The Diencephalon

The diencephalon is the one region of the adult brain that retains its name from embryologic development. The etymology of the word diencephalon translates to “through brain.” It is the

Figure 1. The Diencephalon. The diencephalon is composed primarily of the thalamus, hypothalamus and epithalamus, which together define the walls of the third ventricle. The thalami are two elongated, ovoid structures on either side of the midline that make contact in the middle. The hypothalamus is inferior and anterior to the thalamus, culminating in a sharp angle to which the pituitary gland is attached. The epithalamus is located posterior to the thalamus.

connection between the cerebrum and the rest of the nervous system, with one exception. The rest of the brain, the spinal cord, and the PNS all send information to the cerebrum through the diencephalon. Output from the cerebrum passes through the diencephalon. The single exception is the system associated with olfaction, or the sense of smell, which connects directly with the cerebrum. In the earliest vertebrate species, the cerebrum was not much more than olfactory bulbs that received peripheral information about the chemical environment (to call it smell in these organisms is imprecise because they lived in the ocean). The diencephalon is deep beneath the cerebrum and constitutes the walls of the third ventricle.  The three major regions of the diencephalon are the thalamus, the hypothalamus, and the epithalamus (Figure 1). There are other structures, such as the subthalamus, which includes the subthalamic nucleus that is part of the basal nuclei.

The cranial nerve associated with this area of the brain is the optic nerve (CN II) shown in Figure 3.  The optic chiasm, where some axons of the optic nerves decussate, can be seen on Figure 2.

Thalamus

The thalamus is a collection of nuclei that relay information between the cerebral cortex and the periphery, spinal cord, or brain stem. It has a left and right half, which are connected medially by the intermediate mass (Figure 2).

Sensory input to the thalamus comes from most of the special senses and ascending somatosensory tracts. Each sensory system is relayed through a particular nucleus in the thalamus. The thalamus is a required transfer point for most sensory tracts that reach the cerebral cortex, where conscious sensory perception begins. The one exception to this rule is the olfactory system. The olfactory tract axons from the olfactory bulb project directly to the cerebral cortex, along with the limbic system and hypothalamus.

The thalamus is a collection of several nuclei that can be categorized into three anatomical groups. White matter running through the thalamus defines the three major regions of the thalamus, which are an anterior nucleus, a medial nucleus, and a lateral group of nuclei. The anterior nucleus serves as a relay between the hypothalamus and the emotion and memory-producing limbic system. The medial nuclei serve as a relay for information from the limbic system and basal ganglia to the cerebral cortex. This allows memory creation during learning, but also determines alertness. The special and somatic senses connect to the lateral nuclei, where their information is relayed to the appropriate sensory cortex of the cerebrum.

The thalamus does not just pass the information on, it also processes that information. For example, the portion of the thalamus that receives visual information will influence what visual stimuli are important, or what receives attention.  In this way, the thalamus helps the cerebrum to focus on specific stimuli while ignoring others.

The cerebrum also sends information down to the thalamus, which usually communicates motor commands. This involves interactions with the cerebellum and other nuclei in the brain stem. The cerebrum interacts with the basal nuclei, which involves connections with the thalamus. The primary output of the basal nuclei is to the thalamus, which relays that output to the cerebral cortex. The cortex also sends information to the thalamus that will then influence the effects of the basal nuclei.

Hypothalamus

Figure 2. Sagittal view through the diencephalo and structures immediately superior to this region (septum pellucidum and corpous callosum).

Inferior and slightly anterior to the thalamus is the hypothalamus (Figure 1,2), the another major region of the diencephalon.  The hypothalamus is attached to the pituitary gland by a stalk called the

infundibulum.  Inferior to the hypothalamus are a pair of bumps which are the mammillary bodies.  While the function of the mammillary bodies is not well understood, it is possible that they play a role in memory formation.  The hypothalamus is a collection of nuclei that are largely involved in regulating homeostasis. Functions of the hypothalamus to this end are numerous, and include:

• Processing of signals from thermoreceptors, and the generation of responses to warm (shivering) or cool (sweating) the body.  For this reason, the hypothalamus is sometimes referred to as the body’s thermostat.
• Acts as the executive region in charge of the autonomic nervous system by exerting control over cardiovascular activities, exocrine gland secretions in the digestive tract and skin, and smooth muscle contractions in visceral organs.  This is achieved due to communication by the hypothalamus with brainstem regions that directly stimulate autonomic responses in the body.
• Regulates most of the hormone producing activity of the endocrine system through its control of the pituitary gland.
• Produces feelings of hunger and satiation by receiving signals from the stomach that indicate if the stomach is stretched and full, or if it is shrunken and empty.
• Generates feelings of thirst when osmoreceptors in the hypothalamus detect the concentration of solute in the blood as it flows past the receptors.  High solute levels indicate low water content in the blood and produce feelings of thirst, while low solute levels indicate more water which decreases the desire to drink.
• Working with the hippocampus to form new memories.
• Generating a physical reaction in the body due to emotional responses.  This occurs due to close communication with the limbic system of the cerebrum, where basic emotions are formed.  This is why an emotion such as anger creates immediate changes in blood pressure and heart rate.
• Controls sleep/wake circadian rhythm, so that one feels more alert and awake when exposed to light, and one experiences more feelings of sleepiness when in the dark.  This occurs due to exposure to light as a stimulus, as well as because of communication between the hypothalamus and the pineal gland of the epithalamus.
• Produces activity of reward pathways in the brain so that certain actions cause an experience of pleasure.  This acts to motivate activities such as eating, or engaging in sexual activity.

Figure 3. Superficial view of the diencephalon and brainstem.

Epithalamus

Posterior to the thalamus is the epithalamus (Figure 1,2), which is best known for being the region that possesses the pineal gland.  The pineal is a small lentil-shaped gland that secretes the hormone melatonin in response to signals from the hypothalamus.  When the hypothalamus receives signals that the body is detecting light, it inhibits the secretion of melatonin from the pineal gland and causes a feeling of wakefulness.  Low or no light exposure is also relayed to the hypothalamus, which then stimulates the pineal gland to secrete melatonin, which makes one feel sleepy.