Sunday, November 13, 2011

Cell Types of the Neocortex

Among the nerve cells of the neocortex, two types of neurons can be distinguished: projection neurons with long axons (excitatory glutamatergic pyramidal cells), and interneurons with short axons (inhibitory GABAergic interneurons). The pyramidal cell is characterized by one apical dendrite, which ascends to the molecular layer and branches there, and numerous basal dendrites. Its descending axon gives off numerous recurrent collaterals. The cell-deficient molecular layer (layer I) contains Cajal–Retzius cells with tangentially running axons. The different types of granule cells or stellate cells are predominantly interneurons and are found in all layers at various densities. They include Martinotti’s cells, the vertically ascending axons of which ramify in various cortical layers and reach as far as the molecular layer. The cellules à double bouquet dendritique of Cajal, cells with two vertically oriented dendritic trees (primarily in layers II, III, and IV), possess long ascending or descending axons. The axon of some stellate cell types arborizes after a short course, or it bifurcates and terminates with basketlike networks (basket cells) on adjacent pyramidal cells. Axon bifurcations may run horizontally and terminate on distant pyramidal cells. Their inhibitory function has been confirmed by detection of GABA in the synapses of basket cells.

The results of histological and electrophysiological studies have made it possible to design models in which the described cell types are organized in a functional group. The vertical column is conceived as a module, that is, as a group of elements forming a functional unit. The efferent elements of the column are the pyramidal cells. Their axons either run to other cortical columns, where their terminal ramifications end at the spines of other pyramidal cells, or they run to subcortical groups of neurons. The numerous axon collaterals terminate at the pyramidal cells of nearby columns.

There are two kinds of afferent fibers: the association fibers from other columns and the specific sensory fibers from peripheral sensory areas. In every layer the association fibers give off branches that terminate at the spines of pyramidal cells. They ascend to the molecular layer, where they branch into horizontally running fibers. The latter have synaptic contacts with apical dendrites within a radius of 3mm. The excitation transmitted by them reaches far beyond the column; however, it remains weak because the number of synaptic contacts is limited. The specific fibers terminate in layer IV on interneurons, primarily on the cells with two dendritic trees. The axons of the latter ascend vertically along the apical dendrites of pyramidal cells and form synapses with their spines. These series of synapses result in powerful transmission. The basket cells, which are inhibitory interneurons, send their axons to the pyramidal cells of adjacent columns and inhibit them, thereby restricting the excitation. The basket cells themselves are activated by recurrent collaterals of the excitatory pyramidal cells. The axons of Martinotti’s cells ascend to the molecular layer where they form branches. The number of neurons per column is estimated to be 2500, approximately 100 of which are pyramidal cells. It should be considered, however, that a vertical column is not a clearly defined histological entity. Possibly, it does not represent a permanent morphological unit but rather a functional unit, which forms and disintegrates according to the level of excitation.