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The great Spanish neuroanatomist Santiago Ramón y
Cajal, who made fundamental contributions to the study of nervous system
anatomy at the turn of the 20th century, wrote, "Like the entomologist in
search of brightly colored butterflies, my attention hunted, in the garden of
gray matter, cells with elegant and delicate forms, the mysterious
butterflies of the soul." Cajal's "cells with elegant and
delicate forms" are neurons, the fundamental building blocks of the brain and
nervous system. If we wish to understand how the workings of the brain
relate to our perceptions, thoughts, feelings and memories, the neuron is as
good a place as any to start.
The neuron's exquisite architecture is evident in the picture to the right, which shows one of Cajal's drawings, from 1899, of a Purkinje neuron from the pigeon cerebellum. One of the first things to notice about the neuron is that it's different at the top than at the bottom. That's because the neuron is like a one-way street; it is designed by evolution to convey something from its top end, which consists of numerous dense branches called dendrites, through its roughly spherical cell body and down its long thin wire-like extension called the axon. That "something" is information. Neurons are information-processing cells. They move information from one part of the nervous system to another.
The dendrites of a neuron are like antennae, extended out to receive small pulses of information, in the form of chemical signals, from other neurons. These signals are passed from one neuron to another at specialized sites on the dendrites called synapses. A single Purkinje neuron has thousands of synapses on its dendrites. At the synapses, the neuron is relentlessly peppered by chemical signals, each carrying information from other neurons. The neuron converts this information into a decision, made on a moment-to-moment basis: whether or not to send its own signals, in the form of electrical impulses called action potentials, down its axon to other synapses on other neurons in other parts of the nervous system. When we consider that the typical human brain contains around 50 billion neurons and perhaps 100 trillion synapses, we can begin to appreciate how the amazing properties of the human mind emerge from this staggeringly complex, interconnected network of information-processing cells, constantly talking to each other.
Nowadays, most of us are familiar with the general idea that big, complicated things are made of smaller, simpler things. Galaxies, for example, are made of stars and planets which are composed of molecules and atoms. But is it really possible to understand our subjective experiences, things like the memory of a friend's face, the sight of a blue sky or the feeling of a warm breeze, in terms of chemical and electrical conversations between billions of tiny neurons? The 18th century German philosopher Immanuel Kant said that two things filled him with wonder and awe - "the starry heavens above and the moral law within." Modern neuroscience is leading us to the conclusion, unimaginable to Kant or anyone before his time, that these are simply two different configurations of the same basic stuff. As the British computer scientist Steve Grand put it, "Matter flows from place to place and momentarily comes together to become you." No wonder Cajal was so blown away, staring through his microscope at the mysterious butterflies of the soul.