The pyramidal neurons in layer III of cat primary auditory cortex (AI)

The neuronal architecture of pyramidal cells in layer III of the primary auditory cortex (AI) of adult cats was examined as a prelude to connectional and fine structural studies; in a further paper, the results of parallel studies of non-pyramidal layer III cells are presented. Layer III is about 400 micron thick, comprises about one-quarter of the thickness of AI, and lies some 400-800 micron deep to the pial surface. It is distinguished in Nissl, fiber, and Golgi preparations from layers II and IV, and also on connectional grounds, since its neurons are one of the principal inputs to the contralateral AI. Layer III may be divided into two roughly equal tiers on the basis of its neuronal and cytoarchitecture. Layer IIIa is populated by small cells with oval somata and many tiny pyramidal cells; the fiber architecture is dominated by radial bundles of medium-sized axons interspersed among columns of apical dendrites arising from deeper-lying pyramidal cells. In layer IIIb medium-sized and large pyramidal cells are more numerous, and the fiber architecture has a different, much denser texture, including extensive lateral components which invade layer IV, and large contingents of descending, probably corticofugal, axons. Five kinds of pyramidal neurons occur in Golgi preparations. Most numerous are the small, medium-sized, and large pyramidal cells; the two types of star pyramidal neurons are less common. The small pyramidal cell has a limited dendritic field and rather delicate dendrites; all but the apical one usually end in layer III. The medium-sized pyramidal cell is the most common neurons, and its rich basilar dendritic arbors are conspicuous, with their many dendritic appendages, in the layer III neuropil; their distal dendrites spread into layer IV. The largest pyramidal cells lie mainly in layer IIIb, and their lateral dendrites often mark the layer IIIb-IVa border. The apical dendrites of medium-sized and large pyramidal cells often extend to layer Ib, where they branch obliquely. The axons of these cells branch laterally after descending through layer III and toward the white matter. Often secondary or tertiary branches reascend to layer IV and more superficially; there is considerable stereotypy in this branching pattern. These numerous secondary branches contribute heavily to the layer IIIb-IVa lateral fiber plexus. The fourth variety of pyramidal cell has a round soma and a stellate dendritic field whose distal branches extend from layer V to layer I, but whose axon is chiefly in layer III. Finally, a star pyramidal cell with long lateral basilar arbors but rather smooth dendrites completes the picture.(ABSTRACT TRUNCATED AT 400 WORDS)     

Ultrastructural characteristics of callosal neurons in deep layers of the primary auditory cortex (AI) in cat]

An electron microscope study of retrogradely labelled nonpyramidal neurons has been carried out in layers V-VI of the primary auditory cortex (AI) after HRP injections into the contralateral AI of cats. From 2 to 9 synapses were usually revealed on somatic profiles of these callosal neurons. Synapses occupied 15.8 +/- 1.7% (on the average) of the somatic surface of these neurons. All of the revealed synapses on the somata of these callosal neurons had symmetric contacts and were formed by axon terminals with small elongated synaptic vesicles. An average length of these synaptic contacts in sections was 1.6 +/- 0.1 mm. HRP-labelled axon terminals of callosal fibres in layers V-VI contained round synaptic vesicles and formed asymmetric synapses on spines and dendrites. Possible functional significance of axo-somatic synapses in formation of impulsation patterns of the callosal neurons is discussed.     

Commissural neurons in layer III of cat primary auditory cortex (AI): pyramidal and non-pyramidal cell input

The types of layer III neurons in cat primary auditory cortex (AI) projecting to the contralateral AI were studied with horseradish peroxidase or horseradish peroxidase conjugated to wheat germ agglutinin. Injections between the anterior and posterior ectosylvian sulci retrogradely labeled both pyramidal and non-pyramidal somata in contralateral cortical layers III, V, and VI in AI, and in the ventral nucleus of the ipsilateral medial geniculate body. Three-quarters (72%) of the retrogradely labeled cells were found in layer III and one-quarter (28%) lay in layers V and VI. Every part of AI was innervated by commissural neurons. The topographical distribution of the labeled cells varied systematically. Injections in the caudal part of AI labeled cells in the caudal part of the opposite AI, while more rostral injections labeled cells in the contralateral, rostral AI. Injections covering the rostro-caudal extent of AI labeled cells throughout the opposite AI. Each part of AI thus projects most strongly to a contralateral, homotypic area, and less strongly to other, adjacent sectors of AI. The types of labeled cells were distinguished from one another on the basis of size, somatic and dendritic morphology, laminar distribution, and nuclear membrane morphology. Their somatodendritic profiles were compared to, and correlated with, those in Golgi-impregnated material from adult animals. Among the pyramidal cells of origin were small, medium-sized, and large neurons, and star pyramidal cells. The non-pyramidal cells of origin included bipolar and multipolar cells. Thus, at least six of the 12 kinds of neurons, as defined by morphological methods, participate in the interhemispheric pathway. Pyramidal cells comprised 65% of the cells of origin, 14% of the labeled cells in layer III were non-pyramidal, and 21% of the neurons could not be classified. It is unknown if these different types of commissural neurons have the same laminar or cytological targets in AI, or if they represent more than one functional or parallel pathway within AI. In any case, cytologically diverse layer III neurons contribute to the commissural system.     

The morphological characteristics of the callosal neurons of the first auditory area of the cortex (AI) in the cat]

Cortical stratification of callosal neurons in the primary auditory cortex (AI) of cat was studied by means of horseradish peroxidase (HRP). Two main groups of callosal neurons were revealed. The first group comprising 60% of all AI callosal neurons consisted predominantly of layer III large pyramidal neurons. Average area of these pyramidal neuron perikaryon profiles was 261.8 +/- 8.8 microns2. The number of HRP-labelled callosal neurons in layer III was 22% of all cells in this layer. The second group comprising 27% of all AI callosal neurons consisted mainly of large cells of layers V and VI which could not be classified as pyramidal neurons. Average area of these nonpyramidal neuron perikaryon profiles was 250.3 +/- 8.4 microns 2. In layer I callosal neurons were not revealed, in layers II and IV accordingly 6% and 7% of AI callosal neurons were located.     

Covariation of distributions of callosal cell bodies and callosal axon terminals in layer III of cat primary auditory cortex

Primary auditory cortex in the cat is both the source and target of callosal fibers. Injection of horseradish peroxidase (HRP) in the high frequency representation of AI in one hemisphere retrogradely labels callosal cell bodies and anterogradely labels callosal axon terminals in AI of the opposite hemisphere. In tissue sections cut through layer III parallel to the cortical surface, elongated patches composed of dense aggregates of callosal cell bodies and callosal axon terminals alternate with regions containing lower concentrations of these elements. Labeling in AI is most dense in regions corresponding to the frequency representation of the injected site. In layer III of the densely labeled region, patches of high concentrations of labeled callosal axon terminals correspond with high concentrations of labeled callosal cell bodies. On the other hand, little correspondence is apparent between the distributions of the two elements in layer III in the sorrounding area of lighter labeling. Layers V and VI contain relatively few labeled callosal axon terminals and cell bodies, and our data do not suggest whether the two distributions covary in these layers.     

The non-pyramidal cells in layer III of cat primary auditory cortex (AI)

The form and location of non-pyramidal neurons in layer III of the primary auditory cortex (AI) of adult cats is described in Golgi, Nissl, and other material. The cells were compared to the profiles of retrogradely labeled, commissurally interconnected cells. A principal finding is that certain non-pyramidal and pyramidal cells project interhemispherically to AI; a second conclusion is that the retrogradely labeled commissural cells form small clusters or narrow strips separated by unlabeled patches even after massive injections in the opposite AI. The non-pyramidal cells of origin have not yet been conclusively identified, but they must include one (or more) of the following six types of cells observed in Golgi-impregnated material: tufted or bitufted cells with a radially elongated dendritic arbor; sparsely spinous stellate neurons with thin, smooth dendrites and vertically disposed axonal branches; small stellate cells with varicose dendrites, a restricted dendritic field, and a profusely branched local axon; bipolar neurons with long, thin dendrites; medium-sized multipolar cells with radiating, sparsely branched dendrites; and small stellate neurons with smooth dendrites and a tiny dendritic field. These non-pyramidal cells are found throughout layer III but are more numerous in the upper part, layer IIIa, where they mingle with the small pyramidal neurons. As a rule the axonal branches of non-pyramidal cells are more numerous than those arising from layer III pyramidal neurons, and although they have many axonal collaterals, most project locally and vertically in narrow radial strips. In contrast, pyramidal cell axons have ascending and descending components which invade large, lateral territories in many cortical layers. Layer III non-pyramidal neurons are similar to those in layer IV in certain respects, although their dendritic fields are more spherical and less tufted than those of layer IV cells, and their axons have more local, limited targets. These axons appear to contribute but little to the conspicuous, lateral fiber striae in layer III. The primary intrinsic targets of non-pyramidal cell axons appear to be the apical dendrites of medium-sized and large layer III pyramidal cells, and recurrent branches to the parent cell; their fine, distal branches fortify the vertical plexus in layer III, and certain axons may descend into layer IV. Since layer III in AI receives both commissural and thalamic input, it is possible that these parallel, afferent channels are to some degree segregated, and to some degree convergent, onto particular types of cells.(ABSTRACT TRUNCATED AT 400 WORDS)     

Connections between pyramidal neurons in layer 5 of cat visual cortex (area 17)

The structural features of two physiologically-characterised pyramidal neurons (PC1 and PC2) closely situated in layer 5b in the visual cortex (area 17) of a single cat were studied using a combination of electrophysiological and anatomical techniques. Both PC1 and PC2 had exceptionally large somata (30-40 microns in diameter). On the basis of this and other morphological features cell PC1 was classified as a Meynert cell. PC1 possessed a very large (2.75 degrees X 4.50 degrees) binocularly driven standard complex receptive field. PC2 was also binocularly driven with a small, B-type receptive field. Both cells had the same preference for the direction and orientation of visual stimuli. PC1 and PC2 could be antidromically activated from stimulating electrodes positioned above the dorsal lateral geniculate nucleus with a response latency indicating that these cells probably innervated the visual tectum or pretectum. In addition to corticoefferent axons, the two neurons possessed extensive intracortical axon arbors that ramified extensively in layers 5 and 6 of the medial and lateral banks of the lateral gyrus in area 17. Axon collaterals from both PC1 and PC2 also innervated a small common target region in area 18. A total of 313 boutons from the axonal arbors of PC1 and PC2 were examined in the electron microscope. All of the identified synaptic junctions were found to establish Gray type 1 asymmetrical contacts. The combined ultrastructural data for both neurons indicated that 80% of boutons were onto dendritic spine heads, with 14%, 6%, and 1% onto small-, medium-, and large-calibre dendritic shafts, respectively. The spectrum of postsynaptic targets showed little variation with respect to lamina, distance from somata, or cortical area. Other large pyramidal neurons in layer 5 and spiny neurons in layer 6 were identified as receiving synaptic input from either PC1 or PC2. Using a computer graphics system, rotations of the bouton distributions revealed the existence of a clustered innervation of layers 5 and 6 in areas 17 and 18 derived from the two identified neurons. The bouton distributions strongly resembled the tangential pattern described previously for the functional slab-like organisation of the cortex. The results provide a morphological basis for the clustered intrinsic connectivity of pyramidal cells in layers 5 and 6 of the cat visual cortex. Furthermore, the results indicate the widespread excitatory influence of large pyramidal neurons on other cells projecting subcortically to sites dealing with visually guided behavior.     

The characteristics of the synaptic apparatus of the primary auditory area (A1) of the cat cerebral cortex

Quantitative and qualitative comparative studies of the synaptic apparatus in different layers of the primary auditory cortex (AI) in cat were performed using an electron microscope. The total average density of axonal terminal profiles in this area was 255 terminals per 1000 microns2 of the slice area. These profiles occupied 8.9% of the total studied area of slices. 45.3% of axonal terminals in area AI formed synapses on spines, 48.5%--on dendrites and 6.2%--on neuronal somata. 83.9% of synapses were asymmetric, 16.1%--symmetric. The number of synapses in 1 mm3 of the neural tissue in area AI estimated by stereological methods was 322.8 x 10(6).     

Structure of layer II in cat primary auditory cortex (AI)

The cytoarchitecture, myeloarchitecture, neuronal architecture, and intrinsic and laminar organization of layer II were studied in the primary auditory cortex (AI) of adult cats. The chief goal was to describe the different types of cells and axons to provide a framework for experimental studies of corticocortical connections or of neurons accumulating putative neurotransmitters. A further goal was to differentiate layer II from layer III. Layer II extends from 150-200 micron to about 400 micron beneath the pia and has two subparts. The superficial stratum, layer IIa, has many small, chiefly non-pyramidal neurons, primarily with round or oval perikarya, and a sparse, fine, and irregularly arranged axonal plexus. Layer IIb somata are larger and more densely packed and there is a more developed vertical and lateral axonal plexus. The border with layer III was marked by numerous large pyramidal cells with a thicker apical dendrite with more developed basal dendritic arbors than those of layer II pyramidal cells. Eight varieties of neurons were recognized in Golgi-impregnated material. These included small and medium-sized pyramidal cells, whose apical dendrites often ramified in layer I; bipolar and bitufted cells with polarized, sparse dendritic arbors; small smooth or sparsely spinous multipolar cells with radiating dendrites and small dendritic fields; spinous multipolar cells, whose large dendritic fields had more extensive apical than basal arbors; large sparsely spinous multipolar cells with smooth, robust apical dendrites; tufted multipolar cells with highly developed apical dendrites and some dendritic appendages; and extraverted multipolar cells with a broad, candelabra-shaped dendritic configuration, and with most dendrites oriented at right angles to the pia. The axons of the different cell types had the following general dispositions: those arising from the pyramidal cells could often be traced into the white matter but had many local branches as well; those of the other neurons had more or less extensive local axonal collateral systems and fewer branches which appeared to be corticofugal. However, the complete trajectory of the axons was not always impregnated in the adult material.(ABSTRACT TRUNCATED AT 400 WORDS)     

Binaural columns in the primary field (A1) of cat auditory cortex

Responses of single neurons and neuron clusters were studied at short intervals along penetrations into the high-frequency (4-25 kHz) representation of A1. The monaural and binaural sensitivities of neurons at best frequency were studied. With the exception of some neurons responsive only to binaural stimulation, the monaural responses of most were classified as contralateral dominant, ipsilateral dominant or equidominant. Binaural interactions of most neurons were classified as summation (binaural response size greater than the monaural response size) or suppression (dominant ear response size greater than the binaural response size). Most neurons arrayed in a column perpendicular to the cortical surface display the same aural dominance and binaural interaction. Summation columns occupy about two-thirds of the area sampled; suppression columns, about one-third. Within most suppression columns, the contralateral ear was dominant. Within summation columns, aural dominance varied. Summation columns appear to be composed of smaller columns differing in aural dominance. The sizes of binaural interaction columns vary considerably; some occupy several square millimeters of cortical surface. At least some binaural interaction columns occupy strips of cortex oriented orthogonal to isofrequency contours.     

Long-term potentiation and depression in layer III and V pyramidal neurons of the cat sensorimotor cortex in vitro

Synaptic plasticity of the cat sensorimotor cortex was examined intracellularly in vitro. After tetanic stimulation of the white matter, layer III and V pyramidal neurons showed long-term potentiation (LTP) of EPSPs in high incidence without GABA_A antagonist. The incidence and magnitude of LTP were very conspicuous in layer V cells. After an NMDA receptor antagonist application, the synaptic potentiation was blocked completely in layer III but not in layer V cells. Long-term depression (LTD) of the evoked EPSPs was also induced by the same stimulation in some layer III cells, where a transient hyperpolarization of the membrane potential was observed during tetanus.© 1997 Elsevier Science B.V. All rights reserved.     

Morphology of layer V pyramidal neurons in the cat somatosensory cortex: an intracellular HRP study

Pyramidal tract (PT) or corticopontine neurons of the cat somatosensory cortex (SI) were identified with antidromic activation on stimulation of the bulbar pyramid or pontine nuclei (PN) and stained intracellularly with HRP after examining the electrophysiological properties. Comparison of the conduction velocity of the stem axons and the soma-dendritic morphology revealed that in the cat SI, there exists two types of layer V pyramidal neurons, i.e. one has smooth apical dendrites with larger soma (51.6 +/- 9.5 x 22.7 +/- 2.8 micron) and the other has richly spinous apical dendrites with smaller soma (34.0 +/- 8.8 x 15.3 +/- 3.3 micron). The former group responded antidromically at latencies shorter than 1 ms by PT stimulation or 1.5 ms by PN stimulation, respectively. These values were consistent with the borderline latencies between two similar groups of layer V pyramidal neurons in the motor (fast and slow PTNs) and parietal (aspiny and spiny layer V corticopontine neurons) cortices in the cat.     

Sensitivity of cat primary auditory cortex (Al) neurons to the direction and rate of frequency modulation

Responses of 65 single auditory cortex (AI) neurons to frequency-modulated (FM) sweeps with different rates and direction of frequency change were examined quantitatively. Most units responded differentially depending on the characteristics of the FM sweep stimulus. Sixty-five percent of the units encountered responded at least twice as well for one direction of the FM sweep as for the other direction. Of these direction selective neurons, 67% preferred downward-directed FM sweeps (i.e. changing from high to low frequencies) while only 33% preferred upward-directed FM sweeps. The preference for downward-directed FM sweeps was especially clear in EI cells. In addition, cortical neurons often displayed sensitivity to the rate of frequency modulation (speed sensitivity).     

Corticocortical connections of cat primary auditory cortex (AI): laminar organization and identification of supragranular neurons projecting to area AII

The laminar distribution and structure of the supragranular cells projecting from primary auditory cortex (AI) to the second auditory cortex (AII) in the cat were studied with horseradish peroxidase. Injections in AII retrogradely labeled somata in ipsilateral cortical layers I-VI of AI. A bimodal laminar disposition was found, with approximately 40% of the labeled cells in layer III, 25% in layer V, and 10-15% each in layers II, IV, and VI; only a few cells were found in layer I. The labeled cells were scattered in small aggregates between which unlabeled neurons were interspersed. There was some, though not a strict, topographical distribution of the labeled cells according to the locus of the injection in AII. Injections in the caudal part of AII labeled cells in more rostral AI, while rostral AII injections labeled cells in more caudal AI. Ventral AII injections labeled more ventrally located AI cells, while more dorsal AII injections labeled more dorsally situated AI cells. AII injections also labeled cells in other auditory cortex subdivisions, including the posterior ectosylvian, ventroposterior, temporal, and dorsal auditory zone/suprasylvian fringe cortical areas, and in some non-auditory cortical areas. In layers II and III, both pyramidal and non-pyramidal cells were labeled. More pyramidal cells were labeled in layer III than layer II (80% vs. 62%), and the proportion of non-pyramidal cells in layer II was more than twice that in layer IV (27% vs. 12%). The types of labeled cells were distinguished from one another on the basis of size, somatic and dendritic shape, and laminar distribution. The profiles of labeled cells in these experiments were compared to, and correlated with, those in Golgi-impregnated material. In layer II, the classes of corticocortical projecting cells consisted of small and medium-sized pyramidal, bipolar, and multipolar cells. Those in layer III included small, medium-sized, and large pyramidal neurons, and bipolar and multipolar cells. The average somatic area of the labeled cells did not differ significantly from that of the unlabeled cells, and both were about equal in somatic size to neurons accumulating tritiated gamma-aminobutyric acid in layers II and III. These findings suggest that there is convergent, ipsilateral input onto AII from every layer in AI, and from other cortical auditory and non-auditory areas. A morphologically heterogeneous population of cells in AI contributes to these projections. Diversity in the cytological origins of corticocortical projections implies functional differences between layers II and III since the latter also projects commissural, while layer II in the cat, does not.     

Layer V in cat primary auditory cortex (AI): cellular architecture and identification of projection neurons

The cytoarchitectonic organization and the structure of layer V neuronal populations in cat primary auditory cortex (AI) were analyzed in Golgi, Nissl, immunocytochemical, and plastic-embedded preparations from mature specimens. The major cell types were characterized as a prelude to identifying their connections with the thalamus, midbrain, and cerebral cortex using axoplasmic transport methods. The goal was to describe the structure and connections of layer V neurons more fully. Layer V has three sublayers based on the types of neuron and their sublaminar projections. Four types of pyramidal and three kinds of nonpyramidal cells were present. Classic pyramidal cells had a long apical dendrite, robust basal arbors, and an axon with both local and corticofugal projections. Only the largest pyramidal cell apical dendrites reached the supragranular layers, and their somata were found mainly in layer Vb. Three types departed from the classic pattern; these were the star, fusiform, and inverted pyramidal neurons. Nonpyramidal cells ranged from large multipolar neurons with radiating dendrites, to Martinotti cells, with smooth dendrites and a primary trunk oriented toward the white matter. Many nonpyramidal cells were multipolar, of which three subtypes (large, medium, and small) were identified; bipolar and other types also were seen. Their axons formed local projections within layer V, often near pyramidal neurons. Several features distinguish layer V from other layers in AI. The largest pyramidal neurons were in layer V. Layer V neuronal diversity aligns it with layer VI (Prieto and Winer [1999] J. Comp. Neurol. 404:332--358), and it is consistent with the many connectional systems in layer V, each of which has specific sublaminar and neuronal origins. The infragranular layers are the source for several parallel descending systems. There were significant differences in somatic size among these projection neurons. This finding implies that diverse corticofugal roles in sensorimotor processing may require a correspondingly wide range of neuronal architecture.     

Synaptic processes in neurons of the cat pericruciate cortex evoked by pyramidal tract stimulation]

The influences of pyramidal tract stimulation on the activity of neurons in the pericruciate cortex were investigated on 423 neurons (81 neurons were studied intracellularly and 342--extracellularly), 78 of them having background activity. Pyramidal stimulation is shown to evoke not only antidromic spikes (0.5-16.0 ms latency) in the pyramidal cells, but also lateral and recurrent PSPs in the pyramidal and unidentified units of all cortical layers. IPSPs were observed in 46.7% of the investigated neurons, EPSPs--in 21.0%, mixed responces--in 26.0%. The latency of IPSPs was 1.5-14.0 ms, their amplitude ranged from 1.3 to 17.0 mV, the duration of the rising phase varied from 4 to 18 ms and the whole duration was 18-120 ms reaching sometimes 250-500 ms. In 30% of cases it was possible to divide the IPSPs into two phases: a fast one with a duration of 10-20 ms and a slow one. The latency of IPSPs was 2.6-19.0 ms, their amplitude--1.0--7.8 mV and duration--from 10.0 to 50.0 ms. The antidromic discharge in the pyramidal tract inhibited the background activity for 200-400 ms in 51.2% of spontaneously active units; acceleration was observed in 19.5% and mixed effect in 7.4% of units. The participation of pyramidal axonal collaterals and cortical interneurons in generation of the described processes is discussed.     

Quantitative analysis of lamina III pyramidal neurons in the parietal cortex of newborns]

The stage of neuronal development in the parietal (postcentral) cortex of human newborns was studied quantitatively in postmortem cases of different gestational (37th or 40th week) and postnatal age (2 hours or 3 weeks). In GOLGI-impregnated tissue obtained by autopsy, layer III pyramidal neurons were investigated. Comparatively, data were collected about dendrite parameters such as dendritic branching, length and spine density. The spine distribution (number and density) at the apical main dendrite, the apical tuft dendrites and in single basal dendritic fields as well, typically for the human layer III pyramidal neuron at the end of the gestational period, is described. Dendritic parameters from basal dendritic fields were compared in the three cases investigated (37th week with 2 hours or with 19 days of survival; 40th week of gestation with 2 days of survival). The large increase in spine number and density, especially in the second and higher orders of dendritic branching was obvious in the 19 days old infant brain. Special pathological aspects of the cases influencing the neuronal development are discussed. By means of a computerized method, quantitative data characterizing the human lamina III pyramidal neuron at the end of gestation are provided.     

Development of axonal arbors of layer 6 pyramidal neurons in ferret primary visual cortex

We followed the development of axonal arbors of layer 6 pyramidal neurons in ferret striate cortex to determine whether early developing axon collaterals are formed specifically in the correct target layers from the outset or achieve their adult specificity by the elimination of initially exuberant projections. These neurons were chosen for study because they are amongst the first to be generated in the developing ferret's striate cortex, and, in mature animals, these cells have axonal arbors that are highly specific for layer 4 and to a lesser extent layers 2/3 but have few collateral branches in layer 5. The axonal arbors of individual layer 6 pyramidal neurons were reconstructed following labeling in living slices prepared from the striate cortex of ferrets aged 13–35 days postnatal (P13–35). At the earliest ages (P13–15), axonal arbors consisted of a simple axon extending from the base of the cell body into the subplate or white matter and usually forming a few collateral branches but never ascending into layer 5. By P19–20, about one-half of the cells had extended axon collaterals into layer 5 or higher, and these already appeared to branch preferentially in layer 4. All of the cells from older animals had substantial axonal arbors in layers 2–4. By P26–28, there were approximately ten times as many axonal branches in layer 4 as in layer 5. Between P26–28 and P35, there was no significant change in the number of branches in layer 5, but the numbers of both branches and of axon collateral terminations in layer 4 approximately doubled. Thus, the extent of axonal arborization in layer 4 increases dramatically between P13 and P35, and growth is highly specific for correct target layers, with few branches formed in layer 5. © 1996 Wiley-Liss, Inc.     

Morphological features of layer V pyramidal neurons in the cat parietal cortex: an intracellular HRP study

Layer V pyramidal neurons in the cat parietal cortex (areas 5 and 7) were investigated with intracellular HRP staining. Antidromic responses were recorded intracellularly as well as extracellularly with pontine stimulation under Nembutal anesthesia. The relationship between the latency of antidromic responses and the morphology of HRP-stained neurons was analyzed. A total of 65 neurons were stained with HRP, and sixteen of these neurons were activated antidromically with pontine stimulation. Two distinct groups of layer V pyramidal neurons were detected morphologically by intracellular HRP staining; i.e., one (F type) consisted of neurons with relatively large somata (58.4 +/- 8.1 micron X 24.5 +/- 5.1 micron, N = 11) and aspiny or sparsely spinous apical dendrites, and the other (S type) consisted of neurons with smaller somata (44.6 +/- 7.6 micron X 19.3 +/- 3.9 micron, N = 22) and richly spinous apical dendrites. These two groups showed different electrophysiological properties; i.e., the former responded antidromically to pontine stimulation at a latency shorter than 1.5 ms (namely, with a conduction velocity faster than 18 m/second) and the latter responded at a latency longer than 1.5 ms. The two neuronal types in the parietal cortex corresponded respectively to fast and slow pyramidal tract neurons (PTNs) investigated in the sensorimotor cortex. Although their morphological features were almost similar to those of PTNs, the branching pattern of apical dendrites of the F-type pyramidal neuron seemed to be different from that of fast PTNs. In the parietal cortex, apical dendrites of F-type neurons showed rather frequent branching in layer I. This was similar to the pattern of branching in slow PTNs. Such a characteristic branching pattern suggested that, in the cat parietal cortex, layer V pyramidal neurons of both types are adapted to receive cerebellar inputs through the ventroanterior (VA) thalamic nucleus to the superficial cortical layers.     

Proposed inhibitory neurons of the auditory cortex in the cat

A complex of properties of inhibitory neurons in the auditory cortex is defined according to the peculiarities of IPSPs recorded in many cortical neurons. 12 units with such properties were found in a group consisting of 54 cells. These low-threshold units were localized predominantly in layers III-IV, had no background activity and responded to afferent volleys by short-latent barbiturate-resistant discharges (1-4 spikes).