Sperm, as seen under a microscope.
______________________________ Up to this point in the chapter, we have looked at the various parts of the male sexual
system, but we have not described their functioning in much detail. In the following pages, we examine two of these functions: erection and ejaculation.
Erection
An erection is a process coordinated by the parasympathetic division of the autonomic nervous system (Ryan-Berg, 2011). When a male becomes sexually excited, the nervous system sends out messages that cause expansion of the arteries leading to the three erectile cylinders in the penis. As a result, the rate of blood flow into these
chapter 4
parallel cylinders increases rapidly. Because blood flowing out of the penis through veins cannot keep up with the inflow, it accumulates in the spongelike tissues of the three erectile cylinders, causing erection. The penis remains erect until the messages from the nervous system stop and the inflow of blood returns to normal.
The capacity for erection is present at birth. It is common and quite natural for infant boys to experience erections during sleep or diapering, from stimulation by clothing, and later by touching themselves. Nighttime erections occur during the rapid eye movement (REM), or dreaming, stage of sleep (Silverberg, 2008b). Erotic dreams can play a role, but the primary mechanism seems to be physiological, and erections often occur even when the dream content is clearly not sexual. Often a man awakens in the morning just after completing a REM cycle. This explains the phenomenon of morning erections, which were once erroneously attributed to a full bladder.
Although an erection is basically a physiological response, it also involves psychological components. In fact, some writers distinguish between psychogenic (from the mind) and physiogenic (from the body) erections, although in most cases of sexual arousal, inputs come simultaneously from both thoughts and physical stimulation.
How great an influence does the mind have on erections? We know that it can inhibit the response: When a man becomes troubled by erection difficulties, the problem might be psychological, as we will discuss in Chapter 14. Also, extensive evidence shows that men can enhance their erection (as reflected in increased penile tumescence) by forming vivid mental images or fantasies of sexual activity (Smith & Over, 1987).