Neuroscience
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Following a set of studies concerning the intrinsic electrophysiology of mammalian central neurons in relation to global brain function, we reach the following conclusions: (i) the main difference between wakefulness and paradoxical sleep lies in the weight given to sensory afferents in cognitive images; (ii) otherwise, wakefulness and paradoxical sleep are fundamentally equivalent brain states probably subserved by an intrinsic thalamo-cortical loop. From this assumption, we conclude that wakefulness is an intrinsic functional realm, modulated by sensory parameters. In support of this hypothesis, we review morphological studies of the thalamocortical system, which indicate that only a minor part of its connectivity is devoted to the transfer of direct sensory input. ⋯ These considerations lead us to challenge the traditional Jamesian view of brain function according to which consciousness is generated as an exclusive by-product of sensory input. Instead, we argue that consciousness is fundamentally a closed-loop property, in which the ability of cells to be intrinsically active plays a central role. We further discuss the importance of spatial and temporal mapping in the elaboration of cognitive and perceptual constructs.
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Comparative Study
Aspartate-like immunoreactivity in primary afferent neurons.
There is now good evidence that amino acids act as neurotransmitters in primary afferent neurons of dorsal root ganglia. Glutamate is the primary candidate for such a role, and there are reasons to believe that release of glutamate may be accompanied by the release of other neuroactive substances. Using immunocytochemical techniques, we have tested the hypothesis that some dorsal root ganglion neurons contain elevated levels of aspartate as well as glutamate. ⋯ The presence of high levels of aspartate in terminals located in the superficial laminae of the dorsal horn was verified by pre- and post-embedding immunocytochemistry with the electron microscope. Aspartate was demonstrated in scalloped terminals, including dark scalloped terminals believed to be associated with unmyelinated fibers of nociceptors. This evidence supports the hypothesis that aspartate as well as glutamate is present in the cell bodies and terminals of nociceptive primary afferents, and may be released by the terminals of these afferents to activate neurons in the superficial laminae of the dorsal horn.
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The efferent projections of the core and shell areas of the nucleus accumbens were studied with a combination of anterograde and retrograde tract-tracing methods, including Phaseolus vulgaris-leucoagglutinin, horseradish peroxidase and fluorescent tracers. Both the core and shell regions project to pallidal areas, i.e. ventral pallidum and entopeduncular nucleus, with a distinct topography in the sense that the core projection is located in the dorsolateral part of ventral pallidum, whereas the shell projects to the medial part of the subcommissural ventral pallidum. Both regions of the accumbens also project to mesencephalon with a bias for the core projection to innervate the substantia nigra-lateral mesencephalic tegmentum, and for the shell projection to reach primarily the ventral tegmental-paramedian tegmentum area. ⋯ The shell, however, has additional features that are reminiscent of the recently described extended amygdala [Alheid G. F. and Heimer L. (1988) Neuroscience 27, 1-39; de Olmos J. S. et al. (1985) In The Rat Nervous System, pp. 223-334]; in fact, the possibility exists that the shell represents a transitional zone that seems to characterize most of the fringes of the striatal complex, where it adjoins the extended amygdala.