The limbic system

Abstract

“The hypothalamus, the anterior thalamic nucleus, the cingulate gyrus, the hippocampus and their interconnections, constitute a harmonious mechanism which may elaborate the functions of central emotion as well as participate in the emotional expression.” James Papez, 1937 The limbic system consists of the phylogenetically old limbic lobe and other subcortical structures and their connections. Although not empirically proven, the limbic system is a functional concept which may be employed to explain various brain functions.[1] HISTORY Paul Pierre Broca in 1878 spoke of ‘le grand lobe limbique’ or the great limbic lobe and applied the term “limbic” (from the Latin limbus for border) to the curved rim of the cortex which incudes the cingulate and the parahippocampal gyri. However, its putative role in emotion was elaborated by the American physician, James Papez in 1937 in the seminal paper titled ‘A proposed mechanism of emotion’. This anatomical model is referred to as the Papez circuit.[2] Yakovlev in 1948 proposed Yakovlev's circuit in the control of emotion involving the orbitofrontal, insular and anterior temporal lobe cortex, the amygdala and the dorsomedial nucleus of thalamus.[3] In 1952, Paul D. MacLean coined the term “limbic system” to describe Broca's limbic lobe and related subcortical nuclei as the collective neural substrate for emotion.[1] MacLean was also instrumental in proposing and defining the Triune concept of the brain. MacLean's evolutionary “Triune brain theory” proposed that the human brain was in reality three brains in one: the R-complex (reptilian complex), the limbic system and the neocortex.[4] The concept of the limbic system has since been further expanded and developed by Nauta, Heimer and others. COMPONENTS OF THE LIMBIC SYSTEM There is no universal agreement on the total list of structures, which comprise the limbic system. The brain regions that constitute the limbic system are: Limbic cortex Cingulate gyrus Parahippocampal gyrus Hippocampal formation The dentate gyrus Hippocampus Subicular Complex Amygdala Septal area Hypothalamus These structures form a complex network for controlling emotion.[5] Limbic lobe The limbic lobe situated at the inferomedial aspect of the cerebral hemispheres, consists of two concentric gyri surrounding the corpus callosum. Broca proposed that the larger outer gyrus be named “limbic gyrus” and the smaller inner one “the intralimbic gyrus”. The limbic gyrus (limbic lobe) consists of the isthmus of the cingulate gyrus, the parahippocampal gyrus (both of which are continuous via a bundle of white matter called “cingulum”) and the subcallosal area.[6] The cingulate gyrus (Latin = Belt ridge) dorsal to the corpus callosum is heavily interconnected with the association areas of the cerebral cortex. The parahippocampal gyrus in the medial temporal lobe contains several distinct regions, the most important being the entorhinal cortex (ERC). The ERC funnels highly processed cortical information to the hippocampal formation and serves as its major output pathway.[5] The hippocampal formation Hippocampal formation in the temporal lobe has three distinct zones: The dentate gyrus The hippocampus proper The subiculum Embryologically, the hippocampal formation is an extension of the medial edge of the temporal lobe. The entire hippocampal formation has a length of about 5 cm from its anterior end at the amygdala to its tapering posterior end near the splenium of the corpus callosum.[5] Dentate gyrus The dentate gyrus is composed of three layers: an outer acellular molecular layer, a granular middle layer and an inner polymorphic layer. Hippocampus The word hippocampus means “Sea Horse” in Greek. Hippocampus is a trilaminate structure with an outer molecular layer, a middle pyramidal layer and an inner polymorphic layer. On the basis of differences in cytoarchitecture and connectivity, the hippocampus has four fields (named by Lorente de No in 1934): CA1, CA2, CA3 and CA4 (CA: Cornu Ammonis). To embryologic enthusiasts, CA4 is not part of the Cornu Ammonis but is a separate structure—the dentate hilus. The thin layer of fibers adjacent to the polymorphic layer of the hippocampus is known as the alveus. These fibres coalesce to form the fimbria and subsequently, the crura of the fornix (main efferent pathway of the hippocampal formation). The crura of the fornix converge to form the body of the fornix, which later forms the columns of the fornix and pass through the hypothalamus into the mammillary bodies.[57] Subicular complex The subicular complex has three components: the presubiculum, the parasubiculum and subiculum. Subiculum is the transitional zone between the six-layered, entorhinal cortex and the three-layered hippocampus.[5] Amygdala Identified by Burdach in the early 19th century, amygdala, an almond-shaped structure deep within the temporal lobe, is a collection of nuclei lying beneath the uncus. Lying at the anterior end of the hippocampal formati