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Limbic System

Main Text

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 3 arches

Extends from temporal to frontal lobes and composed 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 (parolfactory 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 and composed of

– Hippocampus proper (Ammon horn)

– Dentate gyrus

– Supracallosal gyrus [indusium griseum, thin strip of gray matter that extends from dentate/hippocampus around corpus callosum (CC) to paraterminal gyrus]

– Paraterminal gyrus (below CC rostrum)

Curves over CC, below callosal sulcus
• Inner arch

Smallest arch

Extends from temporal lobe to mammillary bodies

Composed of fornix, fimbria

IMAGING ANATOMY

Overview

• Hippocampus

Curved structure on medial aspect of temporal lobe that bulges into floor of temporal horn

Consists of 2 interlocking U -shaped gray matter structures

– Hippocampus proper (Ammon horn) forms more superolateral, upside-down U

– Dentate gyrus forms inferomedial U

Has 3 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 CC
• Ammon horn (hippocampus proper)

Subdivided into 4 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, alveus

– Alveus borders temporal horn of lateral ventricle

– Forms fimbria → crus of fornix
• Fornix

Primary efferent system from hippocampus

4 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, mammillary 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)

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Mar 13, 2021 | Posted by admin in NUCLEAR MEDICINE | Comments Off

Main Text

TERMINOLOGY

Definitions

GROSS ANATOMY

Overview

IMAGING ANATOMY

Overview

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