Reticular Activating System

For a long time neuroanatomists have described a loose network (reticulum) of cells in the central core of the upper spinal cord, medulla, pons, and midbrain called the reticular formation. However, the functional significance of this system has been elucidated only recently. Working with monkeys, Magoun and his associates (37, 39) observed that direct electrical stimulation of this reticular core in the brain stem produced changes in the electrical activity of the hemispheres similar to those seen when an animal is awakened from sleep or alerted to attention. The typical high voltage, slow waves of the EEG are replaced by low voltage, fast waves. It was observed later that stimulating naturally sleeping monkeys in these areas by chronically implanted electrodes brought about behavioral awakening or arousal. Further, it was observed that large experimental lesions in these areas produce loss of wakefulness, the animals appearing anesthetized or comatose. The animals are not aroused by even the most vigorous peripheral stimulation, and electrocorticograms are of the coma type. Thus, both behaviorally and in terms of electrical activity, the animals appear comatose despite the fact that classical afferent pathways between peripheral sense organs and the cortex are intact. These observations suggest that the central reticular core exercises a profound modulating influence on the sensory input to the organism.

Recent neuroanatomic studies have revealed neural connections between this central core and the afferents of all sensory modalities. Other neuroanatomic investigations (19) have revealed projections from sensory cortical areas to the central reticular core. In fact, projections have been described also between portions of the reticular formation and motor and association areas of the cortex and many of the structures subsumed under the limbic system.

The reticular formation of the midbrain is continuous with thalamic and hypothalamic areas - reticular nucleus of the thalamus and lateral hypothalamic area - which appear to be part of the same system. The latter is sometimes called the diffuse thalamic system but for the present discussion the term reticular activating system or R.A.S. will be used for both parts.

The nicety of the modulation of sensory input by the reticular activating system can be demonstrated by technics of evoked potentials. Electrical spikes may be recorded from the medial geniculate body of the thalamus, part of the sensory pathway for hearing, in response to clicks. However, the amplitude of these spikes may be increased or decreased by simultaneous stimulation of one or another area of the rostral reticular core (36). The R.A.S. appears to exert an analogous modulating influence on efferent (motor) outflow. The various brain mechanisms concerned with postural regulation appear to exert their influence on the peripheral motor neurons subserving skeletal muscle responses largely by means of the reticular formation (10).

The R.A.S. is presumed to permit shifts in the focus of attention of various sensory inputs by its modulation of afferent impulses as well as reciprocal influence with neocortical areas. Presumably this is what permits us to select out of the myriad of stimuli with which we are continuously bombarded from the environment those which are immediately important to us. Without such a device it would be impossible for us to make sense of our environments and function in an integrated, adaptive fashion. Related to this may be our ability to react very selectively to certain classes of stimuli. Recall the sleeping mother who reacts to her child's cry but remains somnolent in the presence of other sounds of equal intensity. At other times it is important for us not to respond with arousal to a continuous stimulus such as the sound of an air conditioner or the roar of a ship's engine. Experiments of Hernandez-Peon and Scherrer (17) suggest that the R.A.S. is involved in "habituation" of this kind. They observed that the usual reduction over time of potentials evoked at the dorsal cochlear nucleus (part of the auditory pathway) by a repeated click is blocked by lesions of the brain stem reticular formation.

Finally, the rich connections between parts of the R.A.S. and various limbic system structures make possible the participation of reticular influences in the integration of endocrine, autonomic, and emotional aspects of behavior. Indeed, there is much experimental evidence in support of such functional interrelationships.