Habenula

At a glance

English

Habenula

Latin (TA2)

Habenula

System

Brain and spinal cord

Region

Head

Side

Single, on the midline

About the habenula

The habenula (from the Latin for a little rein) is a pair of small cell masses in the epithalamus, the upper and back part of the diencephalon. The name first meant the pineal stalk, and only later passed to the nucleus that lies beside it. On each side the habenula forms a slight swelling, the habenular trigone, on the upper medial surface of the thalamus just in front of the pineal gland and above the posterior wall of the third ventricle. The two sides are joined across the midline by the habenular commissure, in the upper lamina of the pineal stalk.

In mammals each habenula divides into medial and lateral nuclei, and each of these into smaller subnuclei. Most of the input arrives through the stria medullaris thalami, a white band running back along the junction of the medial and upper surfaces of the thalamus, which ends in the habenula. The lateral nucleus receives from the lateral hypothalamus, the lateral preoptic area, the ventral pallidum and the inner pallidal segment. The medial nucleus is fed mainly by the septal nuclei, with further dopaminergic, noradrenergic and GABAergic fibres. Output leaves in the fasciculus retroflexus (habenulo-interpeduncular tract), which descends through the midbrain to end in the interpeduncular nucleus and neighbouring midline cell groups of the tegmentum.

Through those connections the habenula sits in a position to restrain or release almost every brainstem monoamine system. The lateral habenula projects to the dopamine cells of the ventral tegmental area and substantia nigra compacta, largely through an inhibitory relay, and to the serotonin cells of the raphe nuclei. Its neurons fire when an expected reward fails to arrive or when something unpleasant happens, so it acts as a signal of disappointment that teaches avoidance and shapes choice. The habenula also takes part in pain processing, sleep-wake timing, feeding and stress responses.

This 'anti-reward' role has drawn attention in psychiatry. Reduced habenular volume has been reported in depression, and in animal models excessive burst firing of the lateral habenula accompanies depressive behaviour; the anaesthetic ketamine, which has a rapid antidepressant effect, blocks that firing. Deep brain stimulation aimed at the stria medullaris has been tried in severe, treatment-resistant depression.

Function and supply

Function

Relays signals from the limbic forebrain and basal ganglia to the midbrain, regulating dopamine and serotonin release; involved in reward-prediction error, avoidance learning, pain, sleep and stress.

Blood supply

Medial posterior choroidal branches of the posterior cerebral artery.