Baseline Flaccid State
Cavernosal muscle stays tonically contracted until a signal arrives.
- Contracted: Resting muscle tone (sympathetic dominance)
- Low: Sinusoidal blood volume (collapsed spaces)
- ~2-5 mL/min: Arterial inflow (baseline trickle)
- Unrestricted: Venous outflow (open drainage)
Resting smooth muscle tone
Cavernosal muscle stays contracted by default, day and night.
The normal nerve pathway
Arousal usually signals via pelvic nerves releasing nitric oxide.
Why this pathway can fail
Nerve damage, vascular disease, or psychogenic causes can block it.
Injection Administered
A fine needle delivers alprostadil directly into erectile tissue.
- Corpus cavernosum: Injection site (lateral proximal shaft)
- 27-30G: Typical needle gauge (insulin-syringe scale)
- 5-20 min: Onset of action (dose dependent)
- PGE1 analog: Drug class (alprostadil)
Where the needle goes
Injected into the lateral corpus cavernosum, away from vessels and nerves.
Why intracavernosal, not oral
Local delivery skips gut absorption and systemic dilution entirely.
Dose titration
Clinicians start low and adjust to avoid prolonged erections.
Direct Smooth Muscle Relaxation
Alprostadil relaxes muscle locally, no nerve signal required at all.
- EP receptors: Receptor target (prostaglandin E1)
- cAMP ↑: Second messenger (via adenylate cyclase)
- NO / cGMP: Bypassed pathway (nerve-dependent route)
- 5-20 min: Onset window (to full relaxation)
The cAMP pathway
PGE1 binds EP receptors, raising cAMP and relaxing muscle fibers.
Skipping nitric oxide entirely
This route works even when nerves cannot release nitric oxide.
Why this suits neurogenic ED
Spinal injury or prostatectomy patients often keep this pathway intact.
Rapid Arterial Inflow
Relaxed sinusoidal spaces open and flood rapidly with blood.
- 20-40×: Inflow surge (over resting baseline)
- Rapid: Sinusoid expansion (seconds to minutes)
- Rising: Cavernosal pressure (toward systolic level)
- 5-15 min: Time to rigidity (typical response)
Sinusoidal filling
Relaxed trabecular spaces expand and rapidly fill with arterial blood.
Pressure builds toward rigidity
Cavernosal pressure climbs as inflow far outpaces outflow.
Compression begins
Expanding sinusoids start pressing against the tunica albuginea.
Sustained Erection
Compressed veins trap blood, sustaining rigidity for thirty to sixty minutes.
- 30-60 min: Typical duration (dose dependent)
- Subtunical veins compressed: Veno-occlusion (traps inflow)
- Low-dose protocol: Priapism risk (reduces overdose risk)
- Gradual: Detumescence (as drug effect wanes)
The venous trap
Swollen sinusoids compress subtunical veins, locking blood inside.
Duration depends on dose
Higher alprostadil doses extend rigidity but raise priapism risk.
Return to baseline
As the drug clears, muscle tone returns and outflow reopens.