I just love the feeling of forming new neural pathways, don’t you? 🙂
Please note that some of this info is lifted directly from the books listed at the end. These notes are for myself (because writing it down helps me learn it). If reading it helps you too, you are welcome to it, but please don’t quote me. Go to the original sources listed below.
The human brain is a massive network of approximately 100 billion neurons (nerve cells). Proper functioning of the nervous system depends on communication between neurons which act kind of like a 250 mile/hour bucket brigade, passing an electrochemical message from neuron to neuron until the message gets to where it needs to go. A neuron looks more or less like a large, many-legged spider on one end, attached to a long string which branches out into little stumps on the other end. The spider legs (called dendrites) are the receiving end of a message. As dendrites receive a new message, an electrical charge gets built up among them collectively, and when there is enough of a charge, it kicks off a ripple of charge swapping (positive/negative) down the channels of ions that run the length of the axon (the long string) down to the terminal boutons (the little stumps). Now, every terminal bouton has many little craters (called vesicles) in which messenger molecules (aka neurotransmitters) can park. And when the charge hits the terminal boutons, the vesicles expel their contents into the surrounding empty space where they’ll be picked up by the receptors sites on the neighboring dendrites and the whole thing starts over again. The empty space between a terminal bouton of one neuron and the dendrite of another is called a synapse. We’ve got about 100 to 1000 synapses for 100 billion neurons in our brains.
These neurons run all throughout our bodies, not just our brains, and they tend to hang together in functional groups. In fact, what we call a nerve in a non-brain part of the body is really a fiber made up of a bunch of axons. When we were each just four weeks in utero, our little embryonic bodies were producing a half million neurons every minute. Over the following weeks, these neurons got distributed into brain cells or other kinds of cells, based on the architectural blue print in our genetic code. During the first and second trimester, the neurons began to reach tentacles out to each other, establishing synapses (the empty space between in which the brain chemicals are to be passed). Three months before birth, we had more brain cells than we ever would have again. In fact, there were far more than we’d ever need to survive as a fetus or an adult.
At this point, our brains started organizing their cells into functional components (listed below) and those that didn’t get picked for any team, died off. Then those circuits started learning–mostly about the sound of our mother’s voice and the kind of food she liked to eat–and tuning themselves to the world beyond. Once we were born, we went into over-time learning mode. The structures in our brains slowly matured, adding to our abilities to remember and build on those memories. But first, for the 18 months following birth, we were phenomenal learning machines. While older brains need some reason (such as a reward) for learning (paying attention to one stimulus over another), baby brains soak up everything coming through their sense. (Babies may seem to just sit and stare blankly at things, but in fact they are taking in a hell of a lot of information.) Neural circuits that received repeated stimulation developed stronger synaptic connections while those that laid dormant lost responsiveness to their particular kinds of inputs. At about age 11 for girls and 12 1/2 for boys, just before puberty, we experienced a second growth spurt in grey matter which was followed by another period of thinning that lasted up until about age 25 when our executive functions final matured. (For more on this, see the prefrontal cortex, below.) We continue to learn throughout our lifetimes and different parts of our brains grow bigger based on the inputs that are repeated, but it is not known whether growth volume is due to reorganization of existing circuits (thereby decreasing other areas), an addition to the number of neural connections, or the birth of actual new brain cells.
Different groupings of neurons serve different functions; they respond to particular kinds of inputs in particular kinds of ways. Seeing a spider, for instance, involves the visual cortex (occipital lobe) to gauge its color and orientation, the temporal lobe to determine its shape, the parietal lobe for its position, the hippocampus to take all this input and crossreference it with memories to recognize it as a spider, and the amygdala (which kicks in before all of these) generates a fear response (based on a crude impression of the input) which later gets checked against whatever information the hippocampus came up regarding whether or not what we saw is something that warranted caution.
Let’s take a quick, selective tour of some of those functional groupings of neurons in the brain that I find interesting …
- Cortex (or Cerebrum) – The wrinkly, sausage looking stuff of the outer layer. (Ever wonder about the wrinkles? The more wrinkles the more surface area and the more room for neurons.) This controls higher cognitive processing, such as analytical thought. The cortex is divided up into sections called “lobes” which shake out as follows …
- Parietal Lobe – (towards the top and slightly to the back) associated with movement, orientation, recognition, perception of stimuli. One part of the parietal lobe, called the somatosensory cortex, is responsible for processing touch.
- Occipital Lobe – (at the back) associated with visual processing. Interestingly, when oxygen levels decrease across your vision processing neurons (for example, during near death experiences) , the neurons activate and their output is the same as if you were seeing a white light. The tunnel perception is created because the neurons that are not yet deprived of oxygen do not activate, and you perceive their output as black. As more neurons are deprived of oxygen, you see the white spot grow larger. This occurs even for people who have been blind from birth.
- Frontal Lobe – (goes from just behind the forehead to the top of the head) associated with reasoning, planning, parts of speech, movement, emotions, and problem solving. The frontal lobes are important for providing the judgment and flexibility of thought that underlies talent.
- Prefrontal cortex – The front part of the frontal lobe (just behind your forehead). It is responsible for impulse control, weighing alternatives, social judgments, plans for the future, attention, and organization of complex information. In order to recognize you as your “self” (for example, in a mirror, or as separate from others), you need the prefrontal cortex. Circuits in the prefrontal cortex start to develop around age 2. Before these comes on line, a child with a smudge on her cheek will try to wipe the spot off her reflection in a mirror, rather than understand that the image in the mirror is herself, and wipe her own cheek. Interestingly, this is also the age at which children often develop a sense of themselves as separate from their mothers (which leads to fond use of the words “No” and “Mine”) and a sense of their mothers as separate from them (which can be anxiety and frustration producing). A understanding that one’s self persists through time (e.g., being able to recognize yourself in photos and relate to the events therein) doesn’t kick in until about age 3. The memory you are forming right now is taking place in your prefrontal cortex. It will remain in your consciousness as long as neurons containing the information remain active. In order to not lose the memory, the information has to be passed on to your hippocampus before these neurons go inactive or displace the thought. (See the hippocampus for more on memories and time.) The prefrontal cortex is also the last area of the brain to reach maturity. So, at puberty, we’ve got adult passions, sex drive, energy, and emotion, but the ability to rein these in, restrain impulses, and exercise good judgment hasn’t matured until about age 25. Damage to the prefrontal cortex can lead to loss of speed, strength, and controllability of the movements of the limbs, hands, and eyes; impaired ability to strategize and plan, especially in new situations; difficulty using cues from the environment to alter one’s behavior (which can lead to inappropriate social and sexual behavior and increased impulsivity, rule breaking, and compulsive behavior); and short term memory loss and judgments about recency.
- Temporal Lobe – (on the bottom of the cortex, which essentially places it in the middle of the brain) associated with perception and recognition of auditory stimuli, memory, and speech and the now famous “God spot” which has been shown to light up with activity during self-reported, profoundly spiritual experiences and also when religious persons are shown words invoking spiritual belief. Structures in the temporal lobe (and the limbic system) supply drive and motivation.
- Hippocampus – A structure in the temporal lobe (and a member of the limbic system in that middle layer of brain matter), it handles
- New learning.
- Spatial relationships. An MRI study published in 2000 by scientists at University College, London, showed that in London taxi drivers (who must go through a difficult and thorough examination on London streets before getting their permit) the rear portion of the hippocampus was enlarged compared with those of control subjects.
- Impulse and emotional control. The hippocampus is also helpful in that calms us down after stress by shutting off a stress response (see the HPA axis, below) and reducing arousal.
- Movement of memories from temporary to permanent storage. The hippocampus can hold on to memories for from hours to days. Permanent storage of information, however, requires physical changes in the neurons and connections between them. It is thought that this process is done during sleep, during which time it has been observed that the hippocampal neurons reproduce the earlier firing pattern and neurons in the prefrontal lobes of the neocortex parallel this firing pattern. (It’s as if the mind is replaying the events of the day.) The hippocampus doesn’t mature enough to store cohesive memories until about age 4, hence most of us don’t remember much in our lives from before then. However, that doesn’t mean we don’t carry around some sort of memory from before then. But we need to talk about the amygdala to explain that ….
- Amygdalae – Little almond-shaped blobs at the back of the temporal lobe which are members of the limbic system, more or less behind the bridge of your nose. They elicit and control aggression, appraise threats, and are able to add fear to permanent memories. Neuronal impulses exiting the amygdalae elicit a fight-or-flight response (independent of the cerebral cortex). Emotionally-charged memories can get etched into the amygdalae, but until the hippocampus gets involved, they are without context. So, although they may not be accesible by the conscious mind, they might still influence the way we act and feel beyond our awareness. Emotions are the least plastic part of the brain. Once an emotional association is made with an input, it is difficult, if not impossible, to unmake it. However, we can learn to manage our emotions by learning what triggers them. (Some emotional triggers are universal. For example, a sudden invasion of your field of vision triggers fear. But most are learned and therefore unique to an individual.) Learning these triggers shortens the period the time between the onset of an emotion and when we become consciouslsy aware of it and therefore we are more likely to double-check to see if the emotion is appropriate to the situation. Since perception at the amygdala level is often rapid, crude, and inexact, there are many false alarms (for example, misperceiving a piece of rope for a snake). When a stressor is perceived, the amygdalae activates the hypothalamus. Evolutionarily speaking, the amygdalae is an ancient part of the brain and part of our evolutionary heritage is to automatically fear certain kinds of life-threatening things. I say “automatically” but this is not quite right. The groundwork is there, but the switch has to be flipped by social experience. Studies at the University of Wisconsin-Madison in the 1980’s showed that lab-raised monkeys would become afraid of snakes only after shown a video of wild monkeys being afraid of snakes. However, they didn’t become afraid of flowers after being shown a video altered to make the wild monkeys appear to be afraid of flowers.
- Hippocampus – A structure in the temporal lobe (and a member of the limbic system in that middle layer of brain matter), it handles
- Thalamus – A relay station for all information on its way to the cortex. During dream sleep, blood flow to the thalamus increases while flow decreases to the complex thinking areas in the frontal brain, reducing our sense of self-awareness during dreaming and making it difficult to remember dreams on awakening. (Incidentally, sensory deprivation is thought to increase the number of serotonin receptors (see neurotransmitter below) in the thalamus, causing excessive sensory impulses to reach the prefrontal lobes even to the point of inducing hallucinations.)
- Hypothalamus – A little blob right smack in the core which regulates sleep cycles, hunger, and sex drive. It is part of what’s known as the HPA axis (hypothalamic-pituitary-adrenal pathway) which is responsible for the release of steroid hormones during stress.
Just to make matters more complicated, all of these pieces of the cerebrum have a right half and a left half (for example, there is a right hippocampus and a left one, a right frontal lobe and a left one), which are connected by bundles of fibers, notably the corpus collosum (a superhighway of axons). Each half of the brain shoulders different parts of the processing load.
- Right hemisphere – The right brain does random, intuitive, wholistic, divergent (the tearing apart of a topic to explore its various components), and subjective kinds of thinking. It is the home of negative emotions and imagination. It perceives experiences in the moment but has no sense of how an experience fits into a context of prior experiences. So, for example, it is the right temporal lobe that lights up when we have an “Aha” moment of sudden insight or when we understand a joke or a metaphor. (Also, just before a sudden insight, some visual information stops going to that part of the brain, kind of like the way we close our eyes to concentrate. For more info about this, go here.) The right hemisphere is also responsible for discriminating shapes (including geometric learning and recognizing faces), reading and expressing emotions, understanding metaphors and music, and receiving the inputs from the left half of the body. For example, violin players will have a larger right hemisphere somatosensory cortex than on their left hemisphere because the left hand fingers move about the neck playing notes, whereas the right hand merely holds the bow. Damage to the right side of the brain may result in an ability to tell the difference between melodies, in the identification of faces, or locating objects in space.
- Left hemisphere – The left brain does logical, sequential, analytical, convergent (the combining of ideas based on their commonality), and objective kinds of thinking. It attempts to place experiences into a context of existing knowledge. As it attempts to do this, it is endeavouring to ascribe meaning to experiences and place them in an overall context. It is the home of language processing, speaking, skilled movement, and positive emotions. Studies at the University of Wisconsin-Madison have shown that while people who are inclined to fall prey to negative (destructive) emotions displayed a pattern of persistent activity in regions of ther right prefrontal cortex, those with more positive temperaments had more activity in the left prefrontal cortext. Studies of Tibetan monks show their baseline of activity to be much farther to the left than average. And controls groups who received eight weeks of meditation training showed a pronounced shift in brain acitivity toward the left, “happier”, frontal cortex and a healthier immune response to a flu shot. The left hemsiphere is also responsible for the movements of and inputs from the right side of the body.
- Serotonin – An inhibitory neurotransmitter, it affects mood, appetite, memory, and learning. Too much causes anxiety, rumination, irritability, and aggression. Too little causes depression.
- Dopamine – An inhibitory neurotransmitter, it regulates the flow of information from other parts of the brain to the prefrontal lobes. Too little causes incoherent and delusional thoughts. Too much causes hyperalertness. Too little in the prefrontal lobes will make it hard for you to remember a phone number long enough to write it down. Too much in the limbic system and not enough in the neocortex can cause anxiety and paranoia. Dopamine also affects the part of the brain that controls muscle movement. (One element of Parkinson’s disease is insufficient dopamine, which results in muscle tremors.)
- Norepinephrine (a derivative of dopamine) – Too much makes us feel like we are on amphetamines, too little leads to impaired attention.
- Epinephrine (a derivative of norephinephrine) – when this monoamine is in the central nervous system, we call it adrenaline.
Not all brains, or even sides of the brain, are created equal. The right and left halves, different in function, also differ in physical attributes. For example, most of us are born with a larger left temporal lobe (necessary for speech) than a right temporal lobe. And, on the left side of the brain and in women in general, there tends to be more grey matter (indicating more dendrites and an increased capacity for computation at the cost of a lower capacity for speed of getting messages from neuron to neuron). Men tend to have more white matter (indicating more axons and therefore more superhighways for getting info from place to place leaving less room in the brain for computational types of activities). But this isn’t unchangeable, exposed to high levels of certain sex hormones (anrogens) at the right stage of development, girls have develop better spatial skills than other females. And, with all that said, it appears that folks who have had a hemispherectomy from their brains as children (as treatment for severe, widespread epilepsy) can get along just fine with the cells of the remaining hemisphere picking up the jobs of the lost hemisphere. Having “half a brain” isn’t a founded epithet.
But, roughly speaking, the more neurons, the more information we can process. The number and types of neurons we each get are initially dictated by genetics. But the opportunity for us to reach our genetic potential is influenced by nutrition and stimulation, most importantly during the initial few years of life. Exposure to toys and sounds of all sorts, such as music and speech, causes the parts of our brains that process that information to grow larger. If we are unfortunate enough to be malnourished and understimulated or overly stressed from conception through the first few years of life, we may grow to be less “intelligent” or adaptable than someone with less genetic potential but better nutrition and more appropriate levels of stimulation. (That said, it is important to keep in mind that numerous studies have shown that success in life does not correlate with test scores; it correlates with attitude.) Another interesting side note is that the neural networks of dyslexics (currently, a culturally perceived weakness) may take longer to develop but result in an increased ability to solve problems visually (e.g., completely in their minds before committing anything to paper). (Da Vinci, Edison, Tesla, and Einstein were a few famous dyslexics.)
So, I was kind of hand-wavy back there about those “messenger molecules”. While a message is traveling the length of the neuron, it is the form of an electric charge, but when it goes from neuron-to-neuron (across a synapse), it is in a chemical form (called a neurotransmitter). There are 30-plus neurotransmitters. And each receptor on a dendrite of a neuron is configured to receive only one kind of neurotransmitter. Problems crop up when there isn’t enough neurotransmitter available to do the job or when there aren’t enough of the right kind of receptors available to soak up a neurotransmitter (and therefore to build up enough charge to pass on a signal). There are also enzymes floating in the empty space around the neurons whose job it is to eat up any leftover neurotransmitter that didn’t get soaked up by the dendrites (so that the neurotransmitters don’t keep stimulating the neuron after it has already picked up the signal). Another problem arises when those enzymes do too good of a job and don’t leave enough around for the neurotransmitter to sufficiently drench the receptors on the dendrites of a target neuron.
There are actually two kinds of neurotransmitters: excitatory and inhibitory. For simplicity’s sake, I’ve only been talking about the excitatory ones (the ones that cause a neuron to fire). The inhibitory neurotransmitters use the same mechanisms to inhibit a neuron from firing.
The major neurotransmitters in the brain are of a type called monoamines. They are constantly being produced by way of amino acids (which we take in to our system mostly from the proteins we eat.) Without proper protein, our brains become foggy, we get depressed, etc. (Incidently, one element in making a person more susceptible to brain-washing is to feed them a diet low in protein and high in carbohydrates, such as drinks with high sugar content, cookies, or fruits and vegetables. This impairs their ability to reason and makes them susceptible to guilt, self-doubt, humiliation, and information overload. Just think about how you feel around Thanksgiving and Christmas time when you’ve been eating large quantities of carbs.)
Let’s take a quick tour of the major neurotransmitters in the brain:
I mentioned earlier that each receptor is set up to receive only one kind of neurotransmitter (like a lock and a key). But some drugs contain chemicals that look enough to the receptors like the neurotransmitter it wants that the receptor will accept it. In other words, we can fake out the brain into certain excitatory or inhibitory states depending on what kinds of chemicals we give it. Anybody who has ever taken mood altering substances knows exactly what I mean. (Cilicin, a chemical found in certain mushrooms, is similar enough to serotonin as to be able to bind to the serotonin receptors, in this case in the thalamus. Anything that disrupts the thalamus can disrupt the flow of sensory information to the prefrontal lobes, causing abnormal thoughts to occur.) Other chemicals can interfere with the reabsorption process (of the enzymes that oxidize the leftovers) of a neurotransmitter, leaving too much in the system. (Cocaine blocks the uptake of dopamine, for example, leaving an excess of it in the system. Take a look above at the symptoms of having too much dopamine in your system.) If a person is having trouble with overabsorption of a neurotransmitter, a psychiatrist might prescribe them an MAOI (monoamine oxidase inhibitor–“monoamine” is the neurotransmitter and “oxidation” is how the extra neurotransmitter is eaten up or absorbed).
Neuronal activity changes with our mental state. When we are in the waking state, we usually have a high degree of beta activity (that is, the brain oscillating at a frequency of 13 to 19 Hz or cycles per second). During periods of learning and perception, the brain cranks it up to gamma levels (20 to 100 Hz) with synchronized activity of clusters of neurons. Passing from being awake to being asleep involves passing through a series of brain states. Although there is some overlap, the process begins with alpha (8 to 12 Hz) just before sleep (yum num num), passes through theta (4 to 7 Hz), and ends with delta (3 Hz and lower) in the deepest sleep state. REM sleep, associated with dreaming, uses a wave pattern similar to post alpha or beginning sleep state.
A trance is an altered state of consciousness, like sleepwalking, which involves the alpha state. Typical signs of being in an alpha trance are body relaxation, dilated pupils, and a high degree of suggestibility. A relaxed feeling results from an alpha-related release of opiate-like molecules, introducing an addictive element to the alpha trance. During a trance state, the right brain hemisphere (which deals with emotions and imagination without the capacity to link present experiences to the past or the future) is much more active than the left brain hemisphere. Regular meditation for at least an hour every day, for at least a few weeks, is very likely to cause a prolonged state of alpha with little strong beta. The description of the Buddhist state of Nirvana seems to match the detachment from reality and freedom from desire that would result from stifling the left hemisphere (of rational thought) by way of continual meditation.
The suggestibility within this state can be an advantage when one desires to retrain one’s mind. However, if alpha trances are induced involuntarily and someone alters our beliefs, we call it brain-washing. So, if you’d like to be brainwashed, eat them a high carb diet (see neurotransmitter above) and allow yourself to be lulled into a state of high suggestibility, much like a significant portion of the American population has already achieved watching TV after an unhealthy, overly proportioned dinner.
Sources of information:
Giovannoli, J. (1999). The biology of belief: How our biology biases our beliefs and perceptions. New Jersey: Rosetta Press.
Preston, J.D., O’Neal, J.H., & Talaga, M.C (2002). Handbook of clinical psychopharmacology for therapists (3rd ed.). Oakland, CA: New Harbinger Publications.
Shreeve, J. (2005, March). Beyond the Brain. National Geographic, 2-31.