Part 6: The Neuron Doctrine and the Synapse: Cajal, Golgi, and Sherrington

As the 19th century drew to a close, scientists turned their attention from the brain's large-scale organization to its microscopic architecture. The question that dominated this era seemed deceptively simple: how were the billions of cells in the nervous system connected to one another? The answer would revolutionize our understanding of brain function and lay the foundation for modern neuroscience.

Two competing theories emerged to explain neural organization. The reticular theory, championed by Camillo Golgi, proposed that the nervous system was a continuous network, with nerve cells physically fused into a single interconnected web. This view suggested that neural signals flowed through the brain like electricity through a wire, with no interruptions or boundaries between cells. The alternative—the neuron doctrine—proposed that the nervous system consisted of individual cells that came close together but remained distinct entities. This seemingly technical anatomical dispute had profound implications for understanding how information flowed through the nervous system.

The resolution came through the meticulous work of Santiago Ramón y Cajal, whose beautiful drawings of neural tissue remain iconic today. Using Golgi's own silver staining technique, Cajal painstakingly traced individual neurons and demonstrated that they were indeed separate cells. His detailed observations revealed that neurons had a characteristic structure: a cell body with branching dendrites that received signals and a single axon that transmitted signals to other cells. Cajal's drawings showed that while neurons came into close contact, they never actually merged. His work was so compelling that both he and Golgi shared the 1906 Nobel Prize, despite their fundamental disagreement about neural organization.

If neurons were separate entities, how did signals pass between them? Charles Sherrington provided the answer through his ingenious experiments on reflexes. He noticed that neural transmission showed properties inconsistent with direct electrical connection: signals traveled in only one direction, showed delays, and could be inhibited or facilitated. Sherrington proposed that neurons communicated across tiny gaps, which he named "synapses" (from the Greek word meaning "to clasp together"). His work revealed that synaptic transmission was not simply electrical but involved complex processes that could modify and regulate neural signals.

The neuron doctrine transformed our understanding of the nervous system from a continuous network to a collection of discrete computational units. This insight opened new avenues for understanding how the brain processes information, learns, and adapts. The synapse emerged as the crucial site where neural signals could be modulated, providing a mechanism for the flexibility and plasticity that characterizes brain function. These discoveries set the stage for the 20th century's explosion of knowledge about neural communication, from the discovery of neurotransmitters to modern computational models of neural networks.