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Limbic system

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  • 1. LIMBIC SYSTEM sherlockradiology.com Review
  • 2. Terminology • Definitions
  • 3. Limbic lobe • Phylogenetically older cortex • Fewer layers than neocortex • Major role in memory, olfaction, emotion • Composed of subcallosal, cingulate, parahippocampal gyri + hippocampus, dentate gyrus, subiculum, entorhinal cortex
  • 4. Limbic system • Limbic lobe • Plus some subcortical structures (e.g., amygdala,mammillary bodies, septal nuclei, etc.)
  • 5. Gross Anatomy • Overview
  • 6. Limbic lobe • formed by nested "C-shaped" arches of tissues surrounding diencephalon, basal ganglia
  • 7. Outer arch • Largest of the three arches • Extends from temporal to frontal lobes, comprised of - Uncus (anterior end of parahippocampal gyrus) - Parahippocampal gyrus (swings medially at posterior temporal lobe, becomes isthmus of cingulate gyrus) - Cingulate gyrus (anterosuperior continuation of parahippocampal gyrus) - Subcallosal (paraolfactory area) is anteroinferior continuation of cingulate gyrus • Curves above callosal sulcus (continuous with hippocampal sulcus of temporal lobe)
  • 8. Middle arch • Extends from temporal to frontal lobes, comprised of - Hippocampus proper (Ammon horn) - Dentate gyrus - Supracallosal gyrus (indusium griseum, a thin strip of gray matter that extends from dentate/hippocampus all the way around corpus callosum to paraterminal gyrus) - Paraterminal gyrus (below corpus callosum rostrum) • Curves over corpus callosum, below callosal sulcus
  • 9. Inner arch • Smallest arch • Extends from temporal lobe to mamillary bodies • Comprised of fornix, fimbria
  • 10. Imaging Anatomy • Overview
  • 11. Hippocampus • Curved structure on medial aspect of temporal lobe that bulges into floor of temporal horn • Consists of two interlocking "U-shaped" gray matter structures - Hippocampus proper (Ammon horn) forms more superolateral, upside-down U - Dentate gyrus forms inferomedial U • Has three anatomic subdivisions - Head (pes hippocampus): Most anterior part, oriented transversely; has 3-4 digitations on superior surface - Body: Cylindrical, oriented parasagittally - Tail: Most posterior portion; narrows then curves around splenium to form indusium griseum above corpus callosum (CC)
  • 12. Ammon horn (hippocampus proper) • Subdivided into four zones (based on histology of main cell layers) - CA1 (Sommer sector): Small pyramidal cells (most vulnerable; commonly affected by anoxia, mesial temporal sclerosis) - CA2: Narrow, dense band of large pyramidal cells ("resistant sector") - CA3: Wide loose band of large pyramidal cells - CA4 (end-folium): Loosely structured inner zone,enveloped by dentate gyrus • Blends laterally into subiculum - Subiculum forms transition to neocortex of parahippocampal gyrus (entorhinal cortex) • Covered by layer of efferent fibers, the alveus - Alveus borders temporal horn of lateral ventricle ventricle - Forms fimbria → crus of fornix
  • 13. Fornix • Primary efferent system from hippocampus • Four parts - Crura (arch under CC splenium, form part of medial wall of lateral ventricles) - Commissure (connects crura) - Body (formed by convergence of crura, attached to inferior surface of septum pellucidum) - Columns (curve inferiorly to mammillary bodies,anterior thalamus, mamillary bodies, septal nuclei)
  • 14. Amygdala • Large complex of gray nuclei medial to uncus, just in front of temporal horn of lateral ventricle • Tail of caudate nucleus ends in amygdala • Major efferent is stria terminalis - Stria terminalis arches in sulcus between caudate nucleus, thalamus - Forms one margin of choroid fissure (other is fornix)
  • 15. Imaging Pitfalls • Normal variant is incomplete fusion of hippocampal sulcus → CSF-containing "cysts" along medial hippocampus
  • 16. Reference • Diagnostic and surgical imaging anatomy. Brain, head & neck, spine /H. Ric Harnsbcrger... [et al.] ; managing editor, Andre Macdonald. - - 1st ed. p.76-85 ; I cm.