The diencephalon is central to the human brain, just above the brainstem. It is divided into four main structures: thalamus, hypothalamus , epithalamus, and subthalamus.
The diencephalon is involved in many critical bodily functions, including coordinating with the endocrine system to release hormones, transmitting sensory and motor signals to the cerebral cortex, and regulating circadian rhythms (the sleep-wake cycle).
The diencephalon structures are also involved in regulating body temperature, appetite, and more. Dysfunction in any of these important parts of the human brain can lead to serious illness .
During fetal development, the diencephalon arises from the anterior neural tube vesicle, which also gives rise to the anterior and terminal brains. The diencephalon is divided into four structures: the thalamus, epithalamus, subthalamus, and hypothalamus, which form around the eighth week of pregnancy .
In adults, the diencephalon is located in the center of the brain, at the top of the brainstem, above the midbrain, and below the brain . It is part of the third ventricle of the brain. Together with the brain, the diencephalon is part of the forebrain. The optic nerve connects the eyes to various structures located in the diencephalon.
The thalamus looks like two paired "bulbs" consisting mainly of gray matter, which lie on the sides of the third ventricle of the brain. These bulbs are located at the apex of the subthalamus and are also associated with the epithalamus. The two parts of the thalamus are linked by an interthalamic bond.
Several nerve fibers connect the thalamus to the cerebral cortex. It receives blood through four arteries: the tuberothalamic artery, the paramedian artery, the thalamogenic artery, and the medial and lateral posterior choroidal artery .
The epithalamus forms the back of the diencephalon and is made up of many structures, including the gabenula, the brain line, and the pineal gland . Gabenules are connected to the diencephalon by a group of nerve fibers called the gabenular commissure.
The cerebral fringe is also a group of nerve fibers that connect the nuclei of the septum, the lateral preoptic region of the hypothalamus, and the anterior nuclei of the thalamus with the gabenula. The pineal gland is unpaired and is about 5-7 millimeters (mm) in size .
The subthalamus lies ventrally from the thalamus. The main structure within the subthalamus is the subthalamic nucleus, which is divided into three subsections: the dorsolateral motor territory, the ventromedial associative territory, and the medial limbic territory.
Other important structures within the subthalamus include the internal zone, the reticular nucleus, and the perigenicular nucleus.
The hypothalamus is a very small cone-shaped structure located under the thalamus. Despite its small size, it is one of the most important structures in the human brain. The hypothalamus ends with a pituitary pedicle (also called a pedicle funnel) that connects it to the pituitary gland .
The median eminence is a collection of neurosecretory cell nerve endings that run the entire length of the pituitary gland to the pituitary gland. Other adjacent structures include the mammillary bodies and the optic chiasm .
Each of the four structures located in the diencephalon plays an important role in the normal functioning of the brain, which we will discuss separately.
The thalamus also receives sensory and motor signals from the body and transmits this information to the cerebral cortex. It plays a crucial role in regulating awareness and alertness.
With the exception of smell, all of our sensory information is processed by the thalamus, including visual information from the retina , auditory information, pain, touch, and temperature. The thalamus is also associated with mood and motivation (via the limbic system ) and plays a role in motor language function and cognition .
The epithalamus contains the pineal gland, which secretes melatonin . Melatonin is responsible for the sleep-wake cycle (circadian rhythm) in humans. Neural pathways also connect the epithalamus with the limbic system and the basal ganglia .
Like other parts of the diencephalon, the subthalamus houses many groups of nerves that connect different parts of the brain. It is also the control center of the peripheral nervous system and connects the endocrine system with the nervous system and the limbic system. The subthalamus is made up of different parts, including:
- The uncertain zone, which stimulates the thalamus (responsible for smooth muscle, heart and gland function, focus, reflexes, and more)
- Reticular nucleus, responsible for the regulation of the thalamocortical pathway and consciousness.
- The perigenicular nucleus, which plays an important role in vision.
- Subthalamic nucleus responsible for somatic motor function
The hypothalamus is known to maintain homeostasis. Homeostasis is, in a sense, a state of balance, well-being, or proper functioning. This 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 (mainly through the pituitary gland). Regulates key hormones such as oxytocin and antidiuretic hormone (ADH) .
The wide range of bodily functions involved and the processes by which the hypothalamus achieves homeostasis is too complex to fully describe here, but some of the physiological functions directly associated with this important part of the brain include:
- Body temperature
- Emotions, behavior, memory.
- Cardiac rhtyms
- Regulate fluid and electrolyte levels in the body, which in turn supports functions such as blood pressure.
- Sexual drive
- Oxytocin plays an important role in labor and breastfeeding.
The hypothalamus communicates with the pituitary gland, which is often called the main gland. Signals 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.
Due to the wide range of physiological functions performed in the diencephalon, dysfunction due to a genetic condition, disease (such as injury or tumor), or trauma (such as lack of oxygen, stroke, or traumatic accident ) that occur anywhere. in this part of the brain it can be destructive.
In addition, any alteration in the components of the diencephalon and the neurotransmitters or hormones that regulate these regions can contribute to the development of mental illnesses such as schizophrenia or clinical depression.
While some related states may overlap, we will attempt to break down the potential effects and related conditions in the affected areas of the diencephalon.
Associated with thalamic dysfunction
The following conditions are associated with alteration of the thalamus:
Associated with epithalamic dysfunction
Any injury or problem that disrupts the proper functioning of the epithalamus, or especially the pineal gland, can lead to psychiatric problems such as mood disorders, problems with the sleep-wake cycle, and more. The following conditions are associated with impaired epithalamic function:
- Sleep disorders
- Tumors in the pineal gland can lead to hydrocephalus.
- Visual disability
- Parino syndrome
- Pineal calcification, which can be associated with Alzheimer's and migraines.
- Abnormal regulation of melatonin. which has been associated with neuropsychiatric disorders, including autism spectrum disorder and attention deficit hyperactivity disorder (ADHD )
Associated with dysfunction of the subthalamus
Some of the following conditions are associated with trauma or dysfunction of the subthalamus:
Associated with hypothalamic dysfunction
Due to the wide range of hormones regulated by the hypothalamus and the pituitary gland, there are many conditions or diseases that can result from brain injury or dysfunction of any kind in this area of the brain. They may include:
- Altered regulation of body temperature, appetite, or the sleep-wake cycle
- Uncontrolled eating and subsequent obesity (hypothalamic obesity)
- Suprarrenal insufficiency
- Hypothyroidism or other thyroid disorders.
- Decreased sex drive, inability to breastfeed, vaginal dryness, hypogonadism, or other problems with the regulation of sex hormones.
- Abnormal growth
- Diabetes insipidus
A wide range of tests are available to check brain function, including problems in the diencephalon. These may include, but are not limited to:
- Medical imaging tests, such as magnetic resonance imaging (MRI) to visualize parts of the brain.
- Blood hormone tests or levels of certain neurotransmitters in the blood to check the function of the hypothalamus, pituitary, or pineal gland, for example.
- An electroencephalogram (EEG) studies the electrical function of the brain and can identify things like seizures.
- Cognitive tests
- Genetic testing