The Cavernous Nerves
The cavernous nerves are delicate bundles of nerve fibers, part of the autonomic nervous system, situated within the pelvic cavity. These nerves originate from the inferior hypogastric plexus and travel toward the base of the penis. Their function is to carry the involuntary signals controlling blood flow to the erectile tissues. They are fundamental to achieving penile rigidity.
The Mechanism of Erection
The process of achieving an erection is a neurovascular event that begins with the cavernous nerves transmitting signals to the penis. These nerves release chemical messengers, known as neurotransmitters, directly into the erectile tissue. The primary messenger released is nitric oxide (NO), facilitated by acetylcholine from the nerve endings.
Once released, nitric oxide diffuses into the smooth muscle cells that line the arteries and the spongy tissue chambers of the penis, called the corpora cavernosa. This chemical signal causes the smooth muscle cells to relax. The relaxation allows a large volume of blood to flow rapidly into the corpora cavernosa, trapping the blood within the penile structure.
This rapid influx of blood, combined with the subsequent compression of the veins that normally drain the organ, leads to the physical expansion and rigidity of the penis, known as tumescence. The sympathetic nervous system typically maintains the smooth muscles in a contracted state. The parasympathetic signals from the cavernous nerves must override this contraction to initiate and maintain the erection.
Susceptibility to Surgical Injury
The cavernous nerves are particularly vulnerable to damage because of their fragile structure and close anatomical relationship to other pelvic organs. These nerves form the neurovascular bundle, which runs along the posterolateral surface of the prostate gland. They are encased in the thin fascia surrounding the prostate, making them difficult to distinguish visually from surrounding tissue during surgery.
The primary context for damage is during a radical prostatectomy, the surgical removal of the prostate to treat cancer. Because the prostate is the central point of the nerve’s course, surgical dissection in this area places the nerves at high risk. Even when surgeons employ meticulous nerve-sparing techniques, the nerves can still suffer injury, leading to neurogenic dysfunction.
Injury can occur through direct transection, where the nerve is cut, or indirectly through excessive stretching or traction applied during the removal of the prostate. Thermal damage from electrocautery devices used to control bleeding is another common mechanism of injury, as the heat can destroy the delicate nerve fibers.
Recovery and Rehabilitation Post-Damage
Injury to the cavernous nerves during pelvic surgery often results in neurogenic erectile dysfunction because nerve signals for blood flow are interrupted. The lack of nerve-induced blood flow can cause the penile tissues to become chronically under-oxygenated, leading to structural changes like scar tissue buildup, or fibrosis, in the corpora cavernosa.
To counteract these adverse changes and promote functional recovery, penile rehabilitation is often initiated immediately after surgery. The main goal is to preserve the smooth muscle content of the penis and maintain tissue oxygenation while the damaged nerves attempt to heal. This preservation creates a more favorable environment for eventual nerve regeneration.
One common treatment is the use of phosphodiesterase type 5 (PDE5) inhibitors, such as sildenafil or tadalafil, taken chronically at a low dose. These oral medications enhance the effects of residual or newly regenerated nitric oxide, helping to relax the smooth muscles and encourage blood flow. Although the evidence is mixed on whether chronic use accelerates the return of natural function, they are widely used to maintain tissue health.
When oral medications are insufficient, second-line therapies are employed to mechanically or pharmacologically force blood into the penis.
Treatment Modalities
Vacuum erection devices (VEDs) create a negative pressure around the penis to draw blood into the corpora cavernosa. Intracavernosal injections involve administering a vasoactive medication directly into the penile tissue. Both methods provide a temporary erection and maintain the necessary tissue oxygenation, preventing the atrophy and fibrosis that can make later recovery more difficult.