Habenula

The habenula is a small, paired structure of the epithalamus, situated dorsomedially near the posterior end of the thalamus and closely related to the third ventricle and pineal gland. Each habenula contains medial (MHb) and lateral (LHb) habenular nuclei, which differ in their cytoarchitecture and connections.

The habenular nuclei lie deep to the habenular trigone, a small triangular elevation on the dorsomedial aspect of the epithalamic region. The stria medullaris thalami, an important afferent fibre bundle, approaches the habenula anteriorly and conveys fibres from forebrain and limbic regions to the habenular complex. The right and left habenular regions are connected across the midline by the habenular commissure.

The habenular commissure is closely related to the pineal stalk, occupying its superior part. The pineal gland projects posteriorly from the epithalamus, while the posterior commissure is related to the inferior part of the pineal stalk. Thus, the habenular commissure and posterior commissure form important superior and inferior relationships, respectively, of the pineal attachment.

Habenular connections

The medial habenula receives prominent input from the septal and diagonal band regions of the basal forebrain and projects predominantly to the interpeduncular nucleus. The lateral habenula has broader connections, receiving inputs from cortical, limbic, hypothalamic and basal-ganglia regions and projecting toward several brainstem and monoaminergic centres.

Efferent fibres from the habenula descend mainly in the fasciculus retroflexus. Medial habenular axons principally form its core, whereas lateral habenular axons occupy its outer shell and continue toward more widespread brainstem targets.

Functional correlates

Functionally, the habenula acts as a forebrain-to-brainstem relay involved particularly in reward and aversion, motivation, and affective behaviour.