Neuroscience For Dummies
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The brain is amazing because of what it is (a switchboard of billions of neurons), and because of what it does (the memory and artistic skills of some savants almost defy belief). Here are seven attributes of the human brain that, when you think about it, are pretty amazing.

The brain has 100 billion cells and a quadrillion synapses

Your brain is built from an enormous number of neurons, on the order of 100 billion (plus possibly 1 trillion glial cells). Each of these neurons makes about 10,000 connections, called synapses, with other neurons, yielding about a quadrillion connections. In fact, your brain has enough neurons and neuronal connections that theoretically it could store each and every experience in your life, including all the visual, auditory, tactile, and other sensations associated with those experiences. The number of distinct brain states expressed this way far exceeds the number of atoms in the known universe.

Most of the cells in your brain are in the neocortex and the cerebellum. The cerebellum uses lots of cells to allow high precision in coordinated movement. The neocortex uses lots of cells for high precision in sensory discrimination and for planning complex behavior.

Consciousness doesn't reside in any specific area of the brain

Many people misunderstand how the brain works because of the popular — but incorrect — notion that certain functions reside solely in particular parts of the brain. One part of the brain is thought to house your ability to taste, another your ability to see, another your ability to move your right hand, and so on. Given this perception, it's easy to see why people tend to think that consciousness resides in a particular area.

The brain doesn't have any consciousness neuron, and no particular brain area, by itself, serves as the seat of consciousness. Nor is consciousness just a function of brain size; otherwise, elephants would be conscious and humans not. No place in the brain receives the results of all the neural processing in the rest of the brain, so there's no "top" to the neural hierarchy, and nothing in the brain "looks at" images formed in other parts of the brain.

Instead, several areas of the brain are necessary for consciousness. Areas of the brain that seem to be necessary for and activated by consciousness include the thalamus, the prefrontal cortex, and portions of the parietal and medial temporal lobes. Still, these areas are not unique to the human brain, although the prefrontal cortex is larger (as a percentage of body weight) in humans than any other animal. In addition, damage to the reticular formation in the brain stem produces unconsciousness, but that brain area exists in non-mammalian vertebrates such as lizards and frogs.

The brain has no pain receptors

Although the experience of pain is dependent on brain areas such as the anterior cingulate cortex, the brain tissue itself actually has no receptors for pain. The brain "experiences" the pain reported by receptors elsewhere in the body, mostly in the skin.

Because brain tissue has no pain receptors, surgery can be done on the brain with the patient fully awake (although tranquilizers and analgesics are typically given to reduce anxiety, and local anesthetics are used for the early phase of the surgery where the scalp skin is cut in order to remove a piece of the skull).

When you have a headache, then, it is not usually because something in your brain hurts, but because a pain message from somewhere in your body reaches the brain. For example, you can get a headache because you actually have mild indigestion or some other body pain of which you are not directly aware.

This doesn't mean, however, that brain dysfunction can't be felt as pain. Migraine headaches appear to be due to transient vascular problems in the brain that lead to abnormally high neural activity, which is felt as pain not because pain receptors are activated, but because some unknown brain circuit interprets that excessive activity as painful. Tumors and strokes may induce excessive brain activity that the brain similarly interprets as painful. In some cases, pain receptors may be activated but referred to the wrong place (as when trigeminal pain receptors are activated).

Another example is the pain associated with looking directly at a very bright light. There are no pain receptors for bright light in the eye, but the brain interprets something about the firing of ganglion cells going from the eye to the brain as indicating that the light level is high enough to be damaging, and it's felt as pain.

Cutting the largest fiber tract in the brain produces few side effects

The largest fiber tract in the nervous system is the corpus callosum, which connects the left and right brain hemispheres. This fiber tract contains about 200 million axons (slightly more in women than men).

This tract has been severed surgically many times to stop epileptic seizures from spreading from one cerebral hemisphere to the other, becoming amplified, spreading back, and producing a whole brain grand mal seizure.

When this procedure was tested on a few patients, the results were remarkable. Several of the patients experienced a significant reduction in seizures, and none of the patients showed any obvious neurological deficit from the surgery.

Adults lose several hundred thousand neurons a day with no noticeable effect

Neuronal count peaks at about birth in humans; then, over the lifespan, about 10 percent of this is lost, which amounts to losing several hundred thousand neurons a day, more or less. Although some may argue male behavior during midlife crises is proof of the negative impact losing so many neurons a day can have, the fact is that this loss shouldn't compromise mental capacity that much.

The loss of so many neurons that occurs in normal aging without any apparent mental compromise is one reason most neuroscientists don't believe that single neurons store single memories. If they did, death of individual neurons would produce sudden, irretrievable loss of specific memories, which doesn't happen.

Instead, information is spread out over many cells and synapses. When neurons in the brain die, it almost certainly weakens but does not erase any memory they were involved in storing. Other neurons in the network undoubtedly compensate to restrengthen the memory. It may be that senility occurs when so many neurons are lost that not enough are left to compensate for the ongoing neural death.

Pound for pound, the brain takes a lot of energy

The brain has the highest metabolic activity per mass of any part of the body. It constitutes about 5 percent of your body mass but consumes about 20 percent of its energy. This also means that about 20 percent of the total blood flow in the body is coursing through the brain, supported by blood vessels whose length, including capillaries, is hundreds of miles.

The high energy use by the brain has been taken advantage of in brain imaging. Brain scan techniques like fMRI (functional magnetic resonance imaging) actually detect either the increased blood flow in brain areas that are highly active or the level of blood oxygenation in active versus less active areas.

The high metabolic demand created by big brains has been used to argue that animals with large brains, such as dolphins, must be highly intelligent, even though their intelligence may be fundamentally different from human intelligence; otherwise, there would be no fitness advantage of supporting such a large brain, and the forces of natural selection would have favored smaller rather than larger brain size.

Adult brains can grow new neurons

The central nervous systems of babies and non-mammalian vertebrates such as fish readily regenerate damaged brain areas and tracts following injury. This isn't the case with adults. Overall, people lose neurons as they age. In addition, after brain injuries such as strokes, even when there is some recovery of function, the area of the brain damaged doesn't heal; it still consists of scar tissue and vacant spaces filled with fluid.

However, the idea that the adult brain is fixed and incapable of neural regeneration is undergoing profound change. In the last quarter of the 20th century, study results began showing that some regions of the adult human brain sometimes grow new neurons. Particularly exciting was that discovery that one of the regions where new neurons can be added is the hippocampus, a medial temporal lobe area crucial for forming long-term memories, and that the growth of new neurons in the hippocampus is associated with this learning.

Another place that new neurons grow in adults is in the olfactory system; specifically, olfactory receptors are constantly being replaced. Researchers hypothesize that these receptors, in detecting odors, are exposed to toxins and that the way to maintain function is to replace them routinely.

Recent research has exploded on the capabilities of stem cells, which divide and differentiate into the different final cell types during an organism's development. Very recently, techniques have been found to convert adult, differentiated cells into stem cells, which offers the possibility of pharmacologically triggering neural regeneration where it might be needed.

About This Article

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Frank Amthor is a professor of psychology at the University of Alabama at Birmingham, where he also holds secondary appointments in the UAB Medical School Department of Neurobiology, the School of Optometry, and the Department of Biomedical Engineering. His research is focused on retinal and central visual processing and neural prostheses.

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