Genes, Brains, and Human Potential The Science and Ideology of Intelligence

160 INTELLIGENT DEVELOPMENT

Hormones Interact
Th at hormones interact and do not usually act as in de pen dent switches has
been known for a long time. All major functions depend on their inte-
gration and coordination: digestion, metabolism, respiration, sensation,
sleep, excretion, lactation, response to stress, growth and development,
heart function and blood circulation, reproduction, motivation, mood,
and so on. One example is thyroxine from the thyroid which helps regu-
late temperature and general metabolism. Another is the wide range of
ste roid hormones, produced in the adrenal glands or in the gonads.
Th ey are involved in an even wider range of anti- infl ammatory, anti- stress,
and sexual functions, as well as integrating general cell/tissue metabo-
lism. Some of them regulate sodium and potassium levels and the state
of body hydration (and thereby— e.g., through thirst— infl uence be hav ior).
As with other intercellular signaling, the integration of hormones is
mediated through receptors on cell membranes, to which the hormones
bind before activating internal transduction pathways. But they meet
with interactive rather than in de pen dent eff ects. Th e eff ects depend on
wider bodily states and events in the outside world.
Most common are synergistic eff ects in which two or more hormones
produce eff ects greater than the sum of their separate eff ects. One hor-
mone may enhance the target organ’s response to another hormone,
even one secreted at a later time. Conversely, there are antagonistic
eff ects in which one hormone inhibits the eff ects of another. For example,
insulin lowers blood glucose level, while glucagon, produced by the liver,
raises it. Ordinarily these will be well orchestrated, but the eff ects of hor-
mone imbalances are well known in medicine and can be diffi cult to di-
agnose and treat.
Th e neuroendocrine stress axis—or, to give it its full name, the hypo-
thalamic- pituitary- adrenal axis—is a key physiological system. It regu-
lates responses to stress, either from internal or external sources. As such
it aff ects, and is aff ected by, many body functions, such as digestion, the
immune system, energy metabolism, and emotional aspects of psy chol ogy.
Part of the classic stress response consists of secretion of corticotropin-
releasing hormone from the hypothalamus in the midbrain. It passes
quickly to the nearby pituitary gland, which actually releases the hor-

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