Circuitry of the neocortex

 

The human neocortex is a sheet of neural tissue approximately 1,000 cm2 in area and 2mm thick. To visualize this sheet, think of a cloth dinner napkin, which is a reasonable approximation of the area and thickness of the neocortex. The neocortex is divided into dozens of functional regions, some related to vision, others to audition, and others to language, etc. Viewed under a microscope, the physical characteristics of the different regions look remarkably similar.

 

There are several organizing principles seen in each region throughout the neocortex.

 

Layers

The neocortex is generally said to have six layers. Five of the layers contain cells and one layer is mostly connections. The layers were discovered over one hundred

years ago with the advent of staining techniques. The image above (from Cajal)

shows a small slice of neocortex exposed using three different staining methods.

The vertical axis spans the thickness of the neocortex, approximately 2mm. The left side of the image indicates the six layers. Layer 1, at the top, is the non-cellular

level. The “WM” at the bottom indicates the beginning of the white matter, where

axons from cells travel to other parts of the neocortex and other parts of the brain.

 

The right side of the image is a stain that shows only myelinated axons. (Myelination is a fatty sheath that covers some but not all axons.) In this part of the image you can see two of the main organizing principles of the neocortex, layers and columns. Most axons split in two immediately after leaving the body of the neuron. One branch will travel mostly horizontally and the other branch will travel mostly vertically. The horizontal branch makes a large number of connections to other cells in the same or nearby layer, thus the layers become visible in stains such as this.

Bear in mind that this is a drawing of a slice of neocortex. Most of the axons are coming in and out of the plane of the image so the axons are longer than they appear

in the image. It has been estimated that there are between 2 and 4 kilometers of axons and dendrites in every cubic millimeter of neocortex.

 

The middle section of the image is a stain that shows neuron bodies, but does not show any dendrites or axons. You can see that the size and density of the neurons also varies by layer. There is only a little indication of columns in this particular image. You might notice that there are some neurons in layer 1. The number of layer 1 neurons is so small that the layer is still referred to as a non-cellular layer. Neuro-scientists have estimated that there is somewhere around 100,000 neurons in a cubic millimeter of neocortex.

 

The left part of the image is a stain that shows the body, axons, and dendrites of just a few neurons. You can see that the size of the dendrite “arbors” varies significantly in cells in different layers. Also visible are some “apical dendrites” that rise from the cell body making connections in other layers. The presence and destination of

apical dendrites is specific to each layer.

 

In short, the layered and columnar organization of the neocortex becomes evident when the neural tissue is stained and viewed under a microscope.