Again addressing the oscillating, cause-and-effect relationship that has illuminated the development of knowledge of brain anatomy and physiology, we direct attention to three great systems on which the integrated behavior of higher animals depends: the ancient limbic system, the classical sensory or thalamocortical system, and the nonspecific ascending reticular system. Named in the order in which they were recognized and will be discussed, in each system an anatomic substrate was described before its “use,” or function, became apparent. The English-American neurophysiologist well known for her historical writings on the nervous system, M. A. B. Brazier, equated this triad of structures with the functions necessary to maintain the conscious state (1963, pp. 748–749). First, “transmission of the sense-labeled impulse bearing the message from the periphery to the brain” is achieved by “the classical afferent system, ascending laterally through the specific thalamic nuclei to specific cortical sites.” Second, “awareness that the message has arrived” occurs because “the ascending sensory systems in the midbrain core and medially placed thalamic nuclei are profoundly implicated.” Receiving and storing the message, the final requisite of the conscious state, “is served by the third of the three systems named: the limbic system, and in particular the hippocampal system.” This chapter considers the manner by which each of those integrated anatomic circuits was recognized as a functional entity. They cannot be envisioned as neatly demarcated nor are they competitive; rather, they lack agreed-on boundaries and it is clear that they interact profusely, each feeding into and receiving information from the others.
KeywordsBrain Stem Limbic System Reticular Formation Thalamic Nucleus Amygdaloid Body
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