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  • N-acetylglutamate:: a coenzyme that assists in processing the first step of the urea cycle and activates carbamoyl phosphate synthetase I.
  • Narcotic analgesics: painkillers that contain an opiate and therefore may be addicting with prolonged use. Names include hydrocodone, codeine, morphine, oxycodone, meperidine (Demerol), methadone, fentanyl (Duragesic), butorphanol (Stadol) and others. They appear alone or in combination with aspirin, ibuprofen or acetaminophen. Tradenames for the compound drugs include Vicodin, Percocet, Norco, Tyco, and OxyContin.
  • Negative feedback: when the end-product of a pathway shuts down the initiation of product formation.
  • Nervous system: the system of cells, tissues, and organs that regulates the body's responses to internal and external stimuli. In humans it consists of the brain, spinal cord, nerves, and ganglia.
  • Neuroendocrine system:  the system of cell that release hormones that affect the nervous system.
  • Neurological: pertaining to the nerves and nervous system, including the disorders and diseases that affect them.
  • Neurotoxin:  a substance that is poisonous or damaging to the nerves.
  • Neurotransmission: communication among nerve cells through signaling processes.
  • Neurovisceral: refers to the nerves of the internal organs.
  • Nitrogen: an atmospheric gas that forms many compounds in the body (such as proteins) and is a component of all living cells.
  • Nucleic acid: DNA is a string of nucleotides, which contain genetic information and is in the cells of all living things and some viruses.
  • Nucleotides: specific molecules that, when joined together, make up the structural units of DNA and RNA. And also play important roles in metabolism.