Crus of the Penis

The crus of the penis refers to the two extensions of the corpora cavernosa, the erectile tissues that form the shaft of the penis. These paired structures are located at the base of the penis, where each crus anchors the penis to the pubic bone, specifically attaching to the ischial and pubic rami, which are parts of the pelvic bone. The crura (plural of crus) run along the inferior sides of the pelvis and are covered by the ischiocavernosus muscles. This connection provides structural support to the penis, ensuring stability during erection. The crura are positioned deep within the pelvis and form the root of the penis.

Structure and Anatomy

The crus of the penis is a critical anatomical structure located at the base of the penis, where it forms part of the root of the penis. The crura (plural of crus) are extensions of the corpora cavernosa, the erectile tissues responsible for penile rigidity during erection. Below is a detailed description of the anatomy of the crus of the penis.

Location

The crura are located at the base of the penis, on the left and right sides. They extend from the base of the corpora cavernosa and attach to the ischial and pubic rami, which are part of the bony pelvis. Each crus runs along the lower lateral sides of the pubic bones, deep within the pelvis, and forms the root of the penis. The crura are positioned beneath the skin and other tissues of the perineum, attaching the penis securely to the skeleton for support.

Attachment to the Pelvis: The crura firmly anchor the corpora cavernosa to the ischial tuberosities and the pubic symphysis, which are the bony structures that form part of the pelvic girdle. This attachment ensures that the penis remains stable during erection and sexual activity.

Structure and Composition

The crus of the penis is composed of erectile tissue, similar to the rest of the corpora cavernosa. Each crus is a tapered extension of the corpora cavernosa, which are cylindrical structures filled with spongy, vascularized tissue. The erectile tissue within the crura is made up of:

• Trabeculae: The inner framework of the erectile tissue, known as trabeculae, consists of smooth muscle and connective tissue. These trabeculae form a mesh-like structure that supports the cavernous spaces (lacunae) within the corpora cavernosa. During erection, these spaces fill with blood, leading to the expansion of the erectile tissue.
• Cavernous Spaces: The crura contain numerous vascular spaces that fill with blood during sexual arousal, similar to the corpora cavernosa in the penile shaft. These spaces are lined with endothelial cells and surrounded by smooth muscle, which relaxes to allow increased blood flow during erection.

Tunica Albuginea

Each crus is covered by a tough, fibrous layer of connective tissue known as the tunica albuginea, which surrounds the entire corpus cavernosum. The tunica albuginea is a dense, white fibrous sheath that provides structural integrity to the erectile tissue and helps maintain the shape and rigidity of the corpora cavernosa during erection.

• Outer Layer: The tunica albuginea has an outer longitudinal layer that runs parallel to the length of the penis and crura. This layer provides tensile strength, allowing the corpora cavernosa to expand during erection without overstretching.
• Inner Circular Layer: Beneath the outer layer is the inner circular layer, which wraps around the corpus cavernosum and provides additional support. The inner layer helps maintain the structural integrity of the erectile tissue, ensuring that the crura can withstand the pressure changes during erection.

Ischiocavernosus Muscles

Each crus is enveloped by the ischiocavernosus muscles, which are important muscles in the pelvic region that assist with the function of the penis during erection. These muscles originate from the ischial tuberosities and surround the crura.

• Muscle Attachment: The ischiocavernosus muscles cover the outer surface of the crura and attach to the ischial ramus, forming a muscular sheath around the erectile tissue. These muscles play a key role in maintaining the rigidity of the penis by compressing the veins that drain the corpora cavernosa, helping to trap blood within the erectile tissue during sexual arousal.
• Contraction and Relaxation: During sexual arousal, the ischiocavernosus muscles contract, increasing the pressure within the corpora cavernosa and aiding in the maintenance of an erection. This muscular action supports the function of the crura by reinforcing the blood-trapping mechanism that is essential for sustained erection.

Blood Supply

The crura, like the rest of the corpora cavernosa, receive their blood supply from the internal pudendal artery, a major branch of the internal iliac artery. Blood flow to the crura is delivered by the deep arteries of the penis, also known as the cavernosal arteries.

• Deep Arteries of the Penis: These arteries run through the center of each crus and supply the erectile tissue with oxygenated blood. During sexual arousal, the cavernosal arteries dilate, increasing blood flow into the cavernous spaces within the crura, contributing to penile erection.
• Helicine Arteries: The deep arteries give rise to smaller, coiled vessels called helicine arteries, which deliver blood directly to the erectile spaces. These arteries are responsible for regulating the blood flow into the erectile tissue, ensuring that the crura fill with blood during sexual activity.

Venous Drainage

The venous drainage of the crura follows the same pathway as the rest of the penis, with blood being drained through the emissary veins that pass through the tunica albuginea and into the deep dorsal vein of the penis. During erection, the venous outflow from the crura is reduced due to compression by the ischiocavernosus muscles, which helps to maintain the engorgement of the erectile tissue.

Veno-Occlusive Mechanism: The tunica albuginea and the ischiocavernosus muscles play key roles in trapping blood within the crura during erection. This mechanism is essential for sustaining an erection by limiting the outflow of blood from the corpora cavernosa.

Nerve Supply

The crura are innervated by both autonomic and somatic nerves, which regulate their function during erection and sexual activity.

• Parasympathetic Nerves: The parasympathetic fibers of the autonomic nervous system are responsible for initiating the erectile response by triggering the release of nitric oxide, which causes relaxation of the smooth muscle in the walls of the blood vessels supplying the crura. This vasodilation allows blood to flow into the erectile tissue, leading to engorgement.
• Somatic Nerves: The somatic nerve supply, primarily from the pudendal nerve, innervates the ischiocavernosus muscles. These muscles contract during sexual arousal, aiding in the veno-occlusive mechanism that maintains the erection.

Relationship to Surrounding Structures

• Corpus Cavernosum: The crura are the proximal extensions of the corpora cavernosa, the erectile structures that run the length of the penile shaft. The crura form the root of the penis, anchoring the corpora cavernosa to the pelvic bones and providing structural support.
• Bulb of the Penis: The bulb of the penis, which is part of the corpus spongiosum, is located between the two crura at the base of the penis. The bulb is responsible for surrounding the urethra and is involved in the expulsion of semen and urine. The crura and the bulb together form the root of the penis.
• Pelvic Bones: The crura are attached to the ischial and pubic rami, the bony structures that form part of the pelvis. This attachment is crucial for stabilizing the penis during erection and sexual activity.

Developmental Anatomy

During fetal development, the crura form from the genital tubercle, the same structure that gives rise to the erectile tissues of the penis. The crura develop as part of the corpora cavernosa and are responsible for anchoring the penis to the body.

Variations in Anatomy

The size and shape of the crura can vary between individuals. In some men, the crura may be longer or more prominent, which may influence the degree of rigidity and stability of the penis during erection. These anatomical variations are generally within the range of normal human development and do not typically affect function.

Relationship to Erectile Function

The crura, as part of the corpora cavernosa, are directly involved in the erectile process. The blood flow into the crura during sexual arousal contributes to the overall engorgement and rigidity of the penis. The anchoring of the crura to the pelvic bones ensures that the penis remains stable during erection, allowing for proper function during intercourse.

Function

The crus of the penis plays a crucial role in the structural and functional aspects of penile erection, support, and stability. As part of the corpora cavernosa, the crura contribute to several key functions, particularly related to erection and sexual performance. Below is a detailed explanation of the functions of the crus of the penis.

Anchoring the Penis to the Pelvic Bones

One of the primary functions of the crus of the penis is to anchor the penis securely to the pelvic bones, providing structural stability to the organ. The crura attach to the ischial and pubic rami of the pelvis, ensuring that the penis remains firmly supported during both flaccid and erect states.

• Stability During Erection: The attachment of the crura to the pelvic bones helps stabilize the penis during erection. As blood flows into the corpora cavernosa and the penis becomes erect, the crura act as a foundation, preventing excessive movement or shifting of the penile shaft. This stability is essential for maintaining the functional rigidity required for penetration during sexual intercourse.
• Preventing Displacement: The firm anchorage provided by the crura ensures that the penis does not become displaced or bent during physical activity or sexual intercourse. Without this firm attachment, the penis would be unable to sustain the necessary stability during sexual performance.

Supporting Penile Rigidity During Erection

The crura are integral to the erectile function of the penis. As extensions of the corpora cavernosa, the crura contribute to the blood engorgement that leads to the rigidity of the penis during sexual arousal.

• Blood Engorgement and Expansion: During sexual arousal, blood flows into the cavernous spaces within the corpora cavernosa, including the crura. The crura, like the rest of the corpora cavernosa, expand as they fill with blood. This expansion is essential for creating the pressure needed to achieve and maintain penile rigidity. The increased blood flow and engorgement of the crura contribute to the overall erection, ensuring that the base of the penis is firm and stable.
• Pressure Regulation: The tunica albuginea, the fibrous layer surrounding the crura, plays a key role in regulating the pressure within the erectile tissue. By providing a strong, supportive sheath, the tunica albuginea helps maintain the internal pressure required for penile rigidity. The crura, as part of this system, help generate the pressure needed to keep the penis erect.

Contribution to the Veno-Occlusive Mechanism

The veno-occlusive mechanism is a key process in maintaining an erection. The crura contribute to this mechanism by helping trap blood within the corpora cavernosa, ensuring that the penis remains engorged and rigid during sexual activity.

• Venous Compression: During an erection, the ischiocavernosus muscles, which surround the crura, contract. This contraction compresses the veins that drain the corpora cavernosa, reducing the outflow of blood from the erectile tissue. By trapping blood within the crura and the rest of the corpora cavernosa, this mechanism maintains the internal pressure that keeps the penis rigid.
• Sustaining the Erection: The veno-occlusive mechanism supported by the crura ensures that the penis remains erect for the duration of sexual activity. The crura, by assisting in venous compression, help prevent the premature loss of rigidity, allowing for sustained erection and successful sexual performance.

Structural Support for Penetration

The crura provide crucial structural support during penetration by stabilizing the base of the penis. This function is important for effective sexual intercourse, where stability and rigidity are required to facilitate penetration.

• Enhancing Penile Strength: The crura, as part of the root of the penis, form the foundation that supports the rest of the organ. This strength at the base is essential for the penis to withstand the mechanical forces involved in penetration. Without the support provided by the crura, the penis would lack the stability needed for firm and controlled movements during intercourse.
• Prevention of Bending or Buckling: The crura help prevent the penis from bending or buckling during penetration, particularly during vigorous sexual activity. By anchoring the penis to the pelvic bones and providing a firm base, the crura ensure that the shaft of the penis remains aligned and rigid.

Role in Erection Mechanics and Blood Flow Regulation

The crura play an important role in the overall mechanics of penile erection by regulating the flow of blood into the erectile tissues and supporting the smooth muscle relaxation required for erection.

• Vasodilation and Blood Flow: The deep arteries of the penis, which supply the corpora cavernosa, including the crura, dilate during sexual arousal. This vasodilation allows for increased blood flow into the helicine arteries and subsequently into the cavernous spaces of the crura. The crura, along with the rest of the corpora cavernosa, engorge with blood, contributing to the overall rigidity of the penis during erection.
• Smooth Muscle Relaxation: The crura contain trabecular smooth muscle, which relaxes in response to parasympathetic nerve stimulation. This relaxation allows the cavernous spaces within the crura to fill with blood, contributing to the engorgement of the corpora cavernosa and the erection of the penis. Without the smooth muscle relaxation in the crura, the blood flow required for a full erection would be limited.

Interaction with the Ischiocavernosus Muscles

The ischiocavernosus muscles, which envelop the crura, play a crucial role in enhancing the function of the crura during erection and sexual activity.

• Compression of the Crura: When the ischiocavernosus muscles contract during sexual arousal, they compress the crura, increasing the pressure within the erectile tissue. This compression helps trap blood within the corpora cavernosa, maintaining the rigidity of the penis.
• Stabilization of the Penis: In addition to helping with venous compression, the ischiocavernosus muscles provide further stabilization to the crura during erection. This muscular support ensures that the penis remains properly aligned and firm during penetration.

Role in Sexual Response and Arousal

The crura, as part of the corpora cavernosa, play a key role in the male sexual response by facilitating erection, which is necessary for sexual intercourse.

• Engorgement and Sexual Arousal: During sexual arousal, the crura fill with blood as part of the overall erectile response. This engorgement is a fundamental component of the male sexual response, allowing the penis to become rigid enough for penetration. The crura’s involvement in this process ensures that the base of the penis remains as engorged and firm as the shaft, contributing to the strength of the erection.
• Sensory Feedback: Although the crura are not as richly innervated as the glans, they still contribute to sensory feedback during sexual activity. The pressure changes within the crura during erection and sexual stimulation provide sensory information to the nervous system, contributing to the overall sexual experience.

Stability During Flaccid State

Even when the penis is flaccid, the crura provide structural support by anchoring the penis to the pelvic bones. This support ensures that the penis maintains its proper position relative to the body, even when not erect.

Maintaining Penile Position: The crura help maintain the position of the penis relative to the pelvis, even when the organ is not in an erect state. This positional stability is important for everyday activities, as it prevents excessive movement or displacement of the penis during physical activity.

Role in Erectile Dysfunction Mechanisms

In cases of erectile dysfunction (ED), the ability of the crura to fill with blood or support the veno-occlusive mechanism may be impaired. Conditions affecting blood flow, such as vascular disease or nerve damage, can impact the function of the crura, leading to difficulties in achieving or maintaining an erection.

• Impairment of Blood Flow: In some forms of erectile dysfunction, the arteries supplying the crura may not dilate sufficiently, limiting blood flow into the corpora cavernosa and preventing the crura from becoming fully engorged. This reduced blood flow can lead to insufficient penile rigidity.
• Veno-Occlusive Dysfunction: In cases where the crura fail to trap blood effectively due to poor venous compression, the erection may not be sustained. This failure of the veno-occlusive mechanism can lead to premature loss of erection, affecting sexual performance.

Clinical Significance

The crura of the penis are crucial for the structural integrity and erectile function of the penis. Several clinical conditions may affect the crura, influencing sexual health and penile stability:

• Erectile Dysfunction (ED): Any impairment in the blood flow to the crura, such as vascular disease, can result in insufficient engorgement of the corpora cavernosa, leading to difficulty achieving or maintaining an erection. Problems with the veno-occlusive mechanism in the crura may also prevent the penis from retaining blood, causing premature loss of erection.
• Penile Fracture: Although rare, trauma to the penis during erection can cause damage to the tunica albuginea of the corpora cavernosa, including the crura. This injury may result in pain, swelling, and erectile dysfunction, often requiring surgical intervention.
• Surgical Considerations: The crura are sometimes involved in surgeries for penile implants or reconstruction following trauma or congenital abnormalities. Surgeons must carefully preserve the crura’s attachment to the pelvis to ensure proper function.
• Pelvic Trauma: Injuries to the pelvic region, such as fractures or blunt force trauma, can impact the crura’s connection to the pelvic bones, potentially affecting penile stability and erectile function.