Multiple layer 5 pyramidal cell subtypes relay cortical feedback from secondary to primary motor areas in rats.

Higher-order motor cortices, such as the secondary motor area (M2) in rodents, select future action patterns and transmit them to the primary motor cortex (M1). To better understand motor processing, we characterized "top-down" and "bottom-up" connectivities between M1 and M2 in the rat cortex. Somata of pyramidal cells (PCs) in M2 projecting to M1 were distributed in lower layer 2/3 (L2/3) and upper layer 5 (L5), whereas PCs projecting from M1 to M2 had somata distributed throughout L2/3 and L5. M2 afferents terminated preferentially in upper layer 1 of M1, which also receives indirect basal ganglia output through afferents from the ventral anterior and ventromedial thalamic nuclei. On the other hand, M1 afferents terminated preferentially in L2/3 of M2, a zone receiving indirect cerebellar output through thalamic afferents from the ventrolateral nucleus. While L5 corticopontine (CPn) cells with collaterals to the spinal cord did not participate in corticocortical projections, CPn cells with collaterals to the thalamus contributed preferentially to connections from M2 to M1. L5 callosal projection (commissural) cells participated in connectivity between M1 and M2 bidirectionally. We conclude that the connectivity between M1 and M2 is directionally specialized, involving specific PC subtypes that selectively target lamina receiving distinct thalamocortical inputs.

Models of cortical networks with long-range patchy projections.

The cortex exhibits an intricate vertical and horizontal architecture, the latter often featuring spatially clustered projection patterns, so-called patches. Many network studies of cortical dynamics ignore such spatial structures and assume purely random wiring. Here, we focus on non-random network structures provided by long-range horizontal (patchy) connections that remain inside the gray matter. We investigate how the spatial arrangement of patchy projections influences global network topology and predict its impact on the activity dynamics of the network. Since neuroanatomical data on horizontal projections is rather sparse, we suggest and compare four candidate scenarios of how patchy connections may be established. To identify a set of characteristic network properties that enables us to pin down the differences between the resulting network models, we employ the framework of stochastic graph theory. We find that patchy projections provide an exceptionally efficient way of wiring, as the resulting networks tend to exhibit small-world properties with significantly reduced wiring costs. Furthermore, the eigenvalue spectra, as well as the structure of common in- and output of the networks suggest that different spatial connectivity patterns support distinct types of activity propagation.

Cytoarchitectonic organization and morphology of the cells of hippocampal complex in strawberry finch, Estrilda amandava.

Neurons in the hippocampal complex (dorsomedial forebrain) were described and located following Golgi impregnation. Five fields were recognized in the hippocampal complex: medial and lateral hippocampus, parahippocampal area, central field of the parahippocampal area and crescent field. In the medial hippocampus three layers have been observed: suprapyramidal towards the pial surface, pyramidal at the central and infrapyramidal adjacent to the ventricle. Neurons of the hippocampal complex were classified in to two main cell groups: predominant projection neurons with spinous dendrites and local circuit neurons. Projection neurons were further sub classified into three main types: pyramidal, pyramidal like, and multipolar neurons. In addition to these neurons, monotufted and bitufted neurons were also observed in the medial and lateral hippocampus with low frequency. The pyramidal neurons were dominant neuronal types in the pyramidal layer-II of the medial hippocampus, mixed with pyramidal like and multipolar neurons. Pyramidal and pyramidal-like neurons were found restricted in the pyramidal layer II of the medial hippocampus while the multipolar neurons were uniformly distributed in all subfields of the hippocampal complex. In the lateral hippocampus irregular shaped radial glial cells were present near the ventricular wall and projecting their dendrites towards the pia. Second group of local circuit neurons with local arborization of their projections were present in the medial hippocampus and in parahippocampal area.

Microcircuits in visual cortex.

The microcircuitry of the neocortex is bewildering in its anatomical detail, but seen through the filters of physiology, some simple circuits have been suggested. Intensive investigations of the cortical representation of orientation, however, show how difficult it is to achieve any consensus on what the circuits are, how they develop, and how they work. New developments in modeling allied with powerful experimental tools are changing this. Experimental work combining optical imaging with anatomy and physiology has revealed a rich local cortical circuitry. Whereas older models of cortical circuits have concentrated on simple 'feedforward' circuits, newer theoretical work has explored more the role of the recurrent cortical circuits, which are more realistic representations of the actual circuits and are computationally richer.

Dendritic architecture of the von Economo neurons.

The von Economo neurons are one of the few known specializations to hominoid cortical microcircuitry. Here, using a Golgi preparation of a human postmortem brain, we describe the dendritic architecture of this unique population of neurons. We have found that, in contrast to layer 5 pyramidal neurons, the von Economo neurons have sparse dendritic trees and symmetric apical and basal components. This result provides the first detailed anatomical description of a neuron type unique to great apes and humans.

Specializations of the granular prefrontal cortex of primates: implications for cognitive processing.

The biological underpinnings of human intelligence remain enigmatic. There remains the greatest confusion and controversy regarding mechanisms that enable humans to conceptualize, plan, and prioritize, and why they are set apart from other animals in their cognitive abilities. Here we demonstrate that the basic neuronal building block of the cerebral cortex, the pyramidal cell, is characterized by marked differences in structure among primate species. Moreover, comparison of the complexity of neuron structure with the size of the cortical area/region in which the cells are located revealed that trends in the granular prefrontal cortex (gPFC) were dramatically different to those in visual cortex. More specifically, pyramidal cells in the gPFC of humans had a disproportionately high number of spines. As neuron structure determines both its biophysical properties and connectivity, differences in the complexity in dendritic structure observed here endow neurons with different computational abilities. Furthermore, cortical circuits composed of neurons with distinguishable morphologies will likely be characterized by different functional capabilities. We propose that 1. circuitry in V1, V2, and gPFC within any given species differs in its functional capabilities and 2. there are dramatic differences in the functional capabilities of gPFC circuitry in different species, which are central to the different cognitive styles of primates. In particular, the highly branched, spinous neurons in the human gPFC may be a key component of human intelligence.

Estimators of the precision of stereological estimates: an example based on the CA1 pyramidal cell layer of rats.

Because of the complex and dynamic structure of the brain, there is perhaps no other organ system in which the application of stereological methods can contribute so much with regard to understanding normal and pathological processes. In order to design the studies in an optimal manner, with regard to the number of individuals, sections, probes, and to be able to critically evaluate the stereological studies made by others, it is important for neuroscientists to have an understanding of the precision or reproducibility of a stereological estimation procedure. This precision or reproducibility is often referred to as the coefficient of error of the estimate, which is a statistical expression for the size of the standard error of the mean of repeated estimates, relative to the mean of the estimates. Like the 'margin of error' associated with public opinion polls, it indicates how much the estimate would vary if it were repeated numerous times. It is difficult and time consuming to empirically derive the coefficient of error of estimates made of features observed in histological preparations. To overcome this obstacle, it is common practice to try to get a feeling for the precision of an estimate by estimating the coefficient of error itself. In this paper, we will compare and discuss the coefficient of error of estimates of volume and cell number made with different numbers of sections and probes in the CA1 pyramidal cell layer of the rat hippocampus. The conclusions drawn from this analysis indicate that, using practically feasible and anatomically sensible sampling schemes, the Gundersen-Jensen coefficient of error estimator or the 'Split-Sample' coefficient of error estimator can provide useful information about the precision of stereological estimates even in highly irregular brain regions and requires little work.

Morphological characteristics and electrophysiological properties of CA1 pyramidal neurons in macaque monkeys.

Little is known about the morphological characteristics and intrinsic electrophysiological properties of individual neurons in the nonhuman primate hippocampus. We have used intracellular recording and biocytin-labeling techniques in the in vitro hippocampal slice preparation to provide quantitative evaluation of the fundamental morphological and intrinsic electrophysiological characteristics of macaque monkey CA1 pyramidal neurons. These neurons have previously been studied in the rat in our laboratory. Monkey CA1 pyramidal neurons have an average soma volume of 3578 microm3, 4.71 basal dendrites with 53 terminal branches for a dendritic length of about 10,164 microm, 1.13 apical dendrites with 47 terminal branches for a dendritic length of about 10,678 microm. In comparison, rat CA1 pyramidal neurons have an average soma volume of 2066 microm3, 3.35 basal dendrites with 29 terminal branches for a dendritic length of about 4,586 microm, 1.43 apical dendrites with 62 terminal branches for a dendritic length of about 8,838 microm. The basic intrinsic electrophysiological properties of CA1 pyramidal cells are similar in monkeys and rats. Monkey CA1 pyramidal neurons have a resting membrane potential of about -62 mV (rat: -62 mV), an input resistance of 35 MOmega (rat: 34-49 MOmega), a rheobase of 0.17 nA (rat: 0.12-0.20 nA) and an action potential amplitude of 83 mV (rat: 71-89 mV). Although morphological differences such as the increased dendritic length may translate into differences in neural processing between primates and rodents, the functional significance of these morphological differences is not yet clear. Quantitative studies of the primate brain are critical in order to extrapolate information derived from rodent studies into better understanding of the normal and pathological function of the human hippocampus.

Comparative morphology of three types of projection-identified pyramidal neurons in the superficial layers of cat visual cortex.

The morphology and dendritic organization of corticocortical neurons in the superficial layers of area 18 that project to area 17 were studied by intracellular injection of lucifer yellow in the fixed-slice preparation. This corticocortical population contains primarily standard pyramidal cells, but occasional nonpyramidal, modified, fusiform, star, and inverted pyramidal cells were also seen. All cell types were present throughout layer 2 and in the upper and middle parts of layer 3. Standard pyramidal cells were found exclusively in lower layer 3. The mean somatic area of the area 17 projecting neurons was 251 microns 2. The width of basal dendritic fields was correlated to cell size for standard pyramidal cells but not for the other cell types. Next, the morphology and dendritic organization of the area 17 projecting neurons were compared to the pyramidal cells of the local horizontal patch networks and of the callosal system. The depth profile of the area 17 projecting and callosal pyramidal groups was virtually identical, peaking at 400 microns from the pial surface, whereas the local patch pyramidal group peaked at 281 microns. The local patch, area 17 projecting, and callosal pyramidal cells displayed increasingly larger mean somatic areas and basilar dendritic field width measurements. The number of basal dendritic branch points was greatest for callosal cells, and it was indistinguishable between local patch and area 17 projecting neurons. In the tangential plane, circular dendritic fields were observed on all callosal cells, but they were found on only approximately half of the local patch and area 17 projecting neurons. The remaining local patch and area 17 projecting neurons displayed mediolaterally and anteroposteriorly elongated basal dendritic fields, respectively.

Morphology and distribution of electrophysiologically defined classes of pyramidal and nonpyramidal neurons in rat ventral subiculum in vitro.

Intracellular electrophysiological recordings were made from 210 ventral subicular neurons in rat brain slices. Recordings were classified as burst-firing or nonburst-firing. Eighteen burst-firing neurons were filled with Neurobiotin, and all had pyramidal morphology. Nine of these recordings were made from intrinsically burst-firing (IB) cell bodies, and nine were made from burst-firing dendrites (BD). Twelve nonburst-firing neurons were also filled with Neurobiotin. Eight were regular spiking (RS) and had pyramidal morphology, four were fast spiking (FS) and nonpyramidal. Additional electrophysiological parameters distinguished IB from BD, RS from FS, and RS from IB recordings. The distribution of IB and RS neurons was examined by using 180 recordings. Information from the first series of experiments was used to distinguish between somatic and dendritic recordings. The deep-superficial axis (alveus-hippocampal fissure) was divided into four equal rows. RS neurons accounted for 12%, 28%, 58%, and 50% of presumed somatic recordings in successively more superficial rows. The proximal-distal (CA1-perforant path) axis was divided into five equal columns. RS cells accounted for 52% of presumed somatic impalements in the central column compared with 16% in the most proximal and 10% in the most distal columns. Thus, two electrophysiological classes of pyramidal neuron were localized to particular regions of the ventral subiculum. In the light of existing knowledge of the topography of subicular inputs and outputs, our results are consistent with the hypothesis that the ratio of RS to IB pyramidal neurons will be different in different transhippocampal circuits.

Neural dynamics in cortex-striatum co-cultures--I. anatomy and electrophysiology of neuronal cell types.

An in vitro system was established to analyse corticostriatal processing. Cortical and striatal slices taken at postnatal days 0-2 were co-cultured for three to six weeks. The anatomy of the organotypic co-cultures was determined using immunohistochemistry. In the cortex parvalbumin-positive and calbindin-positive cells, which resembled those seen in vivo, had laminar distributions. In the striatum, strongly stained parvalbumin-positive cells resembling striatal GABAergic interneurons and cholinergic interneurons were scattered throughout the tissue. The soma area of these interneuron classes was larger than the average striatal soma area, thus enabling visual selection of cells by class before recording. Cortical neurons with projections to the striatum showed similar morphological features to corticostriatal projection neurons in vivo. No projections from the striatum to the cortex were found. Intracellular recordings were obtained from 94 neurons. These were first classified on the basis of electrophysiological characteristics and the morphologies of cells in each class were reconstructed. Two types of striatal secondary neurons with unique electrophysiological dynamics were identified: GABAergic interneurons (n = 17) and large aspiny, probably cholinergic, interneurons (n = 15). The electrophysiological and morphological characteristics of cortical pyramidal cells (n = 27), cortical interneurons (n = 1), as well as striatal principal neurons (n = 34), were identical to those reported for similar ages in vivo. Organotypic cortex-striatum co-cultures are therefore suitable as an in vitro system in which to analyse corticostriatal processing. The network dynamics, which developed spontaneously in that system, are examined in the companion paper.

Effects of dendritic morphology on CA3 pyramidal cell electrophysiology: a simulation study.

We investigated the effect of morphological differences on neuronal firing behavior within the hippocampal CA3 pyramidal cell family by using three-dimensional reconstructions of dendritic morphology in computational simulations of electrophysiology. In this paper, we report for the first time that differences in dendritic structure within the same morphological class can have a dramatic influence on the firing rate and firing mode (spiking versus bursting and type of bursting). Our method consisted of converting morphological measurements from three-dimensional neuroanatomical data of CA3 pyramidal cells into a computational simulator format. In the simulation, active channels were distributed evenly across the cells so that the electrophysiological differences observed in the neurons would only be due to morphological differences. We found that differences in the size of the dendritic tree of CA3 pyramidal cells had a significant qualitative and quantitative effect on the electrophysiological response. Cells with larger dendritic trees: (1) had a lower burst rate, but a higher spike rate within a burst, (2) had higher thresholds for transitions from quiescent to bursting and from bursting to regular spiking and (3) tended to burst with a plateau. Dendritic tree size alone did not account for all the differences in electrophysiological responses. Differences in apical branching, such as the distribution of branch points and terminations per branch order, appear to effect the duration of a burst. These results highlight the importance of considering the contribution of morphology in electrophysiological and simulation studies.

Thalamocortical projection from the parafascicular nucleus to layer V pyramidal cells in frontal and cingulate areas of the rat.

Thalamocortical projections originating from the parafascicular nucleus were reinvestigated using biocytin or biotylinated dextran amine as anterograde tracers in the rat. After stereotaxic injection of the marker in the lateral part of the parafascicular nucleus, labelled ascending fibres were observed running ipsilaterally to the frontal motor and anterior cingulate areas. Labelled fibres gave rise in layer VI to a plexus of thin ramifications ending in layer V, where sparse boutons en passant and terminaux were seen in close apposition to pyramidal cells. Few retrogradely labelled pyramidal somata, contacted by labelled varicosities, were also observed. Electron microscopy demonstrated the synaptic nature of the labelled contacts, displaying asymmetrical junctions and a round vesicular content. The direct loop parafascicular-motor cortex-parafascicular may be of great functional significance in motor control.

The hippocampus makes a significant contribution to experience-dependent neocortical plasticity.

Rats received unilateral hippocampal lesions before being placed in complex environments or standard lab housing. Three months later the brains were prepared for Golgi-Cox staining. Pyramidal cells in layer III of parietal cortex were analyzed bilaterally. The basilar dendrites of the parietal pyramidal cells in the intact hemisphere showed the expected changes in experience-dependent changes, including an increase in dendritic branching and length as well as spine density. In contrast, the basilar dendrites of the parietal neurons in the lesion hemisphere showed no significant effect of experience on dendritic branching or length and showed a decrease in spine density in the same neurons. The apical fields failed to show an effect of experience in either hemisphere. The hippocampus plays a crucial role in neocortical experience-dependent plasticity.

A structurally detailed finite element human head model for simulation of transcranial magnetic stimulation.

Computational studies of the head utilizing finite element models (FEMs) have been used to investigate a wide variety of brain-electromagnetic (EM) field interaction phenomena including magnetic stimulation of the head using transcranial magnetic stimulation (TMS), direct electric stimulation of the brain for electroconvulsive therapy, and electroencephalography source localization. However, no human head model of sufficient complexity for studying the biophysics under these circumstances has been developed which utilizes structures at both the regional and cellular levels and provides well-defined smooth boundaries between tissues of different conductivities and orientations. The main barrier for building such accurate head models is the complex modeling procedures that include 3D object reconstruction and optimized meshing. In this study, a structurally detailed finite element model of the human head was generated that includes details to the level of cerebral gyri and sulci by combining computed tomography and magnetic resonance images. Furthermore, cortical columns that contain conductive processes of pyramidal neurons traversing the neocortical layers were included in the head model thus providing structure at or near the cellular level. These refinements provide a much more realistic model to investigate the effects of TMS on brain electrophysiology in the neocortex.

The Sholl analysis of neuronal cell images: semi-log or log-log method?

Although the Sholl analysis is a quantitative method for morphometric neuronal studies and its application provides many benefits to neurobiology since it is obvious, common and meaningful, there are many unresolved theoretical issues that need to be addressed. Nevertheless, it can be practiced without much background or sophistication. The two different methods of the Sholl analysis--log-log and semi-log--have been applied previously without a clear basis as to what to use. To make an adequate choice of the method, one should try and accept that one which gives a better result. We consider that some of the underlying principles, assumptions and limitations for the Sholl analysis can be found in basic mathematics. In order to compare the results of applications of the semi-log and log-log methods to the same neuron, we introduce the concept of determination ratio as the ratio of the coefficient of determination for the semi-log method and that for the log-log method. If the semi-log method is better as related to the log-log method, the value of this parameter is larger than 1. Having in mind that dendrites exhibit enormously diverse forms, we point out that the semi-log Sholl method is more frequently utilizable in practice. Only the neurons, whose dendritic trees have one or a few sparsely ramified dendrites being much longer than the rest ones, could be successfully and exactly analysed using the log-log method. We also compare the Sholl analysis with fractal analysis for the characterization of neuronal arborization patterns and found that between the Sholl and fractal analysis exist various and important analogies.

Frequency and dendritic distribution of autapses established by layer 5 pyramidal neurons in the developing rat neocortex: comparison with synaptic innervation of adjacent neurons of the same class.

Synaptic contacts formed by the axon of a neuron on its own dendrites are known as autapses. Autaptic contacts occur frequently in cultured neurons and have been considered to be aberrant structures. We examined the regular occurrence, dendritic distribution, and fine structure of autapses established on layer 5 pyramidal neurons in the developing rat neocortex. Whole-cell recordings were made from single neurons and synaptically coupled pairs of pyramidal cells, which were filled with biocytin, morphologically reconstructed, and quantitatively analyzed. Autapses were found in most neurons (in 80% of all cells analyzed; n = 41). On average, 2.3 +/- 0.9 autapses per neuron were found, located primarily on basal dendrites (64%; 50-70 microns from the soma), to a lesser extent on apical oblique dendrites (31%; 130-200 microns from the soma), and rarely on the main apical dendrite (5% 480-540 microns from the soma). About three times more synaptic than autaptic contacts (ratio 2.4:1) were formed by a single adjacent synaptically coupled neuron of the same type. The dendritic locations of these synapses were remarkably similar to those of autapses. Electron microscopic examination of serial ultrathin sections confirmed the formation of autapses and synapses, respectively, and showed that both types of contacts were located either on dendritic spines or shafts. The similarities between autapses and synapses suggest that autaptic and synaptic circuits are governed by some common principles of synapse formation.

Contributions of individual layer 6 pyramidal neurons to local circuitry in macaque primary visual cortex.

We have studied the contributions of individual layer 6 pyramidal neurons in macaque primary visual cortex to local cortical circuitry by intracellular labeling and analysis of the morphologies of 58 neurons. These neurons are separated based on the laminar specificity of axonal and dendritic arbors into two classes, class I and class II, and into several types within these classes. Class I neurons project axons heavily and predominantly to layer 4C, whereas class II neurons have axonal projections primarily to layers other than 4C. Only 16 of the 58 neurons in our sample (28 percent) project to the white matter. Class I projection neurons are found at the top and bottom of layer 6, suggesting that they project to the lateral geniculate nucleus, whereas class II projection neurons are located in the middle of layer 6, suggesting that they project to the claustrum. The different types of class I neurons are distinguished from one another based on the sublaminar specificity of their axonal and dendritic arbors within layer 4, where they are biased toward compartments dominated by input from different functional streams. They are also distinct in their distributions within the depth of layer 6. The distinctive characteristics of the neuronal types we have identified suggest that each receives input from different sources and projects to a set of targets that is functionally appropriate. Thus, each type is likely to contribute uniquely to computations within V1 and extrinsically.

The pyramidal neuron in occipital, temporal and prefrontal cortex of the owl monkey (Aotus trivirgatus): regional specialization in cell structure.

Recent studies have revealed marked regional variation in pyramidal cell morphology in primate cortex. In particular, pyramidal cells in human and macaque prefrontal cortex (PFC) are considerably more spinous than those in other cortical regions. PFC pyramidal cells in the New World marmoset monkey, however, are less spinous than those in man and macaques. Taken together, these data suggest that the pyramidal cell has become more branched and more spinous during the evolution of PFC in only some primate lineages. This specialization may be of fundamental importance in determining the cognitive styles of the different species. However, these data are preliminary, with only one New World and two Old World species having been studied. Moreover, the marmoset data were obtained from different cases. In the present study we investigated PFC pyramidal cells in another New World monkey, the owl monkey, to extend the basis for comparison. As in the New World marmoset monkey, prefrontal pyramidal cells in owl monkeys have relatively few spines. These species differences appear to reflect variation in the extent to which PFC circuitry has become specialized during evolution. Highly complex pyramidal cells in PFC appear not to have been a feature of a common prosimian ancestor, but have evolved with the dramatic expansion of PFC in some anthropoid lineages.

Contributions of individual layer 2-5 spiny neurons to local circuits in macaque primary visual cortex.

We studied excitatory local circuits in the macaque primary visual cortex (VI) to investigate their relationships to the magnocellular (M) and parvocellular (P) streams. Sixty-two intracellularly labeled spiny neurons in layers 2-5 were analyzed. We made detailed observations of the laminar and columnar specificity of axonal arbors and noted correlations with dendritic arbors. We find evidence for considerable mixing of M and P streams by the local circuitry in VI. Such mixing is provided by neurons in the primary geniculate recipient layer 4C, as well as by neurons in both the supragranular and infragranular layers. We were also interested in possible differences in the axonal projections of neurons with different dendritic morphologies. We found that layer 4B spiny stellate and pyramidal neurons have similar axonal arbors. However, we identified two types of layer 5 pyramidal neuron. The majority have a conventional pyramidal dendritic morphology, a dense axonal arbor in layers 2.4B, and do not project to the white matter. Layer 5 projection neurons have an unusual "backbranching" dendritic morphology (apical dendritic branches arc downward rather than upward) and weak or no axonal arborization in layers 2-4B, but have long horizontal axonal projections in layer 5B. We find no strong projection from layer 5 pyramidal neurons to layer 6. In macaque V1 there appears to be no single source of strong local input to layer 6; only a minority of cells in layers 2-5 have axonal branches in layer 6 and these are sparse. Our results suggest that local circuits in V1 mediate interactions between M and P input that are complex and not easily incorporated into a simple framework.