A brief synopsis of the information on this page is as follows: You need to be relaxed in order to obtain an erection. Once you have overcome that obstacle, you must increase your heart-rate, and breathing in order to actually reach the point of orgasm. If you aren't doing these things, then your body will have a very hard time reaching the point of orgasmic release.

      Erection is a necessary precursor to normal male orgasm. In order to form an erection smooth muscle fibers present in the corpus cavernosum and arteries destined to the penis relax in response to the release of non adrenergic non cholinergic neuromediators synthetized by postganglionic parasympathetic nerve fibers present in the cavernous nerves. (That boils down to the following happening when your brain decides that you are aroused) [2]. This causes the corpora (bulbous) cavernosa and the corpus spongiosum to fill up with blood. The filling of these chambers with blood results in the penis getting larger and hard when erect. Nitric oxide is the main proerectile neuromediator. Noradrenaline released by sympathetic fibers contracts penile smooth muscle fibers [2]. Noradrenaline seems to play a major role in the termination of erection, because the contraction of these smooth muscle fibers allows the blood that has filled the corpora to leave the penis.

(courtesy of www.mhhe.com)

      Parasympathetic arousal is arousal of all of the systems in your body that you want to be working for you when you aren't stressed. The human stress response originally evolved for the purpose of dealing with situations that evoke the fight or flight response. So the parasympathetic nervous system is in charge of things like healing, your immune system, digestion, hair growth, and erections. An example: If a Saber-toothed tiger jumps out from behind a boulder, than you don't want your body wasting precious energy digesting breakfast, and fighting off your cold, or giving you an erection; you want it cranking up your heart-rate, blood pressure, and respiratory system to get fuel to your muscles and sensory apparatus so that you can either club it in the nose, or get back to your cave before the tiger eats your liver. Your body is smart enough to do this for you. However, in this day and age, we aren't always smart enough to tell our bodies to stop the stress response. Instead of running from tigers for a few minutes, and then collapsing in the safety of our caves, relieved and letting our parasympathetic nervous systems take over, many people are constantly worried about bills, work, relationships, etc. so they don't fully turn off their stress response (their parasympathetic nervous systems). Since our bodies are smart enough to not waste precious energy on erections when we're stressed (sympathetic nervous system is active), this can lead to erectile difficulties. It is for these reasons that parasympathetic arousal is a necessary precursor to an erection.

      A number of important physiological changes must take place once sexual arousal has begun in order to reach orgasm. Interestingly enough, most of them are subsets of the Sympathetic (stress response) mentioned in the Necessary Precursors section above. Symptoms of the sympathetic response that are necessary to reach orgasm once an erection has been produced [3]: