Glossary G-17and contributes to the elastic properties
of the lungs.
sympathoadrenal (sim 9 pa-tho-a-dre 9 nal) system
The sympathetic division of the autonomic system
and the adrenal medulla as they function together to
promote the body changes of the “fight-or-flight”
response.
symport (sim 9 port) A form of secondary active
transport (coupled transport) in which a molecule or
ion is moved together with, and in the same direction
as, Na^1 ions; that is, into the cell.
synapse (sin 9 aps) The junction across which a
nerve impulse is transmitted from an axon terminal
to a neuron, a muscle cell, or a gland cell either
directly or indirectly (via the release of chemical
neurotransmitters).
synapses en passant The type of synapses
formed by autonomic neurons with their target cells.
Neurotransmitters are released into the extracellular
fluid from a number of regions of the autonomic
axons as they pass through the target tissue.
synapsin (s ̆ı-nap 9 sin) A protein within the
membrane of the synaptic vesicles of axons.
When activated by the arrival of action potentials,
synapsins aid the fusion of the synaptic vesicles
with the cell membrane so that the vesicles may
undergo exocytosis and release their content of
neurotransmitters.
synaptic plasticity (s ̆ı-nap 9 tik plas-tis 9 ̆ı-te)
The ability of synapses to change at a cellular or
molecular level. At a cellular level, plasticity refers
to the ability to form new synaptic associations. At
a molecular level, plasticity refers to the ability of
a presynaptic axon to release more than one type of
neurotransmitter and to exhibit synaptic facilitation
and depression.
syncytium (sin-sish 9 e-um) The merging of cells
in a tissue into a single functional unit. Because the
atria and ventricles of the heart have gap junctions
between their cells, these myocardia behave as
syncytia.
synergistic (sin 0 er-jis 9 tik) Pertaining to regulatory
processes or molecules (such as hormones) that have
complementary or additive effects.
systemic (sis-tem 9 ik) circulation The circulation
that carries oxygenated blood from the left ventricle
via arteries to the tissue cells and that carries blood
depleted of oxygen via veins to the right atrium; the
general circulation, as compared to the pulmonary
circulation.
systole (sis 9 -t ̆o-le) The phase of contraction in
the cardiac cycle. Used alone, this term refers to
contraction of the ventricles; the term atrial systole
refers to contraction of the atria.T
tachycardia (tak 0 ̆ı-kar 9 de- ̆a) An excessively rapid
heart rate, usually applied to rates in excess of 100
beats per minute (in contrast to bradycardia, in which
the heart rate is very slow—below 60 beats per
minute).
target organ The organ that is specifically affected
by the action of a hormone or other regulatory
process.
T cell A type of lymphocyte that provides cell-
mediated immunity (in contrast to B lymphocytes,
which provide humoral immunity through
the secretion of antibodies). There are three
subpopulations of T cells: cytotoxic (killer), helper,
and suppressor.
telomere (tel 9 ̆o-m ̄er) A DNA sequence at the
end of a chromosome that is not copied by DNAthat mature to become spermatozoa without further
division.
spermatocyte (sper-mat 9 ̆o-s ̄ıt) A diploid cell of
the seminiferous tubules in the testes that divides by
meiosis to produce spermatids.
spermatogenesis (sper 0 m ̆a-to-jen 9 ̆ı-sis) The
formation of spermatozoa, including meiosis and
maturational processes in the seminiferous tubules.
spermatozoon (sper 0 m ̆a-to-zo 9 on) pl., spermatozoa
or, loosely, sperm. A mature sperm cell formed from
a spermatid.
spermiogenesis (sper 0 me- ̆o-jen 9 ̆ı-sis) The
maturational changes that transform spermatids into
spermatozoa.
sphygmomanometer (sfig 0 mo-m ̆a-nom 9 ̆ı-ter)
A manometer (pressure transducer) used to measure
the blood pressure.
spindle fibers Filaments that extend from
the poles of a cell to its equator and attach to
chromosomes during the metaphase stage of cell
division. Contraction of the spindle fibers pulls the
chromosomes to opposite poles of the cell.
Starling forces The hydrostatic pressures and the
colloid osmotic pressures of the blood and tissue
fluid. The balance of these pressures determines
the net movement of fluid out of or into blood
capillaries.
stem cells Cells that are relatively undifferentiated
(unspecialized) and able to divide and produce
different specialized cells.
stereoisomers (ster 9 e-o-iso-mers) Molecules with
the same atoms in the same sequence, but which
differ in the three-dimensional arrangement of their
atoms.
steroid (ster 9 oid) A lipid derived from cholesterol
that has three six-sided carbon rings and one five-
sided carbon ring. These form the steroid hormones
of the adrenal cortex and gonads.
stretch reflex The monosynaptic reflex whereby
stretching a muscle results in a reflex contraction.
The knee-jerk reflex is an example of a stretch reflex.
striated (stri 9 ̄at-ed) muscle Skeletal and cardiac
muscle, the cells of which exhibit cross banding, or
striations, because of the arrangement of thin and
thick filaments.
stroke volume The amount of blood ejected from
each ventricle at each heartbeat.
substrate (sub 9 str ̄at) In enzymatic reactions, the
molecules that combine with the active sites of
an enzyme and that are converted to products by
catalysis of the enzyme.
sulcus (sul 9 kus) A groove or furrow; a depression
in the cerebrum that separates the folds, or gyri, of
the cerebral cortex.
summation (s ̆u-ma 9 shun) In neural physiology,
the additive effects of graded synaptic potentials.
In muscle physiology, the additive effects of
contractions of different muscle fibers.
suppressor T cells A subpopulation of T
lymphocytes that acts to inhibit the production
of antibodies against specific antigens by B
lymphocytes.
suprachiasmatic (soo 0 pr ̆a-ki 0 az-mot 9 ik) nucleus
(SCN) The primary center for the regulation of
circadian rhythms. Located in the hypothalamus, the
SCN is believed to regulate circadian rhythms by
means of its stimulation of melatonin secretion from
the pineal gland.
surface tension A property of water wherein
the hydrogen bonding between water molecules
produces a tension at the water surface.
surfactant (sur-fak 9 tant) In the lungs, a mixture
of phospholipids and proteins produced by alveolar
cells that reduces the surface tension of the alveoliskeletal muscle pump A term used with reference
to the effect of skeletal muscle contraction on the
flow of blood in veins. As the muscles contract, they
squeeze the veins and in this way help move the
blood toward the heart.
sleep apnea A temporary cessation of breathing
during sleep, usually lasting for several seconds.
sliding filament theory The theory that the thick
and thin filaments of a myofibril slide past each other
during muscle contraction, decreasing the length
of the sarcomeres but maintaining their own initial
length.
slow waves Pacemaker depolarizations in the
intestine produced by pacemaker cells, the interstitial
cells of Cajal; these produce action potentials and
resulting smooth muscle contractions.
smooth muscle A specialized type of nonstriated
muscle tissue composed of fusiform single-nucleated
fibers. It contracts in an involuntary, rhythmic
fashion in the walls of visceral organs.
SNARE complex Proteins—including
synaptobrevin-2, syntaxin, and SNAP-25)—that
function to anchor synaptic vesicles in axon
terminals to the presynaptic plasma membrane.
These aid the fusion of the vesicle membrane
and plasma membrane, formation of a pore, and
the exocytosis of neurotransmitter in response to
depolarization.
sodium/potassium (so 9 de-um/p ̆o-tas 9 e-um) pump
An active transport carrier with ATPase enzymatic
activity that acts to accumulate K^1 within cells and
extrude Na^1 from cells, thus maintaining gradients
for these ions across the cell membrane.
somatesthetic (so 0 mat-es-thet 9 ek) sensations
Sensations arising from cutaneous, muscle, tendon,
and joint receptors. These sensations project to the
postcentral gyrus of the cerebral cortex.
somatic (so-mat 9 ik) motor neuron A motor
neuron in the spinal cord that innervates skeletal
muscles. Somatic motor neurons are categorized as
alpha and gamma motoneurons.
somatomammotropic (so 9 m ̆a-t ̆o-mam 0 ̆o-trop 9 ik)
hormone A hormone secreted by the placenta
that has actions similar to the pituitary growth
hormone and prolactin; also called chorionic
somatomammotropin (hCS).
somatomedin (so 0 m ̆a-t ̆o-med 9 n) Any of a group
of small polypeptides that are believed to be
produced in the liver in response to growth hormone
stimulation and to mediate the actions of growth
hormone on the skeleton and other tissues.
somatostatin (so 0 m ̆a-t ̆o-stat 9 n) A polypeptide
produced in the hypothalamus that acts to inhibit
the secretion of growth hormone from the anterior
pituitary. Somatostatin is also produced in the islets
of Langerhans of the pancreas, but its function there
has not been established.
somatotropic (so 0 m ̆a-t ̆o-trop 9 ik) hormone Growth
hormone. An anabolic hormone secreted by the
anterior pituitary that stimulates skeletal growth and
protein synthesis in many organs.
sounds of Korotkoff (k ̆o-rot 9 kof) The sounds
heard when blood pressure measurements are taken.
These sounds are produced by the turbulent flow
of blood through an artery that has been partially
constricted by a pressure cuff.
spastic paralysis (spas 9 tik p ̆a-ral 9 ̆ı-sis) Paralysis
in which the muscles have such a high tone that
they remain in a state of contracture. This may be
caused by inability to degrade ACh released at the
neuromuscular junction (as caused by certain drugs)
or by damage to the spinal cord.
spermatid (sper 9 m ̆a-tid) Any of the four haploid
cells formed by meiosis in the seminiferous tubulesfox36375_glo_G1-G19.indd 17fox36375_glo_G1-G19.indd 17 2/4/15 12:29 PM2/4/15 12:29 PM