The anatomy of the penis is a complex biological structure designed to fulfill two primary physiological roles: the expulsion of urine and the delivery of sperm-containing fluid for reproduction. This dual functionality is made possible by a sophisticated arrangement of external skin, internal erectile tissues, and an extensive network of blood vessels and nerves. Understanding this structure requires examining the visible features, the core internal components, and the mechanisms that govern its function.
External Structures and Features
The visible, outer portion of the organ is composed of the shaft and the glans. The shaft, or body, is covered by loose, stretchy skin that accommodates the change in size during engorgement. The underside of the shaft features a visible ridge known as the penile raphe, which continues across the scrotum and perineum. Hair typically becomes denser toward the base.
The distal end of the shaft culminates in the glans, a sensitive, cone-shaped structure often called the head. This region is rich in specialized nerve endings, making it the most sensitive external part. The circumference of the glans’ base forms a projecting border called the corona, which overhangs the shaft.
Located at the tip of the glans is the meatus, the opening of the urethra, which serves as the exit point for both urine and semen. A fold of skin called the prepuce, or foreskin, may partially or fully cover the glans. The prepuce connects to the underside of the glans via the frenulum, a highly sensitive elastic band of tissue.
Internal Cylinders and Supportive Framework
The core structure is defined by three cylindrical columns of specialized tissue running the length of the shaft. These columns are encased by the tunica albuginea, a tough, fibrous layer of connective tissue providing structural support. The arrangement includes two paired upper columns, the corpora cavernosa, and a single lower column, the corpus spongiosum.
The two corpora cavernosa are the primary erectile tissues, positioned side-by-side on the upper aspect. Each consists of a sponge-like network of vascular spaces (sinusoids) separated by smooth muscle and connective tissue. These columns are responsible for the rigidity and expansion during erection.
The corpus spongiosum is positioned beneath the corpora cavernosa, extending from the base to the glans. The urethra runs through its entire length. This column prevents the collapse of the urethra during arousal, ensuring the channel remains open for the unobstructed passage of semen during ejaculation.
The Vascular and Nervous System
The transition from a soft to a rigid state is governed by the nervous system and local vasculature. Erection is initiated by the parasympathetic nervous system (S2 to S4 spinal segments). Upon stimulation, nerve signals trigger the release of nitric oxide (NO) in the local tissues.
Nitric oxide causes the smooth muscle surrounding the arteries within the corpora cavernosa to relax. This leads to a rapid increase in blood flow into the sinusoidal spaces, causing the chambers to expand.
The expansion of the corpora cavernosa compresses the small veins (venules) against the tunica albuginea. This veno-occlusive mechanism traps the blood within the erectile tissue, causing sustained rigidity. Detumescence occurs when the sympathetic nervous system causes the smooth muscle to contract, allowing the trapped blood to drain.
Sensation and motor control are managed by the somatic nervous system, primarily through the pudendal nerve. This nerve branches into the dorsal nerve, which carries sensory information (touch, pressure, temperature) from the skin and glans back to the spinal cord. The density of nerve endings in the glans and frenulum is responsible for intense sensory input.
Ejaculation is a reflex action involving a shift in autonomic control. The sympathetic nervous system controls the emission phase, propelling semen into the posterior urethra. The expulsion phase is managed by the somatic nervous system via the pudendal nerve, which triggers rhythmic contractions of the bulbospongiosus and ischiocavernosus muscles, forcing the semen out through the meatus.