During puberty, the brain, along with the body, matures fast, but there’s a specific set of systems that mature very dramatically called the corticostriatal system.
The corticostriatal system is made of two sets of brain regions; the prefrontal cortex, and the striatum. The prefrontal cortex, essentially the entire front third of the grey matter surface of your brain. The striatum is a cluster of grey matter nuclei near the middle. The two are connected by a series of white matter loops making them able to communicate back and forth (grey matter is formed from neuron cell bodies, white matter is the axons which connect them, axons are covered in a fatty substance called myelin, which is what makes it white in colour). In teenagers, these two regions develop asynchronously, so the striatum increases in activity first, during early and mid-adolescence, and then over time, the frontal regions and the white matter loops catch up.
The corticostriatal system, overall is responsible for a huge number of functions including movement, emotional regulation, value judgements, error monitoring, planning, motivation, self-control and working memory. One specific part of the striatum called the ventral striatum (also called the nucleus accumbens) specifically responds to rewards. When you see something desirable or rewarding, or something that might lead to a reward, activity in this nucleus will increase its activity. If you then don’t get the reward you expected, there will be a little dip in its activity. The ventral striatum is connected to (among other things) parts of the frontal lobe called the orbitofrontal cortex, which calculates the value of options and the dorsolateral prefrontal cortex, which exerts self-control over behaviour.
During adolescence teenagers become impulsive. Even very risk-averse teenagers are typically more impulsive and more prone to risk taking than they were as children or as they will be as adults.
This increase in impulsivity is believed to be due to the imbalance in maturity between the more active and mature striatum and the relatively immature prefrontal cortex. This is generally called the dual systems theory of adolescence. This is a very well established models which has provided testable hypotheses which have been supported by lots of studies, including in very large samples. It seems to be a very sound model of adolescent brain development, and the effect it has on adolescent behaviour.
Impulsivity is actually composed of a number of different behaviours. One element of impulsivity which is especially important to how we think about teenagers is reward sensitivity, which is, in essence, the degree to which the ventral striatum becomes active in response to a given reward. Teens become very reward sensitive, so rewarding, exciting things seem much more rewarding, while at the same time, the frontal regions which compare those risky, rewarding options to safer ones, are still underdeveloped and can’t compensate.
Impulsivity and increased reward sensitivity is considered to be a normal feature of adolescence, but its also often viewed as a dangerous one. Increased impulsivity increases a person’s risk for drug use, binge drinking, risky driving, unprotected sex and general propensity for danger. So its easy to approach reward sensitivity as something to be ameliorated where possible. Most studies either examine impulsivity in isolation in a laboratory setting, or look at risky behaviour specifically, since they’re a health concern. Accidents, made more likely by risk-taking behaviour, are the leading cause of death in teenagers.
But a collection of studies by Dr. Eva Telzer are expanding the dual-systems model by examining how adolescent behaviour is modified by the social context it occurs in. Research by Dr. Telzer’s group has found that risk-taking behaviour in teenagers changes based on the social context it occurs in and that the modulation seems to be driven by differences in how the brain responds to social rewards.
Social rewards are exactly what they sound like, a reward derived from social approval. Its important to remember when we talk about reward and the brain that the brain only has one reward system. The ventral striatum responds to all rewards, which covers everything from food, to achieving a goal, to helping someone, to taking cocaine. And increased reward sensitivity that teenagers experience applies to all forms of rewards. But also that people can still respond differently to different kinds of rewards.
Being sensitive to social rewards means that teenagers will take more risks when they are with their friends than with their mothers, but what is most interesting about this fairly intuitive finding is that they are also find less risky choices more rewarding. Having their mother present during a task choosing between safe or risky choices meant that teenagers also had a greater response in their ventral striatum when they make that safe choice.
What makes this even more interesting is that these social effects seem to extend beyond just whose present in any given moment. Teenager who generally have a closer relationship to their family or more supportive friends also take fewer risks and find risky behaviour less rewarding. And teens who find social rewards and helping others more rewarding seem to become less impulsive and at lower risk for depression over time.
These are all fairly new results, which mostly haven’t yet been replicated in another lab, so its somewhat early to declare this an established finding, but its very interesting for two reasons. The first is that provides a much more nuanced way of looking at teenager’s behaviour. Most studies of teenaged impulsivity looks at it entirely as a risk to be ameliorated or controlled, but increased reward sensitivity, especially social reward sensitivity can also an important mechanism for teens to develop independence and adult relationships and instead of being something dangerous to be suppressed. These findings, if they bear out in later research, suggests that they can be a mechanism for improving teen’s safety and mental health, if they’re allowed an appropriate environment.
More generally, this is just an interesting paradigm. Humans are social creatures and the social context we behave in is hugely important for understanding behaviour. So the brain, which exists to take in and respond to the environment responds to huge quantities of social information. But social neuroscience like this, which attempts to study how the brain uses and responds to social information is a very young field and its very hard to do wellj. So work like this adds an important and previously lacking dimension to how we study what goes into making decisions.
Telzer, E. H., Fuligni, A. J., Lieberman, M. D., & Galván, A. (2014). Neural sensitivity to eudaimonic and hedonic rewards differentially predict adolescent depressive symptoms over time. Proceedings of the National Academy of Sciences of the United States of America, 111(18), 6600–5. http://doi.org/10.1073/pnas.1323014111
Telzer, E. H. (2016). Dopaminergic reward sensitivity can promote adolescent health: A new perspective on the mechanism of ventral striatum activation. Developmental Cognitive Neuroscience, 17, 57–67. http://doi.org/10.1016/j.dcn.2015.10.010
Telzer, E. H., Fuligni, A. J., Lieberman, M. D., Miernicki, M. E., & Galv??n, A. (2013). The quality of adolescents peer relationships modulates neural sensitivity to risk taking. Social Cognitive and Affective Neuroscience, 10(3), 389–398. http://doi.org/10.1093/scan/nsu064
Telzer, E. H., Ichien, N. T., & Qu, Y. (2015). Mothers know best: Redirecting adolescent reward sensitivity toward safe behavior during risk taking. Social Cognitive and Affective Neuroscience, 10(10), 1383–1391. http://doi.org/10.1093/scan/nsv026
Any movie worth watching is worth analyzing, so a brief scene in Captain America: Civil War involving some plums has triggered a lot of discussion plums’ potential health benefits.
So, if your memory has been damaged by years of brainwashing can you actually improve it by eating plums?
Fortunately, two Australian scientists have systematically reviewed* the research of the health effects of plums and their findings were published just two months ago.
They did identify several studies where eating plum extract or powdered plums improved memory or cognitive skills.
So is that a yes?
Well, maybe. Every single one of those studies was done in rats or mice. They didn’t find any studies showing that plums improved cognition in humans.
We do studies in animals because they can tell us a lot more about what will happen in people than a dish full of human cells or a computer model, but they aren’t perfect. When something improves cognition in a rat or a mouse it suggests that it might do the same in a human, but it doesn’t mean that it will for sure. Studies about diet are especially difficult to generalize from, because lab rats eat precisely controlled diets with carefully measured components, while human diets are complicated and variable.
There’s a few more reasons not to get too excited about these studies. First off, none of them looked at actual plums, they used plum powder or plum extract, which is less variable in content than fresh plums, and easier to add to rat chow.
Incidentally, the researchers noted that the human studies they did find (which looked at different health effects of plums), tended to use either dried plums, or plum juice. There are very few studies on fresh plums. Dried, extracted, or juiced plums have slightly different nutrient contents than fresh ones. Some chemicals become more concentrated, and others tend to be lost during processing.
Secondly, the amounts of plums in the rat diets were comically enormous. In a lot of them, the plum component was 2-5% of their entire diet. Imagine putting all the food you eat in a day on a scale, then replacing 5% of that with dried plums and you’ll get some idea of how much plum these rats were eating. Way more plums than you would ever want to eat. Any effect that eating a normal number of plums would have would be much smaller.
Why would you do such an unrealistic study?
These studies aren’t really trying to mimic the effects of a human diet, even a very plum rich human diet at all. They’re simply asking the question can plums affect the brain conditions they were studying, at all, in any way? And when you just want to find that out its best to get the biggest effect you can. A small dose of plums might produce an effect so small that it couldn’t be detected, and that’s much less likely to happen with a giant dose. It also means that you can do a good study with fewer experimental animals, which is a good thing. This is due to a statistical concept called statistical power, in essence, the smaller an effect is the bigger the sample you need to detect it. If plums produce only a small change in brain function, you’d need to study a large group of rats to find it, but if the effect on the brain is large, you can find it with only a small number of rats.
And thirdly, these positive studies, even if they do translate to humans, are not suggesting that plums are magic brain-food. Each of the studies looked at a different condition, one found that plums extract could protect against the effects of a (very) high cholesterol diet. Another found plums improved cognition in elderly rats, and another in rats with diabetes.
What you can take from this variety of studies is that plums appear to protect the brain from various conditions that can impair it (called neuroprotection), not that they necessarily improve cognition or memory in and of themselves. And none of these studies suggest, or even look at, if they can reverse damage that has already occurred, which is really totally different.
But what about brainwashing? Could plums protect you from brainwashing?
Well, some of the sources of damage that happen when your brain is physically injured are the same as when your brain is damaged by aging or illness, so its not impossible that the same sources of neuroprotection might work too, but that would still only apply to damage that was still occurring, or had just occurred, not things that happened months or years ago.
Generalizing research from one illness to another is a little bit like generalizing between species. A finding in one case suggests that another study would be useful, but the only way to prove it, is to do the research and prove it.
*A review is a paper that compares and summarizes earlier studies to draw conclusions based on the totality of many studies, instead of just one. A systematic review is a review where papers are collected in a fixed way so the researchers won’t be biased in which papers they choose to include.
Igwe, E. O., & Charlton, K. E. (2016). A Systematic Review on the Health Effects of Plums ( Prunus domestica and Prunus salicina ). Phytotherapy Research, 30, 701–731.
TW for discussion of rape and trauma
Almost everyone has heard of the fight or flight response. When you get into a scary situation your sympathetic nervous system revs up and pumps out adrenaline and your body prepares to either fight of something or someone threatening you, or, more commonly, to run away at top speed.
The freeze response is a less well known counterpart to the classic fight or flight response, which is also called tonic immobility (TI). When an animal’s ability to fight or flee has been either blocked or exhausted the freeze response is evoked. The reflexive freeze response is believed to be a last-ditch attempt to escape a predator, because a lot of predators will drop prey that suddenly plays dead or goes limp. It is often studied in animals by immobilizing them, to simulate this. During TI animals will become very still and silent (the characteristic freeze) either stiffening up or going limp and tremble. But they also have the same sympathetic nervous response seen in the fight or flight reflex; dilated pupils, rapid heart-rate and breathing and reduced response to pain.
The freeze response has been described very thoroughly in animals, but less well in humans. In people, the freeze response was first described as “rape-induced paralysis” in victims of sexual assault. Because of this, the human freeze response has mostly been studied as a phenomenon associated with sexual assault, but it is actually a general part of the human fight or flight response and can occur in victims of all forms of trauma, including combat and other forms of interpersonal violence, natural disasters and accidents. The freeze response can be evoked in people, just like in animals in laboratory studies meant to model panic. Some people with panic disorders will also experience the symptoms of a freeze response during panic attacks.
When people experience a freeze response during trauma they also report feelings of panic and a second group of symptoms called peritraumatic dissociation which include feelings of unreality and being detached from your body. Dissociation, freeze response and panic are all closely correlated, but panic and dissociation can only be studied in people (a rat cannot tell you whether it feels a sense of unreality, for example), so it isn’t clear right now, if these are three things which often co-occur, or if dissociation and panic are actually components of the freeze response. It is possible that panic reactions occur along a spectrum with the full freeze response as the most intense or complete manifestation and other combinations of panic and dissociation representing a less intense reaction.
Discovering who experiences freezing during trauma and who doesn’t and why is important because experiencing a freeze response as well as dissociation or panic during a traumatic event is a major risk factor for going on to develop post-traumatic stress disorder (PTSD) later. Patients with PTSD report freezing during trauma at higher rates than people who have experienced similar trauma but do not have PTSD and in healthy college students have experienced trauma, those who experienced more freezing symptoms at the time also report more PTSD symptoms later. Patients who experienced freezing and subsequently developed PTSD also have more severe symptoms, on average, and do not respond as well to medication as patients who developed PTSD but did not experience a freezing response during their initial trauma. This is also the case in patients with panic disorder, patients who do have freezing responses during panic attacks have more severe symptoms and experience greater disability than those who don’t.
At least one study found that freezing predicts subsequent PTSD better than dissociation which is a more well-known and better studied risk factor. This would make sense if dissociation is, in fact, a part of the larger freeze response. However, its important to note that the most common way of measuring the occurrence of freezing, the Tonic Immobility Scale, includes questions on dissociation, which means that whether or not these two things are really part of the same neurological response people with dissociative symptoms will, necessarily receive a higher score on measures of freezing as well, which might influence this result (since dissociation by itself does predict PTSD).
Ultimately we just don’t know exactly how the freezing response relates to dissociation or to PTSD, the research is just not complete enough, but we do know a few things about how freezing relates to other factors that determine someone’s risk for PTSD after trauma. A study which attempted to evoke freezing symptoms in a laboratory test (can I just note that I have great respect for anyone who volunteers to participate on a study which aims to cause anxiety) found that people who had more anxiety-sensitivity experienced more freezing during the test. Anxiety-sensitivity is a personality trait that measures how much anxiety bothers people, and by itself it’s a risk factor for PTSD. People with good attentional control (a measure of how well you can control what you think about and focus on) have a lowered risk for PTSD in general and also had less intrusive thoughts (which is one symptom of PTSD) after they had freezing induced in a laboratory test. So the relationship that has been found between PTSD and freezing extends to other aspects of someone’s personality and cognitive skills which affect their risk for PTSD.
But there may also be a socially driven component to the effects that freezing has on developing PTSD. People who freeze during trauma are more likely to feel guilty and ashamed and they may receive less social support from people around them after their trauma, which could make them more vulnerable to developing PTSD. This is especially a problem for victims of sexual assault because victims who cannot provide evidence of fighting back are often disbelieved by the police and their friends and family.
Abrams, M. P., Carleton, R. N., Taylor, S., & Asmundson, G. J. G. (2009). Human tonic immobility: measurement and correlates. Depression and Anxiety, 26(6), 550–6. doi:10.1002/da.20462
Cortese, B. M., & Uhde, T. W. (2006). Immobilization Panic. American Journal of Psychiatry, 163(8), 1453–1454.
Fiszman, A., Mendlowicz, M. V, Marques-Portella, C., Volchan, E., Coutinho, E. S., Souza, W. F., … Figueira, I. (2008). Peritraumatic tonic immobility predicts a poor response to pharmacological treatment in victims of urban violence with PTSD. Journal of Affective Disorders, 107(1-3), 193–7. doi:10.1016/j.jad.2007.07.015
Fusé, T., Forsyth, J. P., Marx, B., Gallup, G. G., & Weaver, S. (2007). Factor structure of the Tonic Immobility Scale in female sexual assault survivors: an exploratory and Confirmatory Factor Analysis. Journal of Anxiety Disorders, 21(3), 265–83. doi:10.1016/j.janxdis.2006.05.004
Hagenaars, M. a, & Putman, P. (2011). Attentional control affects the relationship between tonic immobility and intrusive memories. Journal of Behavior Therapy and Experimental Psychiatry, 42(3), 379–83. doi:10.1016/j.jbtep.2011.02.013
Heidt, J. M., Marx, B. P., & Forsyth, J. P. (2005). Tonic immobility and childhood sexual abuse: a preliminary report evaluating the sequela of rape-induced paralysis. Behaviour Research and Therapy, 43(9), 1157–71. doi:10.1016/j.brat.2004.08.005
Pereira, M. G., Alves, R. D. C. S., Tavares, G., Lobo, I., Rego, V. R., Portella, C. M., … Oliveira, L. De. (2012). Peritraumatic tonic immobility is associated with posttraumatic stress symptoms in undergraduate Brazilian students. Rev Bras Psiquiatr, 34(1), 60–65.
Rocha-Rego, V., Fiszman, A., Portugal, L. C., Garcia Pereira, M., de Oliveira, L., Mendlowicz, M. V, … Volchan, E. (2009). Is tonic immobility the core sign among conventional peritraumatic signs and symptoms listed for PTSD? Journal of Affective Disorders, 115(1-2), 269–73. doi:10.1016/j.jad.2008.09.005
Schmidt, N. B., Richey, J. A., & Maner, J. K. (2009). Exploring Human Freeze Responses to a Threat Stressor. J Behav Ther Exp Psychiatry, 39(3), 292–304.
Volchan, E., Souza, G. G., Franklin, C. M., Norte, C. E., Rocha-Rego, V., Oliveira, J. M., … Figueira, I. (2011). Is there tonic immobility in humans? Biological evidence from victims of traumatic stress. Biological Psychology, 88(1), 13–9. doi:10.1016/j.biopsycho.2011.06.002
It’s Valentine’s day (or at least, it was when I was writing this) and what Valentine’s day would be complete without articles of very dubious quality extolling the virtues of the neurohormone oxytocin (I can’t seem to avoid them anways)? Oxytocin, for those who have been lucky enough to avoid these articles is a peptide (a small protein) and functions both as a neurotransmitter in the brain, and as a hormone circulating around the rest of the body. Oxytocin actually does a lot of different things, but it is most famous for inducing social bonding, both romantic and otherwise. No article about oxytocin is complete without a mention of the prairie vole, a rodent most widely known for forming a long term mated pairs which raise their litters of pups together, these pairs rarely divorce, and if the pairs are split up, voles don’t remarry. Oxytocin is widely credited for making the prairie vole into a small fluffy paragon of romantic bliss.
Except that vole love is rather more complicated than it seems and involves more than just oxytocin. Aratonga et al1, have found that if you really want to make a vole fall in love oxytocin shouldn’t be your target at all, what you want is a different neurotransmitter, called dopamine.
Actually convincing two voles to form a mated pair in the lab isn’t very difficult, just leave a male and a female vole in a cage together for twenty-four hours and they will mate and eventually form a pair bond. The two are considered pair bonded when they both prefer their partner to other voles, and when treat all other voles aggressively. If you don’t have twenty-four hours, you can speed up the process by activating the D2 dopamine receptors.
A quick aside about dopamine and its receptors. Dopamine is a neurotransmitter and once released from a neuron, it can bind to several different kinds of receptors. Which kind of receptor it binds to determines what dopamine makes the post-synaptic cell do, so dopamine binding to a D1 receptor can produce totally different response to dopamine binding to a D2 receptor, even though it’s all the same dopamine.
Male voles (this whole study focused on males) will very rapidly form a preference for a female if the D2 receptors in one part of its brain are activated, even if they haven’t mated yet. For this to happen though the shell (the outer layer) of an area called the nucleus accumbens must be activated specifically. A brain wide activation, or even one that occurs in the central core of the nucleus accumbens won’t work. Activating the D1 receptors in the same area will actually block pair bonding.
This was determined using a set of drugs called D1 and D2 agonists and antagonists, which were injected into the shell of the nucleus accumbens. Dopamine will bind to all the dopamine receptors in the area it is released, but these drugs will bind specifically to one receptor but not the other. An agonist is a drug which turns a receptor on, while an antagonist turns it off, to recap:
D1 agonists activate D1 receptors
D1 antagonists deactivate D1 receptors
D2 agonists activate D2 receptors
D2 antagonists deactivate D2 receptors
D2 agonists can create a pair bond. D1 agonists don’t create pair bonds, and when D1 and D2 agonists are injected together the D1 agonists will actually prevent the D2 agonist from creating a pair bond. When a D2 agonist and D1 antagonist are injected together, however, pair bonding can occur.
So, the activity of different dopamine receptors in the shell of the nucleus accumbens can either promote or prevent pair bonding. But when voles pair bond without the help of drugs they don’t have specific ways of activating just the D1 or just the D2 receptors, dopamine will act on them both. We just saw that when these two are activated together pair bonding is blocked, so how do voles ever fall in love?
Well, the two receptor types aren’t actually activated together. Which receptors a cell has isn’t static. A neuron can alter which receptors are expressed on its surface, where they are expressed and how many of them there are, and this can drastically alter how that cell responds to a signal from one of its presynaptic neuron. This is what happens to the male prairie voles.
Unpaired voles initially have lots of D2 receptors in their nucleus accumbens, and very few D1 receptors, so they initially form pair bonds with females when they are allowed to interact and mate for twenty-four hours. Initially, even though these voles have a preference for their new mates, they aren’t fully pair bonded and will still approach other voles. It takes about two weeks before they develop antagonism towards all other voles and are considered fully pair bonded. Males which are fully pairbonded have about 60% more D1 receptors in their nucleus accumens than unpaired males, and blocking these receptors with D1 agonists blocks their aggression and weakens their bond with their mates.
So here is how voles fall in love. When two unpaired voles meet they have lots of D2 dopamine receptors in the shell of the nucleus accumbens and activity of these receptors cause them to form a preference for their partner above all other voles. As they continue to associate the numbers of D1 dopamine receptors increases. The activity of D1 receptors blocks the activity of D2 receptors. This means that in addition to preferring their mate, the voles also become aggressive towards other voles and this is permanent. Once a vole has a mate, it almost never remarries for any reason because the D1 receptors are still there. Essentially, its ability to form a new pair bond, or ‘fall in love’ has now been removed. Voles have only one true love.
Is this how humans fall in love? Probably not. Despite the endearing resemblance to the first ten minutes of Up voles aren’t tiny fuzzy people and forming a monogamous pair bond is not in the strictest sense falling in love. Also, although this will upset the makers of Valentine’s day cards and most romance novels humans aren’t actually monogamous. If we were, serial dating, polyamory and the various forms of polygamy which have existed throughout human history would all be literally impossible. So we probably don’t use this dopamine system for this purpose.
Animal studies are a wonderful, powerful, versatile research tool. If they weren’t we’d have stopped doing them. But figuring out to what extent the results of an animal study can be applied to people is hard and requires a lot of careful analysis. We study vole pairbonding to learn about humans because there are a large number of behavioural analogues between human relationships and vole mated pairs and there are some shared neurological bases for those relationships, but we probably don’t cement long term relationships the same way voles do. One of the reasons why the role of oxytocin in vole pairbonding gets more attention than the role of dopamine is that its much more directly relevant to people than the system studied in this article.
So remember, you are not a prairie vole, and unless you plan to fall in love with the first person you ever have sex with and spend the rest of your life attacking everyone you aren’t married to, don’t ask small monogamous rodents for relationship advice. Some nice stories of human social bonding can be found here instead. Its less rigorous, and its only behavioural, but since all behaviour is generated by neurotransmitter systems in our brains anyway, you might still pick up some good advice.
Happy Valentine’s Day.
1. Aragona, B. J. et al. Nucleus accumbens dopamine differentially mediates the formation and maintenance of monogamous pair bonds. Nature neuroscience 9, 133–9 (2006).
If the job of the brain is to process information and generate thoughts, feelings and behaviour, how does it work? The brain is made of two kinds of highly specialized cells, neurons and glia. Neurons communicate with each other to process information. Glia, broadly speaking, are support cells, helping neurons function. It is the communication between the neurons that is primarily responsible for what the brain does. Neurons have a very specific structure (Figure 1) which is related to how they function. The bushy dendrites take in information from other neurons. The architecture of the dendrites is carefully organized to allow the neuron to get input from where it needs it, and is referred to as its dendritic tree. Information travels through the neuron in the form of small electrical signals called membrane potentials, these can be either excitatory, making the cell more likely to generate a signal of its own, or inhibitory, making it less likely. These signals travel down the dendrites and through the cell body. At the base of the cell body (the axon hillock) all these membrane potentials are summed together, if the resulting signal excites the cell above a set threshold the then the neuron will fire a signal of its own called an action potential which travels down the axon (the difference between an action potential and a membrane potential is related to their electrical properties). The myelin wrapped around the axon acts like an insulator so the electrical signal can bounce between the nodes of Ranvier, and travels much faster than it would otherwise.
While schematic neurons, like the one here, are a convenient way to see all the parts of a neuron different types of neurons actually all look slightly different and their shape is determined by their function. Neurons that gather input from all over the brain will have immense dendritic trees and small nearly invisible axons (Figure 2). The pyramidal motor neurons which carry the information used to signal the muscles to move have small trees, but very large axons needed to transmit information down the body (Figure 3). Typically neurons which transmit information out to the body are called motor neurons, those which carry information from the body to the brain are called sensory neurons, and those within the brain and spinal cord which process and transfer information are referred to as interneurons.
Signalling between neurons involves first a burst of electricity and then a puff of chemicals called neurotransmitters, so it is referred to, imaginatively, as electrochemical signalling. When the electrical signal reaches the axon terminals, they release a small burst of neurotransmitters. Individual neurons are separated by a small gap called a synapse. The neurotransmitters cross the synapse and bind to special receptors on the dendrites of the next neuron which is what creates the membrane potentials. The combination of a neurotransmitter and a receptor is what determines whether a signal is inhibitory or excitatory each one can bind to multiple receptors, meaning that each transmitter can cause excitation or inhibition by binding to different receptors. Different receptor types occur in different parts of the brain, which help control the effects of neurotransmitters, and determine the function of the different brain regions.
Since you are reading this post it’s a pretty safe assumption that you are a human being, and have a brain.
What is a brain? The brain is a located inside the skull, it weighs approximately 3 pounds, on average, and has the consistency of Jello (a fact I have never personally tested). The large wrinkly surface is called the cerebrum, this is the largest and most complex part of the human brain. The smaller wrinkly knob at the bottom is called the cerebellum, and the stalk is the brainstem, which tapers down into the spinal cord. Nerves from through the body run into and out of the spine (with the exception of a few who run directly into the brain), and carry information in and out. Under the cortex, things get much more complicated. Your brain isn’t a homogenous mass, but, instead, is made of dozens of specialized structures, all with different functions. The cortex is also made of many different functional areas, although they can’t be differentiated by the naked eye).
What does a brain do? The main job of the brain is information processing. All the information about the outside world, and your internal environment is picked up by your nerves and carried to your brain, which interprets it and integrates it. This is where sound waves turn into words and music, photons turn into pictures, and then they both get combined to give you the show you are watching on TV.
The brain is also the source of thoughts and feelings about all the information you feed it by seeing, hearing, smelling, etc. Your brain generates emotions (aided and abetted by your hormones and immune system), remembers things to use later and generates new thoughts and ideas. Finally, the brain also turns thoughts into behaviour, generating movement, speech, facial expressions.
All of your thoughts, sensations, feelings, impressions, memories, ideas and general you-ness, are in your brain. Of course, it’s more complicated than that (in biology, everything is more complicated than that). The body isn’t an army, the brain doesn’t take in information and give out orders, it gives and receives constant feedback to and from the rest of the body more like a conversation and the other participants in the conversation contribute to how the brain functions.