572 GlossaryMuscle tone(MUSS-uhl TONE) The state of slight con-
traction present in healthy muscles (Chapter 7).
Muscular dystrophy(MUSS-kyoo-ler DIS-truh-fee) A
genetic disease characterized by the replacement of mus-
cle tissue by fibrous connective tissue or adipose tissue,
with progressive loss of muscle functioning; the most
common form is Duchenne’s muscular dystrophy
(Chapter 7).
Muscular system(MUSS-kew-ler) The organ system that
consists of the skeletal muscles and tendons; its functions
are to move the skeleton and produce body heat (Chapter
7).
Mutation(mew-TAY-shun) A change in DNA; a genetic
change that may be passed to offspring (Chapter 3).
Myalgia(my-AL-jee-ah) Pain or tenderness in a muscle
(Chapter 7).
Myasthenia gravis(MY-ass-THEE-nee-yuh GRAH-viss)
An autoimmune disease characterized by extreme muscle
weakness and fatigue following minimal exertion
(Chapter 7).
Mycosis(my-KOH-sis) (Pl.—mycoses) An infection caused
by a pathogenic fungus (Chapter 22).
Myelin(MY-uh-lin) A phospholipid produced by Schwann
cells and oligodendrocytes that forms the myelin sheath
of axons and dendrites (Chapter 2).
Myelin sheath(MY-uh-lin SHEETH) The white, seg-
mented, phospholipid sheath of most axons and den-
drites; provides electrical insulation and increases the
speed of impulse transmission (Chapter 4).
Myocardial infarction (MI)(MY-oh-KAR-dee-yuhl in-
FARK-shun) Death of part of the heart muscle due to
lack of oxygen; often the result of an obstruction in a
coronary artery (Syn.—heart attack) (Chapter 12).
Myocardium(MY-oh-KAR-dee-um) The cardiac muscle
tissue that forms the walls of the chambers of the heart
(Chapter 4).
Myofibril(MY-oh-FYE-bril) A linear arrangement of sar-
comeres within a muscle fiber (Chapter 7).
Myoglobin(MY-oh-GLOW-bin) The protein in muscle
fibers that contains iron and stores oxygen in muscle
fibers (Chapter 7).
Myometrium(MY-oh-MEE-tree-uhm) The smooth mus-
cle layer of the uterus; contracts for labor and delivery of
an infant (Chapter 20).
Myopathy(my-AH-puh-thee) A disease or abnormal condi-
tion of skeletal muscles (Chapter 7).
Myopia (my-OH-pee-ah) Nearsightedness; an error of
refraction in which only near objects are seen clearly
(Chapter 9).
Myosin(MY-oh-sin) A contractile protein in the sarco-
meres of muscle fibers; pulls actin filaments (Chapter 7).
Myxedema(MIK-suh-DEE-mah) Hyposecretion of thy-
roxine in an adult; decreased metabolic rate results in
physical and mental lethargy (Chapter 10).—N—
Nail follicle(NAYL FAH-li-kuhl) The structure within the
skin of a finger or toe in which a nail grows; mitosis takes
place in the nail root (Chapter 5).
Narrow spectrum(NAR-oh SPEK-trum) An antibiotic
that is effective against only a few kinds of bacteria
(Chapter 22).
Nasal cavities(NAY-zuhl KAV-i-tees) The two air cavities
within the skull through which air passes from the nos-
trils to the nasopharynx; separated by the nasal septum
(Chapter 15).
Nasal mucosa(NAY-zuhl mew-KOH-sah) The lining of
the nasal cavities; made of ciliated epithelium that warms
and moistens the incoming air and sweeps mucus, dust,
and pathogens toward the nasopharynx (Chapter 15).
Nasal septum(NAY-zuhl SEP-tum) The vertical plate
made of bone and cartilage that separates the two nasal
cavities (Chapter 15).
Nasolacrimal duct(NAY-zo-LAK-ri-muhl) A duct that
carries tears from the lacrimal sac to the nasal cavity
(Chapter 9).
Nasopharynx(NAY-zo-FA-rinks) The upper portion of the
pharynx above the level of the soft palate; an air passage-
way (Chapter 15).
Natural killer cell(NATCH-er-al KILL-er SELL) A type
of lymphocyte that is not specific and is part of innate
immunity; kills pathogens and tumor cells by direct con-
tact (Chapter 14).
Necrotizing fasciitis(NEK-roh-ty-zing FASH-ee-EYE-
tis) An acute, fulminant, and potentially fatal infection
of fasciae often caused by Group A streptococci (Chap-
ter 5).
Negative feedback mechanism(NEG-ah-tiv FEED-bak)
A control system in which a stimulus initiates a response
that reverses or reduces the stimulus, thereby stopping
the response until the stimulus occurs again and there is
a need for the response (Chapter 1).
Neonatal herpes(NEE-oh-NAY-tal HER-peez) A herpes
simplex infection in a newborn; may cause encephalitis
and may be fatal (Chapter 20).
Nephritis(ne-FRY-tis) Inflammation of the kidney; may be
caused by bacterial infection or toxic chemicals (Chapter
18).
Nephron(NEFF-ron) The structural and functional unit of
the kidney that forms urine; consists of a renal corpuscle
and a renal tubule (Chapter 18).
Nerve(NERV) A group of neurons, together with blood
vessels and connective tissue (Chapter 8).
Nerve tissue(NERV TISH-yoo) The tissue specialized to
generate and transmit electrochemical impulses that have
many functions in the maintenance of homeostasis
(Chapter 4).
Nerve tract(NERV TRAKT) A group of neurons that30Scanlon(p3) Glossary 8/17/06 11:08 AM Page 572