Thalamic Systems Intralaminar Nuclei ILN

The intralaminar nuclei are a group of midline systems that receive primarily gluta-matergic projections from the classical lateral reticular systems in the brainstem (see summary of RAS) and also from pontine cholinergic systems. ILN in turn sends primarily glutamatergic connections to specific layers of cortex (typically layers I and II), to the basal ganglia, and to the basal forebrain. The ILN has traditionally been conceptualized as an extension of the RAS and, along with the nRt, part of the nonspecific thalamus. The ILN includes both anterior and posterior groups of nuclei. Because of the complex connectivities of the ILN, these nuclei play a central role in cortical arousal, attention, intention, working memory, and sensorimotor integration, including gaze control, with gaze control being virtually paradigmatic for attentional control in visual animals. Schiff and Plum (2000) propose that anterior ILN groups perhaps have a greater role in working memory/sensory integration, and that conversely, posterior ILN groups likely play a key role in motor integration/gating of voluntary motor processes, and emotion.

ILN lesions can generate (depending on their severity) relatively brief coma, vegetative states, akinetic mutism, delirium, and often various kinds of dementia as an end state (see case studies). Extensive bilateral lesions of the ILN systems (if other components of the extended reticular activating system are relatively undamaged) show a bizarre and fascinating clinical course, as we have seen in the second case. This is a rare syndrome, but it underlines major theoretical challenges to the neuroscience of consciousness, suggesting that the ILN systems may provide integrative functions that neurodynamically link the extended brainstem reticular components and the thalamo-cortical mantle. These linkages appear to be essential for core consciousness. However, unspecified collateral systems are able to progressively acquire these functions over time, in a fashion that is still poorly understood.

Bilateral lesions of the ILN discussed in cases study 2 may be one clinical syndrome that more dramatically than any other underlines the extent of our fundamental ignorance regarding the complex integrative reticular-thalamocortical mechanisms foundational for consciousness. This syndrome, which shows a walk-through of all the major disorders of consciousness, suggests that the fundamental integrative mechanisms for consciousness are "writ large" throughout the brain's connectivities and functional neurodynamics in a fashion still not well mapped. This walk-through syndrome suggests that these fundamental integrative mechanisms cannot be neatly localized to any particular brain system. Rather, they are likely to be heavily instantiated in the extended reticular thalamic activating system (ERTAS) and other anatomically closely related systems. Lesser (or greater) disturbance of integrative mesodiencephalic neurodynamics produces a lesser (or greater) disorder of consciousness, from coma all the way to confusional states, and even including transitions to dementia and milder cognitive deficits. The walk-through syndrome argues for a graded and recursive conceptualization of consciousness, and not the intuitively more appealing all-or-nothing concept of consciousness. It suggests that consciousness involves a progressive or epigenetic layering of poorly understood integrative mechanisms, from its core constitutive elements through to its more extended cognitive aspects. No existing theory of consciousness in the scientific literature adequately explains this walk-through syndrome, and most theorists and researchers are not even aware of its now reasonably well-validated existence.

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