The main input/output to the habenula is governed by the dorsal diencephalic conduction system, that is, the fasciculus retroflexus, and the stria medullaris. However the habenula also makes projections with the pineal gland via the habenulopineal tract and collateral projections with the medial forebrain bundle from typical reward processing areas.
Case Discussion
The habenula complex, comprising both medial and lateral divisions, is classified as a diencephalic structure within the epithalamus, along with the pineal gland. The primary input and output to this complex are regulated by a well-preserved pathway known as the dorsal diencephalic conduction system, encompassing both the stria medullaris and the fasciculus retroflexus.
Signals directed towards the lateral habenula mainly stem from the basal ganglia, basal forebrain, and septal area via the stria medullaris. These signals are then transmitted through the fasciculus retroflexus to the brainstem's monoaminergic nuclei. There is also evidence that suggests that collateral fibers from the stria medullaris merge with the medial forebrain bundle, forming secondary projections to the habenula. Collectively, these pathways oppose traditional reward circuits by exerting inhibitory control over monoaminergic neurons, likely serving as an area for anti-reward processing.
Overstimulation of the habenula, particularly its lateral division, leads to heightened inhibition of dopaminergic and serotonergic outflow to distant reward circuits. This phenomenon is believed to contribute to the pathophysiology of affective disorders and care should therefore be given to this structure when performing neurosurgical procedures.
It is also worth noting that the medial habenula makes projections to the pineal gland via the habenulopineal tract.
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