15 Telencephalon: Neocortex Introduction.........................491 – Dendritic Clusters, Axonal Bundles and Radial Cell Cords As (Possible) Sulcal Pattern ........................498 Constituents of Neocortical Minicolumns . 582 Structural and Functional Subdivision – Microcircuitry of Neocortical Columns .... 586 ofNeocortex.........................498 – Neocortical Columns and Modules: – Structural Subdivision 1: Cytoarchitecture . 498 A Critical Commentary ............... 586 – Structural Subdivision 2: Myeloarchitecture . 506 Comparative Aspects ................... 591 – Structural Subdivision 3: Myelogenesis ....510 Synopsis of Main Neocortical Regions ...... 592 – Structural Subdivision 4: Connectivity .....510 – Introduction....................... 592 – Functional Subdivision ................516 – Association and Commissural Connections . 592 – Structural and Functional Subdivision: – Functional and Structural Asymmetry Overview .........................528 of the Two Hemispheres .............. 599 – Structural and Functional Localization – Occipital Lobe ..................... 600 in the Neocortex: Current Research – Parietal Lobe ...................... 605 and Perspectives ....................530 – TemporalLobe..................... 611 Neocortical Afferents ...................536 – Limbic Lobe and Paralimbic Belt ........ 617 Neocortical Neurons – FrontalLobe ...................... 620 and Their Synaptic Relationships ..........544 – Insula........................... 649 – IntroductoryNote...................544 – Typical Pyramidal Cells ...............544 – Atypical Pyramidal Cells ..............559 – Local Circuit Neurons ................560 Introduction Microcircuitry of Neocortex ..............569 – Introduction.......................569 – Networks of Pyramidal Neuron ..........570 The neocortex is an ultracomplex, six-layered – Interneuronal Systems ................571 structure that develops from the dorsal pallial Neocortical Columns and Modules .........575 sector of the telencephalic hemispheres (Figs. – Introduction.......................575 2.24, 2.25, 11.1). All mammals, including – The Investigations of Lorente de Nó: monotremes and marsupials, possess a neocor- Elementary Units and Glomérulos ........576 tex, but in reptiles, i.e. the ancestors of mam- – The Columnar Organization mals, only a three-layered neocortical primor- of the Somatosensory Cortex ...........576 dium is present [509, 511]. The term neocortex – The Columnar Organization refers to its late phylogenetic appearance, in oftheVisualCortex..................578 comparison to the “palaeocortical” olfactory – The Auditory Cortex .................579 cortex and the “archicortical” hippocampal – TheMotorCortex...................579 – Columnar Patterns Shown by the Cells cortex, both of which are present in all am- of Origin and the Terminal Ramifications niotes [509]. of Cortico-cortical Connections ..........579 The size of the neocortex varies greatly – Minicolumns and the Radial Unit Hypothesis among the various mammalian groups. In of Cortical Developmen ...............581 some insectivores, such as the hedgehog, its 492 Section II Structure of Spinal Cord and Brain Parts Fig. 15.1. Brains of some mammals drawn on the same scale. The neocortex is shown in red 15 Telencephalon: Neocortex 493 Fig. 15.2. The telencephalon of the opossum (A) and human (B). The position and extent of the neocortex (shown in red) and of a number of subcortical centres (marked in A and B by the same hatchings/symbols) are indicated. In the opossum, the neocortex covers the remaining parts of the telencephalic hemispheres like a cap. In humans, the enormously expanded neocortex almost completely surrounds all subcortical centres. amc, amygdaloid complex; aon, anterior olfactory nucleus; olb, olfactory bulb; ot, olfactory tubercle; sep, precommissural septum; str, corpus striatum 494 Section II Structure of Spinal Cord and Brain Parts size does not exceed that of the “older” parts (Fig. 15.4 B). The characterization and determi- of the cortex, but in primates and cetaceans it nation of these layers is generally based on attains remarkable proportions, becoming by Nissl-stained material. Although the study of far the largest centre in the brain (Figs. 15.1, such material, in which the dendritic and axo- 15.2). Stephan and Andy [709] determined the nal processes of the neurons remain largely average index of progression for the neocortex unstained, yields in itself very little insight into (and for many other brain structures) in a the structural and functional organization of number of insectivores, prosimians and simi- the cortex, the results of such cytoarchitectonic ans, including humans. These indices express analyses are important because they provide a how many times larger the neocortex is in a very useful general framework for studies em- particular group of species than in that of a ploying other, more critical techniques. typical basal insectivore of the same size. It Beginning at the surface, the six neocortical was found that in prosimians the neocortex is layers are as follows: (I) lamina molecularis, on average 14.5 times, and in the simians 45.5 (II) lamina granularis externa, (III) lamina times larger than in basal insectivores. In hu- pyramidalis externa, (IV) lamina granularis in- mans, the neocortex appeared to be 156 times terna, (V) lamina pyramidalis interna and (VI) larger than that of basal insectivores. lamina multiformis. As regards the functional differentiation of I. The lamina molecularis or lamina zonalis the neocortex, in many “primitive” mammals contains only very few cell bodies. (marsupials, insectivores and rodents) much of II. The lamina granularis externa is composed the neocortex is occupied by projection areas of small, densely packed cell bodies. The which either receive, via the thalamus, im- name of this layer is misleading because pulses directly related to the various special most of its constituent somata belong to senses, or are concerned with the steering of small pyramidal neurons which, like all motor activity. The sensory fields comprise the typical cortical pyramids, direct their somatosensory cortex, receiving impulses from apices toward the surface. sense organs situated in the skin, the muscles III. The lamina pyramidalis externa is a thick and the joints, then the primary visual cortex layer in which pyramidal somata prevail. and finally the primary auditory cortex. These somata increase progressively in size There is evidence suggesting that already in from superficial to deep. primitive mammals the various projection IV. The lamina granularis interna consists of areas just mentioned are separated from each small, densely packed, pyramidal and non- other by narrow strips of non-projection cor- pyramidal somata. tex (see Cajal [84], p. 830 and figure 531). V. The lamina pyramidalis interna consists What we now see is that these strips expand mainly of medium-sized and large, loosely enormously to form a large area known as the arranged pyramidal somata. parietotemporal association cortex. Another VI. The lamina multiformis is composed of rel- area of association cortex develops in front of atively tightly packed, spindle-shaped so- the motor cortical areas (Fig. 15.3). This region mata. Golgi material reveals that most of is known as the prefrontal association cortex. these somata belong to modified pyramidal All cortical areas maintain afferent and efferent cells. connections with subcortical centres. However, these various association cortical areas are pri- The neocortex is frequently also designated as marily connected with other cortical fields. isocortex [784, 791] or homogenetic cortex.Brod- The neocortex shows a laminated structure mann [70, 71] used the latter term to indicate throughout its extent. Whereas in the piriform that, even though some cortical areas in the adult and hippocampal parts of the mammalian cor- brain may have more or less than six layers, the tex three layers can be distinguished, in the entire neocortex has passed through a basic, six- neocortex six layers are usually recognized layered stage during ontogeny (Fig. 15.4 A). 15 Telencephalon: Neocortex 495 Fig. 15.3. Lateral views of the brains of the hedgehog (A), the prosimian Galago (B) and human (C) to show the evolutionary differentiation of the neocortex. In the hedgehog almost the entire neocortex is occupied by sensory (ss, vis, ac) and motor (m) areas. In the prosimian Galago the sensory cortical areas are separated by an area occupied by association cortex (pa). A second area of association cortex (aa) is found in front of the premotor cortex (pm). In the human the association areas are strongly developed. aa, anterior associa- tion area; ac, auditory cortex; i, insular cortex; m, motor cortex; pa, posterior association area; pm, premotor cortex; ss, somatosensory cortex; vis, visual cortex 496 Section II Structure of Spinal Cord and Brain Parts Fig. 15.4. The laminar pattern of the human neocortex: A in a newborn, diagrammatic; B Nissl preparation, adult; C myelin sheath preparation, adult. After Vogt and Vogt [786] and Rose [637]. Bi, Bo, inner and outer stripes of Baillarger 15 Telencephalon: Neocortex 497 Apart from the neuronal perikarya, several cortical neurons. Many extrinsic afferents also other structural elements in the cortex show a take a radial course after having entered the more or less distinct laminar arrangement. cortex from the deep white matter. Myelin-stained and reduced silver preparations The