epithalamus

The epithalamus is a small region of the diencephalon that forms the dorsal part of the thalamic area near the posterior roof of the third ventricle. Its main components are the pineal gland (epiphysis), the habenular nuclei, and associated tracts such as the stria medullaris thalami. The pineal gland is an endocrine organ that synthesizes melatonin from serotonin. Melatonin secretion follows a circadian rhythm influenced by light input relayed from the retina through the suprachiasmatic nucleus, with a role in sleep regulation and circadian timing; in some species it also participates in seasonal physiological changes. In humans, melatonin levels tend to rise at night, helping to modulate sleep-wake cycles.

The habenulae are paired nuclei that receive input from limbic and basal forebrain regions and project to brainstem areas via the fasciculus retroflexus, notably to the interpeduncular nucleus. Through these connections, the habenulae participate in processing reward, aversion, mood, and modulation of monoaminergic systems in the brain.

The stria medullaris thalami is a fiber tract that carries afferent signals to the habenula. The epithalamus lies near the posterior thalamus and is situated above the third ventricle, close to the posterior commissure.

Clinical relevance: lesions or tumors in the pineal region can cause Parinaud syndrome due to dorsal midbrain compression, leading to vertical gaze palsy and light-near dissociation. Pineal calcification is common with aging, and incidental pineal cysts or tumors may affect melatonin secretion.