Cajal's Neuron Doctrine is a fundamental principle in neuroscience proposed by Santiago Ramón y Cajal in the late 19th century. It states that the nervous system is made up of individual cells called neurons, which are the basic structural and functional units. Cajal emphasized that neurons are separate entities that communicate with each other through specialized connections known as synapses. This doctrine challenged the earlier belief that the nervous system was a continuous network. Cajal's work laid the foundation for modern neuroscience, helping scientists understand how information is processed in the brain and how neural circuits function.