The epithalamus is part of the diencephalon and lies between the thalamus and the wall of the third ventricle. The epithalamus is thought to include the pineal gland or pineal gland, as well as the two “reins” and several connecting cords. It is certain that the pineal gland plays an important role in the control of the circadian rhythm, the day-night rhythm. Through certain structures, the epithalamus is connected to the olfactory centers and to the visual pathway. Certain reflexes such as the pupillary reflex, salivary reflex, and others are most likely controlled by the epithalamus.
What is the epithalamus?
The epithalamus is part of the diencephalon and is located between the thalamus and the third ventricle of the brain. It is a small structure that contains the pineal gland (epiphysis), several connecting tracts (commissures), the reins (habenulae), and the pretectum (area pretectalis), which receives information from the retina via nerve fibers and controls the pupillary reflex. The reins provide the connection to the olfactory brain and the brain stem and receive information from there to control the salivary reflex. The smell of good-smelling food causes salivation to be stimulated and other physiological preparations of the digestive tract for food intake to occur. The pineal gland, which can also be counted among the endocrine glands, is the part of the epithalamus that controls the circadian rhythm, the day-night rhythm, through the synthesis of the control hormone melatonin. Through a very complex system of signal transmission and signal processing, the pineal gland receives light stimuli and other information from the retina of the eyes, which is incorporated into the control of the circadian rhythm.
Anatomy and structure
As a component of the diencephalon, the following structures are attributed to the epithalamus: The pineal gland, also called the pineal gland, the habenulae (reins), the subcommissural organ, the commissura posterior, and the neuronal core areas of the habenulae and area pretectalis. The habenulae are not composed of nerve fiber strands, but of a collection of neuronal nuclei, which means that they are not only concerned with incoming or outgoing nerve signals, but also with the processing of the signals, that is, they are concerned with unconscious decisions for certain regulatory circuits and reflexes. The nucleus accumulations in the bridles most likely form the circuitry between the brain stem and the olfactory centers, so that when “nutritious” odors arrive, a complex preparatory cascade for food intake can be started. The pineal gland contains a large amount of hormone-producing pinealocytes, which are honeycombed by connective tissue cells. Glial cells are present to support the tissue. For the functional control of hormone production as part of the circadian control of many body functions, the pineal gland has a corresponding number of nerve fibers.
Function and tasks
Although some subtasks and functions of the epithalamus are known, especially those of the pineal gland, there still remains a wide field of research that gives hope for further insights into the function and tasks of the epithalamus and its structures. It seems to be assured that the epithalamus in one of its tasks acts as a switching point between the olfactory center (olfactory brain) in the brain stem and the epiphysis, whereby the epiphysis is considered by most authors as part of the epithalamus. This particular function is not only about the salivary reflex, which causes saliva production to be stimulated when pleasant cooking odors are perceived, but it is about other complex preparations of the body for the ingestion of certain foods. The physiological preparation of the body includes, among other things, a specific stimulation of acid production and also of insulin synthesis if the ingested food scent suggests easily digestible carbohydrates. The pineal gland plays an important role in the circadian rhythm and orients itself on the one hand to internal body clocks and on the other hand to the day-night change. At night – assuming darkness – the pineal gland produces hormones that convert the neurotransmitter serotonin into melatonin. Melatonin plays a major role in many physiological processes that are supposed to adjust the body to sleep. Blood pressure and heart rate decrease, the ability to concentrate decreases, and sleepiness sets in. The concentration of stress hormones also decreases, and a number of other physiological processes take place unconsciously in the body.Shift work or frequent changes of time zones can disrupt this regulatory mechanism to such an extent that physical symptoms occur in the long term. On long-haul flights, it has been common practice for some years to set the lighting in the cockpit to a certain brightness level (lux) when it gets dark in order to suppress melatonin production. People who spend only a short time in other time zones can try to maintain the rhythm of the time zone in which they normally stay, if possible. This supports a smooth reacclimation to the ancestral time zone and reduces jet lag symptoms.
Diseases
Diseases and symptoms directly related to the epithalamus are very rare. The most common problems occur from indirect disorders of the epithalamus, when tumors or hemorrhages in the brain exert mechanical pressure on structures of the epithalamus and the pineal gland. If the cause of the impairment can be corrected, the symptoms usually disappear on their own. In the rare cases where the pineal gland is directly affected, pineal cysts are the most common form of disease. These are benign cysts that form in the pineal gland. The disease is often accompanied by symptoms such as headache and nausea. Vision or balance problems may also occur. If the cysts reach a certain size, they can lead to the formation of hydrocephalus if left untreated due to a buildup of cerebrospinal fluid. A very rare tumor of the pineal gland that originates from the melatonin-producing cells is pinealoblastoma. A slightly more common tumor of the pineal gland is a germ cell tumor, which is usually benign (benign tumor) in women but predominantly malignant (malignant tumor) in men.
