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Insula

Long gyrus of insula

Superior temporal	Transverse temporal
gyrus	gyri of Heschl

Short gyri of insula

Insula

Central sulcus of

insula

Long gyrus Superior temporal of insula gyrus

Fig. 9.10 The transverse gyri of Heschl on the superior aspect of the superior temporal gyrus

Fig. 9.11 The insula (revealed by dissection)

Histological Organization of the Cerebral Cortex

The folded surface of the brain is made up of the gray matter of the cerebral cortex, which is gray because of the very high density of neurons within it. The cortex varies in thickness from 1.5 mm (visual cortex) to 4.5­5 mm (precentral gyrus); it is generally thicker on the crown of a gyrus than in the depths of the neighboring sulci.

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Laminar Architecture

The laminar structure of the cerebral cortex is visible to the naked eye in only a few cortical areas, most clearly in the visual cortex, where an anatomical section perpendicular to the brain surface reveals the white stripe of Gennari (or of Vicq d’Azyr) within the cortical gray matter. Microscopic examination of most cortical areas reveals the basic six-layered structure that typifies the cerebral cortex (neocortex), as described by Brodmann. Cortical areas possessing this structure are called isocortex (after O. Vogt), as opposed to the phylogenetically older allocortex, which, in turn, is divided into the paleocortex and the archicortex. The paleocortex includes the olfactory area, while the archicortex includes the fasciolar gyrus, hippocampus, dentate gyrus, and parahippocampal gyrus.

The internal structure of the six-layered isocortex is depicted in Fig. 9.12. In an anatomical section perpendicular to the brain surface, the following layers can be distinguished, from outside to inside (i.e., from the pial surface to the subcortical white matter).

1.Molecular layer (zonal layer). This layer is relatively poor in cells. In addition to the distal dendritic trees (apical tuft) of lower-lying pyramidal cells and the axons that make synaptic contact with them, this layer contains mostly small neurons (Cajal­Retzius cells), whose dendrites run tangentially within the layer. The Cajal­Retzius cells play an essential role in the development of the cortical laminar pattern. Some of them degenerate once this development is complete.

2.External granular layer. This layer contains many granule cells (“nonpyramidal cells”) and a few pyramidal cells whose dendrites branch out both within the external granular layer and upward into the molecular layer. The nonpyramidal cells are mostly GABAergic inhibitory neurons, while the pyramidal cells are excitatory and use glutamate as their neurotransmitter.

3.External pyramidal layer. As its name implies, this layer contains many pyramidal cells, which, however, are smaller than those of the deeper cortical layers. These cells are oriented with their bases toward the subcortical white matter. The axon of each pyramidal cell arises from the cell base and travels down into the white matter. The axon already receives a myelin sheath within the external pyramidal layer. It may function as a projection fiber or, more commonly, as an association or commissural fiber (p. 366ff.). A dendrite emerging from the apex of the pyramidal cell travels upward into the external granular and molecular layers, where it divides into its terminal branches (apical tuft).

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Golgi stain

Cellular stain

Myelin stain

I.Molecular layer

II.Extern

l granular layer

III.External pyramidal layer

IV. Internal

granular layer

V.Internal pyramidal layer

VI. Multiform

layer

Tangential lamina

External band of Baillarger

Internal band of Baillarger

Fig. 9.12 Cytoarchitecture of the human cerebral cortex as revealed by three different staining techniques. (Diagram after Brodmann, from Rauber-Kopsch: Lehrbuch und Atlas der Anatomie des Menschen, 19th ed., vol. II, Thieme, Stuttgart, 1955.)

4.Internal granular layer. Like the external granular layer, this layer contains many nonpyramidal cells. These granule cells mainly receive afferent input from thalamic neurons by way of the thalamocortical projection. The fibers lying in the external pyramidal layer are mostly radially oriented, but those of the internal granular layer are overwhelmingly tangential, forming the external band of Baillarger.

5.Internal pyramidal layer. This layer contains medium-sized and large pyramidal cells. The largest cells of this layer (Betz cells) are found only in the region of the precentral gyrus. The especially thickly myelinated neurites of these cells form the corticonuclear and corticospinal tracts. This layer also contains many tangentially oriented fibers (internal band of Baillarger).

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Histological Organization of the Cerebral Cortex · 361	9

6. Multiform layer. This layer of polymorph cells is subdivided into an inner, less dense layer containing smaller cells, and an outer layer containing larger cells.

Types of Neurons in the Cerebral Cortex

The cerebral cortex thus contains two major types of neurons: the excitatory projection neurons (pyramidal cells) and the other nonpyramidal cells (granule cells or interneurons), which are more commonly inhibitory and tend to make local rather than long-distance connections. But this dichotomy is oversimplified. The interneurons, for example, come in a number of subtypes, such as basket cells, chandelier cells (axo-axonal cells), and double bouquet cells. Furthermore, the pyramidal cells also participate in local regulatory circuits (recurrent inhibition: backward-running local collaterals of the pyramidal cells activate GABAergic inhibitory interneurons, which, in turn, inhibit the pyramidal cells).

The pyramidal cells of the fifth cortical layer give rise to the projection pathways (Fig. 9.13), which travel through the subcortical white matter and the internal capsule to the thalamus, striatum, brainstem nuclei, and spinal cord. The association and commissural fibers traveling to other ipsilateral and contralateral cortical areas, respectively, are derived from the pyramidal cells of the third cortical layer (numbered 4 in Fig. 9.13). The granule cells of the second and fourth cortical layers, as well as the pyramidal cells, receive projection fibers from the thalamus (1), as well as association and commissural fibers from other cortical areas (2).

Variations of the Laminar Pattern

The six-layered laminar pattern just described is called the homotypical pattern. In some cortical areas, however, the full pattern of six layers is barely discernible; these areas are called heterotypical.

In the receptive cortical fields, such as the visual, auditory, and somatosensory cortices, the density of granule cells is increased, while that of pyramidal cells is decreased (“granulization”; “granular cortex”). In the motor cortical fields, on the other hand, there are relatively more pyramidal cells (“pyramidalization”; “agranular cortex”).

Cytoarchitectural cortical fields. As we have seen, cortical areas vary not only in thickness but also in histological structure. The heterogeneous distribution of various types of neurons across cortical areas, and the resulting variations in the cortical laminar pattern, led the neuroanatomists Brodmann, O. Vogt, and

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Fig. 9.13 Simplified diagram of intracortical neural connections (after Lorente de Nó and Larsell). Efferent neurons/neurites are red, afferent ones are blue, and interneurons are black. For details, cf. text, p. 361.

von Economo to subdivide the cerebral cortex into a large number of cytoarchitectural fields. Brodmann’s cytoarchitectural map of the cerebral cortex, which is somewhat simpler than von Economo’s, is now in general use as a system for naming cortical areas. Agranular cortex is found in Brodmann areas 4 and 6 (primary and secondary motor cortical fields, p. 372); the inner granular layer of these areas is rich in pyramidal cell components. Granular cortex (koniocortex), on the other hand, is found in Brodmann areas 3, 1, 2, 41, and especially 17, the striate cortex (primary receptive cortical areas, p. 380). As shown in Fig. 9.14, the cytoarchitectural fields do not coincide with the gyral pattern of the brain surface. They partly overlap with one another and vary across individuals in their shape and extent.

It is possible to subdivide the cerebral cortex histologically, not only according to cytoarchitectural criteria but also on the basis of local variations in myelinated fibers, glial cells, or blood vessels (i.e., according to its myeloarchitecture, glioarchitecture, or angioarchitecture). More recent brain maps have also exploited variations in neurotransmitters, neurotransmitter-related enzymes,

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Histological Organization of the Cerebral Cortex · 363	9

Fig. 9.14 Cytoarchitectural fields of the human cerebral cortex. a Lateral view of left hemisphere. b Medial view of right hemisphere. The cortical fields are numbered. (After Brodmann, from Bargmann W: Histologie und Mikroskopische Anatomie des Menschen, 6th ed., Thieme, Stuttgart, 1967.)

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9364 · 9 Cerebrum

neuropeptides, and calcium-binding proteins, as revealed by immunohistochemical studies using specific antibodies against these substances.

Plasticity of cortical architecture. The microscopic structure of the cerebral cortex is not strictly genetically determined, nor is it immutable. Much current research concerns the question of how environmental influences, by activating specific groups of neurons, can decisively affect the structural differentiation of cortical areas over the course of ontogenetic development. A further question is whether, and by what mechanisms, long-lasting changes in neuronal activity in the mature brain (e. g., through perturbations of the external environment or loss of a sensory organ) can actually bring about changes in the microarchitecture of the cortex, including a changed anatomy of synaptic connections.

Many studies of this kind have been performed on the visual system, because the environmental conditions affecting it (visual stimuli) are relatively easy to manipulate. It has been found that certain “elementary components” of visual stimuli, including their color, orientation, and localization on the retina, are processed separately by distinct groups of neurons, which are distributed over the visual cortex in small, interspersed areas. These specialized cortical areas take on different characteristic shapes, depending on the elementary aspect of visual processing with which they are concerned: color is processed in so-called “blobs,” while the spatial localization and orientation of the stimulus are dealt with by ocular dominance and orientation columns (cf. p. 380f.). Experimental manipulation of a given type of elementary stimulus, for a sufficiently long period of time, can be shown to produce morphological changes in the corresponding processing units.

Input-specific differentiation of cortical microstructures can be demonstrated in other areas as well. The cortical barrels of the rodent somatosensory cortex, composed of annular collections of cells, are a well-known example: each barrel represents a single whisker of the animal.

Thus, a large number of recent studies permit the following general conclusions: (1) Certain cortical areas contain a topical representation of the sensory stimuli that they process. (2) This representation can undergo plastic change.

The diversity of histological structure among cortical fields immediately implies that they must have correspondingly diverse functions. For well over a hundred years, much research has focused on the assignment of function to different cortical fields. The knowledge that has been gained is of vital clinical importance. We will discuss functional localization in detail in the section after next (Section 9.5), but first, as a necessary prerequisite, the fiber connections of the cerebral cortex will be presented in Section 9.4.

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