The Anatomy of the Diencephalon

The diencephalon has a central location within the human brain sitting just above the brain stem. It is divided into four main structures—the thalamus, hypothalamus, epithalamus, and subthalamus.

The diencephalon is involved in many crucial bodily functions including coordinating with the endocrine system to release hormones, relaying sensory and motor signals to the cerebral cortex, and regulating circadian rhythms (the sleep wake cycle).

Structures that make up the diencephalon are also involved in regulating body temperature, appetite, and more. Dysfunction of any of these crucial parts of the human brain can result in serious health conditions.

Anatomy

During fetal development, the diencephalon arises from the anterior vesicle of the neural tube that also gives rise to the prosencephalon and telencephalon. The diencephalon is divided into four structures the thalamus, epithalamus, subthalamus, and the hypothalamus, which forms by the eighth week of gestation.

In adults, the diencephalon is centrally located within the brain sitting at the top of the brain stem above the midbrain and under the cerebrum. It is part of the third ventricle of the brain. Along with the cerebrum, the diencephalon is part of the forebrain. The optic nerve connects the eyes to several structures located within the diencephalon.

Thalamus

The thalamus looks like two paired "bulbs" consisting of mostly grey matter that sit on each side of the third ventricle of the brain. These bulbs sit atop the subthalamus and are also connected to the epithalamus. The two parts of the thalamus are connected via the interthalamic adhesion.

Multiple nerve fibers connect the thalamus to the cerebral cortex. It is supplied with blood via four arteries the tuberothalamic artery, the paramedian artery, the thalamogeniculate artery, and the medial and lateral posterior choroidal artery.

Epithalamus

The epithalamus makes up the posterior portion of the diencephalon and consists of multiple structures including the habenula, the stria medullaris, and the pineal gland. The habenula are connected to the diencephalon by a group of nerve fibers called the habenular commissure.

The stria medullaris is also a group of nerve fibers that connects the septal nuclei, lateral preoptic hypothalamic region, and anterior thalamic nuclei to the habenula. The pineal gland is unpaired and approximately 5-7 millimeters (mm) in size.

Subthalamus

Ventral to the thalamus is the subthalamus. The main structure within the subthalamus is the subthalamic nucleus which is divided into three subsections, the dorsolateral motor territory, ventromedial associative territory, and medial limbic territory.

Other important structures within the subthalamus include the zona incerta, the reticular nucleus, and the perigeniculate nucleus.

Hypothalamus

The hypothalamus is a very small cone-shaped structure that lies beneath the thalamus. Despite its small size, it is one of the most important structures in the human brain. The hypothalamus ends in the pituitary stalk (also called the infundibular stalk) which connects it to the pituitary gland.

The median eminence is a collection of nerve endings from neurosecretory cells that run the length of the pituitary stalk to the pituitary gland. Other adjacent structures include the mammillary bodies and the optic chiasm.

Function

Each of the four structures located within the diencephalon plays a critical role in normal brain functioning which we will discuss individually.

Thalamus

The thalamus also receives sensory and motor signals from the body and relays this information to the cerebral cortex. It plays a crucial role in regulating consciousness and alertness.

With the exception of the sense of smell, all of our sensory input is processed by the thalamus including visual input coming from the retina, auditory information, pain, touch and temperature. The thalamus is also connected to mood and motivation (via the limbic system) and plays a role in motor language function and cognition.

Epithalamus

The epithalamus contains the pineal gland which secretes melatonin. Melatonin is responsible for the sleep-wake cycle (circadian rhythm) in humans. Nerve pathways also connect the epithalamus with the limbic system and basal ganglia.

Subthalamus

Like other parts of the diencephalon the subthalamus is home to many groups of nerves that connect various parts of the brain. It also is a control center for the peripheral nervous system and connects the endocrine system with the nervous system and limbic system. The subthalamus has different parts including:

• The zona incerta which stimulates the thalamus (responsible for the function of smooth muscle, cardiac muscles, and glands, focus, reflexes, and more)
• The reticular nucleus which is responsible for the regulation of the thalamocortical pathway and consciousness
• The perigeniculate nucleus which plays an important role in vision
• The subthalamic nucleus which is responsible for somatic motor function

Hypothalamus

The hypothalamus is known for maintaining homeostasis. Homeostasis is a state of balance, wellness ,or proper functioning in a sense. It can be described as internal stability.

The hypothalamus maintains homeostasis by regulating parts of the autonomic and somatic nervous systems as well as the endocrine system (mostly through the pituitary gland). It regulates major hormones including oxytocin and antidiuretic hormone (ADH).

The wide range of bodily functions affected and processes by which the hypothalamus achieves homeostasis are too complex to completely describe here but some of the physiological functions directly related to this important part of the brain include:

• Body temperature
• Appetite
• Metabolism
• Emotions, behavior, memory
• Circadian rhythms
• Growth
• The regulation of fluid and electrolyte levels in the body, which in turn maintains functions such as blood pressure
• Sex drive
• Oxytocin plays an important role in childbirth and breastfeeding

The hypothalamus communicates with the pituitary gland which is often called the master gland. Signals coming from the hypothalamus cause other endocrine glands in the body to release vital hormones. For example, it signals the adrenal glands to release cortisone or the thyroid gland to release thyroid hormones.

Associated Conditions

Due to the wide range of physiological functions that take place in the diencephalon, dysfunction due to a genetic condition, disease (such as a lesion or tumor) or injury (lack of oxygen, stroke, or traumatic accident for example), occurring in any location of this part of the brain can be devastating.

Additionally any disruption to components of the diencephalon and the neurotransmitters or hormones these areas are involved in regulating may contribute to the development of psychiatric illnesses such as schizophrenia or clinical depression.

While some associated conditions may overlap we will attempt to break down potential effects and related conditions by area of the diencephalon affected.

Related to Dysfunction of the Thalamus

The following conditions involve a disorder of the thalamus:

• Movement disorders including Parkinson's disease
• Central pain syndrome (also called thalamic pain syndrome or Dejerine-Roussy syndrome)
• Aphasia (problems understanding or expressing speech)
• Fatal familial insomnia 
• Alcoholic Korsakoff syndrome
• Creutzfeldt-Jakob disease (causes changes to the posterior thalamus on magnetic resonance imaging)

Related to Dysfunction of the Epithalamus

Any injury or problem that impairs proper functioning of the epithalamus or especially the pineal gland can lead to psychiatric problems such as mood disorders, issues with the sleep-wake cycle, and more. The following conditions are associated with dysfunction of the epithalamus:

• Sleep disorders
• Tumors in the area of the pineal gland can lead to hydrocephalus
• Vision disturbances
• Parinaud syndrome
• Calcification of the pineal gland, which may be associated with Alzheimer's disease and migraine headaches
• Abnormal melatonin regulation. which has been linked to neuropsychiatric disorders including autism spectrum disorder and attention deficit hyperactivity disorder (ADHD)

Related to Dysfunction of the Subthalamus

Some of the following conditions are related to injury or dysfunction of the subthalamus:

• Movement disorders including tremors, dystonia, Parkinson's disease, myoclonus, and choreiform movements.
• Huntington's disease

Related to Dysfunction of the Hypothalamus

Due to the wide range of hormones regulated by the hypothalamus and pituitary gland there are many health conditions or diseases that can result from a brain injury or dysfunction of any kind in this area of the brain. These might include:

• Dysregulation of body temperature, appetite, or the sleep-wake cycle
• Uncontrolled eating and subsequent obesity (hypothalamic obesity)
• Adrenal insufficiency
• Hypothyroidism or other thyroid disorders
• Decreased sex drive, inability to lactate, vaginal dryness, hypogonadism or other problems related to sex hormone regulation
• Abnormal growth
• Diabetes insipidus

Tests

A wide variety of tests are available to test brain function including problems in the diencephalon. They may include but are not limited to:

• Medical imaging tests such as magnetic resonance imagine (MRI) to visualize parts of the brain
• Blood hormone tests or blood levels of certain neurotransmitters to test the function of the hypothalamus, pituitary gland, or pineal gland, for example
• Electroencephalogram (EEG) looks at the electrical function of the brain and can identify things like seizures
• Cognitive tests
• Genetic testing