This book describes a century of research on how nerve cells communicate with one another, beginning with the formulation of the Neuron Theory and proceeding through studies embracing a broad range of disciplines. The Neuron Theory initially depicted discrete nerve cells interacting at their points of contact ("synapses"); since nerve impulse were often indentified as electrical signals traveling along neuronal processes, it seemed plausible that impulses would also pass from cell to electrically. Over the next hundred years, however, ingenious experiments, facilitated by powerful new techniques and interpreted with imaginative new insights, established new accounts rich in scientific detail: communication was generally achieved by releasing chemicals from one neuron to interact with specific receptors on another, thereby initiating complex chains of metabolic alterations as well as eliciting electrical responses; neurotransmitters were stored in vesicles for release onto postsynaptic neurons, and transport back into presynaptic neurons terminated the actions of some neurotransmitters whereas metabolic degradation terminated the actions of others. The formation of specific synapses during embryological development and the alterations in synaptic transmission accompanying learning also required intricate chains of cellular modifications. Disorders of synaptic transmission could result in neurological and psychiatric diseases, whereas drugs affecting particular steps in synaptic transmission could achieve dramatic therapeutic responses.