sure. They are useful for signaling sudden changes in
the environment. Receptors that respond to light or
mechanics are tonic receptors and produce a constant
signal, after an initial amount of high-frequency action
potentials, while the stimulus is being applied.
sepal Apart of the calyx consisting of a whorl of
modified leaves that encloses and protects the flower
bud before it opens.
sequence The order of neighboring amino acids in a
protein or the purine and pyrimidine bases in RNA or
DNA.
See alsoPRIMARY STRUCTURE.
sequence-directed mutagenesis SeeMUTAGENESIS.
serotonin(5-hydroxytryptamine; 5-HT) Abiogenic
monoamine (like histamine, epinephrine, nore-
pinephrine); a neurotransmitter and an important
vasoactive substance. A mediator of immediate hyper-
sensitivity. Serotonin is one of many mediators released
by circulating basophils and tissue mast cells. Found in
blood platelets, in the gastrointestinal tract, and in cer-
tain regions of the brain.
It plays a major role in blood clotting, stimulating
strong heart beats, initiating sleep, fighting depression,
and causing migraine headaches.serum SeePLASMA.sessile Stalkless, stationary, or attached to a substrate.sex chromosomes The pair of chromosomes needed
for determining the sex of an individual. In females,sex chromosomes 305Light micrograph of sectioned Pacinian corpuscle (large circle,
center), a sensory receptor for touch. The corpuscle consists of
nerve endings surrounded by “onion-skin” membrane layers.
The nerve cells are stimulated when the membranes are
deformed by changes in pressure. Pacinian corpuscles are
especially sensitive to vibration. They are found in skin and in
some internal membranes. Magnification unknown.(Courtesy
©CNRIoto Researchers)
Colored scanning electron micrograph (SEM) of human X (center)
and Y (lower right) sex chromosomes. Each chromosome has
replicated to form two identical strands (chromatids). The area
linking the chromatids is the centromere. The sex chromosomes
inherited during fertilization determine a person’s gender. Males
have an X and a Y chromosome (as seen here), while females
have two copies of the X chromosome. The Y chromosome carries
instructions for the development of male characteristics. The sex
chromosomes are one pair of the 23 pairs (present in most cells in
the body) that contain the DNA necessary for growth and develop-
ment. Magnification: ×7,150 at 6 ×7 cm size.(Courtesy © Andrew
Syred/Photo Researchers, Inc.)