Diencephalon

The diencephalon is a region of the brain located between the cerebral hemispheres and the midbrain, forming the central core of the forebrain (the prosencephalon). It comprises four connected structures: the thalamus, hypothalamus, epithalamus, and subthalamus. Developmentally, it arises from the embryonic diencephalon, a part of the neural tube that differentiates into these components and interfaces with both the brainstem and the cerebral cortex.

The thalamus is a paired, egg-shaped collection of nuclei that acts as the primary relay station for most sensory and motor information traveling to the cerebral cortex. It includes specific nuclei such as the lateral geniculate for visual input and the medial and ventral posterior nuclei for auditory and somatosensory signals, respectively. The hypothalamus lies ventral to the thalamus and regulates autonomic function and the endocrine system, chiefly via control of the pituitary gland. It governs homeostatic processes including temperature, thirst, hunger, sleep, and circadian rhythms, with centers such as the suprachiasmatic and arcuate nuclei playing prominent roles. The epithalamus contains the pineal gland, which secretes melatonin and participates in circadian regulation, and the habenular nuclei involved in reward and aversive processing. The subthalamus participates in motor control and is linked to the basal ganglia circuitry.

Functionally, the diencephalon integrates sensory information, coordinates autonomic and endocrine responses, and links the brainstem with the cerebral cortex. Vascular supply comes mainly from perforating branches of the posterior cerebral and internal carotid arteries, with venous drainage through internal cerebral and basal veins. Clinical lesions can produce sensory or motor deficits, autonomic disturbances, sleep and endocrine abnormalities, or thalamic pain syndromes.