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.

This way the thalamus can recognize that there is a hot object in our hands, but without the cortex, it cannot process more detailed information, like the shape and weight of the object or compare it to previous experiences. The only sensory pathway that doesn’t relay through the thalamus is the olfactory system, which enables us to smell.

The thalamus plays a role in modulating movement through its connections with the basal ganglia, cerebellum and frontal lobe. It can also influence motivated behaviors via connections between the hypothalamus and the frontal lobe. The thalamus can even alter levels of consciousness by communicating with the reticular formation of the brainstem. Talk about multitasking!

Now, to better understand its relation to adjacent structures, let’s make a transverse section of the brain. Medial to the thalamus, there’s the lateral wall of the third ventricle. Anterior to the thalamus, there’s the interventricular foramen, or foramen of Monro, through which the cerebrospinal fluid, or CSF, flows from the lateral ventricles to the third ventricle. Lateral to the thalamus, there’s the posterior limb of the internal capsule, while the posterior part of the thalamus, called the pulvinar, is not covered by other structures and can be seen superior to the posterior aspect of the midbrain.

Now let’s switch to a coronal section of the brain. Here we can see that the dorsal surface of the thalamus is free, sitting under the lateral ventricle and the fornix, while ventrally there’s the tegmentum of the midbrain. Like on the transverse plane, the third ventricle lies medially and the internal capsule lies laterally.

Switching to the sagittal plane, once again, inside the third ventricle, we can see the medial surface of the thalamus and the interthalamic adhesion arising from it.

Now let’s take out the thalamus and zoom in on its superior surface. The first structure here is the internal medullary lamina, which is a layer of white matter that looks like the letter Y and divides the thalamus into three parts: medial, lateral and anterior. The gray matter of each part contains the various thalamic nuclei.

Some of the most important nuclei of the lateral part are the ventral posterolateral nucleus, the ventral posteromedial nucleus, and the ventral lateral nucleus. The ventral postero-lateral (VPL) nucleus receives input from the medial lemniscus and the spinothalamic tract and projects to the primary somatosensory cortex. In order to easily remember sensations that this nucleus transmits you can remember that for VPL: V in ventral stands for the vibration; P in posterior for pain, pressure and proprioception; L in lateral for the light touch; then just add the temperature.

The ventral postero-medial (VPM) nucleus receives input from the trigeminal and gustatory pathways and projects to the primary somatosensory cortex as well. To remember the sensations that this nucleus transmits, you can use the mnemonic: Makeup goes on the face. The M in makeup refers to the ventral postero-medial nucleus while the face refers to somatosensations from the face as well as taste.

The ventral lateral nucleus receives input from the cerebellum and the basal ganglia, and it projects to the motor and premotor regions of the cerebral cortex. This nucleus relays motor information and can influence movements.

The last two nuclei are the medial and lateral geniculate bodies. The medial geniculate body is a small bulge under the posterior end of the pulvinar. It receives input from the inferior colliculus via the inferior brachium and from the superior olivary complex, and then projects to the auditory cortex of the temporal lobe. To remember that it is involved in hearing, you can say the M in medial stands for music.

Lateral to the medial geniculate body is another small bulge called the lateral geniculate body. It receives input from the retina, via the optic nerve, optic chiasm and optic tract, and then projects to the primary visual cortex of the occipital lobe via the optic radiation. To remember that it transmits visual information, you can say the L in lateral stands for light.

Let’s take a short break and see if you can identify the main nuclei of the thalamus.

Good, now let’s switch gears and have a closer look at the hypothalamus. Even though it’s small, the hypothalamus is like a mastermind of the brain as it regulates homeostasis, which is the state of the body where conditions are optimal for internal processes to function properly. The hypothalamus achieves this by receiving various inputs, like visceral or somatic afferents, information related to the special senses, as well as input from the cerebral cortex and the limbic system.

Using neural, blood and CSF connections, the hypothalamus can regulate a number of processes, which you can remember using the TAN HATS mnemonic: thirst and water intake; endocrine organs and hormone secretion of the pituitary gland, which consists of the adenohypophysis and neurohypophysis; hunger and food intake; the autonomic nervous system; temperature; and sexual drive and emotional expression.

Now, just like the thalamus, the hypothalamus has many nuclei that serve different purposes. Two of them, namely the lateral and ventromedial nuclei, control appetite. Specifically, the Lateral nucleus serves as a hunger center, increasing appetite and food intake - to remember that, think about a yummy portion of lentil soup! The VentroMedial nucleus, on the other hand, serves as a satiety center, decreasing appetite and food intake, so you can think of a Voluptuous Model to remember it better!

The next two nuclei, called the anterior and posterior nuclei, control the temperature of the body. The Anterior nucleus serves as a Cooling center and uses the parasympathetic system to decrease body temperature by producing sweat and dilating blood vessels in the skin. Think about AC! The posterior nucleus, on the other hand, serves as a heating center that uses the sympathetic system to increase the temperature by constricting blood vessels in the skin, thereby decreasing sweat production and causing shivering. Think of a Hot Pot to remember this one!

Then, there is the suprachiasmatic nucleus, which receives input from the retina about the presence or absence of light and uses the information to regulate many biological circadian rhythms, like the sleep-wake cycle. Therefore, you can remember the suprachiasmatic nucleus as the Sun-censing nucleus.