interneuron One of the three types of neuron net-
works (along with sensory and motor neurons) that
allow information flow by way of impulses and action
potentials to travel through the nervous system. Found
inthe central nervous system, they provide the center
of a loop that receives upstream input from thousands
of motor neurons, then sends the signals to the brain,
processes the returns, and then transmits the down-
stream output to a similar number of sensory neurons
for action.
internode The segment on a plant stem between the
points where leaves are attached; the region or length
of stem between two nodes.
interphase One of the phases of cell division during
the process of mitosis. Interphase is the stage between
two successive cell divisions and the time when DNA is
replicated in the nucleus, followed by mitosis. Inter-
phase itself has several phases: the first gap phase (G1)
isthe time prior to DNA synthesis where the cell
increases in mass; the synthesis or S phase is the time
DNA is actually synthesized; and gap or G2 is the
phase after DNA syntheses and cell protein synthesis
and before the start of prophase.
See alsoMITOSIS.interstitial Refers to the space between cells, air-
ways, blood vessels, alveoli, atoms, molecules, and
even soil particles.interstitial cells Cells that exist in the connective tis-
sues between other tissues and structures; cells among
the seminiferous tubules, tiny tubes of the testis where
sperm cells are produced and that secrete the male sex
hormones testosterone and other androgens.interstitial fluid(intercellular fluid) Fluid that fills
spaces between cells and provides pathways for the
flow of nutrients, gases, and wastes between capillaries
and cells.intertidal zone One of the oceanic zones where the
ocean meets land; landform can be submerged or
exposed; shallow shore area between low and high tide
or water marks.
See alsoABYSSAL ZONE;BENTHIC ZONE;PELAGIC
ZONE.intra- A prefix meaning within or inside.intrinsic activity The maximal stimulatory response
induced by a compound in relation to that of a given
reference compound.
This termhas evolved with common usage. It was
introduced by E. J. Ariëns as a proportionality factor
between tissue response and RECEPTORoccupancy. The
numerical value of intrinsic activity (alpha) could range
from unity (for full AGONISTs, i.e., agonist inducing the
tissue maximal response) to zero (for ANTAGONISTs),intrinsic activity 179nodeinternodenodeThe internode is the segment on a plant stem between the points
leaves are attached. It is the region or length of stem between
two nodes.