User:Graeme E. Smith/Common Architectonic Aspects of Major Brain Areas

{{User:Graeme_E._Smith/GreySmith Article Framework| Heading = Common Architectonic Aspects of Major Brain Areas| Subheading = | Author=Graeme E. Smith| Abstract = In Architectonics of the Human Telencephalic Cortex, written in 1980, Dr. Heiko Braak gives an overview of the major cortex areas in the Telencephalon. Although his study is micro-architectural in nature, he consistently notes three different zones in each major area of the brain, suggesting that the brain might have a consistent sub-architecture between the Lobes, and the micro-architecture from which they are made. In this article, I examine this sub-architecture, looking for evidence of common factors in the micro-architecture that would make similarities between the Lobes at the sub-architecture level, more obvious. Body= Heiko Braak, in his 1980 book on architectonics Quite often refers to areas of the brain as Core Areas, Belts, and Association Areas. Each lobe it seems has it's own Core area, it's own belt, and a number of association areas. It is my thought, that these areas tend to probably have simularities, core with core, Belt with Belt, and Asssociation Areas with Association areas. To look into this assumption, I will discuss the areas within the specific lobes that have the characteristics of being core areas, then the areas that have the characteristics of being belt areas, and finally the areas that have the characteristics of being Association areas, attempting to draw simularities core with core, belt with belt, and association area with association area.

Core areas in the Mature Isocortex

The Occipital Lobe
The Occipital Lobe or Visual Cortex is a small area of the brain at the back of the brain.

The Striate/Core area
The Striate area of the Occipital Lobe, is easily characterized by it's Line of Genari which is easily seen even in fresh preparations, and was commented on before microscopic examination was possible. Examination microscopically had to wait until multi-layer lenses were possible, which reduced spherical distortion. Occipital lobe structure increases with brain sophistication, which means that the human striate is more complex in layers of neurons than any other animals. Despite this complexity we classify the layers using the standard laminae of isocortical tissue, and sub-laminae designations to indicate specialization within the Laminae. This helps somewhat in interpretation of the tissue layers because it allows us to compare layers that have similar functions. The Molecular Layer is virtually devoid of neurons, being mostly Glial cells and Myelinated Fibers from the Terminal Tufts of Pyramidal Neurons in other layers, and a dense matting of such fibers from other areas of the brain in the outer layer. Only a few neurons are scattered throughout the area. The Corpuscular layer is made up of fine pyramidal cells, with short apical dendrites, and delicate axions leading down from the base of the soma. Dendrites are sparsely fitted with spikes. Because some cells do not have apical dendrites and others have apical dendrites skewed from the vertical, it is difficult to distinguish the pyramidal cells from Pentagonal Stellate cells. The pentagonal stellate cells, which have smoothly contoured dendrites, increase in packing density towards the boundary between Laminae II and Laminae III. The Pyramidal Layer has reducing packing density of Stellate cells, and increasing size and density of Pyramidal cells, until Layer IIIc where a significant increase in size and density of pyramidal cells is experienced. As well Spiny Stellate Cells begin to be found, which increase in packing density as we enter Laminae IV. Although Pyramidal cells interpenetrate Layer IVa, The Spiny Stellate cells which are thought to be modified Pyramidal cells, with horizontal spindle shaped somas dominate Laminae IV. A few rarely branching dendrites extend from the poles of the Spindles. These have few spines and tend not to contour. The Axon heads straight down, and at a distance from the soma a few stout collaterals are thrown off. Layer IVb has reduced cell density, with slightly larger "Solitary" Spiny Stellate cells with Horn shaped soma's. The layer is mostly taken up with the Line of Genari which is a thickly myelinated mat of fibers. The upper border of IVc is indicated by a sharp increase in neuron density consisting of Spiny Stellate Neurons, and IVcα Consists mostly of these cells while by IVcβ a modified pyramidal cell with short modestly spiked dendrites, and an axon that loops back up and ramifies in the superficial layers dominates. This cell has a small well pigmented area surrounding a large central nuceus. This layer is characteristic of the Striate area, and is connected via projections from the parvocellular Laminae of the Geniculate Body. The last part of the Laminae, is Layer IVd which consists of small pyramid cells with a tenuous horizontal line of Myelinated fibers. Laminae V which is usually called the Ganglionic Layer, in the striate area is separated into a number of sub layers. The layer that most closely is associated with the Ganglionic Layer in the parastriate fields, Is Layer Va Which is a tenuous layer consisting of numerous tiny pyramidal cells. When the layer extends out to the parastriate fields, it expands to become the ganglionic layer. Layer Vb is cell sparse consisting mostly of spiny stellate cells and small Pyramidal cells, except for occasional Meynard cells which are extremely large pyramidal cells. These Meynard cells have a thin Apical Dendrite and a large number of wide ranging basal dendrites. One of which is an unusually extended process. In humans the Meynard cells have few or no spines on their dendrites. Layer Vb has enough myelinated fibers to form the inner line of Bailinger, but because of the richness of the fibers within the striate, the inner line is obscured which has led the striate to be mistakenly classed as singlostriate. The Laminae VI area is populated with pyramidal neurons that have long thin dendrites that show spiking as they pass through the Laminae V area, and smooth out as they pass through Laminae IV. Axons contribute to the white matter.

The Temporal Lobe
The Temporal Lobe is a flap of cortex that extends from the Occipital Lobe forward along the side of the cortex. It is thought to be where the brain deals with the "What" issues, how to define objects. As well it is thought that this area is involved in the Auditory Sensory Modality, and there is a segment of it that is allocortical in nature, and thought to have to do with long-term memory.

The Superior Temporal Gyrus Granular Auditory Core
One of the things that is so interesting about the auditory core, is how small it is. We like to think of language as being some large capability of the human brain, and yet, the areas that support language are some of the smallest areas in the brain. The auditory granular core is actually only a portion of a gyrus on the top of the temporal lobe. The Molecular Layer consists of only a few Parallel Fibers cells of Cajal, and a Rich matting of Tangentially adjusted Myelinated fibers from other parts of the brain. The Corpuscular Layer is fairly wide and consists of many small pyramidal cells and stellate cells. The Pyramidal Layer consists mostly of small pyramidal cells that do not increase in size and density as they deepen in the layer. Upper Pyramids, tend to show more pigment than lower Pyramids, a pattern that seems constant in core areas. Layer IIIc pigmented pyramids are noted as dotting the lower extreme of the laminae. The Granular Layer is wide and consists of a dense packing of Pyramidal cells, and a dense mat of Myelinated Fibers thought to be connected via Thalamo-Cortical connections to the redirected sensory data routed through the Thalamus. The Ganglionic Layer is split into a cell rich upper zone, and a cell poor lower zone both populated with small sized pyramid cells organized into columns. Myelination shows that there is a rich interlayer set of myleinated fibers and a rich inner Line of Bailarger. Packing density of its richly pigmented pyramidal cells is densest at the top of the layer and reduces with depth causing it to blend in with the white material without a noticeable Layering. Laminae VI is also richly endowed with myelinated fibers.

The Parietal Lobe
The Parietal Lobe sits above and forward of the occipital lobe, it hosts the Somatosensory Cortex which deals with the inputs from the nervous system that is distributed throughout the body but is not a primary sense such as vision, auditory, or olfactory senses. It is thought to deal with signals from three sources, Exteroreceptive sources, such as touch, Priopreceptive sources such as muscle tension, and Interoceptive sources such as blood sugar, or blood pressure.

The Parietal Lobe is heavily involved with the determination of path during movement and may be the source of the information used by the episodal memory to map items to locations. The Parvocellular core extends down the central sulcys to the sub-central sulcys, and takes the form of a body image that is distorted by its adjustment for sensory density in areas such as the hands feet, and lips. The Molecular Layer is medium thickness, and consists of a cell-poor bottom area, and a highly myelin coated strip of fibrous tissue. The Corpuscular layer is well stocked with small pyramid cells, and well pigmented stellate cells The pyramid layer is characterized by small tightly packed pyramids of roughly uniform size. Pigmentation reduces in a downward direction, The outer tenia is therefore broad and has a blurred upper boundary. The Granular layer has a great number of densly packed small pyramids and stellate cells, it can be separated into two sublayers, a palid top, and a weakly pigmented bottom that hosts Perikerya The Ganglionic Layer consists of a tennuous zone of weakly pigmented pyramids, and a narrow internal tenia The layer is filled with myelinated fibers which are especially crowded in the lower parts. This makes the Inner Line of Baillarger stand out. The narrow multiform layer consists of uniformly pigmented nerve cells, with a sharp border near the white material.

The Frontal Ganglionic Core
The somatomotor core is magnocellular in nature. The Posterior limit of the motor core lies along the anterior wall of the Central sulcys close to its floor. On the Medial faces of the hemisphere, the core is bounded by a line which is in continuation with the central sulcys coursing downwards to the upturned end of the cingulate sulcys where it bends back and upwards at about the anterior limit of the paracentral lobule. Superolaterally, the borderline swings over the free surface of a triangular part of the precentral gyrus whence it recedes into the central sulcys. The lower extremity of the core is reached at about the level of the inferior frontal sulcys. The core receives a particular rich supply from the Thalamic radiation. There is evidence for a precise somatotopical arrangement of motor foci not only in the motor core but also in the belt.

The cell-sparse molecular layer contains a number of myelinated fibers located immediately sujfacent to the extrenal glial zone. The Corpuscular layer is dominated by tightly packed small pyramids and a scattered population of pigment laden stellate cells. The Pyramidal Layer accommodates pyramids which form a gradient with the largest ones and the lower border the amount of pigment stored in the pyramids is subjected to a gradual decrease from above downwards. Hence the large pyramids are almost devoid of pigment deposits. The external tenia covers the lower reaches of the pyrmaidal layer, it is extremely broad and displays a blurred upper border Also the lower one is by far not as sharply traced as in the postcentral core. The position of the outer tenia corresponds with that of the outer stripe of Baillarger. The granular layer cannot be outlined in the motor cortex of the adult, however it is apparently present during foetal life. The Ganglionic Layer can be subdivided into a cell-rich and modestly pigmented Va and a cell-sparse Vb each of approximately the same breadth. Sublayer Va contains in abundance small-sized pyramids which tend to invade both tenae. Pigment preparations allow for the separation of Vb into an upper part PVbα which accomodates the conspicuous Betz cells, which amass pigment in large blocks and a cell sparse PVbβ. Both the Ganglionic and Multiform layers are filled with a dense plexus of Myelinated fibers. The homogeneously pigmented Multiform layer is broad and merges with the white matter.

Conclusions about Core areas in general
Laminae I, is a cell sparse area consisting of mostly glial cells, and a rich fiber matting in the top layer immediately under the glial cells. This feeds the terminal tufts of pyramidal neurons from the other layers, and is fed in turn by the Martinotti cells. Laminae II is usually fairly broad and consists of tightly packed small pyramidal cells and scattered stellate cells. Laminae III, consists of tightly packed pyramidal neurons, stellate cells reduce in population as the layer deepens, and if there is a Granular Layer spiky stellate cells begin to increase as we approach the border with laminae IV. If there is any difference in pyramid size it increases with the depth of the layer, often with IIIc having the largest neurons. It has been noted that in this layer pigmentation decreases with the depth of the neurons, so that the largest and deepest neurons are also the neurons with the least pigment. This often has the effect of blurring the upper edge of the Outer Tennae. Laminae IV is usually Tightly packed with pyramidal neurons and spiky neurons. The exceptions are the occipital Lobe where there are 4 different sublayers each with its own unique type of neuron, and the Ganglionic Core in the Frontal Lobe which is missing the layer. Current theory is that the Granular Layer is the entry point for the Thalamocortical fibers that carry the re-directed sensory data. If this is true, then the reason that the laminae is missing in the Frontal Ganglionic Core, is simply because the core gets its signals from a different source. The complexity of the Occipital Lobe might have to do with the complexity of the optical data being fed into that area. The Ganglionic Layer, seems to mostly separate out into two zones, a cell rich pyramidal layer called Va, and less populated area called Vb. It is interesting to note that when there are larger cells, the cells are usually found in Vbα. The Ganglionic Layer usually has a mat of myelinated fibers, suggesting connections to other aress of the brain. (Perhaps the basal Ganglia). The Multiform layer consists of lightly packed pyramidal neurons. There is no standard for pigmentation, but this layer often shows signs of heavy myelinated matting, suggesting connection to other parts of the brain.

The Belts
So called belt areas tend to be wider than their cores, in my model this is because they are actually dealing with more information content than the primary perception areas. It should be no surprise that the gigantic or magnoscopic cells often extend into the belts as the signals they manage in the belts are probably more sophisticated than the signals they manage in the cores.

The Occipital Belt also known as the Parastriate area
The visual core is throughout its circumference accompanied by the parastriate or belt area. The spread of the belt area is almost restricted to the Medial Fascies of the hemisphere, It's anterior extremity is concealed in the common trunk of the parieto-occipital and the calcarine sulcys. The barrier between the belt and core areas is sharply drawn, and most of the layers in the Belt area show alterations except for the superficial laminae II and IIIab remain much the same. The upper reaches of the Pyramidal Layer display a fair number of stout horizontally adjusted myelinated fibers. These form the line of Kaes-Bechterew which gives the visual belt area an extremostriatal character. The lower reaches of the pyramidal Layer IIIc are mainly filled with medium sized to large pyramids. Myelin preparations display thick radially oriented fiber bundles vanishing at about the upper border of IIIc. The existence of a separate layer IIIc distinguishes the belt from the core area.The external Tenia penetrates into the lower parts of the pyramidal layer. The large IIIc pyramids remain therefore almost devoid of pigment. The upper parts of IIIc nevertheless show typically well pigmented IIIc pyramids. A weak radially oriented striation is recognizable in these parts. The Granular layer contains a wealth of densely packed small nerve cells. Although the cells are poorly invested in pigment, their great numbers result in the appearance of a weakly tinged stripe located in the middle of the external tenia. This stripe is in continuation with sublamina IVcβ of the Striate area. Tripartition of the external tenia (PIVa, PIVb, PIVc) is a distinguishing feature of the parastriate areas and does not elsewhere occur in the ISOCORTEX. Pigment staining thus allows definition of the outer border of the area which defies delineation in the other architectonic methods. The ganglionic layer is divisible into a cell rich Va harboring many weakly pigmented pyramids, and a cell-sparse Vb which corresponds to the inner tenia. The outstanding width of the outer tenia, and the narrowness of the internal one, gives the parastriate field a markedly externoteniate character. The lower reaches of the pyramidal layer, the granular layer and in particular the ganglionic layer contain a wealth of myelinated horizontal fibers. The interstriate zone is also richly endowed with fine ground fibers which results in the conjunctostriate type of the parastriate field. The average myelin content is high,(typus dives) The Muliform Layer is accentuated in pigment preparations with distinctly higher pigment absorption in the upper portions of layer PVI than in PIII or PV. Cell density in the layer is highest at the top decreasing in density gradually as it nears the bottom.

The Temporal Lobe - Paragranulous Belt
The area temporalis paragranulosa forma a hook-like border zone around the core. Anteriorly, it abuts on the proconiocortex. In pigment preparations the pallor of the pyramidal layer distinguishes it, from the progranulous field. The external tenia is clearly a step less broad than that in the core field.

The Parietal Paragranulous Belt
The belt extends in a strip-like fashion posterior and parallel to the core field. Starting out at the anterior wall of the postcentral gyrus it stretches out onto the free surface of the brain. It reaches its greatest expansion close to the upper margin of the hemisphere, narrowing down and receding into the central sulcys as its inferior extremity is approached. The belt ends at about the lattitude of the inferior frontal sulcys. The molecular layer is typical The corpuscular layer retains its characteristics as a cell rich band The Pyramidal Layer is of medium breadth and contains, in contrast with the coniocortex, pyramidal cells growing larger the deeper their position is. Also the amount of pigment increases considerably as the layer is descended, a fact which results in a clear cut lower border of PIIIc. A fair number of fibers in the superficial parts of the layer form the line of Kaes-Bechterew. The Granular layer seems attenuated as compared to the forgoing field The outer line of Baillarger appears as a dense and broad stripe. The breadth of the external tenia is diminished, but clearly exceeds that of the internal tenia. The Ganglionic layer is braoder than in the core, in particular the upper part, PVa, is now a clearly recongizeable stripe. The inner tenia remains relatively narrow, showing a blurred upper and a sharp lower border. Besides its usual sonstituants, it contains a scarce population of Betz pyramids. The tterstiate zone appears lightened. The inner line of Baillerger is the same tint as the outer. The Multiform Layer remains unchanged

category=GreySmith/Neuroanatomy|}}