Introduction
The term “reticular formation” is used to refer to one of several so-called reticular structures of the central nervous system. A reticulum is a mesh or network, and reticular formation designates a specific grouping of more than ninety nuclei of interneurons that have common characteristics in the area of the brain stem. The nuclei are clusters of cell bodies of neurons that form a network of their dendritic and axonal cellular processes, those extensions that bring information into the cell and transmit information from the cell.
The mesh reaches throughout the brain stem, as well as to higher and lower regions of the central nervous system as far as the cerebral cortex and spinal cord, serving both sensory and integrative functions. Anatomically, the reticular formation is continuous from the medulla oblongata, the lowest part of the brain stem, through the pons to the midbrain. It connects with the intermediate gray region of the spinal cord and sends processes into the higher brain areas of the thalamus and hypothalamus.
Neurons of the reticular formation contain many dendritic processes, afferent cytoplasmic extensions that carry electrical stimuli toward the cell nucleus, arranged perpendicular to the central axis of the body. Each cell also contains a single long axon, with numerous collateral branches, that extends along the body’s axis, going to the higher or lower regions of the central nervous system. The axon carries impulses away from the nucleus of the neuron toward the synapse, where it passes information on to the next neighboring cell. The axons and dendrites, present in large numbers, make up the mesh, or reticulum, that gives the reticular formation its name. The many aggregated processes make it extremely difficult to identify the clustered groups of neurons (nuclei) to which the individual cells belong.
Information and Arousal
The reticular formation is a portion of an important informational loop in the brain that allows the modification and adjustment of behavior. This loop extends from the cerebral cortex to subcortical areas (lower brain regions), including the reticular formation, and then back to the cortex. The reticular formation makes connections with all the portions of the loop and plays an important role in exciting or inhibiting the functions of the lower motor neuron centers. This loop is important in practically all functions of the nervous system and behavior, particularly sleep/wakefulness, emotional stress, depression and distress, the induction of rapid eye movement (REM) sleep, and even sleepwalking.
The process of arousal appears to take place as the reticular formation sends impulses to an area of the thalamus occupied by the midline thalamic nuclei. These nuclei then pass the information on to the cortex, which is stimulated to become more aware that information is coming and more attentive to receiving the information. This is an oversimplification of the process, however, as other areas of the brain also seem to be involved in arousal. The neurotransmitters involved in the reticular formation’s connection to the cortex are thought to include both cholinergic and monoamine systems in the arousal process, although these are still not well understood.
The basic functions of the reticular formation are twofold: to alert the higher centers, especially in the cortex, that sensory information is coming into the processing areas and to screen incoming information being passed upward on sensory (afferent) pathways toward the higher centers of the brain, blocking the passage of irrelevant information and passing along the information that should be acted on by the higher brain. All sensory information must be passed through the lower regions of the brain before reaching the associative regions of the cerebral cortex. The cortex is unable to process incoming information unless it has been alerted and aroused and unless the information is channeled through the proper lower brain regions. Besides the reticular formation, the thalamus is also involved in this function, taking information from the reticular formation and passing it on to the cortex, where it is then processed and coordinated to produce motor behavior.
Information Inhibition
Because the reticular formation has so many pathways from each cell leading to many other cells, it is very quickly inhibited by anesthetics that act by inhibiting the transfer of information between cells at the synapse. This inhibition of activity leads to unconsciousness from a general lack of sensation and loss of alertness and arousal as polysynaptic pathways are shut down. Under proper medical control, use of anesthesia to turn off the reticular formation can be lifesaving, allowing surgical procedures that could not be tolerated without it.
Lesions of the brain stem may damage the reticular formation, producing the uncontrolled unconsciousness of coma if they occur above the level of the pons on both sides. Coma that results from drug overdose or drug reaction occurs mainly as the result of depression of the reticular formation. Any lesion of the brain stem that affects the reticular formation directly will also have a secondary effect on other structures on the brain stem, causing disappearance of its reflex reactions. Damage to ascending efferent pathways from the reticular formation to the cortex sometimes can also cause coma. Because the reticular formation aids the brain stem in regulating critical visceral vital functions such as breathing and blood circulation, damage to this area may threaten life itself.
The actions of alcohol on behavior also are the result of its effects on the reticular formation. Alcohol blocks the actions of this area, allowing a temporary loss of control over other brain regions. This lack of behavioral inhibition from higher brain centers produces a feeling of excitement and well-being at first. Later effects of continued alcohol intake lead to depression of emotions and behavior, followed by depression of basic body functions that can produce unconsciousness.
The production of unconsciousness through sleep is also associated with the reticular formation, particularly the part that is in the pons and another center in the lower medulla. The lower medullary sleep/waking center seems to work with the basal forebrain to modulate the induction of sleep. Rapid eye movement (REM) sleep may be controlled, at least in part, by specific nuclei in the pontine reticular formation.
Behavioral Effects
Stimulation of the reticular formation and other areas (the hippocampus and amygdala) improves memory retention (memory consolidation) if electrical current is applied directly to the reticular cells immediately after a training session. It is difficult to understand how this stimulation operates, however, since in some cases stimulating these same areas instead produces retrograde amnesia, causing the loss of memory retention. It is thought that the level of electrical stimulation may cause these different results. The highest and lowest stimulation levels reduce memory consolidation in some cases, and intermediate stimulation seems to be the most effective. The nature of the training process is also important in the results, as learning seems to be more difficult with high stimulation levels associated with aversive conditioning.
Another aspect of the reticular formation and its possible effects on behavior is the theory that many (or perhaps most) convulsive epileptic seizures originate there. Since this area can be stimulated by electrical impulses and by convulsive drugs to produce seizures, it is thought that the reticular formation may be the site from which stimulation of the cerebral cortex starts. It is difficult to establish the origins of epilepsy conclusively, since there are no adequate animal models for this disorder, but antiepileptic drugs are shown to depress neuron function in the reticular formation. The actual source of the convulsive behavior is thought to be the nonspecific reticular core of this formation.
Research and Experimentation
The reticular formation influences nearly all aspects of nervous system function, including sensory and motor activities and somatic and visceral functions. It is important in influencing the integrative processes of the central nervous system, acting on the mind and behavior. Included in this influence are the stimulatory aspects of arousal, awakening, and attentiveness, as well as the inhibitory aspects of drowsiness, sleep induction, and general disruption of the stimulatory functions. To understand how this region of the brain can be so important in such contradictory functions, it is important to consider the integration of excitatory and inhibitory inputs and the consolidation of their overall influences. Depending on which type of stimulus has the greatest effect, the net result on behavior can be alertness or drowsiness, active function or the inactivity of sleep.
Research on anesthetized cats in the late 1940s produced an increased understanding of the activities of the reticular formation. It was shown that electrical stimulation of the brain stem caused changes in the cats’ electroencephalograph (EEG) readings that were similar to changes occurring in humans when they were aroused from a drowsy state to alertness. From these observations and others, it has been concluded that the ascending reticular system of the brain stem acts as a nonspecific arousal system of the cerebral cortex.
In the 1950s, Donald Lindsley and his colleagues studied the reticular formation as the source of arousal. They showed that two discrete flashes of light shown to a monkey produced discrete electrical responses (evoked potentials) in the visual cortex. If the pulses were very close together, only one potential was evoked, showing that the cortex could not distinguish both within that time. If two electrical stimulations were applied directly to the reticular formation at the short interval, however, two discrete flashes were expressed in the cortex, showing the influence of the reticular formation on the threshold level of the cortex’s response to stimuli. J. M. Fuster, one of Lindsley’s coworkers, examined the behavioral responses that resulted from electrical stimulation of the reticular formation in monkeys trained to discriminate between two objects. Reducing the time of visual exposure to the objects also reduced the correct responses, but stimulation of the reticular formation at the same time as the visual exposure reduced the error level. This indicated that increased arousal and attentiveness to the visual stimuli were produced by electrical activation of the reticular formation.
J. M. Siegal and D. J. McGinty’s work on stimulation of the reticular formation in cats in the 1970s showed that individual neurons seem to have a role in controlling various motor functions of the body. Other studies show that various autonomic responses, such as vomiting, respiration, sneezing, and coughing, may also originate at least in part from the reticular formation.
It is thought that the period of sleep known as rapid eye movement sleep, or paradoxical sleep, is a time of memory consolidation. During this time, the reticular formation, the hippocampus, and the amygdala are stimulated to activate the higher brain centers, and arousal occurs. REM sleep is considered paradoxical because the brain waves produced during this time are similar to those produced during stimulation of the awake brain. Vincent Bloch and his colleagues have shown that laboratory animals and human subjects deprived of REM sleep display decreased memory consolidation. During this process, short-term memories are converted somehow into long-term memories, which withstand even disruptions of the electrical activities of the brain. The reticular formation is an important part of memory function, but much remains to be discovered about this and other reticular activities.
Bibliography
Carlson, Neil R. Physiology of Behavior. 10th ed. Boston: Allyn, 2009. Print.
Fromm, Gerhard H., Carl L. Faingold, Ronald A. Browning, and W. M. Burnham, eds. Epilepsy and the Reticular Formation: The Role of the Reticular Core in Convulsive Seizures. New York: Liss, 1987. Print.
Hobson, J. Allan, and Mary A. B. Brazier, eds. The Reticular Formation Revisited: Specifying Function for a Nonspecific System. New York: Raven, 1980. Print.
Klemm, W. R., and Robert P. Vertes, eds. Brainstem Mechanisms of Behavior. New York: Wiley, 1990. Print.
Mai, Juergen K., and George Paxinos. The Human Nervous System. Burlington: Elsevier Science, 2011. Print.
Romero-Sierra, C. Neuroanatomy: A Conceptual Approach. New York: Churchill, 1986. Print.
Sadock, Benjamin J., and Virginia Sadock, eds. Kaplan and Sadock’s Comprehensive Textbook of Psychiatry. 8th ed. Philadelphia: Lippincott , 2005. Print.
Siegel, Alln, and Hreday N Sapru. Essential Neuroscience. Philadelphia: Lippincott, 2011. Print.
Steriade, Mircea, and Robert W. McCarley. Brainstem Control of Wakefulness and Sleep. 2d ed. New York: Springer, 2005. Print.
Yogarajah, Mahinda, and Christopher Turner. Neurology. Edinburgh: Mosby, 2013. Print.
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