Tests such as the bulbocavernosus reflex test are used to determine if there is sufficient nerve sensation in the penis. The physician squeezes the glans (head) of the penis, which immediately causes the anus to contract if nerve function is normal. A physician measures the latency between squeeze and contraction by observing the anal sphincter or by feeling it with a gloved finger inserted past the anus.
Sexual stimulation causes the release of neurotransmitters from cavernosal nerve endings and relaxation factors from endothelial cells lining the sinusoids. NOS produces NO from L-arginine, and this, in turn, produces other muscle-relaxing chemicals, such as cGMP and cyclic adenosine monophosphate (cAMP), which work via calcium channel and protein kinase mechanisms (see the image below). This results in the relaxation of smooth muscle in the arteries and arterioles that supply the erectile tissue, producing a dramatic increase in penile blood flow.
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Alteration of NO levels is the focus of several approaches to the treatment of ED. Inhibitors of phosphodiesterase, which primarily hydrolyze cGMP type 5, provided the basis for the development of the PDE5 inhibitors. Chen et al administered oral L-arginine and reported subjective improvement in 50 men with ED.  These supplements are readily available commercially. Reported adverse effects include nausea, diarrhea, headache, flushing, numbness, and hypotension.
When stimulated by the nerves, the spongy tissue arranges itself in such a way that more blood can be stored in the penis. The veins running through the outer sheath of the penis then compress which stops the blood from leaving the penis. As the blood is stopped from flowing out, the penis fills with blood and stretches within the outer casing, giving an erection.
The inflatable type of device consists of cylinders that are implanted in the corpora cavernosa, a fluid reservoir implanted in the abdomen, and a pump placed in the scrotum. The man squeezes the pump to move fluid into the cylinders and cause them to become rigid. (He reverses the process by squeezing the pump again.) While these devices allow for intermittent erections, they have a slightly higher malfunction rate than the silicon rods.
When you become aroused, your brain sends chemical messages to the blood vessels in the penis, causing them to dilate or open, allowing blood to flow into the penis. As the pressure builds, the blood becomes trapped in the corpora cavernosa, keeping the penis erect. If blood flow to the penis is insufficient or if it fails to stay inside the penis, it can lead to erectile dysfunction.
Associated morbidity may include various other male sexual dysfunctions, such as premature (early) ejaculation and male hypoactive sexual desire disorder. The NHSLS found that 28.5% of men aged 18-59 years reported premature ejaculation, and 15.8% lacked sexual interest during the past year. An additional 17% reported anxiety about sexual performance, and 8.1% had a lack of pleasure in sex. 
PDE5i use in PD has not been well studied; however its benefits have been shown. Raffaele performed an open label, prospective study evaluating the efficacy of sildenafil 50 mg on demand and depressive symptoms experienced by the PD patient (73). Erections were improved in approximately 85% of men and 75% noted improvements in their depressive symptoms as well. Sildenafil was well tolerated without significant side effects. Zesiewicz et al., performed a shorter study showing improvements in erectile function but no change in depression and parkinsonisms after ED treatment (74).
PDE5 inhibitors, the primary second-line therapy, have been the mainstay of ED treatment since the release of sildenafil (Viagra) in 1998, with the subsequent development of many others, and still more in the development stage. These medications do improve erectile quality for the majority of men, and they work by enhancing blood flow in the corpora cavernosa. These medications are generally used on demand and need to be taken about an hour before sexual intimacy. Tadalafil (Cialis) is longer acting and does come in a daily preparation potentially eliminating the ‘on-demand’ need. The daily dosing of tadalafil, 2.5–5 mg\day, has also been approved by the FDA for treatment of symptoms of BPH.41 PDE5 inhibitors are contraindicated in men taking nitrates, but otherwise PDE5 inhibitors are very safe and effective. When PDE5 inhibitors are coadministered with nitrates, pronounced systemic vasodilation and severe hypotension are possible. Many patients with ED are elderly and have the same risk factors as patients with CAD, so these drug combinations are commonly considered or encountered in clinical practice.42
Many common medications for treating hypertension, depression, and high blood lipids (high cholesterol) can contribute to erectile dysfunction (see above). Treatment of hypertension is an example. There are many different types (classes) of medications for high blood pressure; these include beta-blockers, calcium channel blockers, diuretics (medications that increase urine volume), angiotensin converting enzyme inhibitors (ACE inhibitors), and angiotensin receptor blockers (ARBs). Patients may use these medications alone or in combination to control blood pressure. Some of these medications can cause troubles with erections. For example, Inderal (a beta-blocker) and hydrochlorothiazide (a diuretic) cause erectile dysfunction, while calcium channel blockers and ACE inhibitors do not seem to affect erectile function. On the other hand, other medications (such as angiotensin receptor blockers [ARB] including losartan [Cozaar] and valsartan [Diovan]) may actually help with erections. Therefore, if possible, you may benefit from changing your medications, but this requires approval by your prescribing health care provider.