rhizae form when the fungus hyphae grow between
and within the cells of the plant roots. Many species
cannot grow without their mycorrhizae connections.
myelin A lipid that forms a multilayered sheath
around some nerve fibers (axons) in the central, auto-
nomic, and peripheral nervous systems.
myelin sheath Multilayered specialized Schwann cells
(nonconducting glial cells) that help provide the efficient
movement of signals by coating and insulating sections
(internodes) of neurons (axons) in the nervous system.
myiasis An infection caused by fly maggots. Primary
myiasis is when a fly deposits eggs on the host and the
maggots feed upon living tissue. Secondary myiasis
occurs first when there is a break in the skin that
allows maggots the access to tissue.
myocrysin SeeGOLD DRUGS.
myofibril A long, cylindrical, contractile filament
within muscle or muscle fiber that makes up striated
muscle. Each myofibril contains intertwined filaments
of muscle proteins, myosin, and actin; called a
sacromere. The myofibril bundles have alternate light
and dark bands (thick and thin) that contain these pro-
tein filaments responsible for the muscle’s contractile
ability, and these give it the characteristic striated look
under a microscope.
myoglobin A monomeric dioxygen-binding heme-
protein of muscle tissue, structurally similar to a SUB-
UNITof HEMOGLOBIN.
myopia (nearsightedness; shortsightedness) An inher-
iteddisorder where a refractive error in the eye means
that the shape of the eye does not bend light correctly,
resulting in a blurred image; inability to see distant
objects clearly because the images are focused in front
of the retina.
myosin A thick contractile protein found in myofib-
rils that interacts with actin, another protein, to create
contraction in a muscle cell. It is the most abundant
protein in muscles fibrils.myriapods Agroup of organisms comprising cen-
tipedes and millipedes, which are long, flattened,
segmented predators; each segment bears a pair of legs.
Centipedes evolved during the Silurian period, and238 myelin
Transmission electron micrograph of part of a striated muscle fiber
(cell) from the human neck. Striated muscle cells are elongated,
hence their description as fibers. Each cell has several nuclei
(not visible) positioned just below the sarcolemma, the name given
to the cell membrane of muscle cells. Running the length of each
fiber are contractile protein filaments (actin and myosin myofila-
ments) arranged in bunches called myofibrils (20–30 myofibrils
are visible here). The arrangement of the two types of myofilament
causes the prominent transverse banding, which gives striated
muscle its name. Magnification: ×8, 100 at 8 ×10-inch size.
(Courtesy © Science Photo Library/Photo Researchers, Inc.)