The neuron doctrine is a fundamental principle in neuroscience that states that the nervous system is made up of individual cells called neurons. These neurons are the basic building blocks of the brain and spinal cord, and they communicate with each other through specialized connections known as synapses. According to the neuron doctrine, each neuron is a distinct entity, and information is transmitted in a unidirectional manner, from one neuron to another. This concept was established in the late 19th century by scientists like Santiago Ramón y Cajal, who used advanced microscopy techniques to visualize the structure of neurons.