Limbic System Overview and Key Components

Limbic System Overview

• The limbic system is involved in emotion, behaviour, mood, and memory.
• Initially, a significant role of the limbic system was thought to be linked to olfaction (sense of smell).
• Key components include the hippocampus, fimbria, fornix, dentate gyrus, mammillary body, uncus, cingulate gyrus, parahippocampal gyri, amygdaloid body, septal areas and piriform areas, and anterior thalamic nucleus.

Hippocampus

• Situated above the anterior part of the parahippocampal gyrus.
• Projects onto the floor of the inferior horn of the lateral ventricle.
• The anterior portion of the hippocampus is called pes hippocampi.
• Alveus is a thin film of white matter that thickens medially to form the fimbria.
• Alveus separates from the hippocampus as the crus of the fornix.
• The dentate gyrus is part of the hippocampus located between the fimbria and the parahippocampal gyrus.

Fornix

• A major efferent pathway from the hippocampus.
• Also functions as an association tract and commissure (connecting different parts of the brain).
• Fibers originating from the anterior column of the fornix extend both anteriorly and posteriorly to the anterior commissure.
• Anterior fibres in the fornix connect to septal nuclei.
• Posterior fibres in the fornix connect to the thalamus and mamillary bodies.
• Mamillo-thalamic tract connects the mamillary bodies to the anterior nucleus of the thalamus.
• Fibers in the fornix run through the internal capsule to reach the cingulate gyrus.

Papez Circuit

• A circuit of interconnected structures involved in emotion and memory.
• Key structures involved in the circuit include the hippocampus, mamillary body, anterior thalamus, cingulate gyrus, and fornix.

Septal Area

• Composed by gray matter above the lamina terminalis, near the anterior commissure.
• A part of the septal area is the septum pellucidum.

Olfactory Pathways

• Olfactory tract runs through the olfactory sulcus beside the gyrus rectus.
• The anterior end expands to form the olfactory bulb.
• Second-order neuron cells are located within this pathway.
• Axons from these cells travel back through the tract.
• At the anterior perforated substance, the tract divides, with lateral striae extending to the uncus and medial striae connecting to other limbic system parts.

Functions and Disorders

• This section discusses the functions of the limbic system and potential associated disorders.

Autonomic Nervous System

• Most of the limbic system output connects to the hypothalamus primarily through the medial forebrain bundle.

Behavior

• Hypothalamic regions are connected to eating, drinking, and sexual behaviour patterns.
• Lesions in the lateral amygdala cause unrestrained eating (bulimia).
• Lesions in the medial amygdala cause anorexia and hypersexuality.
• Electrical stimulation of the amygdala may induce fear and anxiety in humans.

Memory

• The hippocampus plays a vital role in converting short-term memory into long-term memory (up to 60 minutes to several days or longer).
• Bilateral hippocampal damage causes profound anterograde amnesia.

Spatial Problem Solving

• The hippocampus contains "place cells" that encode spatial memory (where you have been).
• Recalling locations and routes to navigate requires hippocampal activation.