Skip to main content
Biology LibreTexts

25.14: Glossary: N

  • Page ID
  • nail

    accessory organ of the skin made of sheets of dead keratinocytes at the distal ends of the fingers and toes

    nail bed

    pink skin under the nail plate that is visible through the nail

    nail matrix

    deep layer of epidermal tissue at the proximal end of a nail where nail growth occurs

    nail plate

    visible part of a nail that is external to the skin

    nail root

    portion of a nail that is under the surface of the skin at the proximal end of the nail

    nasal cavity

    large, air-filled space in the skull above and behind the nose that helps conduct air in and out of the body as part of the upper respiratory tract

    natural gas

    naturally occurring nonrenewable gaseous fossil fuel consisting primarily of methane that is less polluting to burn than other fossil fuels

    natural resource

    something supplied by nature that helps support life

    natural selection

    force of evolution in which some living things produce more offspring than others so the characteristics of organisms change over time

    negative feedback loop

    control mechanism that serves to reduce an excessive response and keep a variable within its normal range


    newborn infant from birth to the age of four weeks


    one of the million tiny structural and functional units of the kidney that filters blood and forms urine


    type of nervous tissue that consists of many cable-like bundles of axons and makes up the majority of the peripheral nervous system

    nerve impulse

    electrical signal transmitted by the nervous system

    nervous system

    human organ system that transmits electrical signals throughout the body to coordinate all of the body’s voluntary and involuntary activities

    nervous tissue

    tissue made up of neurons that carry electrical messages and glial cells that support neurons

    neural tube

    structure that forms in an embryo and eventually develops into the brain and spinal cord


    formation of new neurons by cell division

    neuroimmune system

    part of the immune system that protects the central nervous system

    neuromuscular disorder

    muscle disorder that occurs due to problems with the nervous control of muscle contractions or with muscle cells themselves

    neuromuscular junction

    chemical synapse where a motor neuron transmits a signal to a muscle fiber to initiate a muscle contraction


    functional unit of the nervous system that transmits nerve impulses; also called nerve cell


    type of chemical that carries nerve impulses from the axon of a neuron to another cell across the synapse


    process in which an embryo develops structures that will eventually become the nervous system


    role of a species in its ecosystem that includes all the ways the species interacts with the biotic and abiotic factors of the ecosystem


    highly addictive psychoactive stimulant drug that is found in tobacco and tobacco smoke

    nitrogen cycle

    biogeochemical cycle through which nitrogen is recycled through the biotic and abiotic components of ecosystems

    nitrogen fixation

    process of changing nitrogen gas to nitrates that is carried out by nitrogen-fixing bacteria in the soil or in the roots of legumes


    type of sensory receptor that responds to pain

    node of Ranvier

    one of the regularly spaced gaps between myelin sheaths along an axon that allow nerve impulses to travel very rapidly


    unable to be broken down in the environment by natural processes


    failure of replicated chromosomes to separate during meiosis II, resulting in some gametes with a missing chromosome (or part of a chromosome) and some with an extra chromosome (or part of a chromosome)

    non-essential nutrient

    nutrient that can be synthesized by the body in sufficient quantities for normal functioning so it does not need to be obtained from food

    noninfectious disease

    any disease caused by genetic and/or environmental factors other than pathogens; also called noncommunicable disease

    non-Mendelian inheritance

    inheritance of traits that have a more complex genetic basis than one gene with two alleles and complete dominance

    nonpoint-source pollution

    contamination of the environment by a pollutant that enters the environment from multiple sources

    nonrenewable resource

    natural resource that exists in a fixed amount and cannot be replenished at all or cannot be replenished quickly enough to keep pace with human use of the resource

    non-steroid hormone

    type of endocrine hormone that is made of amino acids and binds with a receptor on the plasma membrane of a target cell

    normal range

    spread of values around the set point of a biological variable such as body temperature that is considered insignificant in terms of health

    nuclear energy

    nonrenewable energy resource produced in a nuclear power plant by fission reactions in a radioactive fuel, usually uranium

    nuclear force

    atomic force holding together protons and neutrons in the nucleus of an atom that is stronger than the electromagnetic force repelling positively charged protons from each other

    nucleic acid

    class of biochemical compounds that includes DNA and RNA


    small molecule containing a sugar, phosphate group, and nitrogen base that is a building block of nucleic acids

    nucleus (plural, nuclei)

    organelle inside eukaryotic cells that contains most of the cell’s DNA and acts as the control center of the cell


    substance the body needs for energy, building materials, or control of body processes

    nutrient density

    how much of a given nutrient is provided by a particular food, relative to the mass of the food or the amount of Calories it provides


    process of taking in nutrients in food and using them for growth, metabolism, and repair

    nutrition facts label

    label on packaged food that lists the nutrient content per serving of the food and also its ingredients