LIMBIC SYSTEM

Review

Terminology

• Definitions

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

Limbic system

• Limbic lobe

• Plus some subcortical structures (e.g., amygdala,mammillary bodies, septal nuclei, etc.)

Gross Anatomy

• Overview

Limbic lobe

• formed by nested "C-shaped" arches of tissues surrounding diencephalon, basal ganglia

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)

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

Inner arch

• Smallest arch

• Extends from temporal lobe to mamillary bodies

• Comprised of fornix, fimbria

Imaging Anatomy

• Overview

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)

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

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)

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)

Imaging Pitfalls

• Normal variant is incomplete fusion of hippocampal sulcus → CSF-containing "cysts" along medial hippocampus

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.