Hi, guys. Here we're gonna continue our tour of the brain's regions by focusing on the limbic system. And this also involves a variety of 3A, 6A, 6B, 6C, and 7A topics. So the limbic system is not one region of the brain but a set of highly connected structures spanning the forebrain. It has many, many connections to other regions of the brain due to its major functions. Read full transcript
It is responsible for our feelings of emotion and for storing memories, which can involve many regions of the brain at once. Think about your last birthday party for example. You might be able to recall sights, sounds, the names of people there, the things you felt, maybe even tastes and smells. All of which are relayed to different areas of the brain.
The limbic system connects these far-flung areas so that knowledge and experiences can be stored and relived including the emotions associated with them. So when you see the limbic system think, emotion and memory. The limbic system has many different parts. More than you're really expected to know for the MCAT. I've highlighted some of the major parts you're expected to know here, here, here, and here.
So first the amygdala is a structure in the medial temporal lobe that is involved with associating stimuli with punishments and rewards. And it also associating memories and emotional reactions. These stimuli can be good or bad things. Potentially things we want to fight or get away from. Therefore the amygdala is heavily involved in aggression and fear responses as well as decision making involving fight or flight.
If you see a passage talking about fear, anger, aggression, and similar negative responses, it's likely it may ask about the amygdala. The prefrontal cortex in the frontal lobe is involved in restraining the amygdala, so damage to this area or hyperactivity in the amygdala itself can result in a person becoming hyper aggressive, anxious, or impulsive. Damage to the amygdala itself, however, can cause a lack of fear and a placid, calm temperament or flat affect.
This might sound like a good thing, but people with amygdala lesions can lose their sense of self-preservation because they don't fear consequences like normal people. The amygdala is also involved in sensing emotions in others. People who have amygdala lesions can lose the ability to discriminate emotion in facial expressions such as smiling or frowning.
Overall though, if you remember one thing, remember that the amygdala is involved in the emotions of fight or flight, like anger and fear, and it's also good to remember that the amygdala is involved association of stimuli with reward or punishment and with emotional memory. The hippocampus is a very important structure that's also in the medial temporal lobe that is key to learning and memory.
The hippocampus has the function of transferring things from a short term memory into long term memory. Anytime you see a passage involving long term memory, think hippocampus. Now note that the hippocampus is involved in forming new long term memories but it is not involved in retrieving old ones. So someone with damage to the hippocampus will be able to access previous memories but they wont be able to form new ones.
This is called anterograde which means a going forward amnesia. If youve ever seen the movie Memento, this is what the main character has. He's always writing notes to himself because he can't form new memories. This is different from retrograde or going backward amnesia, which is the classic sort of who am I, how did I get here amnesia, you might be more familiar with from movies and TV.
The MCAT might ask about certain diseases that can damage the hippocampus and cause anterograde amnesia. One of them is Korsakoff's syndrome. In this disease of vitamin B deficiency which is often caused by severe alcoholism leads to neural death in the hippocampus. Another disease is Alzheimer's disease.
In Alzheimer's disease, neural proteins join together abnormally and form plaques, which can clog up neurons and lead to cell death throughout the brain. Some of you who have aging grandparents may be familiar with this syndrome. Often they can clearly remember things that happened years ago but have trouble remembering what happened five minutes ago. This is due to anterograde amnesia caused by degeneration of the hippocampus.
Overall, if you can only remember one thing about the hippocampus, remember this. Anytime we make a memory or learn something new, the hippocampus will be involved. Now the thalamus is a structure in the forebrain that acts as a relay. All signals from the senses except for olfaction also known as smell, pass through the thalamus before they go wherever they're going in the brain.
So for example, tracts from the optic nerve, synapse in the thalamus, and then another tract carries the signal to the visual cortex in the occipital lobe. If you're curious, olfactory signals pass from the olfactory nerve, to the olfactory bulb, to an area near the hippocampus on the medial temporal lobe called the olfactory cortex. I've never seen this structure tested, but this helps explain conceptually why smells can trigger stronger emotional and memory responses than other senses.
Because olfaction has a much more direct connection to the limbic system. Some connections do go to the thalamus eventually. But the important concept is that olfaction never passes through the thalamus first, unlike the other senses. The thalamus is also involved in regulating sleep and wakefulness. The thalamus disconnects the higher cortical areas from input during sleep to keep them isolated from the world.
The thalamus also relays info between the hippocampus and cortex to help form long-term memories during sleep. Overall though, the major take with the thalamus is this. The thalamus is a relay for sensory data, except for smell. Now, the hypothalamus is a structure right below the thalamus, hence the name hypothalamus.
The hypothalamus is thought to be involved in pleasure, anger, and aversive responses. However, it is much more important to the expression of emotion than it is to the generation of emotion. Particularly the physiological components of emotion. The hypothalamus is also called the body's thermostat as it is involved in a variety of homeostatic functions that maintain the body in a healthy condition.
It helps regulate motivational drives like hunger, thirst, sleep and libido or sex drive. And also helps maintain biological homeostasis for things like body temperature, blood sugar, electrolyte balance, blood pressure, and the sleep cycle or circadian rhythm. The hypothalamus can do this because it integrates input from many different parts of the body.
And also has output to the autonomic nervous system which controls many of these functions as well as to the pituitary gland, which you can see is right next to it. The hypothalamus is intimately involved in control of endocrine functions because it communicates via the hypophyseal portal system with the pituitary gland, which is the master gland of the endocrine system.
A full discussion of this deserves its own video, so here's the main takeaway. The hypothalamus secretes a variety of releasing hormones into the primary bed of hypophyseal capillaries. Which are then carried through the hypophyseal portal vein to a second set of capillaries in the pituitary gland. This stimulates the pituitary gland to release a variety of pituitary hormones into the bloodstream, which then pass into the hypophyseal vein and then go to the rest of the body where they act on their target glands such as the thyroid for example.
Now, the entire pituitary has its own endocrine gland, the posterior portion on the other hand is actually composed of axons from the hypothalamus. So, the hypothalamus also produces oxytocin and vasopressin, also known as anti-diuretic hormone or ADH. ADH is released in response to high blood osmolarity and it increased water resorption in the kidney and constricts blood vessels in order to raise blood pressure.
The other hormone released by the posterior pituitary is oxytocin, which is involved in stimulating uterine contractions in childbirth, in lactation, and in feelings of bonding between people from physical affection. So this may all seem like a lot so here is your main takeaway for the hypothalamus. When you see homeostasis, think hypothalamus. The hypothalamus is the major endocrine control center of the body.
Because it controls the activity of the master gland, the pituitary. One last piece of the brain that's good to know are the basal ganglia. Now the basal ganglia are a set of several nuclei mostly located around the thalamus. And they're not exactly part of the limbic system but they have a lot of limbic connections so I'm including them in this lesson. The basal ganglia extend from the forebrain to an area in the midbrain called the substantia nigra.
As part of their limbic connections, they're involved in reward based learning and in habitual or repetitive activities. As well, they are involved in the selection and execution of motor commands. The basal ganglia can be thought of as a gate for various other critical functions. There are neural loops between the cortex, thalamus, and basal ganglia that are activated for example when you try to begin a movement.
The basal ganglia normally inhibit the major motor pathways. When you begin a movement, the basal ganglia will continue inhibiting unwanted movement and cease inhibiting thalamic pathways for the wanted movement. So, they open the gate, so to speak, for this impulse to pass. This function becomes damaged in Parkinson's disease. With the death of modulating dopaminergic neurons in the substantia nigra causes the basal ganglia to have problems with inhibiting unwanted movement as well as before.
At the same time, they also have difficulty opening the gate for wanted movements as well. This causes the Parkinson's symptoms of resting tremors, along with a symptomatic difficulty with making voluntary movements. Huntington's disease also relates to the basal ganglia. In Huntington's disease, the accumulation of mutant aggregates in the basal ganglia causes neurons in the inhibitory pathway to die, leading to excessive involuntary unwanted movements.
The basal ganglia have similar loops involving limbic and other cortical connections which are involved in emotion, reasoning, and decision making. Dopaminergic neurons in the ventral tegmental area or VTA of the basal ganglia are involved in pleasure, reward, and reward based learning along with goal directed behavior. So additive drugs often stimulate this area.
The basal ganglia's cortical connections also gate thoughts and impulses coming into the cortex that tell our brain we need to fix something in the environment like washing our hands or locking the door. Malfunctioning basal ganglia can therefore result in the nagging, intrusive desire to engage in habitual behaviors that we see in obsessive compulsive disorder also known as OCD.
The major things to remember for the basal ganglia are these. The basal ganglia are involved in gating movement, emotion, and habitual thoughts and behaviors along with rewards-based learning. So here's a question to test your understanding. Which of the following limbic structures is the least involved in learning and memory?
Pause the video for a second if you need to think about this. So the answer is B, the hypothalamus. The hypothalamus is mostly involved in homeostasis, and the physiological expression of the emotion like blushing. The hippocampus is the most important structure for memory, turning short term memories into long term ones.
The amygdala is involved in emotional learning and the basal ganglia is important in rewards-based learning. The amygdala is also involved in associating emotions with memories and associating a stimuli with rewards and punishments. So to sum things up, here's a table of each areas with the major functions listed.
Overall, just remember the limbic system is vital to motion and memory. The individual parts of a limbic system have many, many functions and we've only scratched the surface in this video. The limbic system also includes a couple regions we didn't cover in this video because they're very low yield. However, the MCAT is not a neuroscience test.
If you know the regions in this table, their basic functions, and what potentially goes wrong when they're damaged. You will have what you need to answer the things the MCAT would ask you. If they want you to know anything beyond that, they will give you the information in the passage.