RETICULAR FORMATION (Reticular Activating System, RAS)

A network of nerve cells deep in the middle of the brain (plate 1), extending from lower to higher centers; a structure involved in arousing and alerting the organism.The RAS is made up of nerve cells in the lower brain stem, a structure about the size of the little finger. It is located in a strategic position since all incoming and outgoing sensory and motor impulses pass near it and may stimulate it. The portion of the structure which sends impulses to the cortex, the ascending RAS, helps to bring about and maintain attention; the part which sends impulses to the spinal cord, the descending RAS, affects the musculature and the autonomic nervous system.The reticular formation plays two interrelated roles. It serves a general arousal function which differentiates wakefulness from sleep (Samuels, 1959), keeping the organism alert to sensory signals; and it regulates the level of attention to specific stimuli. The cortex becomes receptive to stimulation only after it has been aroused by the RAS— for example, input from the visceral and surface receptors (stomach, eye, ear, etc.) stimulate the RAS, which in turn activates the cortex (Lindsley, 1951). Its functions have been tested by direct stimulation, which has been found to produce a waking EEG pattern known as alpha blocking. Experiments have also shown that sleeping animals, such as the monkey, are awakened by such stimulation, and the RAS has therefore been designated the “waking center” (Segundo et al., 1955). On the other hand, injury to the RAS results in apathy, lethargy, or drowsiness (Lindsley, 1951).Experiments also indicate that when an individual is asleep, tactile or other stimuli actually reach the cortex, but since the RAS is not active, the arousal state is low and the signals are not appreciated, or “encoded,” by the brain. Similarly, neural activity can be recorded from the brain during deep anesthesia, but the message cannot be detected and understood because of low RAS activity. It is believed that drugs which produce sleep (barbiturates) or alertness (amphetamines) act upon this area of the brain. Other experiments have shown that monkeys will perform discrimination tasks more quickly and accurately than usual during electrical stimulation of the RAS. Also, when human beings are given the tranquilizer chlorpromazine to cut down on RAS arousal, they show impaired discrimination ability (Primac, Mirsky, and Ros- vold, 1957).Sensory deprivation experiments indicate that the loss of normal functions which occurs in this state is due to a low level of RAS activity. Some investigators believe the RAS is involved in pain arousal, and that general anesthetics produce their effects through this structure. It is also believed to account for the ability of the mother to awaken when her baby cries, and to remain asleep through much louder sounds, although the exact mechanism is still unknown. Hemandez-Pe6n, Sherrer, and Jouvet (1956) have recorded the neural activity in a cat’s ear, and have shown that a continuously sounding click ceases to be registered as soon as the cat is shown a mouse, or smells the odor of fish, or is given an electrical shock. Apparently the RAS cuts off the auditory responses when the situation changes so that other stimuli become more important than the task at hand. Experiments of this kind have led to the theory that RAS is a “gating” or “shunting” mechanism which makes it possible for impulses significant for survival to gain dominance over less important stimuli.