Neurons were cells of the nervous system. They received chemical messages and in response, fired action potentials – electrical signals – that transmitted these messages to other neurons, as well as other sorts of cells. The interconnections between neurons in the brain were weighted, and the array of cells and weighted interconnections produced thought. The brain, the organ of thought and being in most species, was made up of neurons and supporting cells called glial cells. Neurons were also found in the vertebrate spinal cord and peripheral nervous system; there, they formed nerves that conducted the brain's instructions to the various parts of the body, and conduct sensory impressions (vision, hearing, pain, pleasure, and so forth) to the brain. There were various specialized kinds of neurons; each has evolved according to the role it fulfills.
In 2267, the crew of the USS Enterprise discovered that neural parasites resembling enormous brain cells were the cause of several outbreaks of mass insanity that had moved linearly through the galaxy. These parasites were interconnected in some fashion (although without physical contact); they formed a collective, single organism. The parasites were equipped with stingers; using these, they could inject a piece of tissue into a victim; this tissue grew tendrils that infiltrated the victim's nervous system very rapidly. A part of the larger organism, this mass of tendrils could exert control over the victim by inflicting intense physical pain - so intense, that the sensation eventually drove the victim insane. (TOS: "Operation -- Annihilate!")