The diencephalon is a small but important part of the brain. It lies between the brainstem and the cerebrum of the brain and consists of three major components: the thalamus, hypothalamus, and epithalamus.
The thalamus (thal’-ah-mus) consists of two lateral masses of nervous tissue that are joined by a narrow isthmus of nervous tissue called the interthalamic adhesion. Sensory nerve impulses (except those for smell) coming from lower regions of the brain and the spinal cord are first received by the thalamus before being relayed to the cerebral cortex. The thalamus provides a general but nonspecific awareness of sensations such as pain, pressure, touch, and temperature. It seems to associate sensations with emotions but it is the cerebral cortex that interprets the precise sensation. The thalamus also serves as a relay station for communication between motor areas of the brain.
The hypothalamus (hi-po-thal i-ah-mus) is located inferior to the thalamus and anterior to the midbrain. It communicates with the thalamus, cerebrum, and other parts of the brain. The hypothalamus is the major integration center for the autonomic nervous system. In this role, it controls virtually all internal organs. The hypothalamus also is the connecting link between the brain and the endocrine system, which produces chemicals (hormones) that affect most cells in the body. This link results from hypothalamic control of the hypophysis, or pituitary gland, which is suspended from its inferior surface. Although it is small, the hypothalamus exerts a tremendous impact on body functions.
The primary function of the hypothalamus is the maintenance of homeostasis, and this is accomplished through its regulation of
- body temperature;
- mineral and water balance;
- appetite and digestive processes;
- heart rate and blood pressure;
- sleep and wakefulness;
- emotions; and
secretion of hormones by the pituitary gland.
The epithalamus (ep-i-thab-ah-mus; epi = above) is a small mass of tissue located superior and posterior to the thalamus forming part of the roof of the third ventricle. The major structure within the epithalamus is the pineal gland. The pineal gland is stimulated to produce a hormone called melatonin when sunlight levels become low during the evening and overnight hours. This hormone induces sleepiness to initiate the night component of a person’s day-night cycle and may assist in regulating the onset of puberty.
The thalamus and hypothalamus are associated with parts of the cerebral cortex and nuclei deep within the cerebrum to form a complex known as the limbic system. The limbic system is involved in memory and in emotions such as sadness, happiness, anger, and fear. It seems to regulate emotional behavior, especially behavior that enhances survival. Mood disorders, such as depression, are usually a result of malfunctions of the limbic system. It also is referred to as the “motivational system” because it provides our desire to carry out the commands created by the cerebrum.