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

REAL GENES, REAL INTELLIGENCE 119

dimensional (three spatial dimensions plus time) shape of those cues every
bit as much as the bat depends on the shape of its echoes. Th is is impor tant
to cells in multicellular bodies as well as to single- cell organisms.
A good example is the G- protein- coupled- receptors (GPCRs) that co-
piously stud the cell membranes of every cell in a multicellular body. Th ey
mediate most physiological responses to hormones, neurotransmitters,
and environmental changes (over a thousand va ri e ties have been iden-
tifi ed in diff er ent tissues). Like other receptors, they function to initiate
or suppress a multitude of biological pro cesses in the cells. Th eir malfunc-
tioning, in humans, is oft en manifest in heart disease and blood pressure,
infl ammatory, psychological, and other disorders. Not surprisingly, then,
more than half of all drugs given to patients work by targeting one or an-
other GPCR on the body cells.
None of this GPCR stimulation takes place in de pen dently, however, as
if a simple stimulus- response trigger. Responses depend on a background
of structured activities. For example, many of the GPCRs have two bind-
ing sites, one that responds when one factor is pres ent in the environ-
ment, the other when another factor is pres ent— but only if a third factor
is also pres ent. Th e order of the appearance of these factors in time and

intracellular response

ligand binds to a

membrane receptor

extracellular fluid

cell membrane

intracellular fluid

ligand-receptor complex

triggers intracellular response

FIGURE 4.4

Cell surface receptors detecting external signals.

This content downloaded from 139.184.14.159 on Tue, 17 Oct 2017 13:52:50 UTC