The Brain

by Gary L Wenk

  How is the cortex organized?

  The neurons within your cortex are organized into columns of cells that are arranged in six layers. Each column of cells is oriented perpendicularly to the cortical surface and consists of about 100 neurons. The cells in each column are highly interconnected with one another. Animals with bigger brains simply have many more of these columns of cells. Think of the cortex as a bed sheet, with a thickness of about three to four millimeters for most mammals; in contrast, however, the width and length of the bed sheet vary from twin size (dogs) to queen size (humans) to king size (whales). The evolution of the cortex led to bigger bed sheets that vary only a little in thickness. The increase in size was accompanied by a buckling and rippling leading to the formation of a cortical landscape characterized by small hills and valleys; this folding process made it possible to fit a large cortical sheet within a small skull. Overall, the larger and thinner the cortex, the more folded it is. For example, the large brains of dolphins and whales have exceptionally thin cerebral cortices, which are remarkably folded with lots of small gyri. Across the many different species that have been studied, the folding pattern has remained surprisingly consistent; for example, motor functions are always up front, while sensory abilities are located in the back half of the brain.

  Why aren’t human brains bigger?

  Neuroscientists have wondered why we never evolved bigger, and perhaps smarter, brains. Why isn’t the cortex thicker? Why didn’t the forces of evolution give us more columns and make a bigger sheet of cortex? Either of those two advances might have allowed humans to be much smarter. The reason the cortex is not thicker is partly because there would not be enough room for all of the axons necessary to connect all of those extra neurons to one another. Fancy wiring has made us a very successful species; however, not having additional room for all of those extra wires (i.e., axons) has limited humans from becoming even smarter. Imagine the brain as a pair of football-shaped hemispheres that are covered by a thin layer of cortex: so what fills the inside of the football? Most of the interior of the brain is filled with the axons (i.e., the wires) that are necessary to connect all of your neurons. Just for a moment, let us assume that we could add another layer to the surface of the cortex. If this happened, it would become necessary to greatly increase the size of the entire brain in order to make room for all of the extra wiring required to allow the additional layer of neurons to communicate with all of your other neurons. That is the problem: the additional wiring might make us smarter, but the wires would require a lot more room inside the brain. This leads to a much bigger problem: bigger brains require bigger skulls; bigger skulls require much bigger birth canals; bigger birth canals are just not an option for a bipedal, vertically symmetrical animal living on dry land, such as a human. Overall, it appears as though the human brain has evolved to be as big as possible, given the anatomical constraints imposed by the fact that it resides inside your skull, which spends time developing inside your mother.

  How does the cortex develop?

  Imagine that you are watching as an embryo begins to grow inside the womb. Focus on just the front of the embryo where the brain is beginning to take shape. At this stage neurons begin their lives as protoneurons resting in the wall of a fluid-filled neural tube. Each protoneuron then slowly crawls outward over the top of its neighbors until it reaches its final resting place. In this fashion, each of the six layers of your cortex is deposited like a layer cake, starting with the lowest layer at the wall of the neural tube; then, progressively newer neurons-to-be crawl over their siblings to deposit themselves on top as a new layer. Once deposited, each protoneuron differentiates into a mature adult neuron and never undergoes another cell division. The ultimate size of a brain is determined by how many cell divisions occur within the walls of the neural tube before the cells start climbing.

  Differences in brain size between most species are largely due to variations in the size of the cortex. This discussion, therefore, is focused on the cortex. Recent research has discovered that in the mouse brain these primordial neurons undergo nine cycles of division, taking about five hours to complete the process. In the monkey brain, these primordial neurons undergo 18 cycles of division and migration, taking about 20 hours to complete the process. In humans, our primordial neurons undergo 20 cycles of division and migration, taking about 22.5 hours to complete. It is amazing how little time it takes to build the basic framework of the cortex. The center of higher mental functions for humans, the thin layer of cortex on the surface of the brain contains at least 100 billion neurons, and 100 million meters of axons that connect these neurons to one another.

  Scientists now know that the size difference between the brains of Australopithecus and Homo erectus was due to only one additional cycle of cell division—just one! Think of the simplicity of this evolutionary process; the way to build a bigger brain is simply to wait and do nothing for a couple of hours, giving the protoneurons a chance to undergo just one more cell division before beginning their differentiation into adult neurons.

  In summary, the evolutionary solution to making a brain with more capabilities was to generate only six layers of cells in the cortex but expand its total surface area. A recent study of many different mammalian species discovered that cortical thickness varies across species by only a few millimeters; of course, there are some interesting exceptions. Manatees and humans both have unusually thick cerebral cortices on average. Does this mean that manatees are as smart as humans? Not at all! Remember, the most important feature underlying intelligence is fancy wiring between neurons, not the size of the brain. Bigger brains do not guarantee greater intelligence.

  How do male and female brains differ?

  Male and female human brains have many anatomical differences—for example, the corpus callosum is thicker in females than in males. This difference is evident even as early as the fetal stages of development. Is this an important difference? The corpus callosum is a large bundle of axon fibers that allows the two hemispheres of the brain to talk to each other. Having a thicker corpus collosum would offer the opportunity for increased cross-talk between brain hemispheres; this additional avenue of communication is thought to underlie better language skills in females and also may underlie the fact that boys have more learning disabilities and dyslexia than do girls.

  Overall, probably related to their thicker corpus callosum, female brains excel at the intercommunication between hemispheres while male brains show more connectivity within each hemisphere. You will recall that the back of the brain is involved in sensory perception and processing while the front half of the brain controls coordinated movements; the greater intrahemispheric connectivity in male brains may explain why some men are generally better in activities that require skilled eye-hand coordination.

  Are bigger brains always better?

  The size of the brain correlates with longevity in mammals. Larger brains have an initially slower growth period and then spend a longer time giving birth to new neurons. Smaller brains have a faster initial growth period and then neuronal development terminates earlier in the development of the organism. Consequently, human babies are born with a brain encased within a soft skull that is just small enough to squeeze through the birth canal and then quickly begins to grow much bigger. Unfortunately, this means that human babies are born quite vulnerable and are not able to thrive without significant and prolonged parental intervention.

  Having a big brain is never enough. The African elephant brain has three times as many neurons as the human brain. However, the African elephant has far fewer neurons where they are truly needed—in the cortex. If the African elephant brain had evolved a few critical modifications, African elephants would be ruling the world and humans would be performing circus tricks for their benefit.

  Chimpanzees and humans begin their lives with about the same number of neurons per cubic inch of cortex, but as the human brain develops, it systematically destroys neurons that get in the way of the growt
h of axons that are connecting the various parts of the brain to one another. It is astonishing to realize that we humans became smarter, and more intellectually agile, than our closest evolutionary relative by simply killing off excess neurons to obtain the benefits provided by more complex wiring between brain regions. The frontal lobe of the human brain performs such astonishing mental exercises because its wiring is much more complicated than what is seen in the temporal or occipital lobes. So what is the take-home message? It would be that when it comes to brains, the complexity of the wiring matters; overall size is generally less important.

  What is neurogenesis?

  It was once believed that all of your neurons were born while you were developing inside the uterus and that only a small percentage were added during the first few years of life. The assumption always had been that adult brains possessed all of the neurons that anyone would ever need. Apparently, this is not true; we actually require a constant supply of new neurons when we are adults, too. Human brains give birth to approximately 1,400 new neurons every day. This process of neuronal birth is called neurogenesis and only occurs within the hippocampus (in humans), a region of the brain within the temporal lobe that is critical for learning and memory. Today, partly due to the above-ground detonations of nuclear bombs that were conducted between 1945 and 1963, we know quite a lot about the birth of new neurons in adult brains. The atmospheric radioactive carbon atoms that were released by those explosions have been incorporating themselves into the DNA of our dividing neurons, providing a time-stamp of when each neuron was born. What do we do with all of those new neurons every day? Their ultimate fate is unknown: some of them are most likely being used by the hippocampus to help incorporate our daily memories.

  How do we think so fast?

  Neurons communicate with one another via their axons. Axons are conduits for an electrical disturbance called an action potential. Action potentials begin within the cell body of a neuron; once this electrical disturbance is released from the cell body, it ripples its way along the length of the axon until it reaches the end of the axon, where it induces the release of chemicals. The speed of the action potential along the axon, up to about 120 meters/second (or about 3,000 miles per hour), is directly related to two independent factors—the diameter of the axon and the presence of a thick insulating layer of tissue, called a myelin sheath, that is wrapped around most axons. Very thin axons are the slowest; thicker axons conduct action potentials much faster. Some species, such as the intelligent octopus, evolved very big axons and large heads relative to their bodies. This works if you are a mollusk living in the salt water ocean. Our small skulls do not provide enough room for these large axons to live. A myelin sheath is analogous to the insulation on electrical wires; it prevents electrical signals on one axon from leaping over to nearby axons. In summary, during the evolution of bigger brains, myelin sheaths allowed axons to remain thinner but maintain a faster conduction speed for the action potential, thus providing faster information processing. All of these abilities developed within a brain small enough to pass through the birth canal.

  What is multiple sclerosis?

  Considering everything that myelin does for brain function, it is probably not surprising that your brain cannot function normally without myelin sheaths wrapped around many of its axons. To fully appreciate why, just imagine the consequences of losing some or all of the insulation on the wires inside your computer, or inside the walls of your home; nothing would work correctly. For example, you might throw a switch hoping to send electricity to your television; the electrical signal might randomly jump to another wire and proceed to a completely unexpected location, such as your toaster. Unfortunately, this misdirection of electrical signals occurs in the brains of people with multiple sclerosis. The immune system of these patients attacks the myelin sheath surrounding axons in their brain and spinal cord. Multiple sclerosis usually begins between the ages of 20 and 50 and is twice as common in women as in men. As the disease progresses, the passage of the action potential along axons becomes slower and slower until sometimes the action potential never makes it to its intended destination. Visual, motor, and sensory problems are the most common reported symptoms. Over time, as more myelin is lost, patients with multiple sclerosis develop problems controlling their thoughts and emotions. Interestingly, the symptoms worsen when the body gets overheated due to exercise, fever, or hot weather; this is likely due to the effect of the heat on the nerve’s ability to conduct an action potential. Clearly, the failure of action potentials to reach their intended destination interferes with normal brain function and makes the lives of these patients quite challenging.

  Does my brain work perfectly all of the time?

  Textbooks often present the brain as working perfectly at each and every step along the way to consciousness and normal thought; in fact, nothing could be further from the truth. Even if the action potential does successfully complete its journey to the end of the axon, there is no guarantee that anything will happen. There is considerable failure to function and outright chaos associated with much of what happens in your brain. Yet, somehow we do achieve consciousness in spite of the inherent errors associated with the challenge of getting all of the right processes to occur at precisely the right times. For example, under normal circumstances the arrival of the action potential at the end of the axon will induce the release of specific chemicals. Sometimes, nothing is released or too few of these chemicals are released. The chemicals released from the ends of neurons are called neurotransmitters; they instruct the next neuron to initiate its own action potential, and so on. This complex and surprisingly slow electrical-to-chemical communication process is happening in your brain while you are reading this sentence: lots of electrical ripples propagated along your visual axons inducing the release of neurotransmitter chemicals in the part of your brain that handles vision. This visual brain region then sent electrical action potentials off into the vast reaches of your brain to inform those brain regions that you are reading words expressed as black lines on white paper, and that these words have meanings that your brain must interpret. The arrival of action potential rippling along an axon is supposed to induce the release of neurotransmitter chemicals onto the next neuron, and so on and so on. This is how information and thoughts are processed. Various aspects of this process are happening, completely or partially, all over your brain, all of the time, and are the basis of your consciousness and everything you will ever experience. The take-home point is that our brain does not function perfectly all of the time; however, ultimately, we manage to endure with what we have evolved.

  What are neurotransmitters and what do they do for me?

  Within the brain, most of the major structures evolved as small clumps of cells, called nuclei or ganglia, which are involved in related functions. Some ganglia control movement, some control body temperature, and some control your mood. Overall, the basic plan, whether you are an octopus or a human, is that neurons communicate with one another in order to facilitate the sensing of the external world and the internal events taking place inside the body. The brain then decides which behavior to elicit in order to improve its chances of survival and propagation of its species. For humans and some other animals, there is sometimes a more ephemeral goal: Do something that brings pleasure. For humans during the 1960s, this was succinctly re-envisioned as sex, drugs, and rock and roll. We experience pleasure thanks to the particular neurotransmitter chemicals that are being released in the brain and to the particular regions of the brain where they are being released.

  The function of each neurotransmitter depends entirely on the function of the structure in which it is located. Let us look at a few examples. Deep within your brain is a region called the basal ganglia. The neurons in the basal ganglia are responsible for producing normal well-controlled smooth movements. The level of the neurotransmitter dopamine in these nuclei is much higher than in most surrounding brain regions. Therefore, scientists have concluded that dopamine
within the basal ganglia is critically involved in the control of movement. Furthermore, if we expose your brain to a drug that impairs the function of dopamine, then your ability to move will be impaired. Dopamine is obviously critical for movement. It would be incorrect, however, to assume that dopamine is only involved with the control of movement—it is not. You also can find dopamine in the retina of your eye and in your hypothalamus, structures that have nothing to do with movement. Dopamine also is released into small regions deep within the frontal lobes; its release produces a feeling of pleasure. Similarly, the neurotransmitter norepinephrine can be found in the hippocampus, a brain region that is critical for forming new memories. Thus, norepinephrine influences the formation of memories. However, norepinephrine also plays a role in other brain regions that have nothing to do with making memories. The take-away point is that there is no such thing as a specifically unique “dopamine function” or an exclusively distinct “norepinephrine function.” The brain region within which the neurotransmitter is found defines its function, not the neurotransmitter itself. In fact, neurotransmitters exhibit such a complex array of actions in various brain regions that we can rarely make a single universal statement about their role in brain function.

  Neurotransmitter chemicals are produced from the contents of your diet; thus, what you eat can, under certain conditions, influence how you think and feel. First, nutrients such as amino acids, sugar, and fats are absorbed from your food and transported across the blood–brain barrier into your brain; these nutrients are then absorbed into your neurons, where specialized enzymes convert them into neurotransmitters. The neurotransmitter molecules are then stored in very tiny spheres, which sit patiently waiting for the arrival of an action potential that instructs the neuron to release them. Once outside the neuron, the neurotransmitter wanders around looking for a way to communicate with the next neuron. The junction at which two neurons communicate is a synapse. The neurotransmitter molecule, now free to wander around within the synapse, will ultimately bump into and connect with a special protein, called a receptor. Receptors are like boats floating on the outer surface of the neuron on the other side of the synapse. Receptors offer comfortable docking ports for the neurotransmitter to insert itself. Once this docking of neurotransmitter and receptor has been achieved, the next stage in the communication process between neurons begins. At this point, lots of different things could occur; ions might move in or out of pores, enzymes might be activated, genes might be turned on or off, and many other possibilities. These secondary processes may have long-term consequences for the neuron’s behavior and ultimately for your thoughts and actions.