Age-related physiological changes contribute to the variability of the sexual function and behavior of older individuals, even when the confounding effects of medical illness and drugs are considered. There is little information on the mechanisms that mediate age-dependent differences in the sexual function of healthy individuals. This chapter provides a brief update on the neurobiology of male sexual function, and discusses the role of neurological, hormonal, and vascular processes possibly involved in sexual changes during nonpathological aging. It also summarizes the results of a multidisciplinary study of healthy aging men conducted in our laboratories.

The biology of sexual function

Sexological research has been oriented by conceptual models that help organize information about processes and mechanisms that underlie behavior. Sexual arousal is an encompassing concept implicit in Kinsey's research (Kinsey, Pomeroy and Martin, 1948) that unifies all physiological phenomena, central as well as peripheral. Masters and Johnson (1966) elaborated this notion further by structuring their observations along a progressive sequence of phases: excitement – plateau – orgasm – resolution, which they labeled the sexual response cycle. Kaplan (1979), based primarily on clinical evidence, incorporated a cognitive/motivational component in what she called the ‘triphasic’ model of sexual desire, arousal and orgasm, postulating that each of these phases is subserved by separate but interrelated physiological systems. Everitt and Bancroft (1991) criticized the validity of this model because of a lack of scientific evidence and the difficulty of operationally distinguishing between sexual desire and arousal.