Basic Circuitry In The Extratelencephalic Forebrain

The whole lateral wall of the rostral and caudal diencephalon is traversed by numerous transverse, longitudinal, and commissural fibers that interconnect the diverse prosomeric centers into interactive circuitry (Figs. 5A and 5B). These interconnecting fiber systems often reach the midbrain alar and basal plates (and extend from there into the rest of the brain stem), and they converge rostrally at the postoptic (so-called supraoptic) commissure or diverge into the telencephalic stalk. The commissures allow the left and right halves of the brain to interact for coordination of both analytical (alar) and motor/neurovegetative/neurohu-moral (basal) functions.

Another pervasive fiber system, the optic tract, is subpial and is found largely in the alar plate (there is also an accessory basal optic tract targeting specific terminal centers in the basal and alar parts of p1). The main optic tract courses longitudinally from the optic chiasm, where half of its fibers decussate (in man), through the prethalamus, thalamus, and pretectum and up to the midbrain roof (tectum)—the superior colliculus. Along its way, the optic tract gives out collaterals or terminal fibers to prethalamic areas (such as the suprachiasmatic nucleus) and to thalamic (ventral and lateral geniculate nuclei, intergeniculate leaflet, and pulvinar nucleus) and various pretectal centers receiving topographically ordered (retinoto-pic) projections (anterior pretectal nucleus and nucleus of the optic tract) or nonordered retinal projections (olivary pretectal nucleus). The retinorecipient posterior pretectal nucleus lies in the midbrain.

There are also many characteristic transverse fiber tracts normally coursing close to a given interproso-meric boundary (Fig. 5A), although other systems of transverse fibers course sheet-like throughout a given portion of the wall. Most of these fibers change course (i.e., become longitudinal) at a given decision point, generally coinciding with entrance into the basal plate; a few proceed into commissures across the roof or floor plate (mainly in pi and p3; Fig. 5B).

Given the specialization of alar centers in analytic signal decoding/locating and associated relay functions, and the specialization of the basal centers in motor patterns, longitudinal alar or basal fibers crossing diverse neuromeres tend to integrate separately such functions (bilaterally, due to the commissures). Conversely, fiber tracts coursing transversally convey the diverse segmentally analyzed (and longitudinally cross-correlated) alar outputs into the responding basal plate net of reticular and motor neurons. Much of our subconscious brain activity and reflex behavior depend on this subtle multimodal neuronal machinery for its precise situation- and aim-dependent adaptations. Note that this system, which is able to respond in a large extent to external and/or internal multimodal stimuli autonomously, is also controlled by the telencephalon for a higher degree of contextual integration, or volitional control, as indicated by separate descending telencephalic projections to many alar and basal plate centers.

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