The Cerebral Cortex (also referred to as the Neocortex), makes up 70% of the CNS and anatomically consists of two large virtually identical Cerebral Hemispheres united by the Corpus Callosum (a fibrous band of tissue which carries over 200 million nerve tracts from one hemisphere to the other).
The surface of the Neocortex is composed almost entirely of a tight mat of dendrites which extend to the surface from their neural cells in the lower layers of the cortex. The functional aspect of the Cerebral Cortex near the surface of all the convolutions of the brain, is a thin layer of neurons (the Plexiform Layer I) 2 to 5 millimeters in thickness. 
The NeöCortical Layers :
On the basis of their slightly different cytoarchitectonic specificities, Neurohistologists have divided the Neocortex into almost 100 different areas. There are, however, Six Major Layers of cells which are present in all of these different areas (except the Hippocampus and Limbic regions which possess only three).
These cellular layers within the Neocortex can be separated into two basic groups based upon their function: The outer four layers receive axons in from other brain areas( Afferents); The two inner layers ( V-Ganglionic and VI-Fusiform ) send axons out to other brain areas ( Efferents ).
(See
Figure C )
On the basis of their anatomical shape, neurons within the Neocortex may be further categorized into two main types:
Pyramidal & Stellate.
Pyramidal Cells; neurons generally shaped like pyramids, represent the major efferents of the Cerebral Cortex and are found within Layer II (External Granular), Layer III (External Pyramidal),
and Layer V (Ganglionic Layer). Pyramidal cells of Layer V are the largest in size and project to the Brainstem and Spinal Cord. Those in Layers II and III project to other regions of the Neocortex.
Stellate Cells are star-shaped cells which are interneurons and represent a collection of different types on neural cells, which are named largely on the basis of the configuration of their axons and dendrites.
Stellate Cells are found within all of the layers of the Neocortex, but the greatest concentration of them is within Layer IV (Internal Granular Layer),
and esepecially within the sensory cortex. Stellate cells receive afferent neuronal axons from subcortical structures, as well as providing interconnections between the cortical afferent and efferent neurons.
Afferent Neurons (neurons which send axons into the Neocortex) are of two general types:
Specific & Nonspecific.
Specific Afferents are those neurons that terminate in relatively discrete regions of the cortex, usually involving only one or two layers. These include projections from the Thalamus as well as those from the Amygdala. Most of these projections terminate in the more superficial layers of the cortex.
Nonspecific Afferents are those neurons that terminate diffusely over large regions of the cortex, in some cases over all of the cortex. The norepinephrinergic projections from the Brainstem, the cholinergic projections from the basal forebrain, and the projections from certain Thalamic nuclei are examples of nonspecific afferents. Nonspecific afferent neurons often terminate in many if not all of the layers of the Neocortex and presumably serve some general function so as to the enhance ongoing cortical activities.
The various regions of the Neocortex are interrelated by three types of axon projections:
(1) Associational fibers - long and short fibers which project between one gyrus and another,
(2) Projection fibers - between one lobe and lower centers, and
(3) Commisural fibers - interhemispheric connections between one hemisphere and the other.
(See
Figure D )
The Thalamus : Our brain is an extremely large memory storehouse. It never functions alone however, but always in association with the lower centers of the nervous system.
Deep within the cerebrum lie other forebrain structures. One of the most important components of which is the Thalamus, located within the middle of the brain directly above the Brain Stem. All areas of the Neocortex have direct afferent and efferent connections with the Thalamus. The Thalamus transfers all information from the sensory organs (except olfaction) to the Neocortex, and sends all of the instructions from the Neocortex out to the body'smuscles.
The Thalamus is divided into two functional areas, dorsal and ventral.
The ventral Thalamus provides a general input into the Neocortex that may modulate the activity of the Neocortex. The Dorsal Thalamus, (or Thalamus proper), is composed of a number of nuclei, each of which projects to a specific area of the Neocortex. These nuclei receive input from the body's different sensory systems or from other brain areas. The Lateral Geniculate Body (LGB) receives the visual projections, the Medial Geniculate Body (MGB) receives auditory projections, and the Ventral-Posterior-Lateral nuclei (VPL) receive touch, pressure, pain, and temperature projections from the body.
In turn, the LGB projects to area 17, the MGB projects to area 41, and the VPL nuclei projects to Brodmann's areas 1, 2, and 3. A large area of the posterior secondary and tertiary cortex sends projections to and receives projections back from the Pulvinar (P). Some of the subcortical motor nuclei, such as the Globus Pallidus, Substantia Nigra, and Dentate nucleus, project to the Ventral-Anterior and Ventral-Lateral nuclei (VA and VL), and these areas project to primary motor area 4 and secondary motor area 5. The Dorsal Medial nucleus (DM) receives projections from the Amygdaloid complex, Temporal Neocortex, and Caudate nucleus and projects to the remainder of the Frontal Lobe.
The Hypothalamus : Below the Thalamus, lies a nerve cluster called the Hypothalamus, which acts as an essential coordinator of the CNS, playing a crucial role in our emotions, and controlling basic life processes. It is the Hypothalamus which acts as the head ganglion of the internal milieu and the chief coordinator of human instincts and drives.
The Basal Ganglia : Crowning the Thalamus is the Basal Ganglia, four neuronal clusters that help to regulate the body's movements. This structure overlaps the Limbic System which largely controls emotions and behavior, along with the circuitry for Short-Term Memory. The Basal Ganglia and Limbic System both share a nerve knot known as the Amygdala , the oblation of which can produce inappropriate emotional behavior in man at the least and manifest at its worst as alexithymia ! 
The Reticular Activating System :
Arousal and attentional mechanisms located deep within the brain tend to activate regions of both hemispheres symmetrically.
One of the principle sources of signals to excite the outer dendritic layer of the Neocortex is the generalized Thalamocortical fibers of the Reticular Formation (the brain's arousal system).
Diffuse electrical stimulation in the mesencephalic and pontine portions of the Reticular Formation causes immediate and marked activation of the Neocortex. Extending upward into the cerebrum from the mesencephalic reticular formation are multiple diffuse pathways that terminate in all areas in the diencephlon and the cerebrum. This entire system is called the Reticular Activating System (RAS).
Consciousness requires both arousal and mental content. The anatomic substrate includes both the Neocortex, and the Reticular Activating System.
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