erections

An erection is the physiological enlargement and stiffening of the penis resulting from increased blood flow into erectile tissues. It most commonly occurs in response to sexual arousal but can also occur during sleep (nocturnal erections). A healthy erection depends on intact vascular, nervous, hormonal, and tissue components.

Anatomy and mechanism: The penis contains two corpora cavernosa and a single corpus spongiosum. The corpora cavernosa are surrounded by tunica albuginea. During arousal, nerve signals and the endothelium release nitric oxide, causing relaxation of smooth muscle in the erectile tissue and allowing arterial inflow to rise. Blood fills the cavernosal spaces, compressing venous channels and reducing outflow, which sustains rigidity. Enzymatic pathways involve cyclic guanosine monophosphate (cGMP); phosphodiesterase type 5 (PDE5) breaks down cGMP, and drugs that inhibit PDE5 enhance and maintain erection in the presence of sexual stimulation.

Neural pathways and phases: Reflexogenic erections arise from physical stimulation via the pudendal nerves, while psychogenic erections arise from brain-centered stimuli with pathways to the spinal cord and pelvic nerves. Erections progress through tumescence (engorgement), maintenance, and detumescence (return to flaccidity), the latter driven by sympathetic activity and normalization of vascular tone.

Clinical considerations: Erectile dysfunction refers to persistent difficulty achieving or maintaining an erection sufficient for sexual activity. Risk factors include aging, cardiovascular disease, diabetes, hypertension, smoking, obesity, certain medications, and psychological factors. Treatments range from lifestyle changes and management of underlying conditions to medications such as PDE5 inhibitors, vacuum erection devices, intracavernosal injections, or surgical penile implants. Nitrates and PDE5 inhibitors can interact adversely, so medical guidance is important. Normal variation in erection frequency and firmness can occur without pathology.