Corpus Spongiosum

The corpus spongiosum is one of the three cylindrical erectile tissues found in the male penis. Unlike the paired corpora cavernosa, the corpus spongiosum is a single structure that runs along the ventral (underside) of the penis. It surrounds the penile (spongy) urethra, which carries urine and semen, and extends from the perineal membrane at the base of the penis to the glans penis at its distal end. The corpus spongiosum starts as the bulb of the penis at its base and expands to form the glans penis at its tip. It plays an essential role in protecting the urethra during erection.

Structure and Anatomy

The corpus spongiosum is one of the three erectile tissues in the penis and is essential for the structural and functional integrity of the male reproductive and urinary systems. Unlike the paired corpora cavernosa, the corpus spongiosum is a single, unpaired structure that surrounds the urethra and extends along the length of the penis. Below is a detailed description of its anatomy.

Location

The corpus spongiosum is located on the ventral (underside) aspect of the penis, running along the length of the penile shaft. It surrounds the spongy (penile) urethra, the portion of the urethra that transports urine and semen. The corpus spongiosum starts as the bulb of the penis at its base, extends along the shaft, and expands at the distal end to form the glans penis. This structure plays a critical role in protecting the urethra and providing an outlet for both urinary and reproductive fluids.

Sections of the Corpus Spongiosum

• Bulb of the Penis
  • The corpus spongiosum begins at the base of the penis as the bulb, which is the enlarged, proximal portion located deep within the perineum. The bulb is attached to the perineal membrane, which helps anchor the penis to the body.
  • The bulb of the penis is situated between the two crura (the base of the corpora cavernosa) and is covered by the bulbospongiosus muscle, which assists in functions such as ejaculation and urination.
• Penile Shaft (Body of the Corpus Spongiosum)
  • As the corpus spongiosum extends from the bulb along the penile shaft, it remains a cylindrical structure, running parallel to the corpora cavernosa but located on the ventral side of the penis.
  • The spongy urethra runs through the center of the corpus spongiosum, which ensures the protection of the urethra during both flaccid and erect states. This part of the urethra is often referred to as the penile urethra.
  • The tissue surrounding the spongy urethra is softer and less rigid than the corpora cavernosa, which allows it to protect the urethra during erection and ensure that the urethral lumen remains open for the passage of fluids.
• Glans Penis
• • At the distal end of the penis, the corpus spongiosum expands to form the glans penis, the sensitive, bulbous tip of the penis. This expansion ensures that the urethral opening (urinary meatus) is positioned at the center of the glans for the proper discharge of urine and semen.
  • The glans is covered by a layer of specialized skin, which may be further covered by the prepuce (foreskin) in uncircumcised males.

Tunica Albuginea of the Corpus Spongiosum

The corpus spongiosum is surrounded by a thinner layer of connective tissue called the tunica albuginea, which is different from the thicker, more rigid tunica albuginea that surrounds the corpora cavernosa. The thinner tunica albuginea of the corpus spongiosum provides flexibility, ensuring that the urethra remains open during erection without becoming compressed. This structural difference is crucial for the passage of fluids through the urethra, even during a fully erect state.

Vascular Structure

The corpus spongiosum is highly vascular, containing numerous blood vessels that contribute to its erectile properties, although it does not become as rigid as the corpora cavernosa during erection. Blood supply to the corpus spongiosum is provided by the bulbourethral arteries and branches of the internal pudendal artery, which supply the entire penile structure.

The blood flow to the corpus spongiosum is controlled by the autonomic nervous system, allowing the erectile tissue to fill with blood during arousal, but without reaching the same rigidity as the corpora cavernosa, ensuring the urethra remains functional.

Innervation

The corpus spongiosum, like the rest of the penis, receives sensory and autonomic innervation. Sensory input comes from the dorsal nerve of the penis, a branch of the pudendal nerve. This nerve provides sensation to the glans and shaft of the penis, including the corpus spongiosum. Autonomic innervation is also provided by the sympathetic and parasympathetic branches, which regulate blood flow and contribute to the erectile process.

Relation to Surrounding Structures

• Corpora Cavernosa: The corpus spongiosum is located ventral to the paired corpora cavernosa, the two primary erectile tissues responsible for achieving rigidity during erection. The corpora cavernosa are separated from the corpus spongiosum by the deep fascia (Buck’s fascia), which provides structural support to the penis.
• Bulbospongiosus Muscle: The bulbospongiosus muscle covers the bulb of the penis and assists in functions like ejaculation and the expulsion of urine. This muscle surrounds the proximal part of the corpus spongiosum and contributes to the compression and emptying of the spongy urethra during these processes.
• Tunica Albuginea: As previously mentioned, the corpus spongiosum has its own tunica albuginea, which is thinner and more flexible than the one surrounding the corpora cavernosa. This allows for expansion without compromising the urethral lumen.

Urethral Passage Through the Corpus Spongiosum

The spongy urethra, which runs through the center of the corpus spongiosum, is the final segment of the male urethra. It extends from the membranous urethra (located within the pelvic region) to the external urethral orifice at the tip of the glans penis. The soft, spongy tissue of the corpus spongiosum provides a protective environment for the urethra, allowing it to function properly during both urination and ejaculation.

• Proximal Urethra: At the base of the penis, the spongy urethra enters the bulb of the corpus spongiosum. This region of the urethra receives contributions from the bulbourethral glands (Cowper’s glands), which secrete pre-ejaculate fluid to lubricate the urethra.
• Distal Urethra: As the urethra passes through the corpus spongiosum toward the tip of the penis, it narrows slightly at the navicular fossa, a small widening just before the external urethral orifice (urinary meatus) in the glans.

Blood Supply and Drainage

The blood supply to the corpus spongiosum is provided by the artery of the bulb of the penis, a branch of the internal pudendal artery. This artery supplies the bulb of the penis and the proximal portion of the corpus spongiosum. The urethral artery, another branch of the internal pudendal artery, supplies the distal part of the corpus spongiosum, including the glans penis. Venous drainage occurs through the deep dorsal vein of the penis, which drains blood from the erectile tissues, including the corpus spongiosum.

Function

The corpus spongiosum plays several crucial roles in the male urinary and reproductive systems, particularly related to the protection of the urethra, facilitation of ejaculation and urination, and support of the glans penis during erection. Below is a detailed description of its functions.

Protection of the Urethra

The primary function of the corpus spongiosum is to protect the spongy (penile) urethra, which passes through the center of the corpus spongiosum. Unlike the corpora cavernosa, which become rigid during erection, the corpus spongiosum remains softer to ensure that the urethral lumen stays open for the passage of fluids. This softer tissue provides a cushion around the urethra, preventing it from being compressed or obstructed during erection.

The flexibility and spongy nature of this erectile tissue protect the urethra during both flaccid and erect states, ensuring that the urethra remains functional regardless of changes in penile rigidity. This protection is especially important during erection when the penis swells and pressure within the erectile tissues increases.

Maintenance of Urethral Patency During Erection

During erection, the corpora cavernosa become engorged with blood, leading to the rigidity required for sexual intercourse. However, the corpus spongiosum does not become as rigid, allowing the urethra to remain open. This function is critical for ensuring that the urethra remains patent (open) during ejaculation, allowing semen to pass through without obstruction.

The corpus spongiosum expands to some degree during erection, but the thinner tunica albuginea surrounding it ensures that it does not become overly rigid like the corpora cavernosa. This balance between expansion and softness allows for both ejaculation and urination while the penis is erect.

Facilitation of Ejaculation

The corpus spongiosum plays a vital role in the process of ejaculation. During sexual arousal and subsequent ejaculation, semen is transported through the spongy urethra, which is embedded within the corpus spongiosum. The softness and elasticity of the corpus spongiosum ensure that the urethra remains unobstructed as semen travels from the ejaculatory ducts, through the prostatic and membranous urethra, and finally through the penile urethra.

Bulbospongiosus Muscle: The bulbospongiosus muscle, which covers the bulb of the corpus spongiosum, assists in this process by contracting rhythmically during ejaculation. These contractions help propel semen through the urethra and out of the urinary meatus at the tip of the penis. The interaction between the corpus spongiosum and the bulbospongiosus muscle ensures that ejaculation is forceful and efficient.

Support of the Glans Penis During Erection

At the distal end of the penis, the corpus spongiosum expands to form the glans penis, which is the sensitive tip of the penis. During erection, the glans penis plays a significant role in sexual intercourse due to its sensory capabilities and its role in sexual arousal.

The corpus spongiosum supports the glans penis by ensuring that it remains slightly engorged with blood during erection, contributing to its shape and sensitivity. Unlike the corpora cavernosa, which are responsible for the rigidity of the shaft, the corpus spongiosum maintains a degree of softness and flexibility in the glans. This allows the glans to remain sensitive to tactile stimuli, which enhances sexual sensation and plays a role in sexual response.

Cushioning and Prevention of Injury

Because of its spongy and flexible structure, the corpus spongiosum acts as a cushion for the penile urethra, preventing it from being crushed or injured during mechanical stresses, such as during intercourse or physical activities. The softer tissue absorbs shocks and distributes pressure away from the urethra, protecting this critical passageway for both urine and semen.

Additionally, by maintaining a soft and pliable state even during erection, the corpus spongiosum helps prevent injury to the urethra from external forces or excessive rigidity. This cushioning function is essential for ensuring that the penis can undergo the physiological changes necessary for sexual activity without compromising the integrity of the urethra.

Contribution to Urination

Although the primary role of the corpus spongiosum is related to sexual function, it also plays a supportive role in urination. By maintaining the structure of the penile urethra, the corpus spongiosum ensures that urine can pass smoothly from the bladder, through the urethra, and out of the body.

During urination, the spongy tissue of the corpus spongiosum allows the urethra to expand slightly, facilitating the passage of urine. This expansion helps reduce resistance within the urethra, ensuring that urine is expelled in a smooth, uninterrupted stream.

Blood Flow and Engorgement

The corpus spongiosum, like the corpora cavernosa, is highly vascularized and contains a network of blood vessels that contribute to its engorgement during sexual arousal. However, unlike the corpora cavernosa, the corpus spongiosum does not become fully rigid during erection. This partial engorgement helps to expand and protect the urethra without obstructing it, maintaining a passage for fluids while still contributing to the overall engorgement of the penis.

The blood flow to the corpus spongiosum is controlled by the autonomic nervous system, which regulates vasodilation and vasoconstriction in response to sexual arousal or cessation of stimulation. The controlled blood flow ensures that the corpus spongiosum can support the urethra while still allowing it to function during the phases of sexual arousal and ejaculation.

Coordination with the Corpora Cavernosa

The corpus spongiosum works in tandem with the corpora cavernosa, the two larger erectile tissues responsible for the majority of penile rigidity during erection. While the corpora cavernosa become fully engorged with blood to create the rigidity necessary for penetration, the corpus spongiosum remains softer to protect the urethra. This coordination between the spongiosum and cavernosa allows the penis to function effectively during sexual activity, providing both rigidity and a functional urethra for ejaculation.

Clinical Significance

The corpus spongiosum is crucial for both sexual and urinary function, and its health is essential for maintaining these processes. Some of the key clinical considerations related to the corpus spongiosum include:

• Urethral Strictures: Injury or scarring to the urethra, which is embedded within the corpus spongiosum, can lead to narrowing (strictures) that obstruct urine flow and ejaculation. These strictures can occur due to trauma, infections, or congenital conditions and often require surgical correction.
• Penile Trauma: Trauma to the penis, such as a penile fracture, can damage the corpus spongiosum, leading to complications like urethral injury or erectile dysfunction. Immediate medical attention and surgical repair are typically necessary to restore function.
• Peyronie’s Disease: Although this disease mainly affects the tunica albuginea of the corpora cavernosa, severe cases may involve the corpus spongiosum, leading to pain, penile curvature, and erectile dysfunction.
• Hypospadias: A congenital condition where the urethral opening is not located at the tip of the glans but along the shaft. This often involves the corpus spongiosum and may require surgical correction.