The Limbic System, Temporal Lobe, and Prefrontal Cortex

Abstract

The neurologist Paul Broca in the latter half of the nineteenth century initially designated all of the structures on the medial surface of the cerebral hemisphere the “great limbic lobe.” This region, due to its strong olfactory input, was also designated the rhinencephalon. The olfactory portion of the brain (rhinenceph-alon or archipallium) comprises much of the telencephalon in fish, amphibians, and most mammals. In mammals, the presence of a large olfactory lobe adjacent to the hippocampus was once considered to be evidence of the important olfactory functions of these regions. However, when a comparative neuroanatomist examined the brains of sea mammals that had rudimentary olfactory apparatus, e.g., dolphins and whales, the presence of a large hippocampus suggested other than olfactory functions for this region. In 1937, Papez proposed that olfactory input was not the prime input for this region, and the experiments of Kluver and Bucy (Am J Phys 119:352, 1937) and Bucy (1952, 1958) demonstrated the behavioral deficits seen after lesions in this zone. In primates, only a small portion of the limbic lobe is purely olfactory: the olfactory bulb, olfactory tract with medial and lateral roots, olfactory trigone at junction of the olfactory tract with a cerebral hemisphere and with olfactory tubercle (behind trigone contains islets of Calleja with GABA interneurons), pyriform cortex of the uncus, and corticoamygdaloid nuclei (Fig. 16.1). The other portions of the temporal lobe—hippocampal formation, parahippocampal gyrus, and cingulate gyrus—are now known to be the cortical regions of the limbic system (Fulton, Yale J Biol Med 26(2):107–118, 1953; Green, Reticular formation of the brain. Boston: Little, Brown and Company, 1958; Papez, Reticular formation of the brain. Boston: Little Brown and Company, 1958; Scheer 1963; Isaaccson, The limbic system. 2nd ed. New York: Plenum, 1982). The olfactory system due to its strong projections onto the medial temporal lobe directly, by passing the thalamus, will be briefly discussed prior to our focus on the limbic system and the temporal lobe.