John O'Kusky

Bio: John O'Kusky is an academic researcher from Laval University. The author has contributed to research in topics: Synapse & Cortex (anatomy). The author has an hindex of 3, co-authored 3 publications receiving 552 citations.

A laminar analysis of the number of neurons, glia, and synapses in the visual cortex (area 17) of adult macaque monkeys

Abstract: The number of neurons, glia, and synapses in the visual cortex of adult macaque monkeys has been estimated by stereological methods. The data is presented separately for 13 sublaminae. For the total cortical thickness, the average numerical density of neurons is approximately 120,000 per mm3 of tissue. This density increases and decreases in the different sublaminae in the direction that one would expect from the classical descriptions of Nissl-stained material (e.g., 137,000/mm3 in IVC alpha; 211,000/mm3 in IVC beta). There are about 200,000 neurons under 1 mm2 or cortical surface: 28% are in layers I-III, 45% are in layer IV, and 27% are in layers V and VI. The total number in area 17 of one hemisphere is close to 160,000,000. For the total cortical thickness the numerical density of synapses is 276,000,000 per mm3 of tissue. Laminae with higher neuronal densities tend to have higher synaptic densities but the correlation is not perfectly concordant. Moreover, the changes in synaptic densities are not as great as those in neuronal densities so that laminae with higher neuronal densities have a lower synapse to neuron ratio and laminae with lower neuronal densities have a higher synapse to neuron ratio. For the total cortical thickness this ratio is 2,300 synapses per neuron (1,400 in layer IVC beta and 2,800 in layers II and III). There are 478 million synapses under 1 mm2 cortical surface: 40% are in layers I-III, 35% are in layer IV, and 25% are in layers V and VI. The numerical densities of astrocytes, oligodendrocytes, and microcytes are 38,000, 17,000, and 4,000 per mm3, respectively. The overall glia/neuron ratio is 0.49.

Postnatal changes in the number of neurons and synapses in the visual cortex (area 17) of the macaque monkey: a stereological analysis in normal and monocularly deprived animals.

Abstract: The surface area of the striate cortex and the thickness of its laminae were measured in a series of newborn, 3-month, 6-month, and adult macaque monkeys. The numerical densities (Nv) of neurons and synapses were measured in individual laminae. The total numbers of neurons and synapses in the striate cortex of one hemisphere were derived from these measures. Normal monkeys were compared at 3 months and 6 months of age to animals having been reared from birth with a monocular eyelid suture. No significant differences were observed between normal and monocularly deprived monkeys. The perceptual deficits and physiological abnormalities that have been reported in monocularly deprived monkeys do not appear to result from a reduction in the amount of neural circuitry in the striate cortex. The combined data from these groups, however, demonstrated several developmental changes. Cortical thickness increased from birth to 6 months and diminished to near-newborn values in the adult. The 6-month cortex was 19% thicker than that of the adult. This overshoot was greatest in layers II and III, which were 43% thicker at 6 months. Cortical surface area demonstrated a similar trend, being 23% greater at 6 months, but the differences were not statistically significant. The Nv of neurons decreased from birth to 6 months and increased to near-newborn values in the adult. The 6-month Nv was 30% less than that of adults and the greatest changes were seen in layers II and III where the Nv was 38% less than adult values. The total number of neurons in the striate cortex of one hemisphere was 16% less in adults than in newborn animals, but statistically significant neuron losses were limited to layers I, II, IVC alpha, V, and VI. The Nv of synapses increased from birth to 6 months and diminished to near-newborn values in the adult. The 6-month overshoot was 34% for the total cortex and 41% for layers I-III. The total number of synapses in the striate cortex was 95% greater at 6 months than in the adult. In layers I-III the synapses were 130-155% more numerous at 6 months. These data demonstrate an increase in neuronal connectivity in the striate cortex from birth to 6 months, especially in layers I-III, and a subsequent decrease in the adult.

Postnatal changes in the number of astrocytes, oligodendrocytes, and microglia in the visual cortex (area 17) of the macaque monkey: A stereological analysis in normal and monocularly deprived animals

Abstract: The numerical densities (Nv) of astrocytes, oligodendrocytes, and microglia were measured in individual laminae of the striate cortex of macaque monkeys ranging in age from newborn to adult. Using measurements of cortical thickness and surface area, the total number of cells in the striate cortex of one hemisphere was derived for each glial cell type. Normal monkeys were compared at 3 months and 6 months of age to animals reared from birth with a monocular eyelid suture. No significant differences were observed between normal and monocularly deprived monkeys. The combined data from these groups, however, demonstrated several significant developmental changes. The Nv of astrocytes decreased from birth to 6 months of age and subsequently increased in the adult. The greatest changes were seen in the more superficial laminae. These changes, however, were only a response to a substantial overshoot in cortical volume at six months: the total number of astrocytes in the striate cortex did not change. There was a tenfold increase in both the Nv and the total number of oligodendrocytes from birth to maturity with a corresponding increase in the density of myelinated axons. The greatest changes were observed in the deeper laminae. The total number of microglia remained relatively constant from birth to 6 months of age. There was a 55% reduction in the number of microglia in the adult, although statistical analysis indicated that this decrease was only of borderline significance. The possible relationships between these postnatal changes in glial cell numbers and the development of neuronal connectivity are discussed.

Neuronal circuits of the neocortex

Abstract: We explore the extent to which neocortical circuits generalize, i.e., to what extent can neocortical neurons and the circuits they form be considered as canonical? We find that, as has long been suspected by cortical neuroanatomists, the same basic laminar and tangential organization of the excitatory neurons of the neocortex is evident wherever it has been sought. Similarly, the inhibitory neurons show characteristic morphology and patterns of connections throughout the neocortex. We offer a simple model of cortical processing that is consistent with the major features of cortical circuits: The superficial layer neurons within local patches of cortex, and within areas, cooperate to explore all possible interpretations of different cortical input and cooperatively select an interpretation consistent with their various cortical and subcortical inputs.

Interpreting the BOLD signal.

Abstract: The development of functional magnetic resonance imaging (fMRI) has brought together a broad community of scientists interested in measuring the neural basis of the human mind. Because fMRI signals are an indirect measure of neural activity, interpreting these signals to make deductions about the nervous system requires some understanding of the signaling mechanisms. We describe our current understanding of the causal relationships between neural activity and the blood-oxygen-level-dependent (BOLD) signal, and we review how these analyses have challenged some basic assumptions that have guided neuroscience. We conclude with a discussion of how to use the BOLD signal to make inferences about the neural signal.

Concurrent overproduction of synapses in diverse regions of the primate cerebral cortex.

Abstract: Synapses develop concurrently and at identical rates in different layers of the visual, somatosensory, motor, and prefrontal areas of the primate cerebral cortex This isochronic course of synaptogenesis in anatomically and functionally diverse regions indicates that the entire cerebral cortex develops as a whole and that the establishment of cell-to-cell communication in this structure may be orchestrated by a single genetic or humoral signal This is in contrast to the traditional view of hierarchical development of the cortical regions and provides new insight into the maturation of cortical functions