THE NEOCORTEX

Perception, cognition, fine-motor expression, and computational processing, is made possible by neurons, the majority of which are pyramidal neurons. Most pyramidal neurons are located in the neocortical mantle of the lobes of the brain, which gives this outer coating its grayish appearance. Over 90% of the gray matter is located in the neocortex. The neocortex ("new cortex") likely first began to evolve between 300 million to 450 million years ago.

Neocortex has also been referred to as isocortex and is organized into vertical columns and horizontal layers (Mountcastle, 1997). Each layer and each column contains cells which perform specific functions (Ferster et al., 2013; Hubel & Wiesel, 1968, 1974; Mountcastle, 1997; Peters & Jones, 2010) and which receive or transmit information to or from adjacent cells or distant regions of the brain (Kaas & Krubitzer 2008; Peters & Jones, 2010; Sereno et al. 1995). For example, each column contains neurons that may respond to the same frequency of sound, or to tactile input to the thumb, or to a visual input on the same region of the retina (Hubel & Wiesel, 1968, 1974; Mountcastle, 1997), depending on if the column is located in the temporal, parietal, or occipital lobe.

Those neurons which project to neurons in the next column or to those neurons in an upper or lower layer, are referred to as local circuit neurons. Those which project from the neocortex to the brainstem, or from one half of the brain to the other, are referred to as long distance neurons (Peters & Jones, 2010).

Neocortical Layers

Classically, the neocortex is said to consist of six to seven layers when in fact it consists of numerous layers which vary depending on brain area (Braak & Braak, 2017; Peters & Jones, 2010; Ramon y Cajal, 1902/1955; Rose, 1926). For example, the deepest layer, neocortical layer VI, consists of two distinct layers (VIa and VIb). In the occipital lobe, three additional layers (i.e. sublayers) can be distinguished within layer IV (which also receives considerable thalamic input and is very thick). By contrast, within the motor areas of the frontal lobe, layer IV is exceedingly thin (as there is comparatively minimal thalamic input), whereas layer V is exceedingly thick, It is layer V of the frontal motor areas which contribute the bulk of axons that form the descending corticbulbar, corticopontine, and corticorubral brainstem pathways which establish contact with cranial nerve and sensory and spinal motor neurons (Brodal, 2011; Catsman-Berrevoets, 2017). Likewise in the temporal neocortex layer V is relatively thick as are layers I and VI (since much of the temporal lobe is association and assimilation cortex). As noted, the entorhinal cortex, the "gate way to the hippocampus" and which is located along the medial surface of the temporal lobe, consists of between 7 and 8 layers (Braak & Braak, 2017; Ramon y Cajal, 1902/1955; Rose, 1926).

Hence, the thickness, layering, and composition of the human neocortex varies from lobe to lobe and actually consists of from 7 to 9 (or more) layers rather than 6. For our purposes (and throughout this book) we will described the neocortex as having 7 layers.

Specifically, layer I is referred to as the Molecular Layer and consists of Golgi II cells and horizontal cells. Layer I receives innumerable dendrites from local circuit neurons located in the lower layers. Layer I, however, actually contains few neurons and is mostly made up of tangentially running axons and horizontally running bifurcating apical dendrites received from the pyramidal cells of the lower layers (Peters & Jones, 2010). Layer II is referred to as the External Granular Layer, and consists of densely packed small pyramidal, stellate, and granule cells. Most of the neurons in layer II are local circuit neurons which project to adjacent columns and adjacent layers.

Layer III is the Pyramidal Layer and consists of medium pyramidal cells which project axons to distant areas of the neocortex. Hence, the neurons of layer III can be considered long distance neurons.

Layer IV, the Internal Granular Layer, has a granular appearance and consists of small pyramidal, granule, and stellate (starshaped) cells and receives massive axonal projections from the thalamus. These neurons are predominantly local circuit, and project to adjacent columns and layers. That is, upon receiving and analyzing thalamic input, the neurons of layer IV transfer this data to adjacent neurons for additional analysis. Because the primary, secondary and association sensory areas receive considerable thalamic input, layer IV is relatively thick--except in the motor cortex.

Layer V is the Ganglionic Layer and consists of large and medium size pyramidal cells, including, in primary motor cortex (Brodmann's area 4) the giant cells of Betz. The pyramidal neurons of layer V are long distance neurons, and give rise to descending axons which form the corticospinal, pyramidal, corticobulbar, corticopontine, and corticorubral brainstem pathways which establish contact with cranial nerve and sensory and spinal motor neurons (Brodal, 2014; Catsman-Berrevoets, 2010). It is these "pyramidal" and cortico-spinal neurons which make purposeful, fine motor movement possible. Approximately 31% of the corticospinal tract arises from the pyramidal cells located in the primary motor areas 4, with the remainder arising from the frontal motor associations areas 6, 8, and the primary somesthetic areas 3,1,2, with a scattering of fibers being contributed by the occipital and temporal lobe, as well as limbic system structures.

Intra-cortical Neurons

Layer VIa is the Multiform Layer and contains pyramidal, fusiform, and spindle shaped cells, whereas Layer VIb consists of predominantly of spindle shaped cells. These are predominantly local circuit neurons, and receive considerable input from the brainstem.

The Cytoarchitextural Neuronal, & Chemical Organization of the Neocortex

Korbinian Brodmann detailed the regional variation in the cytoarchitectural organization of the cortex, and conducted detailed comparative studies of numerous species, each of which displays common as well as varying patterns of cytoarchitexture and gyral folding. Based on these cytoarchitextural differences and commonalities, Brodmann divided the cortex into distinct regions and created cytoarchitextural maps of the brains of a variety of species, including humans. For examples, Brodmann's area 17 is synonymous with the primary visual cortex, whereas Brodmann's areas 3,1,2, denote and identify the primary somesthetic receiving areas.

However, although these area differ in regard to organization, what they share in common is a preponderance of pyramidal cells. As noted, pyramidal cells are also the largest and are more numerous than any other neocortical neuron (Peters & Jones, 2010). Pyramidal neurons account for up to 3/4 of all neocortical cells. Pyramidal neurons also serve as both local-circuit and long-distance neurons and generally receive two types of synaptic contacts referred to as Gray types I and II which differ in synaptic morphology and (respectively) excitatory vs inhibitory influences (Peters & Jones, 2010). However, almost all pyramidal cell are excitatory and use glutamate and aspartic acid as transmitters (Tsmoto, 1990).

Pyramidal cells can also be classified as Golgi I and II cells. However, of all neocortical neurons, only 10% are Golgi type I neurons, the main source of long-distance (excitatory) axons, the majority are interneurons, i.e. local circuit neurons which in turn provide almost 90% of cortical axons and dendrites. Approximately 95% of Golgi type I long distance axons interconnect distant regions within the same hemisphere and only about 5% cross the corpus callosum, the fiber pathways which link the right and left hemisphere (Peters & Jones, 2010).

Non-pyramidal cells also function as local-circuit (inter-) neurons, and connect adjacent cells, layers, and cell columns, and display the greatest degree of morphological diversity. These include stellate cells, bipolar cells, chandelier cells, and basket cells. Many of these cells are inhibitory and may use GABA as a neurotransmitter, though the majority in fact appear to be excitatory and use glutamate (Peters & Jones, 2010). Presumably, non-pyramidal, local circuit (interneurons) act to fine tune information processing via inhibitory filtering and selective excitatory transmission. They also serve to integrate and assimilate information received in adjacent regions of the neocortex.

In addition to glutamate and GABA, neocortical neurons contain and respond to peptides, including substance P, corticotropin releasing factor, and opiates. The peptide containing neurons tend to congregate in layers II, III, and IV (Jones & Hendry, 1986).

Information processing throughout the brain is also dependent on glia and non-neuroglia elements. Glia serve a supportive and nurturing role, and may also act to store information. During embryonic brain development, radial glia fibers act to guide migrating neurons to the neocortex, and some glia also form myeline sheaths which surround axons, thus serving as a form of insulation which promotes information transmission.

Glia and non-neuroglia elements make up almost 70% of the volume of the neocortex. Of the remainder, 22% consists of axons and dendrites, with the body (soma) of the neuron comprising only 8% (Peters & Jones, 2010).

REFERENCES

Copyright: 1996, 2000, 2010, 2018 - Rhawn Joseph, Ph.D.