This is the information I have been looking at. I am hoping this may shed some light on the reflexogenic erections I am getting. I still am at a bit of a loss but I feel like I am on the right track.
Factors that mediate naturally occurring erection
Erection occurs when nerve impulses from the brain (psychogenic erection) and from genital stimulation (reflexogenic erection) combine to cause blood to flow faster into than out of the penis. From this limited perspective, the penis can be viewed as a hydraulic organ. It is composed of three sponge-like cylindrical bodies that run the length of the penis (the two corpora cavernosa and the corpus spongiosum) which are supplied with blood by small branches of the penile artery. These helicine arteries empty into blood spaces (sinusoids) in this spongy tissue. The spaces are lined with vascular epithelial cells and are separated from each other by trabeculae, partitions made of smooth muscle. Blood is carried to the helicine arteries by branches of the pelvic artery and carried away from the sinusoidal spaces by surface veins than run adjacent to the sheath (tunica albuginea) that surrounds each corporal body.
The nervous input that induces erection is delivered to the penis via the pelvic nerve which exits the spinal cord at the lower sacral level (S2-S4) and branches into the cavernous nerve that supplies the corporal bodies. Both parasympathetic fibres that release acetylcholine (cholinergic fibres) and other nerves that release nitric oxide (NO) are involved in this process. It is damage to this nerve supply during some rectal and prostate surgeries that can cause post-treatment ED. Nervous input during sexual arousal causes dilation of the cavernosal arteries and results in a 30-40 fold increase in the rate of flow of blood through these arteries into the sinusoidal spaces. How does this nervous input cause the increased blood flow that initiates erection?