Anatomy of the diencephalon

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Introduction0:00–0:48

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.

Main components0:48–1:45

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.

Thalamus1:45–7:05

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.

Quiz7:05–7:14

Hypothalamus7:14–12:53

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. The next two nuclei, namely the supraoptic and paraventricular nuclei, are in charge of hormone production.
The supraoptic nucleus produces vasopressin, also known as the antidiuretic hormone, or ADH. This hormone causes constriction of blood vessels, increasing peripheral vascular resistance and it also makes the kidneys reabsorb more water from their tubules into the bloodstream.
These two actions ultimately increase blood pressure. The paraventricular nucleus, on the other hand, makes oxytocin, which stimulates contractions of the uterus while in labor, and later stimulates milk secretion while breastfeeding.
To remember these nuclei and their hormones you can use the SAD POX mnemonic, where S stands for supraoptic, AD for ADH, P for paraventricular and OX for oxytocin.
Lastly, there is the preoptic nucleus, which is involved in thermoregulation and sexual behavior. It also produces gonadotropin-releasing hormone that stimulates the adenohypophysis to release the follicle stimulating hormone, or FSH, and the luteinizing hormone, or LH, which are necessary for normal gonad functioning.
Okay, let’s wrap up the hypothalamus by making a mid-sagittal section through the brain. Remember that the hypothalamus creates the floor and the inferior part of the lateral walls of the third ventricle.
Superiorly is the thalamus, which is separated from the hypothalamus by the hypothalamic sulcus. Anteriorly, there’s the optic chiasm.
The hypothalamus also has a region called the preoptic area, which is located rostrally in the hypothalamus near the lamina terminalis, with the optic chiasm inferiorly and the anterior commissure superiorly.
Most inferiorly, there’s the tuber cinereum, which contains a small bump called the median eminence that connects with the stalk of the posterior lobe of the pituitary gland.
Posteriorly, the hypothalamus includes the mammillary bodies and spreads to the border of the interpeduncular fossa of the midbrain, with the floor of this fossa known as the posterior perforated substance.
The mammillary bodies are two rounded structures that are a part of the limbic system. They contain mammillary nuclei that receive input from the hippocampus via the fornix.
The mammillary nuclei then project to the anterior nuclei of the thalamus via the mammillothalamic tracts. There is also a connection between the mammillary bodies and the midbrain tegmentum via the mammillary peduncles.
On the ventral aspect of the brain, the mammillary bodies can be seen anterior to the posterior perforated substance, between the cerebral peduncles and posterior to the tuber cinereum.
Functionally, the mammillary bodies are believed to play a role in learning and memory. Let’s take another short break and see if you can identify the main nuclei of the hypothalamus.

Quiz12:53–13:02

Pituitary gland13:02–16:54

Ok now, let’s take a closer look at the pituitary gland, also known as the hypophysis, which is actually made up of two parts: the neurohypophysis and the adenohypophysis.
The neurohypophysis is the posterior part of the gland, which develops from the ventral side of the hypothalamus. It contains axons of the supraoptic and paraventricular nuclei, which carry ADH and oxytocin to the neurohypophysis to be stored.
When needed, the neurohypophysis releases these hormones directly into the bloodstream via its fenestrated capillaries. The anterior part is called the adenohypophysis and develops from Rathke’s pouch, which is an ectodermal diverticulum from the roof of the primitive oral cavity.
Unlike the neurohypophysis, the adenohypophysis is connected to the hypothalamus through a system of blood vessels called the hypophyseal portal system.
This system starts with the superior hypophyseal artery located on both sides of the gland, which is a branch of the internal carotid artery.
It enters the median eminence and forms the first capillary plexus. Blood from the first plexus is gathered into hypophyseal portal veins that enter the adenohypophysis and form the second capillary plexus.
Then the blood drains into the hypophyseal veins and joins the systemic circulation. The adenohypophysis makes hormones itself, but their release into the bloodstream is controlled by the hypothalamus.
The neurons of the hypothalamic nuclei produce releasing hormones and release-inhibiting hormones, pack them into granules and transport them via their axons to the median eminence where they release them into the first capillary plexus.
These hormones reach the second capillary plexus and instruct the adenohypophysis to increase or decrease hormone release.
The main hormones of the adenohypophysis are: growth hormone, or GH, that stimulates cell growth; thyrotropic or thyroid-stimulating hormone, TSH for short, that stimulate the thyroid gland; adrenocorticotropic hormone, or ACTH, that stimulates the adrenal glands; follicle stimulating hormone, or FSH, and luteinizing hormone, or LH, that stimulate the gonads; and the luteotropic hormone, or LTH, also known as the prolactin, that stimulates milk production and secretion.
On a midsagittal section of the brain, the pituitary gland sits in the middle of the sphenoid bone, in a depression called the sella turcica.
Superiorly, there’s the median eminence of the hypothalamus. Now, the posterior lobe of the pituitary gland has a stalk, called the infundibulum, that connects to the median eminence.
These three structures form the neurohypophysis. Anterior to the pituitary gland there’s the optic chiasm, while posterior to it, there are the mammillary bodies.
The pituitary gland is closely related to the cavernous sinuses, which almost enclose the pituitary gland on each side. Since this is the only visible part of the diencephalon, if we switch to the ventral side of the brain, we can see the infundibulum sitting between the optic chiasm, anteriorly, and the mammillary bodies, posteriorly.
Lastly, let’s cover the second endocrine gland of the diencephalon, which is the pineal gland, named so because of its pine cone shape.

Pineal gland16:54–18:12

The main function of the pineal gland is regulation of the sleep-wake cycle by production of melatonin, a hormone that helps us fall asleep.
It receives information from the retina about the presence or absence of light, which allows it to know if it is day or night.
Once it gets darker, the pineal gland starts releasing melatonin into the bloodstream and the CSF of the third ventricle, which prepares us for sleep.
It is also believed that the pineal gland can influence the hormone secretion of other endocrine glands, like the pituitary, parathyroid, endocrine pancreas, adrenal glands and gonads.
On a midsagittal section through the brain, the pineal gland has a central, midline position. It sits just caudal to the thalamus and is attached to it by a hollow stalk filled with CSF from the third ventricle.
The pineal gland protrudes posteriorly, lying superior to the superior colliculi. It is also a part of the central nervous system that doesn’t have the blood-brain barrier.

Review18:12–20:08

Alright, as a quick recap... The diencephalon is a part of the forebrain that sits between the cerebral hemispheres, connecting the cerebrum with the midbrain.
It contains the thalamus, dorsally, and the hypothalamus, ventrally. These form the lateral walls and floor of the third ventricle.
The thalamus serves as a relay station for almost all sensory pathways and afferent signals that are targeting the cerebral cortex.
Major nuclei of the thalamus are: ventral postero-lateral (VPL), which relays information from the spinothalamic tract and medial lemniscus; ventral postero-medial (VPM), which relays information from trigeminal and gustatory pathways; lateral geniculate body, which relays visual system information; medial geniculate body, which relays auditory information; and the ventral lateral nucleus, which relays information from motor systems.
The hypothalamus regulates homeostasis and controls hormone production of the pituitary gland. It has several nuclei: lateral and ventromedial nuclei that regulate appetite; anterior and posterior nuclei that regulate body temperature; suprachiasmatic nucleus that regulates circadian rhythms; supraoptic and paraventricular nuclei that produce ADH and oxytocin; and the preoptic nucleus that is related to thermoregulation and sexual behavior.
The pituitary gland contains the neurohypophysis, which is a part of the diencephalon and secretes ADH and oxytocin, and the adenohypophysis that develops from Rathke’s pouch.
It secretes hormones that influence other endocrine glands in the body. The pineal gland secretes melatonin and regulates the sleep-wake