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Parasympathetic pathways originate from the intermediolateral cell columns of the 2nd, 3rd and 4th sacral spinal cord segments. Preganglionic fibers pass through the pelvic plexus where they coalesce with sympathetic fibers from the superior hypogastric plexus. The cavernous nerves that innervate the penis arise from the portion of the pelvic plexus. The pelvic plexus also contains nerves that innervate the rectum, bladder and urinary sphincter and the nerve projections can be damaged during radical excision of the bladder, prostate and rectum, leading to iatrogenic ED (4).
The role of the endothelium in ED has been noted for a number of years and the overlapping of ED and other conditions, especially coronary heart disease, CVD, affecting endothelial function/dysfunction, is clearly present. The endothelial cell is now known to affect vascular tone and impact the process of atherosclerosis, and impacting ED, CVD and peripheral vascular disease.16
Cavernosography measurement of the vascular pressure in the corpus cavernosum. Saline is infused under pressure into the corpus cavernosum with a butterfly needle, and the flow rate needed to maintain an erection indicates the degree of venous leakage. The leaking veins responsible may be visualized by infusing a mixture of saline and x-ray contrast medium and performing a cavernosogram. In Digital Subtraction Angiography (DSA), the images are acquired digitally.
Other factors leading to erectile dysfunction are diabetes mellitus, which is a well-known cause of neuropathy). ED is also related to generally poor physical health, poor dietary habits, obesity, and most specifically cardiovascular disease, such as coronary artery disease and peripheral vascular disease. Screening for cardiovascular risk factors, such as smoking, dyslipidemia, hypertension, and alcoholism is helpful.
Once implanted, the pumps become “part of their body,” Montague explains. “No out-of-pocket cost per use. Predictable response. Works every time.” Pills, even when they work, might be less effective if you’ve had more than a couple drinks or are out of sorts for other reasons. Injections are slightly more reliable than pills but, Montague says, are still subject to variability.
The nerves and endothelium of sinusoids and vessels in the penis produce and release transmitters and modulators that control the contractile state of corporal smooth muscles. Although the membrane receptors play an important role, downstream signaling pathways are also important. The RhoA–Rho kinase pathway is involved in the regulation of cavernosal smooth muscle contraction. 
The association between low testosterone and ED is not entirely clear. Although these 2 processes certainly overlap in some instances, they are distinct entities. Some 2-21% of men have both hypogonadism and ED; however, it is unclear to what degree treating the former will improve erectile function.  About 35-40% of men with low testosterone see an improvement in their erections with testosterone replacement; however, almost 65% of these men see no improvement. 
Examples of common neurologic conditions that can lead to ED include cerebral vascular accident, multiple sclerosis, Parkinson’s disease, and spinal cord injury. Microvascular disease associated with diabetes is thought to compound the endothelial and neural injuries associated with this disease. Pelvic surgery may disrupt both neural and vascular pathways, resulting in ED.
If PDE5 drugs don't work or cannot be used because of potential side effects, your doctor can recommend other therapies. The drug alprostadil (Caverject, Edex, Muse) allows blood to flow more freely in the penis, leading to an erection. The drug can be injected with a tiny needle into your penis. Or, a small pellet (suppository) can be inserted into the opening of the penis. Suppositories and injections are effective in the majority of men.
ED can also occur among younger men. A 2013 study found that one in four men seeking their first treatment for ED were under the age of 40. The researchers found a stronger correlation between smoking and illicit drug use and ED in men under 40 than among older men. That suggests that lifestyle choices may be a main contributing factor for ED in younger men.
An occasional problem achieving an erection is nothing to worry about. But failure to do so more than 50% of the time at any age may indicate a condition that needs treatment. About 40% of men in their 40s report at least occasional problems getting and maintaining erections. So do more than half (52%) of men aged 40 to 70, and about 70% of men in their 70s.
Treatment depends on the underlying cause. In general, exercise, particularly of the aerobic type, is effective for preventing ED during midlife. Exercise as a treatment is under investigation.:6, 18–19 For tobacco smokers, cessation often results in a significant improvement. Oral pharmacotherapy and vacuum erection devices are first-line treatments,:20, 24 followed by injections of drugs into the penis, as well as penile implants.:25–26 Vascular reconstructive surgeries are beneficial in certain groups.
ICI Alprostadil may be used as a mixture with two other drugs to treat ED. This combination therapy called "bimix or trimix" is stronger than alprostadil alone and has become standard treatment for ED. Only the Alprostadil ingredient is FDA approved for ED. The amount of each drug used can be changed based on the severity of your ED, by an experienced health professional. You will be trained by your health professional on how to inject, how much to inject and how to safely raise the drug's dosage if necessary.