After my research this week, I have a much better understanding of how information enters and flows through the brain. In this brain dump, I will outline 2 main topics that help the reader understand the brain’s general structure and flow: turning physical stimulus such as sound or vision into electrical signals for our brain to process, and studying some key subcortical structures in the brain in the limbic system. First of all, it is key to understand the difference between gray and white matter. In terms of a neuron, gray matter makes neuron bodies (soma), synapses, dendrites, and axon terminals, while white matter makes up the actual axons, or nerve fibers, which connect different gray matter regions to each other. Think of gray matter as the train station, while white matter represents the train tracks. The actual electric signal would be the train itself. The gray matter is where high level processing is actually done in the brain, and where signals are transferred via the synapses, while the white matter merely connects distant gray matter regions together. The specific chemistry of white matter (incredibly high in lipid content) allows for electric signals to travel insanely fast between two gray matter areas of the brain, no matter the distance. The gray matter makes up the entire outer layer of the brain in the form of the cerebral cortex. As stated, the cerebral cortex is the outer layer of the brain, and consists of the well-known brain lobes: the frontal lobes, the occipital lobes, the parietal lobes, and the temporal lobes. Just as a sanity check, remember the brain is symmetrical, so there is a frontal lobe on the left side of the brain and there is a frontal lobe on the right side of the brain. The frontal lobe is located in the front of the brain and is responsible for higher level executive functions such as decision making and behavior regulation(prefrontal cortex is a big part of this), along with controlling movement (via the motor cortex). The temporal lobe is located on the sides of the brain and is responsible for auditory processing, understanding speech (via Wernicke’s area), and heavily involved in memory (via the hippocampus). The occipital lobe is located in the back of the brain and is responsible for visual processing. The parietal lobe is above the temporal lobe and behind the frontal lobe (essentially the ‘top’ of the brain) and is responsible for our spatial awareness and for our sense of touch (via somatosensory cortex). Underneath all of these lobes are a collection of white matter tracts that connect various gray matter regions of different lobes together. It is always important to remember that complex brain systems are rarely localized in one lobe, and usually various parts of multiple lobes make up a single system. A good example is the limbic system, which is our emotional regulation system. The limbic system involves parts of the temporal lobe and frontal lobe, and the white matter tracts underneath these lobes that connect these regions are key for this system to function. Now we know the general structure of the brain and generally what the different lobes do. But how is information read from our physical world via our senses such as sight or hearing, and then perfectly transferred to the proper lobe. The answer lies in the brainstem and its most important component: the thalamus. All our sensory information is first sent to the appropriate cranial nerve on the brain stem. There are 12 pairs of cranial nerves total, with each one taking a very specific input. For example, there is the optic nerve and the auditory nerve. After the signals enter the brainstem via the cranial nerves, all information except olfaction (interesting case for another paper) is sent to the thalamus, which is essentially the relay station of the brain that takes in sensory information from our sensory organs, processes/filters the information, and then projects this information to the proper region in the cerebral cortex. For each sense besides smell, there is a specific community of thalamic nuclei cells that specialize in processing that sensory information. To show a more complete example, I will discuss how sound is processed. Sound is the vibration of particles (could be gas, liquid, or solid particles) via a source. In simple terms, when a dog barks, the dog is the source, and our shout vibrates the air particles in front of us at a certain frequency. There are two main factors of sound: pitch (which is based on the frequency of which the particles vibrate) and loudness (which is based on the energy put into a sound; the amplitude/height of the air pressure sine waves). This vibration of air particles via the dog park enters our ear, which acts as a cave to push the sound waves in one direction: toward the eardrum. Once soundwaves hit the eardrum, it vibrates this eardrum membrane like a drum, which vibrates three little bones that connect the eardrum to the inner ear. The inner ear is a structure called the cochlea, which looks like a sea shell filled with ionic fluid. The last of the 3 tiny bones that are closest to the cochlea has a blunt end, and when the eardrum vibrates these bones, the last bone hits the cochlea with its blunt end. This causes the liquid particles in the cochlea to begin vibrating with the same frequency as the dog bark. Inside the cochlea is a thin membrane called the basilar membrane with specialized hair cells that are surrounded by the ionic fluid.. When the ionic fluid waves push against the hair cells, the hair cells vibrate and allow ionic solution to rush into their cell bodies, activating depolarization. It is incredibly important to note certain hair cells only vibrate at certain frequencies. So for example, a cat meowing and a dog barking would activate a completely different set of hair cells. Once the specific hair cells are activated, they send a signal to the brainstem via the auditory nerve, which is a huge white matter tract connecting the inner ear to the brainstem. Now, our auditory information is finally at the brain stem, where it is transferred to the thalamus. Remember the thalamus has specific nuclei for specific senses. Since we are discussing the sense of hearing, the auditory signals are transferred to the MGN (medial geniculate nucleus) of the thalamus. Here, unnecessary information is filtered out. For example, if the TV was playing the news for 3 hours and then the dog barked, our thalamus is going to filter the news so that the auditory information picked up from the news is weaker or simply not even allowed to pass through to the temporal lobe. After the information has been filtered, the thalamus uses white matter tracts to transfer this information, the dog’s bark, to the auditory cortex of the temporal lobe. Within the auditory cortex, we now perceive the sound as a ‘high pitch’ sound that is ‘loud’ (completely based on frequency and energy of particle vibrations). Now, we have a basic understanding of how our sensory organs change physical stimuli into electrical signals, and how these electric signals are first filtered, and then transferred to the appropriate cortical gray matter region to be processed. When we say cortical, it means that the information is going to one of the lobes in the cerebral cortex to be processed. To end, I want to talk a little bit about the limbic system. Note that this is an incredibly complex neural pathway and each component of the limbic system could have a 5 page write up. As such, I will be giving a brief overview of the limbic system and its components. The main function of the limbic system is regulating our emotions and motivations, particularly those that revolve around survival: fear, hunger, anger, and sexual drive. However, it is also heavily involved in higher executive functions, such as learning and memory. The main components of the limbic system are subcortical structures, which means they lay below the lobes of the cerebral cortex. They are all near the thalamus, and thus the brainstem. Since they are deep in the midbrain, all of these brain regions have many long white matter tracts connecting to the cerebral cortex and to each other. The actual components of the limbic system are as follows. The amygdala is a small almond shaped part of the brain that lies deep in the temporal lobes. It is known as the ‘emotional center’ of the brain. This is where fear conditioning and fear extinction occur. Essentially, this tells our bodys when a stimulus warrants a ‘fear’ response. If the amygdala is stimulated, a person will exhibit strong emotions of anger, fear, violence, and anxiety. This is what is meant by a ‘fear response. We are biologically designed to send a fear response based on certain stimuli such as pain. However, fear conditioning occurs when we start associating trivial stimuli, such as talking to people, with a fear response such as pain. This creates a neural pathway in our amygdala that evokes a fear response in our body when we experience the stimulus of talking to people. Fear extinction occurs when we strengthen a new neural pathway in the amygdala that sends an inhibitory signal to a fear response when we experience the stimulus of talking to people. Basically, once our fear extinction neural pathway becomes stronger than our fear conditioned pathway, we will no longer feel a fear response when the fear stimulus is experienced ( in our example, talking to people). This is just a quick overview of the amygdala. We will go further into this in another chapter. The hippocampus is a horseshoe-like structure deep in the temporal lobes near the amygdala which plays a key role in forming and storing new memories, and thus determining our behavior. Additionally, this is an area where neurogenesis occurs, even as adults. All of our episodic memories (which are our experiences) are encoded in the hippocampus. When we remember an experience, a specific neural pathway in the hippocampus is activated. This neural pathway has projections that activate certain parts of the cerebral cortex to make us feel as if we are experiencing that memory currently. For example, if you remember a fun camping experience on a cold night under the stars, this memory is encoded by a specific neural pathway in the hippocampus. This neural pathway in the hippocampus projects to the occipital lobe (seeing the stars) and the somatosensory cortex (feeling the cold). Both of these regions are in the cerebral cortex. While memory is far too complex to fully discuss right now and I don’t want to get into how we activate a memory, just know that studies have shown that once a memory pathway in the hippocampus has been activated, the hippocampus sends signals to various areas in the cerebral cortex. This shows us that subcortical structures use their white matter tracts to be in constant communication with the cerebral cortex. Interestingly, if the hippocampus is damaged, anterograde amnesia occurs, which means you remember everything before your injury, but you are no longer capable of making new episodic memories. The fornix is a C-shaped white matter tract that connects many parts of the limbic system. Most importantly, it connects the hippocampus to the hypothalamus. The hypothalamus is a small brain region that is below the thalamus and above the pituitary gland. Its main function is to link the nervous system to the endocrine system (hormone system in the body). Additionally, it regulates the autonomic nervous system, which are automatic processes we don’t think about such as heart rate, breathing, and digestion.The hypothalamus essentially tells your body when to release hormones, and how much of that hormone to release. The hypothalamus is connected to the pituitary gland, which is known as the ‘master gland’. This is due to the fact that the pituitary gland controls the activity of most other hormone-secreting glands. Because it controls hormones, it is also responsible for controlling our sleep cycle. The thalamus as we know, is the main relay station where sensory and motor information is sent/received to/from the cerebral cortex. We know that the thalamus is connected to almost every area of the brain. For the limbic system, it is incredibly important because it connects other parts of the limbic system to the cingulate gyrus Now let’s work through an example of a stimulus traveling throughout the brain. Let’s say we have a conditioned fear response to loud beeps, and we just heard a loud beep. This sound gets transformed into an electrical signal in our ear and gets sent to the thalamus on the brain stem. The thalamus reads this information and sends it to the auditory cortex of the temporal lobe where the sound wave information is processed. Because we have a strong memory associated with this beep, the auditory cortex sends this information back to the deep brain and into the hippocampus. The memory pathway for the loud beep associated with negativity is activated, and this activates the amygdala to elicit a fear response. Our hippocampus basically tells us that based on our past, this stimulus is bad news and that our body needs a fight or flight fear response. The fear pathway in the amygdala projects to the hypothalamus. The amygdala tells the hypothalamus that we are experiencing a dangerous stimulus, so the hypothalamus causes the release of hormones such as cortisol and epinephrine, which activates our sympathetic nervous system (our fight or flight response). This causes physiological changes such as sweating, increased heart rate, and decreased intestinal motility. Now, we have a better idea of the brain’s general structure and how different areas of the brain communicate with each other when we experience a stimulus.