From Normal Thoughts to Seizures:
The Workings of the Brain
The brain is an incredibly complex organ that controls not only what people think and do, but who they are. Ancient philosophers believed that the heart was the seat of the soul, but we now know that it is actually the brain that is the organ of feelings and thoughts, the source of reactions to the world and our interpretations of it. The brain makes us move and speak, see and hear, feel and understand. The brain is also the source of epilepsy, which is probably why epilepsy is such a complicated disease, with manifestations as diverse as the brains from which it arises. Understanding epilepsy, then, begins with an understanding of the workings of the brain.
The brain is an organ of communication, whose job is to process all of the information important to its owner. All of the complicated things that our brain helps us do -- translate the spiky shapes of letters and lines on a page into words and thoughts; steer around a pedestrian who suddenly steps into the street; recognize the face of a child and feel overwhelming love for her -- consist of electrical and chemical signals that pulse between neurons, which are specialized brain cells. It's sometimes helpful to think of the brain as a living computer. Both brains and computers are made up of individual circuits, both run (more or less) on electricity, and both are capable of retaining and processing information. Both can also malfunction: as a computer screen can briefly freeze, so can a human brain pause during a seizure. But there are differences as well. The human brain can regenerate and heal itself. This is fortunate, because although we can upgrade and buy new computers, we have only one brain throughout life. We sometimes feel that computers do things much more efficiently and easily than our own brains. But no computer can match the creativity and depth of experience that a single human brain has.
This chapter is a brief guided tour through the brain, from the relatively simple individual neuron that is either "on" or "off" to the complex networks of thought and feeling that make us human.
The Brain Cell: The Smallest Unit of Thought
The brain is made up of billions of nerve cells called neurons. Akin to the wires and circuits of the brain, neurons are held together by structural cells, called glia, that protect and insulate the circuits of the brain. Glia are like the insulation on a wire and the backing on which the circuits are mounted. Fundamentally, a neuron (or more accurately a group of neurons) in your brain interprets incoming information, mulls it over, then sends off a signal that results in actions. It is also a neuron (or a group of neurons) that, probably beginning in much the same way, causes all kinds of seizures.
Structurally, a neuron has a cell body that holds most of its workings: the genetic material that determined how it was formed, enzymes that make proteins and transmitter substances, and other machinery for producing energy to run the cell (Figure 1-1). The cell also has "dendrites" -- arms of the cell that branch out to the surrounding area, listening to other neurons in the area and taking in-formation back to the cell body. Some neurons have relatively simple "dendritic trees" that connect with only one or two other neurons; others have extensive branches connecting with dozens or more. Neurons also have an "axon": the output arm of the cell. The axon carries each individual neuron's message to other cells. A neuron, then, looks a little like a scorpion: arms out in all directions, a body that runs the show and decides when to respond, and a stinger that carries a single message to the outside.
The neuron is constantly processing information from other neurons, deciding whether it should "fire" (send a message itself) or stay quiet. When a neuron fires, it releases one or more chemicals called neurotransmitters -- serotonin, norepinephrine, epinephrine, and glutamate are examples. Each of these chemicals can either excite -- so that the receiving cell gets a buzz and thinks about firing itself -- or inhibit -- quieting, encouraging the receiving neuron to stay still. When a neuron fires, only other neurons with an appetite for its particular transmitter will be aroused at what are called receptor sites, the place where the neurotransmitter attaches. One neuron may respond to glutamate but not to serotonin. No receptor, and the transmitter bounces off and tries to find another neuron that's listening. So neurons are selective in which other neurons they listen to.
Is a single neuron firing a thought, or can it be a tiny seizure? So far as we understand these, it can be neither. Neuronal firing can only be thought of as a spark. If it falls on ready tinder, a flame can result, but if it falls on water or dust, it fizzles and remains unnoticed.
Neural Networks: From a Spark to a Thought
It may be difficult to imagine how the simple "on-off" system of a neuron can translate into the complex thoughts and reactions of the human brain. The system begins to get more complex when the concept of neural networks comes in. Every brain contains billions of neurons, each of which can talk to one or many other neurons, and each of which listens to other (or the same) neurons. These create hundreds of thousands of neural pathways in a brain, where neurons are connected to other neurons, and there are an infinite number of possible messages. Moreover, neurons can learn. As neurons talk and listen to each other, fire and quiet, they can develop relationships among themselves. Some pathways between neurons become grooved with frequent contact, and like a well-worn path through the woods a memory is formed. Other paths become overgrown and hard to find if only used once or twice ...(Continues...)