“I think, therefore I am” may be great philosophy, but let’s face it, there’s more to life than thinking. Most of us also have a body of some description (some descriptions are better than others), and our bodies and brains working together make us us–which is not to say that there’s any doubt about who gets top billing. The brain may only be a kilogram and a third of pulpy gray, pink and white tissue, but it’s the most important organ in our body, and the body knows it.
Though it only makes up two percent of the body’s weight, the brain receives 20 percent of the body’s blood, and therefore 20 percent of its oxygen. It is encased in eight thin, domed interlocking plates of bone called the cranium, providing maximum strength with minimum weight, and it is cushioned by cerebrospinal fluid. It’s even protected by a brain-blood barrier that allows oxygen, water and glucose in but keeps out many harmful chemicals.
The brain has several parts. At its base are the structures that govern basic bodily functions. The medulla keeps our lungs pumping, our heart beating and our blood pressure under control. The limbic system controls emotions and the most basic drives (food, survival, etc.) and helps maintain the body’s internal environment.
The cerebellum, at the back of the head, and the caudate nucleus help coordinate body movements and maintain balance. The thalamus processes information related to sensation and movement and relays it to the cerebral cortex, and the pons is a communication bridge between the cerebellum and the cerebral cortex.
The cerebral cortex is the what we generally think of as the brain–the wrinkly gray stuff. It makes up about 40 percent of the brain’s total weight, even though it’s only about three millimetres thick. It contains 75 percent of the brain’s 100 billion nerve cells, or neurons–equivalent to the number of stars in our galaxy.
Neurons are the tools that the brain uses to interact with the body. Neurons carry messages from the various parts of the body to the brain, process that information, and then transmit the brain’s instructions back to the body.
Neurons consist of a cell body, dendrites (short feathery fibers that conduct impulses toward the cell body) and an axon (a long, thin projection carrying impulses to other neurons or tissue). Each axon may have sub-branches of its own, and may be wrapped in an insulating substance called myelin. Axons end in feathery branches, finer than dendrites, terminating in tiny knobs that release chemicals called neurotransmitters onto a muscle or the dendrites of the next nerve cell.
The gap between one neuron and the next, a millionth of an inch wide, is called the synaptic cleft. An electrical signal coming from the cell body of one neuron travels to the end of the axon, where it causes the release of neurotransmitters. These drift across the synaptic cleft and stimulate the dendrite of the next neuron to transmit an electrical signal of its own–all in less than one ten-thousandth of a second.
Those electrical signals may be coming to the brain or travelling the other way. The brain and the spinal cord are the central nervous system, but there is also a peripheral system of nerves emerging from the brain and spinal cord. It’s these nerves which keep brain and body in touch.
Most nerves in the peripheral system contain two kinds of fibers, sensory and motor. The sensory fibers bring messages to the central nervous system from the skin, muscles and sense organs; the motor fibers carry instructions from the central nervous system back to the skeletal muscles. This part of the peripheral system is called the somatic nervous system, which controls the body’s relationship to its external environment.
Many peripheral nerves also contain fibers leading to internal organs and glands. This is the autonomic nervous system, which monitors and regulates internal conditions. Though not under conscious control, this system is still controlled by the brain.
Messages to and from the brain travel with amazing speed. The fastest nerve cells (those insulated with myelin) transmit signals at 150 metres per second. (Uninsulated fibers manage only one metre per second.)
The quantity of signals is even more amazing. Consider everything your nervous system is doing as you read this: your eyes are moving back and forth and sending a steady flow of information to the brain, which is processing it and storing some of it in memory while calling up other memories to help you understand it; your nervous system is instructing your skeletal muscles to keep you in a sitting position; your eyes are being instructed to blink about 25 times a minute; you’re receiving messages from all your internal organs and they are being instructed to beat, digest, or do whatever else it is they do; you’re feeling the temperature of the air and the roughness of the chair’s upholstery; maybe you’re smelling dinner cooking or hearing a background conversation. All of this is happening at once; all of it involves countless neurons firing signals to the brain, and, in the brain, other neurons firing messages back.
Every neuron in the brain can send messages to as many as 10,000 other neurons and receive signals from more than 1,000. This means every one of the billions of neurons in our brain can conceivably carry on several million separate “conversations” with other neurons at once.
Which is why Descartes had enough brain power left over from just existing to worry about whether he really did or not.
