360 Self and the Phenomenon of Lifeb2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to Humanity “9x6”Second law of thermodynamics: A natural law that dictates the flow of heat
from an object with a higher heat content to one with a lower content. A corol-
lary of the second law is that, in a closed system, physical entities tend to change
from order to disorder. A measure of disorder is referred to as “entropy.”
Sensory pathways or tracts: Long fiber tracts that carry sensory information
from the body to the brain.
Signal transduction: Transmission of message from one part of the cell to
another, involving serial protein interactions, frequently incurring phospho-
rylation and dephosphorylation, leading to the final endpoint of gene activa-
tion or repression.
Small interfering RNA (siRNA): Short RNA molecule capable of RNA inter-
ference. It is usually referred to a defense mechanism against viral infection.
Somatic: Refers to parts of the body (may include the internal organs).
Steady state: In chemistry, a state in which the concentration of the mole-
cular components remains unchanged despite the presence of constant
flux. A steady state differs from one of equilibrium in that the latter ceases
to be in flux.
Sulcus: The surface of the cerebral cortex that forms a furrow between two
ridges.
Sympathetic nervous system: The component of the autonomic nervous sys-
tem that is activated by emergency or dangerous situations, resulting in such
actions as “fight or flight.” It is frequently active in an emotional outburst.
The feedback sensation from a sympathetic discharge to the sensory areas of
the brain provides a subjective feeling of emotion.
Synapse: A complex structure consisting of a pre-synaptic nerve ending con-
taining neurotransmitters, a post-synaptic component in the downstream
neuron containing receptors to the neurotransmitters, and a space (synap-
tic gap) between the two. The synapse makes possible a one-way chemical
transmission of message from one neuron to the next. Synaptic function is
subject to regulation and modulation and is responsible for learning.
T lymphocytes (T cells): Lymphocytes that do not produce antibodies. One
type of T cell (the killer T cell) destroys cells by direct contact.
TCA cycle: see Tricarboxylic acid cycle.
Terpenoid: A type of volatile organic compound consisting of polyunsaturated
hydrocarbons.b2726_Glossary.indd 360 6/12/2017 8:00:29 PM