Initiation Phase The initial stage in the carcinogenesis process, which consists of the alteration of the DNA (mutation) of a normal cell.  The initiated cell has thus developed a capacity for unregulated growth. Inorganic Compounds Simple molecules that usually consist of one or two different elements.  For example, water (H2O), carbon dioxide (CO2), bimolecular oxygen (O2), and sodium chloride (NaCl). Integumentary System The organ system that serves as a barrier to invading environmental organisms and chemicals and serves in temperature control.  Organs include the skin, hair, nails, and exocrine glands. Interneurons Interneurons are neurons located only in the CNS and provide connections between sensory and motor neurons. They can carry either sensory or motor impulses.  They are involved in spinal reflexes, analysis of sensory input, and coordination of motor impulses.  They play a major role in memory and the ability to think and learn. They are also known as association neurons.