The neurovascular events that ultimately occur result in the inhibition of adrenergic tone and the release of the nonadrenergic, noncholinergic neurotransmitter, nitric oxide. Nitric oxide is believed to be released from nonadrenergic, noncholinergic nerves and endothelial cells. It subsequently stimulates the guanylate cyclase enzyme system in penile smooth muscle. This results in increased levels of cyclic guanosine monophosphate (cGMP) and ultimately in smooth muscle relaxation, enhancement of arterial inflow, and veno-occlusion, producing adequate firmness for sexual activity.
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.
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. 
Psychological Causes of ED – Between 10% and 20% of ED cases have a psychological cause. Because arousal starts in the brain, psychological issues can be a significant contributing factor to erectile dysfunction. Mental health conditions like depression or anxiety can negatively impact your libido, making it more difficult for you to become aroused.
Moemen et al. compared the effectiveness and satisfaction associated with use of several ED therapies including sildenafil alone, intracavernosal injections (ICI) followed by sildenafil after ICI discontinuation and vacuum erections devices (VED) followed by sildenafil therapy after VED discontinuation (60). Seventy percent of men receiving vasoactive medications preferred sildenafil to ICI, even though rigidity was superior in the ICI group. All men using VEDs were dissatisfied with that form of therapy.
The sympathetic pathway originates from the 11th thoracic to the 2nd lumbar spinal segments and goes via the white rami to enter the sympathetic chain ganglia. Subsequently nerves travel through the lumbar splanchnic to inferior mesenteric and superior hypogastric nerves to the pelvic plexus. The T10 through T12 segments are most often the origin of sympathetic fibers, and the sympathetic chain ganglia that innervate the penis are located in the sacral and caudal ganglia (3).
If you can't take one of these oral medications, your physician may have you try Caverject (alprostadil for injection), a hormone that you inject into your penis using a fine needle, or Muse (alprostadil urogenital), a tiny suppository that you insert into the tip of the penis. Both of these will bring on an erection within five to 15 minutes without sexual stimulation.
Medical conditions, such as hypertension, diabetes mellitus, and cardiovascular disease (CVD), and psychological conditions, such as depression and anxiety, also contribute to sexual dysfunction in middle-aged or elderly men. CVD and hypertension cause a narrowing and hardening of the arteries, leading to reduced blood flow to the corporal bodies, which is essential for achieving an erection. Diabetes is a common aetiology of sexual dysfunction, because it can affect both the blood vessels and the nerves that supply the penis. Men with diabetes are four times more likely to experience ED, and on average, experience ED 15 years earlier than men without diabetes.7 Obesity is also correlated to the development of several types of dysfunction, including a decrease in sex drive and an increase in episodes of ED.8
Yohimbine: The main component of an African tree bark, yohimbine is probably one of the most problematic of all natural remedies for ED. Some research suggests that yohimbine can improve a type of sexual dysfunction that is linked with a drug used to treat depression. However, studies have linked yohimbine to a number of side effects, which can include anxiety, increased blood pressure, and a fast, irregular heartbeat. Like all natural remedies, yohimbine should only be used after advice and under supervision from a doctor.
Penile erection is managed by two mechanisms: the reflex erection, which is achieved by directly touching the penile shaft, and the psychogenic erection, which is achieved by erotic or emotional stimuli. The former uses the peripheral nerves and the lower parts of the spinal cord, whereas the latter uses the limbic system of the brain. In both cases, an intact neural system is required for a successful and complete erection. Stimulation of the penile shaft by the nervous system leads to the secretion of nitric oxide (NO), which causes the relaxation of smooth muscles of corpora cavernosa (the main erectile tissue of penis), and subsequently penile erection. Additionally, adequate levels of testosterone (produced by the testes) and an intact pituitary gland are required for the development of a healthy erectile system. As can be understood from the mechanisms of a normal erection, impotence may develop due to hormonal deficiency, disorders of the neural system, lack of adequate penile blood supply or psychological problems. Spinal cord injury causes sexual dysfunction including ED. Restriction of blood flow can arise from impaired endothelial function due to the usual causes associated with coronary artery disease, but can also be caused by prolonged exposure to bright light.
Using the correct size of tension ring is critical in obtaining the best possible result from this type of treatment. If the ring is too tight, it can be uncomfortable. If too large a ring is used, the erection may not last. The tension ring should not be left in place longer than 30 minutes. Leaving the ring in place for longer than 30 minutes can cause damage to the penis and further aggravate the cause of the erectile dysfunction.
Several studies accessed the prevalence of ED. The Massachusetts Male Aging Study reported a prevalence of 52%.2 The study demonstrated that ED is increasingly prevalent with age: approximately 40% of men are affected at age 40 and nearly 70% of men are affected at age 70. The prevalence of complete ED increased from 5% at age 40 to 15% at age 70.2 Age was the variable most strongly associated with ED.
Supplements are popular and often cheaper than prescription drugs for ED. However, supplements have not been tested to see how well they work or if they are a safe treatment for ED. Patients should know that many over-the-counter drugs have been found on drug testing to have ‘bootlegged' PDE 5 Inhibitors as their main ingredient. The amounts of Viagra, Cialis, Levitra or Stendra that may be in these supplements is not under quality control and may differ from pill to pill. The FDA has issued consumer warnings and alerts.