Diencephalon

What? Where?

The diencephalonA brain region that includes the thalamus, hypothalamus, and epithalamus. consists of the epithalamusA part of the diencephalon that includes the pineal gland and regulates sleep., the thalamusThe brain’s relay center, sending sensory information to the cerebral cortex., and the hypothalamusA small but vital brain region controlling hormones, temperature, and autonomic functions..  This roundish structure at the very center of the brain is the thalamus.  The epithalamus hangs of the back of the thalamus and the hypothalamus hangs off the front.  The thalamus and hypothalamus can be hard to locate because there aren’t distinct boundaries between them. 

Epithalamus

The epithalamus is also known as the pineal glandSmall brain structure that secretes melatonin..  In truth, this is an endocrine organ that releases a hormone rather than a nervous systemThe organ system that controls body functions using electrical and chemical signals. organ.  But, it is made of secretory neuronsThe functional cells of the nervous system that transmit signals., technically making it part of the nervous system.  The epithalamus releases melatoninRegulates sleep-wake cycles., which is a hormone that regulates your sleep-wake cycle, also called your circadian rhythm.  Melatonin is made from melaninA brown-black pigment made by melanocytes in the stratum basal and given to keratinocytes as melanos, as the name suggests.  This makes the pineal gland appear as a darkly stained area on images. Therefore, it is used as a landmark on MRI and CT scans. 

Does melatonin make your sleep.  Kinda.  It can, but it needs to be part of a routine. It should signal to your brain that you are ending your day.  This is why bedtime routines are important for kids AND adults.  A bedtime routine can trigger your pineal gland to release melatonin. This increases the chances that you will get to sleep more easily.  However, cortisolA glucocorticoid involved in stress response, metabolism, and immune regulation., a stress hormone also plays a part in your sleep wake cycle.  So, it’s not just about melatonin, but also about cortisol. 

There is a condition known as non-24 disorder.  This is a condition in which your body doesn’t play to a sleep wake cycle.  This can happen to people living in the arctic circle where, for like 9 months, the sun doesn’t set fully.  Blind people also have trouble maintaining a circadian rhythm.  I once read a paper about non-24. They housed some blind people at a very busy Manhattan hospital. It’s pretty much busy day and night there. 

Thalamus

The thalamus is called the gateway to the cerebrumThe largest part of the brain, responsible for thought, memory, and voluntary movements..  Not the gateway to the brain, but the gateway to the cerebrum, or the big brain hat.  Your cerebrum is the seat of your consciousness. The thalamus takes all incoming sensory information from the body and routes it to the correct place in the brain.  Incoming information from taste buds gets routed to the gustatory cortex.  Incoming info about body temperature gets routed to the hypothalamus. 

Let’s highlight the thalamus on all three sections of the brain here, starting on the sagittal section A cut or slice of the body or an organ for study. on the right.  This oblong structure is the thalamus.  On the transverse section in the middle, you can see the thalamus at the dead center of the cut.  On the coronal section of the brain, these little egg-shaped structures are the thalamus under the third ventricleA narrow cavity in the diencephalon that contains CSF..  Also note the choroid plexusStructures within the ventricles that produce cerebrospinal fluid (CSF). on the third ventricle.

The thalamus, along with a bunch of other areas of the brain is part of the reticular formationA network of neurons in the brainstem that regulate alertness and consciousness..  For right now, let’s say that the reticular formation is a bunch of neurons that keep you awake.  This formation determines if you are someone who can study in the cafeteria. It also decides if you need to study in a corner of the library.  This collection of neurons determines if you sleep through the firetrucks coming to save the burning house down the street. Alternatively, it determines if you sleep through it. Then your spouse tells you about it the next day at breakfast. 

Hypothalamus

The hypothalamus does a lot.  A LOT.  It is an organ that defies classification into a organ system.  This is because it is made of neurons but secretes hormones, just like the pineal gland.  As you can see here, there are many nucleiClusters of neurons in the CNS responsible for processing information. or cell bodies in the thalamus. Each one is responsible for monitoring and changing set points in the body.  You don’t need to know the activity of all these nuclei. You don’t even need to know their names.  What matters right now is that the hypothalamus monitors body temperature. It also monitors osmolarityA measure of solute concentration in fluid; affects fluid movement between compartments., which is the balance of salt and waterThe universal solvent essential for life.. 

If you are too hot or too cold, the hypothalamus will respond accordingly. It will make you sweat to cool down or shiver to get warmer. This will help reverse the change in body temperature.  The hypothalamus can sample the blood. It determines if you are dehydrated or over-watered. Yes, there is such a thing as water poisoning.  Parkinson’s is a disease that erodes the function of the hypothalamus.  Patients with Parkinson’s might say, “It doesn’t feel hot to me.” or they may present at the ER with horrible dehydrationA condition in which fluid loss exceeds intake, leading to a decrease in total body water. and say, “I don’t feel thirsty.”  This is because even though they are hot and dehydrated, they no longer feel that way. Their hypothalamus stopped monitoring these conditions.

The hypothalamus has to sample the blood to know the osmolarity or temperature. Therefore, the blood brain barrier is “thin” around the hypothalamus.  This just means that there is less protection around the hypothalamus.  When organs have a thin blood brain barrier, we call them circumventricular organs because they are circumventing the BBB.

Hypothalamus and the Limbic System

The hypothalamus is part of the limbic systemA group of brain structures responsible for emotions and memory.. This system is a collection of structures in the brain that work together on emotions and behaviors.  You can see some of the structures involved with the limbic system. These include the amygdalaA part of the limbic system involved in processing emotions, particularly fear and pleasure., hippocampusA structure in the temporal lobe essential for memory formation., and cingulate gyrus. However, not all of the structures are visible.  These organs work together to produce emotional outputs.  For example, my husband will leave the garbage in the kitchen overnight. The next day the smell will just be horrific in the morning. 

So, he will ask me to tie up the bag before he takes it out.  No way.  I’m not suffering an unholy scent because he didn’t take the garbage out.  So, my husband will start to gag as he walks to the garbage can.  Hasn’t even smelled it yet!  But his limbic system is taking over his judgement response and producing the gag reflex.  His frontal lobeThe brain region responsible for decision-making, personality, and voluntary movements. and limbic system are fighting.  His frontalForehead bone; forms the front part of the skull and roof of the orbits. Smooth and curved. lobe is saying, “Don’t puke in the garbage, it will only make it worse.”  His limbic system is saying, “BLAAAAARRRRGGGGGGH.”  Emotions versus logic, which will win?  This is what is going on with addiction behaviors.  Your frontal lobe is saying, “I can’t do this anymore.” but your limbic system is saying, “I want this.  I need this.  I’ve earned this.”