William H. Calvin and George A. Ojemann, Inside the Brain: Mapping the Cortex, Exploring the Neuron (New American Library, 1980), chapter 2. See also http://WilliamCalvin.com/Bk1/bk1ch2.htm.
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William H. Calvin
University of Washington
Living Better Electrically:
Exploring the Neuron
William H. Calvin and
George A. Ojemann
About this time in midmorning, the electronics technicians get very busy and several more doctors appear. Rather than coming scrubbed into the OR, the doctors wander into the glassed-in gallery upstairs, which looks down into the OR. They are neurologists who specialize in the interpretation of the electroencephalogram, more commonly called EEG or "brain waves." They first analyzed Neil's brain waves months ago by recording minute electrical voltages from sixteen places on the skin of Neil's head.
Epileptic seizures are often preceded by, abnormal EEG voltage patterns; indeed, there are often abnormal EEG patterns in epileptics even when a seizure is not imminent. For the muscles to start jerking, the abnormal electrical activity must involve wide areas of the brain. In many epileptics, seizures originate from only a small region and then disrupt the surrounding brain, which is otherwise trying to go about its normal business. Locating this "epileptic focus" is the big problem.
In Neil's case, the scalp EEG suggested that this epileptic focus was in the left temporal lobe, which is in front of the left ear and below the motor and sensory strips. Since that portion of the brain can often be removed with little obvious effect upon a patient, Neil is willing to trade a potential deficit for the chance that his seizures can be brought under control. Had his epileptic focus been in the middle of the motor strip, it would have posed a dilemma: removing the left motor strip would have produced permanent paralysis of the right side of the body. Better seizures than paralysis.
The neurosurgeon gently places the tips of eight silver wires on the surface of the brain and hooks the wires up to a cable leading to the EEG machine in the gallery. A TV camera above the EEG machine is connected to the TV screen downstairs in the OR, so that the neurosurgeon can see the results too. These recordings from the surface of the brain are much more accurate than the EEG obtained earlier from the skin, where the voltages had to travel through the dura, the bone, some muscle, and then the scalp.
The EEG voltages are a product of the electrical signals from millions of nerve cells, mostly from those near- the surface of the brain. The EEG is an indirect indicator of their overall levels of activity, about the way that one could judge the overall daily activity patterns of the inhabitants of a~city by listening to traffic noises. What, however is the individual nerve cell doing with electricity? Nerve cells have two major tasks: computing, and then speeding the results of the computation to the far end of the nerve cell so that they can be passed on to another nerve cell. Both tasks run on electricity.
The individual nerve cells (also called neurons) are shaped like leafless trees, with branches and roots separated by a long trunk. Sometimes the trunk, called the axon in a nerve cell, is only one millimeter long. Some neurons have an axon so long that it can reach from the tip of the toe to the base of the brain. An electrical signal speeding along this axon tells the brain that something has touched the big toe. This electrical signal is called the impulse. It is only 1/10 of a volt (more than a thousand times smaller than household electricity), it lasts only 1/1000 of a second (quicker than most camera shutters), and it races along the axon at speeds as high as 500 kph (300 mph).
More often, the race is a relay race: there is a chain of nerve cells connecting the transducer in the skin with the brain. The impulse is passed from one nerve cell to another by an electrically triggered squirt of a chemical. In response to the momentary change in voltage, a chemical is secreted from the end of one nerve cell and diffuses to the next nerve cell in the chain. This junction between two nerve cells is called a synapse. The chemical, called a neurotransmitter substance, in turn produces a change in the voltage of the downstream nerve cell. Many types of drugs, such as painkillers and tranquilizers, interfere with this chemical process connecting two nerve cells with each other at the synapse, and thus enhance or reduce the strength of the connection.
But this chemically evoked voltage change in the downstream cell does not usually cause an impulse by itself The second neuron in the chain from transaucer to brain is typically located in the spinal cord. There are hundreds of transducers in the skin and muscle converging upon this cell in the spinal cord, plus thousands of inputs from other neurons in- the brain and spinal cord. Some produce negative voltage changes ("inhibition"), others produce positive changes ("excitation").
As in a checkbook, the balance is what counts. If the voltage balance is big enough in the positive direction, another impulse will be triggered and will speed along the axon of the second nerve cell, often heading up toward the brain. The balance also determines the rate at which impulses can be produced, which varies between 0 and 1,300 impulses each second. Each nerve cell is thus a simple computer, adding and subtracting influences from many inputs, sending its new message on to many other cells. Chemicals are very important in producing the electricity, just as chemical reactions in batteries may be said to run a pocket calculator or a wrist ,watch. Chemicals also may slowly change the strength of the connection between two nerve cells. But it is with electricity that nerve cells add and subtract, and by using electricity that they speed their messages along to the next nerve cell. The EEG and "evoked potentials" are noisy by-products of all this .electrical processing of information, but clinically useful ones.'
The EEG is not, however, an average of the electrical activities of the brain beneath the electrode. The closer that the neurons are to the electrode, the bigger their contribution. The cerebral cortex is only a few millimeters thick (less than two typewriter spaces). It may be divided into layers (usually six, numbered I-VI but sometimes subdivided even further, e.g., layer IVc). The largest neurons of the cortex are often found in the deeper layers, such as layer V. The inputs to the cortex often prefer certain layers, e.g., the messages from the eyes arrive only in layer IVc of visual cortex. The most poorly understood layers are the ones closest to the brain's surface (I, II, and III), and they are the ones which contribute most heavily to the EEG. Yet despite the lack of detailed understanding about the origins of the EEG in terms of the activities of neurons, a practiced eye can recognize normal and abnormal patterns in the electrical fluctuations of the EEG. Such empirical knowledge is essential to the analysis of where Neil's epilepsy originates.
Continue to CHAPTER 3
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