Anatomy of the diencephalon

Our central nervous system is made up of the cerebrum, the cerebellum and the brainstem, which continues inferiorly with the spinal cord. The cerebrum consists of two cerebral hemispheres, which have an external cerebral cortex made up of nuclei which form the gray matter and deep to that, the white matter consisting of axons.

Embedded within the white matter, there are the basal ganglia, or basal nuclei. Removing them reveals a part of the brain hidden between the hemispheres, called the diencephalon. Together, the cerebrum and diencephalon form the forebrain, or prosencephalon. The diencephalon connects the cerebrum superiorly with the midbrain of the brainstem inferiorly.

On a mid-sagittal section through the brain, we can see the cavity of the third ventricle and the diencephalon around it. The two major parts of the diencephalon are the thalamus, which lies more dorsally, and the hypothalamus, which lies more ventrally.

There are actually two thalami, one on each side, flanking the lateral aspect of the space created by the third ventricle. Between the left and right thalami there’s a bridge of gray matter that connects them, called the interthalamic adhesion, or connection. The hypothalamus forms the inferior part of the lateral wall and the floor of the third ventricle. Between the thalamus and the hypothalamus there’s the hypothalamic sulcus, which separates them. The diencephalon contains two endocrine glands as well: the posterior lobe of the pituitary gland, below the hypothalamus, and the pineal gland, near the caudal end of the thalamus.

Okay, let’s take a closer look at the thalamus first, which is an egg-shaped structure made of gray matter that contains neuronal cell bodies. The thalamus is connected with almost all parts of the central nervous system, like the brainstem and the cerebral cortex, enabling it to influence many different processes in the brain. In fact, the thalamus is a part of almost every sensory pathway, where it serves as a major relay station that gathers, combines and processes afferent information before forwarding it to the cerebral cortex.