Epithamalus
The epithalamus sits on the thalamus from behind. Two important structures of the epithalamus are the pineal gland and the area pretectalis. The pineal gland produces melatonin.
This is an important hormone in the mediation of the circadian rhythm and thus the sleep-wake rhythm. The area pretectalis plays a role in the switching of the pupillary reflex, i.e. the narrowing of the pupil when the light comes in. It receives information from the retina via the optic nerve and sends nerve fibers to a nerve cell nucleus (Nucleus Edinger-Westphal) whose neurons then lead to the activation of the muscle that causes the pupil to narrow (Musculus sphincter pupillae).
It is important that the nerve fibers that are supposed to “report” the incidence of light not only lead to the nucleus Edinger-Westphal (nucleus accessorius of the oculomotorius nerve) on the side where the light actually reaches the eye, but also to the nucleus on the opposite side. The incidence of light that is registered in one eye thus ultimately leads to a narrowing of the pupils of both eyes (consensual light reaction). Below the thalamus lies the subthalamus.
It belongs functionally to the basal ganglia, which are part of the cerebrum. It therefore plays a role in the coordination and fine tuning of movements. The basal ganglia are discussed in more detail below.
Hypothalamus
Under the subthalamus is the hypothalamus. It forms the floor of the 3rd ventricle (the thalamus forms its lateral boundary).It contains the pituitary gland and shows from the outside the Corpora mamillaria, which is located at the border between the hypothalamus and the midbrain. The optic nerve, the optic nerve and second cranial nerve, as well as the optic chiasm, the optic chiasma, are also considered to be part of the diencephalon.
The hypothalamus is, so to speak, the center or the highest station for the integration, processing and coordination of vegetative functions including the control of endocrine organs, i.e. those organs that secrete hormones. Thus, the hypothalamus is jointly responsible for processes such as respiration, circulation, body temperature, fluid and food intake behavior, reproductive behavior, sleeping and waking (circadian rhythm) and many others. Some of them will be discussed in more detail here.
The hypothalamus contains various core groups, each of which has its own functions. An important part of the hypothalamus is the pituitary gland. It lies – limited by bones – in the sella turcica (Turkish saddle), which borders on the sphenoid bone cavity.
This is the reason why surgical interventions on the pituitary gland are usually performed via the nose. The pituitary gland is divided into two parts. The posterior lobe of the pituitary (neurohypophysis) and the anterior lobe of the pituitary (adenohypophysis), which is not part of the central nervous system.
It does not consist of nerve tissue but of glandular tissue and is not part of the hypothalamus in the true sense of the word. The neurohypophysis produces the hormones vasopressin (also known as antidiuretic hormone = ADH) and oxytocin. Vasopressin plays a decisive role in the reabsorption of water in the kidney, and also leads to vasoconstriction.
This hormone is thus released when the hypothalamus registers that the body has too little water. In addition, a feeling of thirst is triggered so that drinking more water is added. Oxytocin is an important hormone in pregnant, birthing and breastfeeding women.
It causes the uterus to contract, i.e. induce labor during childbirth, and is responsible for the milk leakage during breastfeeding. The anterior lobe of the pituitary gland lies directly below the optic chiasma, so that tumors on the pituitary gland can lead to visual field defects. The adenohypophysis produces hormones that have an immense influence on the thyroid, adrenal glands, mammary gland, testicles or ovaries and growth.
They are controlled by the hypothalamus as the superior center. The hypothalamus therefore secretes hormones, which in turn ensure that the pituitary produces and releases hormones or not. The hormones of the thalamus, which have a stimulating or inhibiting effect on the hormone production of the adenohypophysis, are produced in the area of the tuber cinereum, another part of the hypothalamus.
They are called releasing hormones and, through their effect on the pituitary gland, have an effect on the metabolism of the thyroid gland and cortisol, among other things. The Corpora mamillaria, which also belong to the hypothalamus, have numerous connections with the hippocampus and thus play a role in behavior and memory. Clinical background: Numerous pathologies can occur due to disorders in the hypothalamus.
For example, two diseases are mentioned here. Central diabetes insipidus develops when the anterior lobe of the pituitary gland is damaged. The hormone ADH, which normally ensures the reabsorption of water in the kidney, is then missing.
As a result, the affected persons excrete up to 20 liters of urine per day (polyuria) and suffer from a constant strong feeling of thirst and drink large quantities (polydypsia). Another form of diabetes insipidus is the renal form (i.e. caused by the kidney). In this form, the anterior pituitary produces sufficient ADH, but the kidney lacks receptors that recognize and bind the hormone. Therefore, ADH is unable to exert its effect. Destruction of the Corpora mamillaria, which occurs mainly due to chronic alcohol abuse, leads to significant behavioral problems and pronounced memory impairment.