glossary G-23systole Contraction phase of the cardiac cycle.
target cell Any cell that has receptors for a specific signaling
molecule (such as a hormone) and that may alter its behavior
in response to the molecule.
taste receptors Chemoreceptors in the taste buds.
T cell T lymphocyte; one of a class of white blood cells that
carry out immune responses. The helper T and cytotoxic
T cells are examples.
T cell receptor Antigen receptor on a T cell.
tectorial membrane Inner ear structure against which sen-
sory hair cells are bent, producing action potentials that travel
to the brain via the auditory nerve.
telophase (tee-low-faze) Of mitosis, the final stage when
chromosomes decondense into threadlike structures and two
daughter nuclei form.
tendon A cord or strap of dense, regular connective tissue
that attaches a muscle to bone or to another muscle.
teratogens Agents that can cause birth defects.
testcross In genetics, an experimental cross to reveal
whether an organism is homozygous dominant or heterozy-
gous for a trait. The organism showing dominance is crossed
to an individual known to be homozygous recessive for the
same trait.
testes (singular: testis) Male gonads; primary reproduc-
tive organs in which male gametes and sex hormones are
produced.
testosterone (tess-toss-tuh-rown) In males, a major sex hor-
mone that helps control reproductive functions.
tetanus Condition in which a muscle motor unit is main-
tained in a state of contraction for an extended period.
TFR See total fertility rate.
thalamus Coordinating center in the forebrain for sensory
input and a relay station for signals to the cerebrum.
theory See scientific theory.
therapeutic cloning Creating a cloned embryo that will be
used as a source of embryonic stem cells.
thermoreceptor Sensory cell that can detect radiant energy
associated with temperature.
thirst center Cluster of nerve cells in the hypothalamus that
can inhibit saliva production, resulting in mouth dryness that
the brain interprets as thirst.
thoracic cavity The chest cavity; holds the heart and lungs.
threshold Of neurons and other excitable cells, a certain
minimum amount by which the voltage difference across
the plasma membrane must change to produce an action
potential.
thymine Nitrogen-containing base in some nucleotides; a
building block of DNA.
thymus A lymphoid organ with endocrine functions; lym-
phocytes of the immune system multiply, differentiate, and
mature in its tissues, and its hormones affect their functions.stimulus [L. stimulus, goad] A specific change in the environ-
ment, such as a variation in light, heat, or mechanical pres-
sure, that the body can detect through sensory receptors; a
form of energy that activates receptor endings of a sensory
neuron.
stop codon Of protein synthesis, a base triplet in a strand of
mRNA that serves as the stop signal for translation, so that no
more amino acids are added to the polypeptide chain.
strength training Intense, short-duration exercise that pro-
duces larger, stronger muscles.
substrate A reactant or precursor molecule for a metabolic
reaction; a specific molecule or molecules that an enzyme can
chemically recognize, briefly bind to, and modify in a specific
way.
substrate-level phosphorylation A metabolic event; the
direct transfer of a phosphate group from a substrate of a
reaction to some other molecule.
succession (suk-sesh-un) [L. succedere, to follow after]
Orderly changes from the time pioneer species colonize a bar-
ren habitat through replacements by various species until the
climax community, when the composition of species remains
steady under prevailing conditions.
surface-to-volume ratio A mathematical relationship in
which volume increases with the cube of the diameter, but
surface area increases only with the square. In growing cells,
the volume of cytoplasm increases more rapidly than the sur-
face area of the plasma membrane that must service the cyto-
plasm. Because of this constraint, cells generally remain small
or elongated, or have elaborately folded membranes.
sustainability An approach for managing human population
growth, resource use, and the preservation of wild habitats
in ways that will help ensure the long-term survival of the
human species.
sympathetic nerve Any of the nerves of the autonomic
nervous system; generally concerned with increasing overall
body activities during times of heightened awareness, excite-
ment, or danger; sympathetic nerves also work in opposition
with parasympathetic nerves to bring about minor adjust-
ments in internal organs.
synaptic integration (sin-ap-tik) The moment-by-moment
combining of excitatory and inhibitory signals arriving at a
trigger zone of a neuron.
syndrome A set of symptoms that may not individually be
notable, but collectively characterize a disorder or disease.
synovial joint Freely movable joint in which adjoining bones
are separated by a fluid-filled cavity and stabilized by strap-
like ligaments. An example is the ball-and-socket joint at the
hip.
synovial membranes Connective tissue membranes that line
the cavities of the body’s movable joints.
systemic circuit (sis-tem-ik) Circulation route in which
oxygenated blood flows from the lungs to the left half of the
heart, through the rest of the body (where it gives up oxygen
and takes on carbon dioxide), then back to the right side of
the heart.
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