratory systems, was clarified by F. V. Ovsiannikov, N. A. Mislavskii, and M. Flour-ens. The role of the cerebellum was elucidated by L. Luciani. The experimental study of cerebrocortical function was initiated soon thereafter by several German scientists, including H. Fritsh and E. Hitzig in 1870, F. Goltz in 1869 and H. Muck. However, the idea that the reflex principle might apply to cortical activity had already been conceived several years earlier by Sechenov in his Brain Reflexes (1863). A systematic experimental investigation of cortical function was begun by I. P. Pavlov, who discovered conditioned reflexes and, consequently, the possibility of objectively recording nervous activity in the cortex. A. A. Ukhtomskii introduced the principle of the dominant into neuropsychology.

Another line of research in neurophysiology focused on the mechanism of activity in neurons and on the nature of excitation and inhibition. This work was aided by the discovery and development of methods for recording bioelectric potentials. The recording of the electrical activity in nerve tissue and in the individual neurons permitted objective and precise determination of where a nerve impulse occurs, how it develops, and to where and at what rate it spreads along the nerve tissue. Among those who made valuable contributions to the study of the mechanisms of nervous activity were H. Helmholtz, E. Du Bois-Reymond, L. Hermann, E. Pfluger, and the Russian N. E. Vvedenskii, who used the telephone to study electrical responses in the nervous system in 1884. W. Einthoven and subsequently A. F. Samoilov accurately recorded brief, weak electrical responses in the nervous system by means of a string galvanometer. In 1924 the American scientists G. Bishop, J. Erlanger, and H. Gasser introduced electronic amplifiers and oscillographs into neurophysiological research. These technical advances were later combined in electromyography to investigate the activity of individual neuromuscular units. Another technical advance is electroencephalography, which records the total electrical activity of the cerebral cortex.

One of the main concerns of modern neurophysiology is the integrative activity of the nervous system. This is studied by transecting and removing different structures, by obtaining these structures’ electric potentials with surface and needle electrodes, and by stimulating the structures with electricity and heat. Other major achievements of neurophysiology include the discovery and detailed elucidation of the activating and inhibitory influences—both ascending and descending—of the brainstem reticular formation; the identification of the limbic system of the forebrain as one of the highest centers of integration for somatic and visceral functions; and the discovery of mechanisms within the hypothalamus for the higher integration of neural and endocrine regulatory systems.

Currently, detailed research using microelectrode techniques is being directed at the cellular mechanisms of nervous system activity. Using these techniques, it is even possible to detect electrical activity in a single neuron within the CNS. A neuron continues to functional normally for a while after intracellular microelectrode implantation. Microelectrodes have been used to discover how excitation and inhibition develop in different kinds of neurons, what the intracellular mechanisms of these processes are, and how an electrical impulse is transferred from one neuron to another. Equally important to the development of neurophysiology has been the introduction of electron microscopy, which has yielded detailed micrographs of the ultrastructure of neurons and interneuronal connections in the CNS.

These technical advances have enabled neurophysiologists to directly study the mechanisms by which information is coded and transmitted in the nervous system. The experimental use of chemical and physical agents for directly manipulating the activity of nerve cells has also been made possible by the introduction of microelectrode techniques, electron microscopy, and other technical advances. Models of individual neurons as well as of networks of nerves are being formulated on the basis of data obtained in direct experiments on the nervous system. Modern neurophysiology is closely allied with other disciplines, for example, cybernetics, neurochemistry, and bionics.

REFERENCES

Beritov, I. S. Obshchaia fiziologiia myshechnoi i nervnoi sistem, 2nd ed., vol. 1. Moscow-Leningrad, 1947.
Eccles, J. Fiziologiia nervnykh kletok. Moscow, 1959. (Translated from English.)
Eccles, J. Fiziologiia sinapsov. Moscow, 1966. (Translated from English.)
Prosser, C., and F. Brown. Sravnitel’naia fiziologiia zhivotnykh. Moscow, 1967. (Translated from English.)
Obshchaia i chastnaia fiziologiia nervnoi sistemy: Rukovodstvo po fiziologii. Leningrad, 1969. Sherrington, C. Integrativnaia deiatel’nost’ nervnoi sistemy. Leningrad, 1969. (Translated from English.)

P. G. KOSTIUK

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