The Limbic System

The Limbic System 

What is the Limbic System?

Structures of the Limbic System: A Brief Introduction

The Limbic System

The limbic system is often referred to as our emotional brain; the structures of the limbic system regulate fear, anger, and sexual behavior – this is also our pleasure center. In addition to the aforementioned functions, the limbic system is responsible for another very important activity – categorizing memories. The limbic system is comprised of the following: amygdala, cingulate gyrus, fornix, hippocampus, hypothalamus, thalamus, prefrontal cortex, and olfactory (smell) cortex. Some of these structures have more influence than others during a limbic system mediated response. 

Amygdala

The amygdala – tiny brown structure in the picture below and to the right, inside of red circle– is responsible for processing emotions such as fear, anger, and pleasure. The amygdala is also in charge of cataloging and filing away memories, where the memory is stored is based on the magnitude of the emotional response that accompanied the event. The amygdala is critical to our fear response (fight or flight) and is the first structure to respond to a potential danger – signaling the hypothalamus. Interestingly, not only is the amygdala directly linked to aggression, it is also larger in males. 

Cingulate Gyrus

The cingulate gyrus – located just superior to the corpus callosum – coordinates sensory input with emotional responses and it regulates aggressive behavior. (Gyri refer to the convolutions, or ridges, created by infolding of the cerebral cortex increasing surface area.) It is also the cingulate gyrus that is responsible for emotional bonding, especially between a mother and infant. The posterior cingulate gyrus plays a role in spatial processing and memory.

Corpus Callosum

The corpus callosum connects the two hemispheres of the brain.

Fornix

The fornix is not well understood but it is hypothesized that it is involved in memory formation and recall. The fornix originates from the hippocampus and travels up and around the thalamus and then forward and down. The fornix then extends through the hypothalamus to form connections with the mammillary bodies.

Hippocampus

The hippocampus is formed as the innermost fold of the temporal lobe. The hippocampus is involved in the storage of long-term memories, especially declarative memories; the type of memories that are purposely recalled such as facts and events. Although the hippocampus is not directly involved in the formation of new memories, it is involved in the memories of people and location of objects. It is one of the first structure affected in Alzheimer’s patients, impacting the creation of new memories, which explains why early signs of Alzheimer's include short-term memory loss. These structures are also involved in mental illness – shrinking in patients with severe depression and in Schizophrenics. Interestingly, the hippocampus is directly affected by estrogen.

Hypothalamus

The hypothalamus influences the expression of emotion. Along with its boss duties, the hypothalamus is heavily involved in fear, rage, and in the autonomic responses to fear and rage. Finally, parts of the hypothalamus are responsible for male sexual behavior.

Prefrontal Cortex

The orbital and medial prefrontal cortex are associated with decision making and executive functioning. 

Olfactory Tract/Cortex

The olfactory cortex has a significant role in the limbic system (olfaction refers to sense of smell). Structures in the limbic system, such as the amygdala, attach an emotional response and/or memory to a particular smell. Smell enters the brain through the olfactory bulbs and then olfactory tracts where the chemical message of scent ends up at the amygdala.

Thalamus

The thalamus  is not part of the limbic system, per se, but it is important to include because it connects the limbic system to the spinal cord and brain. The thalamus is kind of like a gatekeeper, determining what sensory information should go where. 


Let's Do the Limbic System

Limbic System - Pathway

A picture paints a thousand words right? What does the above mess tell you? To me, it says the pathway through the limbic system is incredibly complex and has lots of players. If we revisit the function of each of the limbic system structures and then connect them together, the picture starts to get a bit clearer. 


The limbic association area is located within the temporal lobe and it links emotion with sensory input. An important function of this region is in the processes of learning and formation of memory. The limbic association area communicates directly with the cingulate gyrus and the hippocampus. 

The hippocampus, via the fornix, then communicates with the mammillary bodies and the hypothalamus. The mammillary bodies then communicate with the thalamus and terminate with the cingulate gyrus. The hippocampus also communicates with the amygdala and the amygdala with the hypothalamus.

The hippocampus is associated with which type of memory formation?

  • Long-term
  • Short-term

In your own words, how would you describe the function of the amygdala?