The fasiculus retroflexus, along with the stria medullaris make up the 2 components that govern the input/ouput to the habenula through the dorsal diencephalic conduction system.
Author: Jordan A. Piper
Case Discussion
The fasciculus retroflexus (FR) is a bidirectional white matter tract that governs the major output from the habenula complex to midbrain regions. Unlike the stria medullaris, the fibers from the FR are predominantly efferent, and whilst the specific downstream nuclei influenced by the FR are somewhat unknown, it is generally agreed upon that it influences monoaminergic centers. Specifically, the outer region of the FR, which originates from the lateral division of the habenula, projects to the rostromedial tegmental nucleus (RMTg), whilst the inner region, which originates in the medial habenula, projects to the interpeduncular nucleus (IPN). On its way to the IPN, this tract courses the pretectal area ventrally before taking a significant 90-degree turn to enter the nucleus. It is this sharp turn that gave rise to the nomenclature "flexus".
Drug abuse, notably cocaine, damages the fasciculus retroflexus, impacting deep brain stimulation effectiveness in addiction treatment. Nicotine also harms this tract, highlighting potential alternative targets like IPN and RMTg for anti-addiction medication due to weakened connections with the habenula.
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