Root of the Penis

The root of the penis is the part of the penis that is anchored to the pelvic region, providing stability to the external portion of the penis. It is located deep within the perineum and consists of the crura of the corpora cavernosa and the bulb of the penis, which is the proximal portion of the corpus spongiosum. The crura are attached to the ischial and pubic rami of the pelvis, while the bulb is connected to the perineal membrane. Together, these structures form the base of the penis, anchoring it securely to the pelvic bones and providing structural support.

Structure and Anatomy

The root of the penis is a crucial part of the male genitalia, providing structural support and anchoring the penis to the body. It is made up of various components, including the crura of the corpora cavernosa, the bulb of the penis, and surrounding connective tissues and muscles. Below is a detailed description of the anatomy of the root of the penis.

Location

The root of the penis is located at the base of the penis, deep within the perineum, the region between the anus and the scrotum. It is the part of the penis that is not externally visible, as it lies within the body, and serves as the attachment point to the pelvic bones.

Perineal Location: The root is positioned in the urogenital triangle of the perineum, between the pubic symphysis (front of the pelvis) and the ischial tuberosities (the bones you sit on). It lies below the pelvic diaphragm and is supported by the perineal membrane, which forms the floor of the deep perineal pouch.

Components of the Root of the Penis

The root of the penis consists of three major components: the crura of the corpora cavernosa and the bulb of the penis, which is part of the corpus spongiosum.

Crura of the Corpora Cavernosa

• Crura (Singular: Crus): The crura are the two diverging, elongated extensions of the corpora cavernosa, which are the main erectile tissues of the penis. They form the lateral parts of the root and are located on either side of the corpus spongiosum. Each crus is attached to the ischial and pubic rami, the bony structures of the pelvis.
• Attachment to the Pelvis: The crura firmly anchor the penis to the ischial rami (part of the pelvic bone), providing stability to the base of the penis. This attachment ensures that the penis remains anchored to the body during erection and sexual activity.
• Vascular Erectile Tissue: Like the rest of the corpora cavernosa, the crura are composed of trabecular smooth muscle and connective tissue that form a network of cavernous spaces. These spaces fill with blood during erection, allowing the crura to expand. The crura are covered by a tough fibrous layer called the tunica albuginea, which helps maintain their structure and rigidity.

Bulb of the Penis

• The bulb of the penis is the proximal part of the corpus spongiosum, the spongy erectile tissue that surrounds the spongy (penile) urethra. The bulb is located in the midline, between the two crura, and forms the central part of the root of the penis.
• Position in the Perineum: The bulb is attached to the perineal membrane, a fibrous sheet that separates the deep and superficial perineal pouches. The perineal membrane provides structural support to the bulb and the rest of the root.
• Surrounding the Urethra: The spongy urethra passes through the center of the bulb, which continues into the corpus spongiosum along the shaft of the penis. The bulb’s structure is important for protecting and supporting the urethra.

Muscles Associated with the Root of the Penis

Several muscles surround the structures of the root of the penis and play important roles in its function. These muscles are part of the superficial perineal muscles, which help regulate blood flow and control certain aspects of sexual function.

Ischiocavernosus Muscle

• The ischiocavernosus muscles cover the crura of the corpora cavernosa. They originate from the ischial tuberosities and ischial rami and insert into the crura. These muscles are responsible for compressing the crura during erection, aiding in the maintenance of blood flow and rigidity in the corpora cavernosa.
• Muscle Contraction: Contraction of the ischiocavernosus muscles helps trap blood in the crura by compressing the veins that drain the corpora cavernosa. This is essential for maintaining an erection.

Bulbospongiosus Muscle

• The bulbospongiosus muscle surrounds the bulb of the penis. It arises from the perineal body and the perineal membrane, and its fibers encircle the bulb. The bulbospongiosus muscle plays a key role in compressing the bulb during ejaculation and the final stages of urination.
• Muscle Action: During ejaculation, the contraction of the bulbospongiosus muscle helps expel semen through the urethra by compressing the bulb. It also assists in the final expulsion of urine from the urethra after urination.

Blood Supply

The root of the penis is richly supplied by branches of the internal pudendal artery, which is responsible for delivering oxygenated blood to the external genitalia and perineal structures.

• Artery of the Bulb of the Penis: The artery of the bulb of the penis, a branch of the internal pudendal artery, supplies blood to the bulb of the penis and the corpus spongiosum.
• Deep Arteries of the Penis: The deep arteries of the penis (also known as the cavernosal arteries) supply blood to the crura of the corpora cavernosa. These arteries run through the center of each corpus cavernosum and deliver blood to the erectile tissues during sexual arousal.

Venous Drainage

Venous drainage from the root of the penis occurs via veins that follow the path of the arteries and drain into the internal pudendal veins.

Venous Outflow: Blood from the bulb and crura is drained by the deep dorsal vein of the penis, which collects blood from the erectile tissues and returns it to the systemic circulation through the internal pudendal veins.

Nerve Supply

The root of the penis is innervated by both autonomic and somatic nerves, which regulate its function during sexual arousal, erection, and ejaculation.

• Parasympathetic Innervation: Parasympathetic fibers from the pelvic plexus stimulate vasodilation in the blood vessels supplying the erectile tissues. This vasodilation is essential for increasing blood flow to the crura and the bulb, facilitating erection.
• Somatic Innervation: Somatic nerves, particularly branches of the pudendal nerve, provide motor innervation to the ischiocavernosus and bulbospongiosus muscles, enabling the contraction of these muscles during erection and ejaculation.

Tunica Albuginea

The tunica albuginea is a tough, fibrous connective tissue that surrounds both the crura and the bulb. Its role is to maintain the shape and structural integrity of the erectile tissues during erection.

• Crura: The tunica albuginea surrounding the crura is thick and strong, providing the support necessary for maintaining the rigidity of the corpora cavernosa during erection.
• Bulb: The tunica albuginea surrounding the bulb is thinner and more elastic, allowing for the expansion of the corpus spongiosum without compressing the urethra.

Development and Embryology

The root of the penis develops during embryogenesis from the genital tubercle, a structure that gives rise to the external genitalia. The crura of the corpora cavernosa and the bulb of the penis differentiate from the mesodermal tissues under the influence of male sex hormones (androgens).

Relationship to Surrounding Structures

• Perineal Membrane: The root of the penis is anchored to the perineal membrane, which provides support and stability to the base of the penis. The perineal membrane separates the deep and superficial perineal pouches and is essential for maintaining the structural integrity of the perineal region.
• Pelvic Bones: The crura of the corpora cavernosa are attached to the ischial rami and pubic rami, providing firm anchorage to the pelvic bones. This attachment is critical for stabilizing the penis during erection and sexual activity.

Variations in Anatomy

The size and shape of the crura and bulb of the penis may vary among individuals. These anatomical variations are usually within the normal range and do not typically affect function. In some cases, anatomical differences in the root of the penis may influence the degree of penile rigidity or the stability of the penis during erection.

Function

The root of the penis serves as a foundation for the external portion of the penis, providing structural support, stabilizing the penis during sexual activity, and contributing to the mechanisms of erection and ejaculation. Below is a detailed description of the root of the penis and its key functions.

Anchoring the Penis to the Pelvis

One of the primary functions of the root of the penis is to provide firm anchorage to the pelvic bones, ensuring that the penis remains stable during both flaccid and erect states.

• Crura Anchoring: The crura of the corpora cavernosa attach to the ischial and pubic rami of the pelvis, providing a firm foundation for the penis. This attachment allows the external portion of the penis to move during sexual activity without being dislodged or destabilized.
• Bulb of the Penis Attachment: The bulb of the penis, the proximal part of the corpus spongiosum, is attached to the perineal membrane, anchoring the urethral portion of the penis securely. This attachment is critical for maintaining the position of the penis and ensuring that it does not shift during erection or ejaculation.

Providing Stability During Erection

The root of the penis plays a significant role in stabilizing the penis during erection, enabling it to remain firm and aligned for effective penetration during sexual intercourse.

• Crura and Penile Shaft Stability: The crura, being extensions of the corpora cavernosa, help maintain the position of the penile shaft during erection. The crura are firmly anchored to the pelvic bones, preventing excessive movement or bending of the penis, which ensures that the shaft remains stable during sexual activity.
• Supporting Penile Rigidity: By anchoring the corpora cavernosa to the pelvis, the root provides a stable base that supports the rigid erectile tissue in the penile shaft. This rigidity is essential for penetration during intercourse, as it allows the penis to maintain its form and withstand the mechanical forces exerted during sexual activity.

Supporting Erectile Function

The crura of the corpora cavernosa and the bulb of the penis play vital roles in erectile function, contributing to the process of blood engorgement that leads to penile erection.

• Blood Engorgement in the Crura: The crura contain the same erectile tissue found in the corpora cavernosa of the penile shaft. During sexual arousal, blood flows into the cavernous spaces within the crura, causing them to expand. This blood engorgement is essential for achieving and maintaining an erection, as the expansion of the crura supports the rigidity of the penile shaft.
• Engorgement of the Bulb: The bulb of the penis, part of the corpus spongiosum, also fills with blood during sexual arousal. However, unlike the corpora cavernosa, the bulb remains softer during erection, ensuring that the urethra is not compressed. The engorgement of the bulb helps maintain the shape and structure of the root, supporting the rest of the penis during erection.
• Pressure Regulation: The root of the penis helps regulate the pressure within the erectile tissues by maintaining a balance between blood inflow and outflow. This is particularly important for sustaining an erection, as the veno-occlusive mechanism traps blood within the corpora cavernosa, allowing the penis to remain erect.

Enhancing Penile Rigidity via Muscle Contraction

The ischiocavernosus and bulbospongiosus muscles surrounding the root of the penis play crucial roles in enhancing penile rigidity and supporting the erection.

• Ischiocavernosus Muscle Action: The ischiocavernosus muscles, which surround the crura, contract during sexual arousal, compressing the crura and limiting the outflow of blood. This compression aids in trapping blood within the corpora cavernosa, enhancing the rigidity of the penis and helping to maintain the erection for the duration of sexual activity.
• Bulbospongiosus Muscle Action: The bulbospongiosus muscle, which surrounds the bulb of the penis, contracts to assist in the engorgement of the corpus spongiosum. While the corpus spongiosum remains softer than the corpora cavernosa during erection, the contraction of the bulbospongiosus muscle supports the overall stability of the penis by compressing the bulb and aiding in blood retention.

Assisting in Ejaculation

The root of the penis, particularly the bulb of the penis and the bulbospongiosus muscle, plays a key role in the process of ejaculation.

• Expulsion of Semen: During ejaculation, rhythmic contractions of the bulbospongiosus muscle surrounding the bulb help propel semen through the urethra. These contractions compress the bulb and force semen out of the urethra with sufficient force to ensure its expulsion during climax.
• Clearing the Urethra: After ejaculation, the contraction of the bulbospongiosus muscle helps clear any residual semen from the urethra, preventing retention and ensuring the urethra is fully emptied. This function is critical for maintaining proper hygiene and reducing the risk of infection or discomfort.

Supporting Urinary Function

In addition to its role in sexual function, the root of the penis supports urinary function, particularly through the bulb of the penis and the surrounding muscles.

• Control of Urine Flow: The spongy urethra, which runs through the bulb of the penis, carries urine from the bladder to the external urethral orifice. The bulb helps regulate the flow of urine during urination, and the contraction of the bulbospongiosus muscle assists in the complete expulsion of urine from the urethra.
• Preventing Urine Retention: After urination, the bulbospongiosus muscle contracts to expel any remaining urine from the urethra. This ensures that the urethra is fully emptied, reducing the risk of residual urine, which can lead to urinary tract infections or dribbling.

Facilitating Sexual Sensation and Response

The root of the penis contributes to sexual sensation through the rich innervation of its components, including the crura, bulb, and associated muscles.

• Sensory Input: The pudendal nerve and other nerve fibers provide sensory input to the root of the penis, enhancing the sexual response. This sensory feedback is important for sexual arousal, as it contributes to the pleasure experienced during sexual activity.
• Muscle Contraction Feedback: The rhythmic contraction of the ischiocavernosus and bulbospongiosus muscles during sexual activity also provides feedback that enhances sexual sensation and contributes to the overall experience of orgasm.

Maintaining Penile Position During Erection

The root of the penis helps maintain the position of the penis during erection, ensuring proper alignment and stability throughout sexual activity.

• Crura and Penile Alignment: The crura anchor the penis to the pelvic bones, ensuring that the penile shaft remains aligned with the body. This alignment is essential for effective penetration during intercourse, as it prevents the penis from shifting or bending excessively during movement.
• Supporting the Penile Base: The bulb of the penis supports the base of the penis during erection, providing a stable foundation for the corpus spongiosum and the penile shaft. This support ensures that the penis remains in the correct position for sexual activity.

Role in Overall Penile Function

The root of the penis, by providing structural support, facilitating erection and ejaculation, and assisting in urinary function, is fundamental to the overall function of the penis.

• Foundation for Erection and Sexual Activity: The root provides the base upon which the external part of the penis functions. Without the stable attachment of the crura and bulb, the penis would lack the rigidity and control needed for effective sexual performance.
• Integration with Pelvic Floor Muscles: The root of the penis is integrated with the pelvic floor muscles, which support the organs of the pelvic region and contribute to sexual function. This integration ensures that the penis remains properly supported within the pelvic region during various activities.

Clinical Significance

The root of the penis is clinically significant due to its essential role in erectile function, stability during sexual activity, and its involvement in several medical conditions:

• Erectile Dysfunction (ED): Any impairment in the blood flow to the crura of the corpora cavernosa or the bulb of the penis can lead to erectile dysfunction. Reduced blood supply or nerve damage can prevent the root of the penis from properly supporting an erection.
• Penile Fracture: Although rare, trauma during sexual activity can cause damage to the root of the penis, especially involving the crura or bulb. Injury to the tunica albuginea of the crura or bulb may result in pain, swelling, and erectile difficulties.
• Pelvic Trauma: Injuries to the pelvis, such as fractures, can affect the attachment points of the crura to the pelvic bones, potentially compromising penile stability and function.
• Surgical Procedures: Surgical interventions for penile implants or reconstruction may involve the root of the penis. Proper handling of the crura and bulb during surgery is crucial for preserving erectile function and stability.