Besides being divided into separate lobe
s based on function, the material of the cortex has distinct features that are much smaller and more specialized. Two millimeters of cortical (gray
) matter sits on top of the white matter, and is actually responsible for all cortical functions. The much more voluminous white matter
below does no processing at all -- it's more responsible for transporting oxygen, mollifying damage, and maintaining the blood-brain barrier. Each square millimeter of that cortical surface, however, contains around 148,000 neuron
s; each is involved with the others in the intense communication
that eventually exhibits itself as action
, and thought
The cerebral cortex itself, that crucial layer of gray matter, is subdivided into six layers. While these layers are not perfectly defined, there is usually much more crosstalk between neurons in any given layer and neurons in other cortical layers or brain areas. From the six layers, three general subdivisions can be made:
- Layers I, II, and III are the three outermost layers, often called the superficial layers. They read information from one another (and to some extent the thalamus), and send information to all the other layers. Being the outermost layers, they are the most recently developed on the evolutionary time scale. Collecting and collating all of the cortical input and influencing all of its output, these layers might well be considered the seat of thought itself.
- Layer IV is the primary terminus of all information coming from the thalamus, which processes all of the body's state and perceptions for the cortex. This layer also is partially responsible for sending information back to the thalamus, as a check against cortical overstimulation. This layer's contact with layers II and III, along with the direct thalamus input noted above, gives the cerebral cortex all of its knowledge about its surroundings.
- Layer V picks up information from layers I and II and sends it to sub-cortical brain areas, as well as the and body. Some layer V neurons take information from layers II and III and send it to the corpus callosum so it can reach the other half of the cortex. Others collate information from layer VI and some from layer I, and send it to the pons, superior colliculus, and even the spinal cord itself. If layer IV is the inbox, then layer V can be thought of as the outbox, sending the results of cortical processing wherever they need to go.
- Layer VI, finally, takes the information available in itself and layer IV, and sends it to the thalamus. Interactions between these two areas probably allow for conscious control, and to a lesser extent automatic, of what stimuli we are paying attention to, like glancing toward your cat from the computer monitor.
Surprisingly, this variegation
is constant over the whole cortical surface, and even more surprisingly has the same configuration in all mammal
s. What varies between mammals is the cortical surface area
and ratio of layers II and III to the entire cortical depth. That is, the surface area of the human cerebral cortex is around 2200 cm2
, whereas chimpanzee
s have around 600 cm2
, cats have 220 cm2
, and rats have less than 7 cm2
. More importantly, since percentages don't corelate with animal size, rodent
s have only around 25% of their cortex depth taken up by level II/III neurons, while carnivore
s have about 36% and primate
s 48%. These differences probably arose as evolution created opportunities for animals with ever greater cortical processing power, which is to say, ever more complex thought.
After single neurons, the first level of cortical organization found (in research thus far, at least) is the minicolumn. These are groups of cells between 20 and 65 micrometers in diameter, which cross all six cortical layers in sort of a cylinder shape. Composed of roughly 100 neurons, these are generally separated from one another by 30 micrometers or so. While it is tempting to think of these as "modules," interchangeable on some level, they are each wired differently and each have axons and dendrites that differ both spatially and functionally.
A group of minicolumns together makes up a macrocolumn, which is generally between .4 and 1 millimeters in diameter. Macrocolumns are probably the result of a large neural input to that particular area of the cortex, which during development causes neurons to grow around it. Again, these structures are not functionally interchangeable -- each one is different from all the others. While macrocolumns usually penetrate all six cortical layers, in the visual cortex they only exist in layers I-III, and are known when found in that area as color blobs.