Interlaminer connections of rat visual cortex: an ultrastructural study

Local intra- and interlaminar connections in mouse barrel cortex

Focal injections of horseradish peroxidase (HRP) in dimethylsulfoxide (DMSO) were targeted into mouse somatosensory cortex, in vitro, with a template. Injections were made at different depths and in different locations in the whisker-barrel-defined somatosensory map in order to determine quantitative connectivity patterns within and between barrel-defined cortical columns. Cortices were sectioned in a plane parallel to the pia at 75 microns. Data were collected directly from microscope slides by computer. Data are presented as: 1) Plots of computer-mapped HRP reaction product density in neurons and cell locations for each section in relation to barrel boundaries; 2) histograms of label in cortical layers related to individual barrel-defined columns; 3) polar plots of relative amounts of label within individual barrel columns in sections through each barrel column; 4) vectors which represent HRP reaction product density as a function of direction and distance from the injection site; 5) statistical analysis of the shape of the label distribution pattern in the plane of the cortex as a function of injection site depth; and 6) probability of labeling of any other barrel column given a labeled barrel column. The principal findings are: 1) The pattern of label distribution, after an injection directly above or directly below an individual barrel, is hour-glass shaped with the waist of the hour-glass in layer IV. 2) Connections within barrel cortex are asymmetrical. Barrel-related columns within a row are more strongly interconnected than those in different rows. 3) Connections of the small barrels associated with whiskers on the upper lip are strongest with other small barrels, but strong connections also exist between these small barrels and the larger barrels. 4) The pattern of intracortical connections in SII is not asymmetrical; interlaminar connections in SII are fundamentally different from those in barrel cortex. 5) Quantitative intracortical projection patterns are highly consistent with functional data on intracortical processing of whisker information. As such, the quantitative data clearly indicate the spatial extent and relative magnitude of populations of neurons involved in intracortical processing of sensory information. The spatial arrangements of these intracortical connections, in conjunction with known developmental events, make it highly likely that the distribution of intracortical axons in mouse barrel cortex is sculpted in part by experience.

Intrinsic connections of rat primary visual cortex: laminar organization of axonal projections

The organization of local projections within the rat primary visual cortex (area 17) was investigated by tracing fibers with HRP in in vitro brain slices. The projections from different layers showed distinct laminar patterns. Layer 4 made a strong, topographically precise, projection to lower layer 2/3; weaker projections extended laterally and terminated diffusely in layer 2/3 but also ran vertically to layers 5 and 6. The connections of lower and upper layer 2/3 were reciprocal and point-to-point. Within layer 2/3, a large number of fibers ran horizontally and terminated at variable distances from the injection site without making terminal clusters. The main output from layer 2/3 was to layer 5. The most prominent projections from the upper half of layer 5 were to layers 2/3 and 6; lower layer 5, in contrast, made wide-ranging, clustered projections to layer 1, the bottom of layer 2/3, and the top of layers 4 and 5. The patches were 130-160 micron wide and spaced apart by 230-260 micron. The main projection that arose from the superficial layer 6 terminated in layer 4 above the injection site. In contrast, lower layer 6 made clustered projections to the layer 3/4 border, extending up to 2 mm in the coronal plane. The patches were 190-220 micron wide and spaced apart by 320-390 micron. Additional projections went to the layer 5/6 border and layers 1 and 2. These results indicate that geniculocortical input is processed through interlaminar connections that are topographically precise, widespread, or patchy. These connectivity patterns suggest a role for these connections in the transformation of functional maps between layers; focused projections preserve the architecture of the layers of origin, and diverging or patchy projections rearrange this organization and form new maps in the target layers (Lund: Annu. Rev. Neurosci. 11:253-288, '88). However, only a few interlaminar connections show one of these patterns in isolation, making it difficult to assign a single function to a particular connection. We, therefore, tentatively conclude that projections terminating in layers 1-4, with the possible exception of the connection between upper layer 6 and layer 4, transform functional maps. In contrast, the topographically precise projections from upper to lower layers preserve functional maps. The specific role of these connections in the construction of receptive field properties, however, is not known.(ABSTRACT TRUNCATED AT 400 WORDS)

A quantitative Golgi study in the occipital cortex of the pyramidal dendritic topology of old adult rats from social or isolated environments

It has been previously established that the housing condition influences the appearance of the dendritic tree in young animals. The purpose of this investigation was to determine the extent of the influence that the housing condition exerts on morphometric studies involving old animals. Specifically, we found superficial pyramidal cells from the visual cortex of socially aged 20-month-old rats had more extensive oblique dendritic trees than neurons from isolated rats of the same age. The basal dendritic tree of neurons from socially reared rats was denser nearest layer IV while the basal dendritic tree of neurons from isolated rats appeared to shift away from layer IV. Our results indicate that the external environment is an independent variable and cortical dendritic morphology is a dependent variable in studies on old adult rats. Thus, the external environment should be considered in reporting results of aging studies on dendritic topology.

The neural circuitry of the neocortex examined in the in vitro brain slice preparation

The in vitro brain slice technique has been applied to the study of the neocortex. Cortical blocks were removed from adult rats deeply anesthesized with halothane, sectioned coronally at 400-700 micrometer, and placed in a brain slice chamber. Cortical slices typically showed spontaneous and evoked potential activity and normal histology for 8 h or longer. Single units and evoked potential recordings were made from different layers of the cortex using micropipettes. The evoked potentials to electrical stimulation of differing intensity, frequency, and from different cortical layers were analyzed. Evoked potential from all but the most superficial layers of the cortex showed a characteristic 6-component response to stimulation of nearby white matter. This evoked potential closely resembled cortical responses recorded in vivo by other investigators following afferent stimulation. The response amplitude of all components increased as stimulus intensity was raised. Radial movement of the recording electrode showed that components 1-3 had their largest amplitudes in the deepest cortical layers, component 4 reached its greatest amplitude and shortest latency in layer IV, and components 5 and 6 reached their greatest amplitudes in layers IV to II. The frequency following for various components was measured showing greater decline in amplitude for components 4-6 than 1-3. This, together with the results of previous investigators, suggests that the first 3 components represent afferent fiber input, while component 4 represents the first cortical response (layer IV). Components 5 and 6 represent later, additional cortical responses. Further support for the intracortical origin of component 4 was provided by lateral intracortical stimulation within layer IV, giving an evoked potential composed mostly of component 4. With lateral movement of the recording electrode in layer IV the evoked potential disappeared in under 1 mm, suggesting a fairly restricted afferent input to the cortex. The present results encourage the use of the cortical brain slice preparation as an appropriate model system in which to study cortical neural circuitry.

Transplantation of embryonic occipital cortex to the tectal region of newborn rats: a light microscopic study of organization and connectivity of the transplants

Occipital cortex was taken from fetal rats and transplanted to the tectal region of newborn rats, where it developed a specific structural identity reflecting in part its cortical origin. The implants showed locally distributing intratransplant connections, and the majority formed connections with defined regions of te host cerebral cortex and the brainstem. A sparse afferent projection from the host had its origin in visual, somatosensory, and cingulate areas of te cortex, pretectum, superior colliculus, central gray, hypothalamus, pontine reticular formation, raphe nuclei, and the locus coeruleus. No input was identified from either the retina or the dorsal thalamus. Efferent fibers were observed in normal fiber preparations as compact bundles running through the host brainstem along two main routes, one group of bundles in a dorsal position and a second group more ventral. Efferent fibers traveling rostrally along the first pathway distributed in the lower part of the stratum griseum superficiale and in the intermediate laminae of the superior colliculus, and in some cases they reached the pretectum and the lateral posterior thalamic nucleus. Deep efferent fibers ran rostrally and caudally in the central gray, and in some cases laterally directed fibers were seen to distribute in the midbrain tegmentum and reticular formation, in one case reaching the pontine gray. The finding that most afferent and many efferent connections of cortical transplants are uncharacteristic of normal cortex stands in marked contrast to retina and tectum, which, when transplanted to the same region, make relatively normal connections.

Pigment-filled appendages of the small spiny neurons: a severe pathological change of the striatum in neuronal ceroid lipofuscinosis

In neuronal ceroid lipofuscinosis, the small spinous nerve cells of the striatum show a conspicuous pathological change in that they develop spindle-shaped and pigment-filled appendages of the soma. The axon emerges from the tip of these expansions, the volume of which often exceeds that of the cell body. The aspiny neurons of the striatum do not show this alteration. The pigment-filled expansions close to the axon may be considered an early sign of neuronal degeneration. The small spinous nerve cell is the predominant cell type of the striatum which receives inputs from various sources. Its slowly progressive destruction might account for extra-pyramidal motor disturbances in the course of neuronal ceroid lipofuscinosis.

The projection of the lateral geniculate nucleus to area 17 of the rat cerebral cortex. V. Degenerating axon terminals synapsing with Golgi impregnated neurons

The sites of termination of afferents from the lateral geniculate nucleus to layer IV and lower layer III in area 17 of the rat visual cortex have been determined by use of a combined degeneration--Golgi/EM technique. Degeneration of geniculocortical axon terminals was produced by making lesions in the lateral geniculate body. After the animals had been allowed to survive for two days, the ipsilateral visual cortex was removed and impregnated by the Golgi technique. Suitably impregnated neurons and their processes in layer IV and lower layer III were then gold-toned and deimpregnated for examination in the electron microscope. A search was made for synapses between degenerating axon terminals and the gold-labelled postsynaptic neurons. Geniculocortical synapses were found to involve: (1) the spines of basal dendrites as well as those of proximal shafts and collaterals of apical dendrites of layer III pyramidal neurons; (2) the spines of the apical dendritic shafts and collaterals of layer V pyramidal neurons; (3) the perikaryon and dendritic spines of a sparsely-spined stellate cell; and (4) the perikaryon and dendrites of a smooth, bitufted stellate cell. In view of this variety of postsynaptic elements it is suggested that all parts of the perikarya and dendrites of neurons contained in layer IV and lower layer III which are capable of forming asymmetric synapses can be postsynaptic to the thalamic input. Finally, an analysis of the known neuronal interrelations within the rat visual cortex is presented.

Pigmentoarchitectonic pathology of the isocortex in juvenile neuronal ceroid-lipofuscinosis: axonal enlargements in layer IIIab and cell loss in layer V

Pigment preparations of 800 micrometer thickness and Golgi studies of the isocortex in juvenile neuronal ceroid-lipofuscinosis, morphologically proven by electron microscopy, revealed: 1. giant axonal dilatations of IIIab-pyramids. These expansions exceeded by far those found in pyramidal cell axons of other isocortical layers. 2. severe numerical reduction of neurons in the ganglionic layer.

Interlaminar connections of tree shrew visual cortex

The intracortical projections of neurons in layers II and upper III of tree shrew visual cortex were studied after terminal lesions in the supragranular layers of area 17. Examination for terminal degeneration was made using ultrastructural techniques. The majority of degenerating terminals were found in layers V and, to a lesser extent, VI, and were presynaptic to neural profiles in the following distribution: 80.5% on spines of small to medium size dendrites, 19% on dendrite shafts, and less than 1% on neuronal perikarya. Degenerating axons coursed in vertical bundles through layers III, IV, V and VI. These findings are similar to those previously described in rat visual cortex.

Connections of striate cortex in the prosimian, Galago senegalensis

Efferent and afferent connections of primary visual cortex, Area 17, were determined in a prosimian, Galago senegalensis, by autoradiographic methods after injections of 3H-proline or 3H-HRP. The cortical connections of Area 17 with Areas 18 and MT were homotopic and reciprocal. Projections from Area 17 terminated largely in layer IV and somewhat in layer III of both Areas 18 and MT. Most of the cells projecting to Area 17 were located in layer V of Area 18 and layer VI of MT. Subcortical projections included the reticular nucleus of the thalamus, where columns of label corresponding to injection sites were found in the caudal fourth of the nucleus. Projections to the lateral geniculate nucleus were along lines of isorepresentation and were in register with the cells projecting back to the injection site. The parvocellular layers were less densely labeled than other layers by the transport of 3H-proline, while concentrations of label were noted on the dorsal and ventral margins of the nucleus and in interlaminar regions between the internal parvocellular and magnocellular layers and between the two magnocellular layers. The pattern of terminations in the pulvinar complex suggested functional subdivisions. We have divided the inferior pulvinar into a large central nucleus, IPc, with topologically organized input from Area 17; a smaller medial nucleus, IPm, with a second pattern of input from Area 17; and a dorso-posterior nucleus, IPp, without input from striate cortex. The superior pulvinar likewise appears to have several subdivisions. One of these, a "central" nucleus of the superior pulvinar, SPc, receives topologically organized projections from Area 17. SPc is about the same size as IPc and is organized as a mirror image of IPc. Thus, both IPc and SPc represent the lower visual quadrant medially and the upper visual quadrant laterally; central vision is represented along the common border for both nuclei, while peripheral vision is represented dorsorostrally in SPc and ventrocaudally in IPc. Finally, the superficial grey of the superior colliculus receives topologically organized input from Area 17.