The diencephalon is the region of the brain that sits on top of the brainstem, but below the telencephalon. It is not considered to be part of the brainstem, but it does look like the top of the popsicle stick. The reason that it is not part of the official "brainstem" is just that it is more deeply embedded within the brain. You will see in lab that the brainstem regions can be cut off from the forebrain as an independent unit; but the diencephalon cannot be eaily removed because of its deep investment in the brain.
There are four brain regions of the diencephalon that you should learn about. The first two are the most important diencephalic regions. Each will be described in more detail below.
In addition, all visual information enters the brain at the level of the diencephalon. You will see that in a bit more detail in the "cranial nerves" web page.
The thalamus is a large, oval-shaped pile of gray matter on each side of the brain. This large region is a relay station. The main type of information that gets relayed here is sensory information. All sensory information going to the brain (except for olfactory information) has to make a pit stop at the thalamus in order to be relayed appropriately.
By having all of our sensory information pass through the same brain region, we can filter out unnecessary sensory information and synchronize important sensory information.
The hypothalamus lies below ("hypo") the thalamus. For some reason, this is a difficult brain region for students to locate on models and sheep brains. The hypothalamus is an extremely important brain region because it helps us control all of our autonomic functions, those things that our bodies do involuntarily and regularly. So, for example, the hypothalamus helps us regulate our body temperature, hunger, hormonal output, and our sleep/wake cycle.
The sleep/wake cycle portion is kind of interesting. Our bodies have a circadian rhythm , which is another way of saying that we conform to a 24 hour day/night cycle. We wake up with the sunrise or after a regular number of sleeping hours. We can stay awake for the remainder of the day. We are most active at certain times of day. These things are regulated by an internal clock that lies within the hypothalamus (in a nucleus of the hypothalamus called the suprachiasmatic nucleus, but you don't have to know the name). The neurons in this circadian rhythm nucleus fire more at certain times of day than at other times. Certainly, their firing pattern feeds in a bit to the reticular formation that you have already learned about.
You will be able to see this little gland in lab. It is teeny, and sits just behind where the eyes are. It is thought to synchronize, or re-set, the circadian rhythm to the day/night cycle. For example, when you travel to California, it is three hours earlier there. You have to readjust your circadian rhythm to match the day/night cycle around you. The pineal gland is thought to help us with this adjustment.
This gland is a very odd, and very important gland. Only half of this gland is diencephalic; the other half is made from tissues that also made the roof of our mouths. So I have referred to the pituitary gland as "odd" because its structure (half diencephalic, half roof of the mouth) is odd. But, this gland is an extremely important part of our endocrine (hormonal) system. You will learn about the specific functions of the pituitary gland next semester when we study the endocrine system.
This gland hangs down off the brain from the diencephalon. It hangs like a ball on a string. This would seem to make the pituitary gland extremely delicate and susceptible to injury. However, the pituitary gland is protected by the sella turcica of the sphenoid bone. You will see this in lab.
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