The habenula (diminutive of Latin habena 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.

[1] The fasciculus retroflexus pathway between the habenula and the interpeduncular nucleus is one of the first major nerve tracts to form in the developing brain.

[6] The MHb shows a high level of nicotinic acetylcholine receptors (nAChRs), that are involved in many forms of addiction.

[7] The medial habenula (MHb) receives connections from posterior septum pellucidum and diagonal band of Broca; the lateral habenula receives afferents from the lateral hypothalamus, nucleus accumbens, internal globus pallidus, ventral pallidum, and diagonal band of Broca.

[8] Neurons in the lateral habenula are 'reward-negative' as they are activated by stimuli associated with unpleasant events, the absence of the reward or the presence of punishment especially when this is unpredictable.

[16] The medial habenula (MHb) receives connections from the posterior septum pellucidum and diagonal band of Broca.

The medial habenula sends outputs of glutamate, substance P and acetylcholine to the periaqueductal gray via the interpeduncular nucleus as well as to the pineal gland.

In many fishes and amphibians, the habenula on one side is significantly larger and better organized into distinct nuclei in the dorsal diencephalon than its smaller pair.

[19][8][20] In some fish (lampreys and teleosts), mitral cell (principal olfactory neurons) axons project exclusively to the right hemisphere of the habenula in an asymmetric manner.

[22][23] The habenular nuclei are involved in pain processing, reproductive behavior, nutrition, sleep-wake cycles, stress responses, and learning.

[26] Deep brain stimulation of the major afferent bundle (i.e., stria medullaris thalami) of the lateral habenula has been used for treatment of depression where it is severe, protracted and therapy-resistant.

[31] Recent exploration of the habenular nuclei has begun to associate the structure with an organism's current mood, feeling of motivation, and reward recognition.

[34] Therefore, researchers speculate that inhibition or damage to the LHb resulting in a failure to process such information may lead to random motivation behavior.

[38] nAChRs are made up a number of α and β subunits and are found in both the LHb and MHb, where research suggests they may play a key role in addiction and withdrawal behaviors.