nanograms per milliliter: A small quantity of a substance; equivalent to one-billionth of a gram (454 grams make 1 pound) in one-thousandth of a liter (1 liter is approximately 1 quart). Abbreviated as ng/ml.

narcolepsy: A sleep disorder that causes extreme sleepiness and uncontrollable sleep attacks, making a person fall asleep at inappropriate times during the day.

nasolabial folds: Lines in the skin leading from the nose to the outer corners of the mouth. Also known as smile or laugh lines.

natural killer cells: A type of white blood cell of the immune system. These cells destroy other cells that fail to display the right chemical flags signaling that they are normal cells.

natural recoverers: People who overcome addiction without treatment or formal self-help programs.

nebulizer: A device that converts a liquid medicine into a mist that can be breathed in.

necrosis: The premature death of living cells or tissues.

needle biopsy: Use of a hollow needle to remove a small sample of tissue for examination.

neoadjuvant therapy: A helper treatment given before a primary treatment is started, such as when chemotherapy is done before surgery in order to shrink a tumor.

neonatal: Relating to an infant younger than 4 weeks of age.

neoplasm: An abnormal growth of tissue, either cancerous or benign.

nephritis: Inflammation of the kidneys.

nephropathy: Kidney disease.

nerve block: Injection of a medication into one or more nerves to relieve pain.

nerve growth factor: A molecule in the body that promotes the growth and repair of nerve cells.

nerve sparing: When referring to prostatectomy, the surgical procedure that preserves the nerves needed to allow the penis to become erect.

neuralgia: A burning or stabbing pain that follows the path of a nerve.

neuritic plaques: Clumps of sticky proteins found in the brains of people with Alzheimer's disease.

neurofibrillary tangles: Twisted strands of proteins that are found inside the dead or dying nerve cells of people with Alzheimer's disease.

neuroleptic agents: Powerful tranquilizing drugs used to treat schizophrenia.

neurologist: A physician trained to diagnose and treat disorders of the nervous system, including the brain and spinal cord.

neuromuscular junction: The tiny space between the end of a nerve and the surface of a muscle.

neuron: A nerve cell.

neuropathy: Nerve damage and resulting loss of sensation, movement, or other function.

neuropeptides: Small proteins that aid in transmitting signals between nerve cells.

neurosyphilis: A rare infection of the brain or spinal cord that occurs when syphilis goes untreated for many years.

neurotransmitter: A chemical messenger released by nerve cells that transmits messages to nearby other nerve cells.

neurotransmitter receptors: Cell structures (usually proteins) that recognize specific neurotransmitters and bind to them. Once bound, a receptor often changes shape, causing a cascade of chemical events within the cell. These events can alter which genes are turned on or off and can make the cell more or less likely to release its neurotransmitters.

neutral alignment: Keeping the body in a straight line from head to toe except for the slight natural curves of the spine.

neutral posture: A standing or seated position in which the chin is parallel to the floor; the shoulders, hips, and knees are at even heights; and the knees and feet point straight ahead.

neutral spine: A position in which the back is straight except for the slight natural curves of the spine.

neutropenia: An abnormally low number of white blood cells.

neutrophils: White blood cells that seek out and engulf foreign cells.

nitrates: Medications that widen blood vessels; usually used to treat chest pain from angina and other heart problems.

nitric oxide: A compound produced by the endothelium (the lining of the interior walls of arteries) that helps widen blood vessels and counteract high blood pressure. Also called endothelium-derived relaxing factor.

nitroglycerin: A drug that relaxes blood vessels and increases the supply of blood and oxygen to the heart while reducing its workload. It is commonly used to treat angina.

NK: Abbreviation for natural killer cells, a type of white blood cell of the immune system. These cells destroy other cells that fail to display the right chemical flags signaling that they are normal cells.

NMDA receptor: Abbreviation for the N-methyl-D-aspartame receptor, a molecule on the surface of a brain cell that admits calcium when activated by the chemical messenger glutamate.

N-methyl-D-aspartame receptor: A molecule on the surface of a brain cell that admits calcium when activated by the chemical messenger glutamate.

NO: Abbreviation for nitric oxide, a compound produced by the endothelium (the lining of the interior walls of arteries) that helps widen blood vessels and counteract high blood pressure. Also called endothelium-derived relaxing factor.

nociceptors: Nerve endings that detect pain and transmit pain information to the brain and spinal cord.

nocturia: Waking up more than once during the night to urinate.

nodule: A small rounded bump or knot of tissue.

nomogram: A chart or graph of mathematical calculations of risk; used in making treatment recommendations and predicting outcomes.

non-HDL cholesterol: The sum of all cholesterol types other than high-density lipoprotein (HDL). These include very-low-density lipoprotein, low-density lipoprotein, and intermediate-density lipoprotein.

non-insulin-dependent diabetes: Now called type 2 diabetes. A disease in which levels of blood sugar (glucose) are too high initially because cells can't properly use insulin (a hormone that helps cells absorb glucose from the bloodstream). Over time the production of insulin may decline.

noninvasive test: A test that does not require any medical instruments to break the skin or enter the body.

nonketotic hyperglycemic hyperosmolar syndrome: A rare and very serious condition associated with type 2 diabetes. Symptoms include extremely high (more than 800 mg/dl) blood sugar levels, severe dehydration, and changes in mental status, ultimately resulting in coma.

nonproliferative retinopathy: A condition in which the walls of the small blood vessels in the retina leak serum and tiny pockets of swelling form in the walls of blood vessels. Also called background retinopathy.

non-REM sleep: The sleep phase that includes deep sleep, the type considered most important for preventing daytime sleepiness.

nonsteroidal anti-inflammatory drug: A drug that reduces swelling and pain.

noradrenaline: A hormone produced by adrenal glands that puts the body on heightened alert when a threat is perceived (the fight-or-flight response). Also known as norepinephrine.

norepinephrine: A hormone produced by adrenal glands that puts the body on heightened alert when a threat is perceived (the fight-or-flight response). Also known as noradrenaline.

normal-pressure hydrocephalus: A buildup of fluid in the brain that causes the brain to swell, and leads to slowing of mental function, trouble walking, and a loss of bladder control.

NSAID: Abbreviation for nonsteroidal anti-inflammatory drug, a drug that reduce swelling and pain.

nuclear tests: Tests that use tiny amounts of short-lived radioactive substances that can target particular organs or cell receptors to diagnose disease.

nucleus accumbens: Part of the brain's reward pathway that is most tightly and consistently responsive to pleasure. Also known as the pleasure center.

nucleus pulposus: The gel-like shock-absorbing central portion of each spinal disc.

nutraceutical: Dietary supplement containing concentrated forms of a presumed bioactive substance originally derived from food and used to enhance health in dosages exceeding those normally obtainable from food.

nutrients: Substances in foods that the body needs to survive.