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PubMed1980

[Comparative studies of the microcirculation in the region of the odontoblastic layer of the teeth of rats. Part 1: Odontoblastic layer (author's transl)].

The article deals with the capillary system of odontoblasts in the rat incisor. Differences in capillary structure between molars and incisors are traced back to functional causes. The odontoblasts of developing molars and incisors possess a specially constructed capillary structure, in as much as the capillaries form a connected double arcade system above and below the odontoblasts. The author hypothesises that odontoblast activity in the rat is tied to this the odontoblasts. The author hypothesises that odontoblast activity in the rat is tied to this double capillary arcade. From the standpoint of germination and mineralisation, it seems important that the supplying capillaries are between the predentine and the odontoblasts.

Animals↗
PubMed1992

Development of axonal arbors of layer 4 spiny neurons in cat striate cortex.

Spiny neurons in layer 4 of cat striate cortex are the primary recipients of geniculocortical afferents and provide crucial links to other cortical layers for processing visual information. Using intracellular staining, we examined the development of the local axonal projections of these neurons to determine (1) whether the laminar specificity of their projections emerged specifically or was sculpted from transient exuberant projections and (2) whether the emergence of excitatory connections from layer 4 to layer 2/3 could contribute to the activity-dependent development of clustered horizontal connections of layer 2/3 pyramidal neurons. Differences in the extent of projections to infragranular (layers 5 and 6, which receive sparse projections) versus superficial layers (layers 2/3 and 4, which receive extensive projections) developed specifically from the outset. By postnatal day 15 (P15) projections to infragranular layers matured and were indistinguishable from those in the oldest animal studied (P33). In contrast, projections to superficial layers continued to increase in complexity after P15. Projections within layer 4, which were the most elaborate at all ages studied, reached maturity at about P20, while those to layer 2/3 continued to increase in complexity through P33. No evidence for exuberant projections to any of these cortical layers was observed. At very early postnatal ages (P5) projections to the subplate region were evident. These disappeared by P8-P11, suggesting the presence of transient connections from layer 4 spiny neurons to subplate neurons. Binocular deprivation did not prevent the emergence of projections from layer 4 spiny neurons into layer 2/3 or development of normal laminar differences in projection density. Connections from layer 4 to layer 2/3 emerged after horizontal connections in layer 2/3 were crudely clustered, but in synchrony with the later refinement of clusters. Collaterals from layer 4 cells first crossed into layer 3 at P11, but were extremely short (extending only 50-200 microns beyond the laminar boundary) and uncommon (only 4 of 19 cells). Since by P8 horizontal projections of layer 2/3 pyramidal neurons are already crudely clustered, the emergence of crude clustering is probably independent of layer 4 to layer 2/3 excitatory projections. The proportion of cells projecting to layer 2/3 and the complexity of their arbors both increased in the subsequent weeks, closely matching the timing of both the refinement of crudely clustered horizontal connections and the emergence of visual responsiveness in layer 2/3.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗
PubMed1996

Layer I neurons of rat neocortex. I. Action potential and repetitive firing properties.

1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study action potential (AP) and repetitive firing properties of layer I neurons in slices of rat neocortex. 2. Layer I neurons had resting membrane potentials (RMP) of -59.8 +/- 4.7 mV (mean +/- SD) and input resistances (RN) of 592 +/- 284 M Omega. Layer II/III pyramidal neurons had RMPs and RNs of -61.5 +/- 5.6 mV and 320 +/- 113 M omega, respectively. A double exponential function was needed to describe the charging curves of both neuron types. In layer I neurons, tau(0) was 45 +/- 22 ms and tau(1) was 5 +/- 3.3 ms whereas in layer II/III pyramidal neurons, tau(0) was 41 +/- 11 ms and tau(1) was 3 +/- 2.6 ms. Estimates of specific membrane resistance (Rm) for layer I and layer II/III cells were 45 +/- 22 and 41 +/- 11 k omega cm2, respectively (Cm was assumed to be 1 microF/cm2). 3. AP threshold was -41 +/- 2 mV in layer I neurons. Spike amplitudes, measured from threshold to peak, were 90.6 +/- 7.7 mV. AP durations, measured both at the base and half maximal amplitude, were 2.5 +/- 0.4 and 1.1 +/- 0.2 ms, respectively. AP 10-90% rise and repolarization times were 0.6 +/- 0.1 and 1.1 +/- 0.2 ms, respectively. In layer II/III pyramidal neurons, AP threshold was -41 +/- 2.5 mV and spike amplitude was 97 +/- 9.7 mV. AP duration at base and half maximal amplitude was 5.4 +/- 1.1 ms and 1.8 +/- 0.2 ms, respectively. AP 10-90% rise and decay times were 0.6 +/- 0.1 ms and 2.8 +/- 0.6 ms, respectively. 4. Layer I neurons were fast spiking cells that showed little frequency adaptation, a large fast afterhyperpolarization (fAHP), and no slow afterhyperpolarization (sAHP). Some cells had a medium afterhyperpolarization (mAHP) and a slow afterdepolarization (sADP). All pyramidal cells in layer II/III and "atypical" pyramidal neurons in upper layer II showed regular spiking behavior, prominent frequency adaptation, and marked sAHPs. 5. In both layer I neurons and layer II/III pyramidal neurons, changes in membrane potential did not greatly alter AP properties. The duration of APs evoked from -50 to -60 mV was only slightly longer, from -80 to -90 mV. The latency to first spike also was not solely dependent on membrane potential. 6. During repetitive firing, APs broadened in both layer I neurons and layer II/III pyramidal neurons. This was most prominent in pyramidal cells. Broadening was dependent on spike frequency and appeared to result from partial inactivation of both outward potassium and inward sodium currents. 7. In layer I neurons, removing Ca2+ from the bathing solution slightly prolonged spike duration and modestly increased AP firing frequency. These results indicate minimal involvement of Ca2+-dependent K+ currents in AP repolarization. fAHPs were reduced whereas sADPs were abolished. In layer II/III pyramidal neurons, removing Ca2+ reduced or blocked mAHPs and sAHPs and decreased or abolished frequency adaptation. 8. Low concentrations (50 microM) of 4-aminopyridine (4-AP) prolonged APs and induced burst-like firing in layer I neurons. In the presence of 4-AP, the spiking behavior of layer I neurons resembled that of regular spiking layer II/III pyramidal cells. At high concentrations (4 mM), 4-AP could induce a delayed depolarization (DD) after each spike in layer I neurons and in a minority of pyramidal neurons. 9. All layer I neurons had a prominent fAHP that was absent or very small in layer II/III pyramidal neurons. fAHP amplitude was inversely related to AP duration. The reduction of fAHPs by 4-AP or during repetitive firing was accompanied by AP prolongation, suggesting that the current underlying fAHP played an essential role in AP repolarization. The fAHP of layer I neurons could be effectively blocked by 4-AP but only slightly reduced by removing Ca2+ from bathing solution, indicating that the fAHP was mediated primarily by a voltage-dependent transient outward current.(ABSTRACT TRUNCATED)

Action Potentials↗
PubMed2004

Formation of the corneous layer in the epidermis of the tuatara (Sphenodon punctatus, Sphenodontida, Lepidosauria, Reptilia).

The formation of the stratum corneum in the epidermis of the reptile Sphenodon punctatus has been studied by histochemical, immunohistochemical, and ultrastructural methods. Sulfhydryl groups are present in the mesos and pre-alpha-layer but disappear in the keratinized beta-layer and in most of the mature alpha-layer. This suggests a complete cross-linking of keratin filaments. Tyrosine increases in keratinized layers, especially in the beta-layer. Arginine is present in living epidermal layers, in the presumptive alpha-layer, but decreases in keratinized layers. Histidine is present in corneous layers, especially in the intermediate region between the alpha- and a new beta-layer, but disappears in living layers. It is unknown whether histidine-rich proteins are produced in the intermediate region. Small keratohyalin-like granules are incorporated in the intermediate region. The plane of shedding, as confirmed from the study on molts, is located along the basalmost part of the alpha-layer and may involve the degradation of whole cells or cell junctions of the intermediate region. A specific shedding complex, like that of lizards and snakes, is not formed in tuatara epidermis. AE1-, AE2-, or AE3-positive alpha-keratins are present in different epidermal layers with a pattern similar to that previously described in reptiles. The AE1 antibody stains the basal and, less intensely, the first suprabasal layers. Pre-keratinized, alpha- and beta-layers, and the intermediate region remain unlabeled. The AE2 antibody stains suprabasal and forming alpha- and beta-layers, but does not stain the basal and suprabasal layers. In the mature beta-layer the immunostaining disappears. The AE3 antibody stains all epidermal layers but disappears in alpha- and beta-layers. Immunolocalization for chick scale beta-keratins labels the forming and mature beta-layer, but disappears in the mesos and alpha-layer. This suggests the presence of common epitopes in avian and reptilian beta-keratins. Low molecular weight alpha-keratins present in the basal layer are probably replaced by keratins of higher molecular weight in keratinizing layers (AE2-positive). This keratin pattern was probably established since the beginning of land adaptation in amniotes.

Journal Article↗
PubMedDate not supplied

Interlaminar connections of the superior colliculus in the tree shrew. III: The optic layer.

These experiments were designed to test the idea that the optic layer in the tree shrew, Tupaia belangeri, is functionally distinct and provides a link between the visuosensory superficial and the premotor intermediate layers of the superior colliculus. First, cells in the optic layer were intracellularly labeled with biocytin in living brain slices. Compared to cells in the adjacent lower part of the superficial gray layer, which have apical dendrites that ascend toward the tectal surface, optic layer cells have dendritic fields that are restricted for the most part to the optic layer itself. The differences in dendritic-field location imply that superficial gray and optic layer cells have different patterns of input. The axons of optic layer cells terminate densely within the optic layer and, in addition, project in a horizontally restricted fashion to the overlying superficial gray and subjacent intermediate gray layers. This pattern also is different from the predominantly descending interlaminar projections of lower superficial gray layer cells. Next, cells in the intermediate gray layer were labeled in order to examine the relationships between optic layer cells and these subjacent neurons that project from the superior colliculus to oculomotor centers of the brain stem. Neurons in the upper part of the intermediate gray layer send apical dendrites into the optic layer and therefore can receive signals from the superficial gray layer either directly, from descending axons of lower superficial gray layer cells, or indirectly, through intervening optic layer cells. In contrast, lower intermediate gray layer cells have more radiate dendritic fields that are restricted to the intermediate gray layer. Thus, these lower cells must depend on descending projections from optic or upper intermediate gray layer cells for signals from the superficial gray layer. Together, these results support the idea that the optic layer is a distinct lamina that provides a link between the superficial and intermediate gray layers. They also are consistent with the traditional view that descending intracollicular projections play a role in the selection of visual targets for saccades.

Animals↗
PubMed1996

Specificity in the axonal connections of layer VI neurons in tree shrew striate cortex: evidence for distinct granular and supragranular systems.

Pyramidal neurons in layer VI of striate cortex are the source of descending projections to the lateral geniculate nucleus (LGN) as well as a major source of axon terminals in the layers of striate cortex that receive LGN projections. This study examines how the connections of layer VI neurons are arranged with respect to the functionally distinct classes of neurons that compose their cortical and subcortical targets. By placing injections of biocytin into layer VI of tree shrew striate cortex, we identified two sublayers that differ in their intracortical and thalamic connections. Neurons in the upper part of layer VI, layer VIa, terminate in cortical layer IV, whereas those in the lower part of layer VI, layer VIb, terminate throughout the supragranular layers, layers I-III. The selectivity of layer VI subdivisions for the granular and supragranular layers is also evident in their descending projections. Neurons in layer VIa terminate preferentially in the LGN layers that supply layer IV (LGN layers 1, 2, 4, and 5), whereas neurons in layer VIb terminate in the LGN layers that supply layers I-III (LGN layers 3 and 6) and in the pulvinar nucleus. Additional subclasses of layer VIa neurons were identified based on the restriction of their terminal fields to narrow subtiers within layer IV. By influencing the activity of distinct populations of cortical neurons and the thalamic neurons that supply them, layer VI neurons could exert a powerful influence on the flow of activity in functionally distinct cortical circuits.

Animals↗
PubMed1986

A fundamental study of normal layer structure of the gastrointestinal wall visualized by endoscopic ultrasonography.

The gastrointestinal wall could be separated into five layers or nine layers by means of the ultrasonic endoscope, and the histological structure of these layers was ascertained by comparing endoscopic ultrasonograms of resected specimens of the gastrointestinal tract with their corresponding histology. The results were as follows: With five layers of the gastrointestinal wall, the first and the second layer corresponded to the mucosa, the third layer was the submucosa, and the fourth layer corresponded to the muscularis propria. The first layer was a border echo demonstrated inside the mucosa. The fifth layer consisted of the serosa and a border echo visualized outside the serosa. When a thin layer was visualized at the same time in both the second and the fourth layers, the gastrointestinal wall was separated into nine layers in total. With nine layers of the gastrointestinal wall, the muscularis mucosae was composed of a thin layer in the second layer and a narrow layer between a thin layer in the second layer and the third layer. A thin layer in the second layer was a border echo visualized inside the muscularis mucosae. A thin layer in the fourth layer of the gastrointestinal wall consisted of a border echo and a connective tissue between the inner circular muscle and the outer longitudinal muscle.

Animals↗
PubMed1979

Evidence that the different classes of relay cells of the cat's lateral geniculate nucleus terminate in different layers of the striate cortex.

Small electrophoretic injections of horseradish peroxidase (HRP) were made with recording microelectrodes in different layers of area 17 of the cat. The sizes and locations of labeled relay cells in the lateral geniculate nucleus (LGNd) were then studied. Layer I was found to receive inputs mainly from small LGNd cells (13--21 micrometer). These cells were located in the parvocellular C laminae and the MIN. Layer II and the upper portion of layer III were not found to receive LGNd afferents. Lower layer III was found to receive inputs mainly from small to medium-sized relay cells. These cells were located in the A laminae and the parvocellular C laminae. Layer IVab and the layer III--IV border region were found to receive afferents from predominantly large LGNd neurons (greater than 20 micrometers). These cells were located in alminae A, A1, C, and the MIN. Layer IVc was found to receive inputs from mostly small to medium-sized cells (15--23 micrometer). Nearly all of these cells were in the A laminae although some small cells in laminae C were observed following layer IVc injections which involved the layer IV--V border region. Layer V, away from the layer IV--V border region, was not found to receive LGNd afferents. Finally, layer VI was found to receive inputs from both large and small relay cells. These cells were all located in the A laminae. In the light of previous findings, these results suggest that the terminals of W, X, and Y cell axons are segregated within the cat's striate cortex: W cells projecting to layer 1, the deeper portion of layer III and the layer IV--V border region; X cells projecting heavily to layer IVc with a minor projection to layer VI and possibly also to lower layer III; Y cells projecting heavily to layer IVab and the layer III-IV border region with a smaller projection to layer VI.

Animals↗
PubMed2001

Development of visual cortical axons: layer-specific effects of extrinsic influences and activity blockade.

During normal cortical development, individual pyramidal neurons form intracortical axonal arbors that are specific for particular cortical layers. Pyramidal neurons within layer 6 are able to develop layer-specific projections in cultured slices of ferret visual cortex, indicating that extrinsic influences, including patterned visual activity, are not required (Dantzker and Callaway [1998] J Neurosci 18:4145-4154). However, when spontaneous activity is blocked in cultures with tetrodotoxin, layer 6 pyramidal neurons fail to preferentially target their axons to layer 4. To determine whether mechanisms that regulate the development of layer 6 pyramidal neuron arbors can be generalized to pyramidal neurons in other layers, we examined the development of layer 5 and layer 2/3 pyramidal neurons in cultured slices of ferret visual cortex prepared on postnatal day 14 or 15. Layer 5 pyramidal neurons developed layer-specific axonal arbors during 5-7 days in vitro. However, unlike layer 6 pyramidal neurons, layer 5 pyramidal neurons formed layer-specific axonal arbors in the presence of tetrodotoxin. In contrast to layer 5 and layer 6 pyramidal neurons, layer 2/3 pyramidal neurons did not form appropriate layer-specific projections during 5-7 days in vitro. Taken together, these data suggest that the development of layer-specific axons is regulated by different mechanisms for neurons in different layers and cannot be categorically classified as either activity-dependent or independent. Instead, the type of pyramidal neuron, the layers targeted, and the type of activity must be considered.

Action Potentials↗
PubMed1984

Anatomy of layer IV in cat primary auditory cortex (AI).

The structure of neurons and axons in layer IV was studied as part of a larger inquiry into the organization of primary auditory cortex (AI) in the cat. Tissue from the convexity of the middle ectosylvian gyrus between the anterior and posterior ectosylvian sulci was studied in Golgi, Nissl, Bodian, plastic-embedded, and other preparations from adult animals. Layer IV is defined as a strip about 200-250 micron thick consisting predominantly of small non-pyramidal neurons intercalated between the pyramidal somata of layers III and V, and in which few commissurally projecting cells occur. Lying some 800-900 micron beneath the pia, layer IV has six types of neurons, as seen in Golgi-stained material from anatomically and physiologically defined AI. These include three varieties (small, medium-sized, and large) of tufted neurons with intracortically branching axons and vertically polarized, cylindrical dendritic fields. Besides the tufted cells, which are the most numerous neurons in layer IV, large multipolar, double bouquet, and spiny stellate cells are scattered through layer IV. Each has a characteristic neuronal architecture and intracortical axonal branches. Smaller tufted cell somata dominate the upper half of layer IV (IVa), larger tufted cells are more common deep in layer IV (IVb). The average somatic area in midnuclear , plastic-embedded sections is 158 micron2. Layer IV (and layer IIIb) receive thick, probably ascending fibers, forming narrow, vertical terminal fields. These axons may be of thalamic origin and overlap with alternating, 50-75-micron-wide columns of somata and neuropil in layer IV. Layer III pyramidal cell axons often project to layer IV and ramify vertically and horizontally. The average height-width ratio of the dendritic domains of layer IV cells is about 3.6:1. The vertically disposed dendrites of layer IV cells, the columnar arrangement of their local axonal branches, and the polarized form of intrinsic and extrinsic axons collectively reinforces the columnar pattern in layer IV. Many layer IV cells structurally resemble neurons in layer IV in the primary visual and somatic sensory cortex. However, most AI cells have a pronounced columnar arrangement of their somata and an elongated, narrow form. The axons of many layer IV cells preserve this vertical arrangement and often branch in layer III and, to a lesser degree, in layer V.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1992

Lateral geniculate projections to the superficial layers of visual cortex in the tree shrew.

Our recent studies of tree shrew striate cortex have focused on the organization of lateral geniculate projections to layer IV and the projections from IV to layer III. Although these pathways play an important role in determining the response properties of layer III neurons, there are additional pathways from the lateral geniculate nucleus (LGN) that terminate directly in layer III. Previous studies provided evidence that these projections originate from layers 6 and 3 of the LGN and terminate in different subdivisions of layer III. In this study we used injections of biocytin to examine the projections of layers 6 and 3 to the cortex in more detail. Consistent with earlier work, we found that LGN layer 6 projects heavily to lower IIIc, while LGN layer 3 terminates densely in layer IIIb and sparsely throughout layers IIIa-I. In addition, we found that neurons in layers 6 and 3 have collateral projections: neurons in LGN layer 6 project to the bottom of layer IVb and sparsely to I-IIIb; neurons in LGN layer 3 project sparsely to layers V and VI and to the middle of IV. These patterns of projections are significant in the light of our studies of the connections from cortical layer IV to layer III. LGN projections to the superficial layers are organized into parallel pathways that exert selective influence over different populations of neurons in layers I-III and on the layer IV neurons that supply them.

Animals↗
PubMed2004

Receptive field scatter, topography and map variability in different layers of the hindpaw representation of rat somatosensory cortex.

We recorded neurons extracellularly in layers II/III, IV, and V of the hindpaw representation of primary somatosensory cortex in anesthetized rats and studied laminar features of receptive fields (RFs) and representational maps. On average, RFs were smallest in layer IV and largest in layer V; however, for individual penetrations we found substantial deviations from this rule. Within the hindpaw representation, a distinct rostrocaudal gradient of RF size was present in all layers. While layer V RFs were generally largest independent of this gradient, layer IV RFs recorded caudally representing the proximal portions of the paw were larger than layer II/III RFs recorded rostrally representing the digits. The individual scatter of the locations of RFs across laminar groups was in the range of several millimeters, corresponding to about 25% of the average RF diameter. The cutaneous representations of the hindpaw in extragranular layers were confined to the areal extent defined by responsive sites in layer IV. Comparison between RFs determined quantitatively and by handplotting showed a reliable correspondence. Repeated measurements of RFs revealed spontaneous fluctuations of RF size of no more than 5% of the initial condition over an observation period of several hours. The topography and variability of cortical maps of the hindpaw representation were studied with a quantitative interpolation method taking into account the geometric centers of RFs and the corresponding cortical recording sites. On average, the overall topography in terms of preservation of neighborhood relations was present in all layers, although some individual maps showed severe distortions of topography. Factors contributing to map variability were overall position of the representation on the cortical surface, internal topography and spatial extent. Interindividual variability of map layout was always highest in the digit representations. Local topographic orderliness was lowest in layer V, but comparable in layers II/III and IV. Within layer IV, the lowest orderliness was observed in the digit representations. Our data emphasize a substantial variability of RF size, overlap and position across layers and within layers. At the level of representational maps, we found a similar degree of variability that often co-varied across layers, with little evidence for significant layer specificity. Laminar differences are likely to arise from the specific input-output pattern, layer-specific cell types and the connectivity between different layers. Our findings emphasizing similarities in the variability across layers support the notion of tightly coupled columnar interactions between different layers.

Action Potentials↗
PubMed2005

A histochemical and immunohistochemical analysis of the subependymal layer in the normal and Huntington's disease brain.

Previous studies in the rodent brain have characterised the cell types present in the subependymal layer, however the general organisation and cellular morphology of the adult human subependymal layer has not been demonstrated previously. In this study, we have demonstrated that the normal human brain subependymal layer contains three morphologically distinct types of cells, A, B and C type cells. The type A cells resembling migrating neuroblasts were located in the superficial part of the subependymal layer, type B cells resembling glial cells were evenly distributed throughout the subependymal layer and caudate nucleus, and type C cells that resembled progenitor cells were located in the deeper regions of the subependymal layer close to the caudate nucleus. We also examined the subependymal layer in the Huntington's disease brain to determine whether neurodegenerative pathology of the caudate nucleus (adjacent to the subependymal layer) altered the cellular composition of the subependymal layer. In the Huntington's disease subependymal layer there was a significant increase in the thickness of the subependymal layer compared with the normal subependymal layer (p < 0.01) and there was a 2.8-fold increase in the number of cells present in the Huntington's disease subependymal layer compared with the normal subependymal layer but the density of cells remained unchanged. As the grade of Huntington's disease increased, so did the overall number of cells in the subependymal layer. An increase in the number of type B cells was responsible for most of the increase demonstrated, however there was also an increase in the numbers of type A and C cells. To further characterise the human normal and Huntington's disease subependymal layer we used immunohistochemistry and antibodies against a range of projection neuron markers, interneuron markers, glial cell markers and GABAA receptor subunits. The results demonstrated the presence of increased numbers of neuropeptide Y positive cells in the Huntington's disease subependymal layer compared with the normal subependymal layer, suggesting that neuropeptide Y neurons may play a role in progenitor cell proliferation. Also there was an increased level of the developmentally active GABAA receptor subunit gamma 2 that indicates that the adult subependymal layer still retains the ability to proliferate. Taken together our results give a detailed description of the adult human subependymal layer and also demonstrate the plasticity of the human subependymal layer in response to Huntington's disease.

Adult↗
PubMed1998

Postsynaptic pyramidal target selection by descending layer III pyramidal axons: dual intracellular recordings and biocytin filling in slices of rat neocortex.

Paired intracellular recordings in slices of adult rat neocortex with biocytin filling of synaptically connected neurons were used to investigate the pyramidal targets, in layer V, of layer III pyramidal axons. The time-course and sensitivity of excitatory postsynaptic potentials to current injected at the soma, and locations of close appositions between presynaptic axons and postsynaptic dendrites, indicated that the majority of contributory synapses were located in layer V. Within a "column" of tissue, radius < or = 250 microm, the probability that a randomly selected layer III pyramid innervated a layer V pyramid was 1 in 4 if the target cell was a burst firing pyramid with an apical dendritic tuft in layers II/I. If, however, the potential target was a regular spiking pyramid, the probability of connectivity was only 1 in 40, and none of the 13 anatomically identified postsynaptic layer V targets had a slender apical dendrite terminating in layers IV/III. Morphological reconstructions indicated that layer III pyramids select target layer V cells whose apical dendrites pass within 50-100 microm of the soma of the presynaptic pyramid in layer III and which have overlapping apical dendritic tufts in the superficial layers. The probability that a layer V cell would innervate a layer III pyramid lying within 250 microm of its apical dendrite was much lower (one in 58). Both presynaptic layer III pyramids and their large postsynaptic layer V targets could therefore access similar inputs in layers I/II, while small layer V pyramids could not. One prediction from the present data would be that neither descending layer V inputs to the striatum or thalamus, nor transcallosal connections would be readily activated by longer distance cortico-cortical "feedback" connections that terminated in layers I/II. These could, however, activate corticofugal pathways to the superior colliculus or pons, both directly and via layer III.

Animals↗