The neocortex is the outer surface of the brain. It is the phylogenically most recent neural development, being responsible for the higher functions such as language and reasoning. Despite having the largest surface area1 of any structure in the brain, the neocortex is only 1.5 to 3.0 mm thick and is organised into six layers of cells.

These stratifications of the neocortex can also be separated, by function, into three groups. The outer three layers generally receive their input from deeper layers and layer 4 in particular. These layers are typically well developed in the cortical regions surrounding the primary areas2. The forth layer receives input from axons which generally come from other areas. In the primary sensory (visual, auditory, somatosensory) areas, large numbers of densely packed cells receive projections from their respective sensory organs. The function of the bottom two layers differs from the others in that it sends axons to other areas of the brain. The cells in these layers are particularly large and distinctive in the motor areas. What this shows is that the morphology of cells present in a particular cortical area is related to the function of the area; these cells will be more abundant, and often larger, in the area they are associated to and less prevalent in other non-related areas. In the neocortex, cell layers 1, 2 and 3 are generally related to association, layer 4 is related to sensation and layers 5 and 6 are related to motor functions. It should be noted that while the cellular organisation of the neocortex is laminar, it is functionally arranged into columns.

Layer 1 is the neuron poor molecular layer consisting of very few intrinsic neurons but mainly fine fibres. These fibres are the dendrites and axons of pyramidal cells originating deeper in the cortex or from subcortical structures. Layer 2 is the external granular layer and layer 4 is the internal granular layer. Both these layers consist of small pyramidal and intrinsic neurons, but get their name from the large number of granule cells present. These granule cells are local circuit neurons with short axons. Layers 3 and 5 are the external pyramidal layer and the internal pyramidal layer. These layers consist of larger pyramidal neurons and longer axons and layer 5 often contains Betz cells, which are a type of giant pyramidal cell. The polymorphic (multiform) cell layer, layer 6, is made up of intrinsic neurons and modified pyramidal cells called spindle cells.

As previously stated, the neocortex is the outer most surface, but there is also the allocortex. The allocortex can be divided into the archicortex and paleocortex, which are evolutionarily older divisions within the brain. The archicortex is the three pre-evolutionary layers of the hippocampus and limbic system and is the oldest division, while the paleocortex also consists of three layers and is connected with the olfactory cortex.

1The neocortex comprises the human brain by 80% and has an area of about 2400 square centimetres. It is the wrinkling gyri and sulci that allows such a large surface area to fit inside the skull.

2This is based on Brodmann's map, a system devised by the German neuroanatomist Korbinian Brodmann. The system divides the cortex into areas differing in their cellular organisation and arbitrarily numbers them. While it is based on cytoarchitecture, it should be noted that there is a high relation between structure and function, thus, making it useful as functional map of the brain.

Phylogenic         Cellular Divisions
             |  I Molecular Layer
             |  II External Granular Layer
NEOCORTEX----|  III External Pyramidal Layer
             |  IV  Internal Granular Layer
             |  V  Internal Pyramidal Layer
             |  VI Polymorphic or Multiform Cell Layer
             |                |II.
             |                |------------------
             |                |III.
             |                 ------------------  
             |                 -------------------
             |                |I.
             |                |-------------------

Log in or register to write something here or to contact authors.