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reticular formation[re′tik·yə·lər fȯr¦mā·shən]
a system of nerve structures located in the central portions of the brain stem: the medulla oblongata, mesencephalon, and thalamus. The neurons that constitute the reticular formation vary in size, structure, and length of axons and have densely intertwined fibers. The term “reticular formation,” introduced by the German scientist O. Deiters, reflects only the formation’s morphological characteristics. The reticular formation is morphologically and functionally related to the spinal cord, cerebellum, limbic system, and cerebral cortex. Both ascending (afferent) and descending (efferent) entering impulses interact in the region of the reticular formation. Impulses may also circulate through closed neuronal chains. Thus, there is a constant level of excitation in reticular formation neurons that maintains tone and a certain degree of readiness in various regions of the central nervous system. This level of excitation is controlled by the cerebral cortex.
Descending influences. Different areas of the reticular formation inhibit or promote the motor reactions of the spinal cord. A relationship between the cerebrospinal reflexes and stimulation of various areas of the brain stem was first observed in 1862 by I. M. Sechenov. In 1944–46, the American neuroanatomist H. Magoun and his associates demonstrated that stimulation of different areas of the medulla oblongata’s reticular formation facilitates or inhibits the spinal cord’s motor reactions. Electric stimulation of the medial part of the medulla oblongata’s reticular formation in anesthetized and decerebrated cats and monkeys results in complete cessation of movements caused by reflex action and by stimulation of the motor areas of the cerebral cortex. All the inhibitory effects are bilateral, but on the side stimulated such an effect is often observed at a lower threshold of stimulation. Some manifestations of the inhibitory influences of reticular formation conform to Sechenov’s description of central inhibition. Stimulation of the periphery of the lateral region of the medulla oblongata’s reticular formation has an inhibitory effect and facilitates the spinal cord’s motor activity. The region of the reticular formation that has a facilitory effect on the spinal cord is not confined to the medulla oblongata but extends forward to embrace the region of the pons and mesencephalon.
The reticular formation can act on different structures of the spinal cord, for example, the alpha motoneurons that innervate the main fibers of the muscles involved in voluntary movements. The latent periods of motoneuronal responses are prolonged by stimulation of the inhibitory regions of the reticular formation. This phenomenon suggests that the inhibitory effects of the reticular structures on the spinal cord’s motor reactions are accomplished by interneurons and possibly by Renshaw cells as well. The mechanism that causes reticular formation to affect muscle tone was discovered by the Swedish neurophysiologist R. Granit. Granit demonstrated that the reticular formation also affects the activity of the gamma motoneurons, whose axons proceed to the intrafusal muscle fibers and help regulate the organism’s posture and its phase movements.
Ascending influences. Various regions of the reticular formation, extending from the diencephalon to the medulla oblongata, exert generalized excitatory influences on the cerebral cortex; that is, they involve all the cortical regions in the excitation process. In 1949, Magoun and the Italian physiologist G. Moruzzi discovered, while studying the brain’s bioelectric activity, that stimulation of the brain stem’s reticular formation changes the slow synchronous high-voltage oscillations characteristic of sleep into the low-amplitude, high-frequency activity characteristic of wakefulness. In animals, the change in the cerebral cortex’ electric activity is accompanied by outward manifestations of arousal.
The reticular formation is closely related anatomically to the classic media of conduction and is excited by the exteroceptive and interoceptive afferent (sensory) systems. Some investigators consequently regard the reticular formation as part of the brain’s nonspecific afferent system. However, the research of P. K. Anokhin led to a different theory. Anokhin used various pharmacologic substances to study the reticular formation’s functions and discovered that chemical preparations act selectively on the reactions effected with the participation of the reticular formation. This led him to postulate the specificity of the reticular formation’s ascending influences on the cerebral cortex. The activating influences of the reticular formation are always of biological importance and are selectively sensitive to various pharmacologic substances. These views were presented by Anokhin in 1959 and 1968. Narcotics introduced into the body inhibit the reticular formation’s neurons, thereby blocking their ascending activating influences on the cerebral cortex.
Hormonal factors, including catecholamines, carbon dioxide, and cholinergic agents, help maintain the reticular formation’s activity and its sensitivity to chemical substances circulating in the bloodstream. The reticular formation thus helps regulate some autonomic functions. The cerebral cortex, when it experiences the tonic activating influences of the reticular formation, may alter the functional state of reticular structures. The cortex may change the rate at which excitation is conducted in the reticular formation and may influence the functioning of individual neurons; that is, the cerebral cortex may control, in I. P. Pavlov’s words, the “blind force” of the subcortex.
The discovery of the properties of the reticular formation and of its relations with other subcortical structures and cortical regions has helped elucidate the neurophysiologic mechanisms of pain, sleep, wakefulness, and alertness, as well as the formation of integrated conditioned reflexes and the development of a variety of motivations and emotions. Research on the reticular formation using pharmacological substances may lead to the treatment of a number of diseases of the central nervous system by means of pharmacotherapy. Such research is creating a new approach to anesthesia and other major areas of medicine.
REFERENCESBrodai, A. Retikuliarnaia formatsiia mozgovogo stvola. Moscow, 1960. (Translated from English.)
Rossi, J. F., and A. Zanketti. Retikuliarnaia formatsiia stvola mozga. Moscow, 1960. (Translated from English.)
Retikuliarnaia formatsiia mozga. Moscow, 1962. (Translated from English.)
H. Magoun. Bodrstvuiushchii mozg, 2nd ed. Moscow, 1965. (Translated from English.)
Anokhin, P. K. Biologiia i neirofiziologiia uslovnogo refleksa. Moscow, 1968.
Granit, R. Osnovy reguliatsii dvizhenii. Moscow, 1973. (Translated from English.)
Moruzzi, G., and H. W. Magoun. “Brain Stem Reticular and Formation Activation of EEG.” In Electroencephalography and Clinical Neurophysiology, vol. 1. Boston, 1949.
V. G. ZILOV