nail: A keratinous structure produced by epithelial cells of the nail root.
narcolepsy: A sleep disorder characterized by falling asleep at inappropriate moments.
nares, external: The entrance from the exterior to the nasal cavity.
nares, internal: The entrance from the nasal cavity to the nasopharynx.
nasal cavity: A chamber in the skull that is bounded by the internal and external nares.
nasolacrimal duct: The passageway that transports tears from the nasolacrimal sac to the nasal cavity.
nasolacrimal sac: A chamber that receives tears from the lacrimal ducts.
nasopharynx: A region that is posterior to the internal nares and superior to the soft palate and ends at the oropharynx.
N compound: An organic compound containing nitrogen atoms.
necrosis: The death of cells or tissues from disease or injury.
negative feedback: A corrective mechanism that opposes or negates a variation from normal limits.
neonate: A newborn infant, or baby.
neoplasm: A tumor, or mass of abnormal tissue.
nephritis: An inflammation of the kidney.
nephrolithiasis: A condition resulting from the formation of kidney stones.
nephron: The basic functional unit of the kidney.
nerve impulse: An action potential in a neuron cell membrane.
neural cortex: An area of gray matter at the surface of the central nervous system.
neurilemma: The outer surface of a neuroglia that encircles an axon.
neurofibrils: Microfibrils in the cytoplasm of a neuron.
neurofilaments: Microfilaments in the cytoplasm of a neuron.
neuroglandular junction: A cell junction at which a neuron controls or regulates the activity of a secretory (gland) cell.
neuroglia: Cells of the central nervous system and peripheral nervous system that support and protect neurons; also called glial cells.
neurohypophysis: The posterior pituitary, or pars nervosa.
neuromodulator: A compound, released by a neuron, that adjusts the sensitivities of another neuron to specific neurotransmitters.
neuromuscular junction: A synapse between a neuron and a muscle cell.
neuron: A cell in neural tissue that is specialized for intercellular communication through (1) changes in membrane potential and (2) synaptic connections.
neurotransmitter: A chemical compound released by one neuron to affect the transmembrane potential of another.
neurotubules: Microtubules in the cytoplasm of a neuron.
neurulation: The embyrological process responsible for the formation of the central nervous system.
neutron: A fundamental particle that does not carry a positive or a negative charge.
neutropenia: An abnormally low number of neutrophils in the circulating blood.
neutrophil: A microphage that is very numerous and normally the first of the mobile phagocytic cells to arrive at an area of injury or infection.
nicotinic receptors: Acetylcholine receptors on the surfaces of sympathetic and parasympathetic ganglion cells; respond to the compound nicotine.
nipple: An elevated epithelial projection on the surface of the breast; contains the openings of the lactiferous sinuses.
Nissl bodies: The ribosomes, Golgi apparatus, rough endoplasmic reticulum, and mitochondria of the perikaryon of a typical nerve cell.
nitrogenous wastes: Organic waste products of metabolism that contain nitrogen, such as urea, uric acid, and creatinine.
nociception: Pain perception.
node of Ranvier: The area between adjacent neuroglia where the myelin covering of an axon is incomplete.
nodose ganglion: A sensory ganglion of cranial nerve X.
noradrenaline: See norepinephrine.
norepinephrine (NE): A catecholamine neurotransmitter in the peripheral nervous system and central nervous system, released at most sympathetic neuromuscular and neuroglandular junctions, and a hormone secreted by the adrenal medulla; also called noradrenaline.
nucleic acid: A polymer of nucleotides that contains a pentose sugar, a phosphate group, and one of four nitrogenous bases that regulate the synthesis of proteins and make up the genetic material in cells.
nucleolus: The dense region in the nucleus that is the site of RNA synthesis.
nucleoplasm: The fluid content of the nucleus.
nucleoproteins: Proteins of the nucleus that are generally associated with DNA.
nucleotide: A compound consisting of a nitrogenous base, a simple sugar, and a phosphate group.
nucleus: A cellular organelle that contains DNA, RNA, and proteins; in the central nervous system, a mass of gray matter.
nucleus pulposus: The gelatinous central region of an intervertebral disc.
nutrient: An inorganic or organic compound that can be broken down in the body to produce energy.
nystagmus: An unconscious, continuous movement of the eyes as if to adjust to constant motion.