habenula

The habenula (diminutive of Latin , meaning "rein") is a small bilateral neuronal structure in the brain of vertebrates, that has also been called a microstructure since it is no bigger than a pea. The naming as "little rein" describes its elongated shape in the epithalamus, where it borders the third ventricle, and lies in front of the pineal gland.

Although it is a microstructure each habenular nucleus is divided into two distinct regions of nuclei, a medial habenula (MHb), and a lateral habenula (LHb) both having different neuronal populations, inputs, and outputs. The medial habenula can be subdivided into five subnuclei, the lateral habenula into four subnuclei. Research has shown morphological complexity in the MHb and LHb. Different inputs to the MHb are discriminated between the different subnuclei. In the two regions of nuclei there is a difference in gene expression giving different functions to each.