Overview of Male Sexual Function and Dysfunction

Libido is the conscious component of sexual function. Decreased libido manifests as a lack of sexual interest in or a decrease in the frequency and intensity of sexual thoughts, whether they arise spontaneously or in response to erotic stimuli. Libido is sensitive to testosterone levels as well as to general nutrition, health, and the effects of medications.

Common causes of decreased libido include hypogonadism (testosterone deficiency), chronic kidney disease, relationship problems, depression, stress, and diabetes, among others. Testosterone deficiency is very common in older men, with a prevalence of between 40 and 50% (1, 23).

prostate cancerbenign prostatic hyperplasia; some antihypertensives; and virtually all medications that are active in the central nervous system (eg, selective serotonin reuptake inhibitors [SSRIs], tricyclic antidepressants, antipsychotics) (4

Erection is a neurovascular response to certain psychological and/or tactile stimuli. Higher cortical input and a sacral parasympathetic reflex arc mediate the erectile response. Neural output travels through the cavernous nerves, which traverse the posterolateral aspect of the prostate. Terminating in the penile vasculature, these nonadrenergic, noncholinergic nerves liberate nitric oxide, a gas. Nitric oxide diffuses into penile arterial smooth muscle cells, causing increased production of cyclic guanosine monophosphate (cGMP), which relaxes the arterial smooth muscle and allows more blood to flow into the corpora cavernosa. As the corpora fill with blood, intracavernous pressure increases, which compresses surrounding venules, causing veno-occlusion and decreased venous outflow. The increased inflow of blood and decreased outflow further increase intracavernous pressure, contributing to erection. Many factors affect the ability to have an erection (see Erectile Dysfunction).

There are 2 types of erection that men can experience: psychogenic and reflexogenic. Psychogenic erection occurs as a result of arousal within the patient’s brain from visual/auditory input and fantasy. Reflexogenic erection occurs as a direct result of physical genital stimulation. Although in most men the 2 types of erection coexist and occur simultaneously, it is possible for psychogenic erection to develop in the absence of genital stimulation and for reflexogenic erection to develop in the absence of brain stimulation. These 2 types of erection are mediated by different nerve pathways. Psychogenic erection depends on intact neurological function at spinal cord levels T11 through L2, while reflexogenic erection depends on function at spinal levels S2 through S4. Specific spinal cord injuries can therefore affect erection (5). For example, men with complete spinal cord injury above T11 do not typically experience psychogenic erections, but they can still experience reflexogenic erections in response to direct genital stimulation.

Emission refers to the deposition of pre-ejaculatory fluid into the prostatic urethra.

Ejaculation refers to the actual expulsion of seminal fluid out of the urethral meatus.

Neural stimulation of the alpha-adrenergic receptors in the male adnexa (eg, penis, testes, perineum, prostate, seminal vesicles) causes contractions of the epididymis, vas deferens, seminal vesicles, and prostate that transport semen to the posterior urethra (emission). Then, rhythmic contractions of the pelvic floor muscles result in pulsatile excretion of the accumulated seminal fluid (ejaculation). At the same time, the neck of the bladder closes, preventing retrograde ejaculation of semen into the bladder.

Orgasm, the pleasurable sensation that occurs in the brain, is generally simultaneous with ejaculation. This is also associated with rhythmic, involuntary muscular contractions within the pelvic region. Anorgasmia may be a physical phenomenon due to decreased penile sensation (eg, from neuropathy) or a neuropsychological phenomenon due to psychiatric disorders or psychoactive medications.