It’s the Limbic System!

What is the limbic system? I mentioned this in another post, but it bears repeating. Limbus means ‘border’. What is the limbic system the border of? It is the edge of the cortex, a mostly 6-layered sheet of cells that covers the wrinkly mass of the brain. The cortex is the ‘thinking matter’, ‘the little gray cells’ that Hercule Poirot often references in Agatha Christie’s mysteries. The brain has two halves, or hemispheres. That great divide along the midline of the brain, between the left and right hemispheres, is called the interhemispheric fissure. Paul Broca, French physician in 1878 described the anatomic structures along the midline associated with the limbus of the cortex as the limbic lobe in which he included the cingulate gyrus, the hippocampus and the olfactory lobe.[1]

Where Is the Edge of the Brain?

If we iron out all the wrinkles in the cortex of one hemisphere, we will have a mostly 6-layered sheet of cells. The 6-layered gray matter is called isocortex. As we approach the edge, it thins out to 4 layers, then to 3 layers. Any cortex with less than 6 layers is called allocortex, since allo is Greek for ‘other’ – in this instance, other than 6 layers.

Imagine that we fold over the 3 layered part of this sheet all around the edge. Furthermore, imagine that before we do this, we put a string inside the pleat, like the string that always seems to get lost inside the pajama bottoms (or does that just happen to me?). When we pull the string, we get a puckered circle, like a lady’s purse, or like pursed lips. Broca, described it in exactly this way:

“As it surrounds the threshold of the hemisphere the mantle forms a border which resembles the circular edge of a purse. Hence I am calling this border the limbus of the hemisphere and the convolution that forms it the ‘limbic convolution’”[2]

The great neuroanatomist Walle Nauta concurs with Broca’s description:

“Little wonder, all in all, that Broca decided he had found the edge of the cerebral cortex, at least in the temporal lobe, and little wonder that he initially chose to call his discovery the great lobe of the hem.”[3]

The hippocampus is the three-layered structure along the midline at the that forms the ‘hem’ on the underside of the brain. Its cross section reveals a spiral structure. This is where the 6 layers on the outer part of the spiral becomes 4, then 3 layers. As the hippocampal structure arches around the superior, or top part of the interhemispheric fissure, it retains that overall topology. but it is not as pronounced as it is in the inferior portion (hippocampus) in humans. In other mammals, however, the spiral-shaped cross section remains obvious along the entire convolution.

Beyond the Edge

What happens beyond the edge of the gray, thinking matter? Different parts of the brain must communicate with each other as well as with the rest of the body: the muscles, organs, etc.  The other type of neuron in the brain appears white in color and is elongated to different extents. The white coating on these cells, called myelin, makes it possible for neural impulses to quickly travel long distances. The gray cells of the brain communicate with each other and the rest of the body through the white matter cabling system.

The white matter connections coalesce at the limbus. Massive tracts are formed including the corpus callosum, which connects the two hemispheres, the fornix, a tract that follows the arch of the limbic convolution, and many others, which interconnect and allow associations of one part of the brain with another.

Function of the Limbic System

The structures that form the limbic system are arranged in a physical circuit (or convolution, as Broca calls it) that follows the edge of the cortex.  In 1937   James Papez wrote a paper called A Proposed Mechanism of Emotion.[4]  He proposed that there was a pathway in the brain for emotions, organized in the same way as those that allow us to sense our environment (such as vision and touch). ‘Papez Circuit’ includes the structures of the limbic lobe identified by Broca, in large part. Subcortical structures such as the amygdala and certain nuclei of the thalamus are included in the pathway. The fact that there are connections of limbic structures with virtually all other parts of the brain[5] gives one the idea that our ‘emotion system’ has an influence on, or can influence every part of brain and our body.

I love that there is a circuit in the brain that is responsible for emotions. I also love that it is surrounded by one big pucker.

Happy Valentine’s Day