perceives a sexual stimulus—it expands. (Smooth muscle, unlike the striated muscles of your arms and legs, is operated by the autonomic nervous system; this is why men can’t simply will themselves erect—or unerect.) The relaxation of the erectile tissue allows blood to rush in and fill out the spongy hollows. Drugs like Viagra enhance the erection process by knocking out a substance nicknamed PDE5, which inhibits smooth-muscle relaxation. They inhibit the inhibitor. (Thus, they’re called PDE5 inhibitors.)
