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PART 1: The Smooth Muscle & the Tunica

Let's start with the tunica. The tunica is a strong tendon-like tissue that surrounds all three corpus chambers—and the smooth muscle within the chambers. The tunica governs the size of the erect penis similar to how a bicycle tire limits the expansion of the inner tube inside. As with a bike tire, once the limit of the tunica is reached, further pressure results in greater stiffness rather than expansion.

Research seems to indicate that, oddly enough, not every man has the same amount of layers of tunica. Most men have two layers, but some men only have one layer in their penis. Others have as much as three layers.

The Penis Smooth Muscle
Now, it's widely believed that the penis has no muscle in it. This is wrong. Your penis isn't a muscle, but it does have muscle in it. Think of it this way. You can enlarge your arms. Are your arms muscle? No way. Do they have muscle in them? Yes way (biceps, triceps, etc.). It's the same thing for your penis, only the type of muscle is different.

A healthy penis is made of roughly 50 percent smooth muscle.

What does the smooth muscle do?
The smooth muscle helps determine the size, shape, and hardness of your penis. Just as your biceps and triceps play a critical role in the size and strength of your arms, the smooth muscle in your penis plays a critical role in the strength and size of your erections.

Your erections are largely caused by complete relaxation of the smooth muscle in your penis. Here's how.

The erection process of a healthy penis, the smooth muscle view
Pre-Erection: Your penis is flaccid. Your smooth muscle, which is largely in the corpus chambers, is contracted. See Figure 1. In the figure, you'll see a cross section of the penis. Inside the three corpus chambers of the penis, you'll see several circular sponge-like spaces known as sinusoidal spaces. Inside these spaces is where the smooth muscle is. There are also some smooth muscle cells in your veins and arteries, but they are different from the ones in the corpus chambers.

The normal erection process:

1. Something turns you on.
2. The smooth muscle relaxes and blood flows into the penis.
3. When the smooth muscle is completely relaxed, it presses against the tunica and cuts off the veins that take blood out of your penis, as in figure 2.
4. An erection occurs

So, when a penis is flaccid, the smooth muscle is contracted. When the penis is erect, the smooth muscle is completely relaxed and pushing on the tunica.

AN IMPORTANT POINT: More Smooth Muscle = Harder Erections
Not all penises have the same amount of smooth muscle. Some penises have more than 50 percent smooth muscle; others have less than 30 percent. This difference is a major reason why some men can’t get an erection and other men can have an erection for hours on end. Says urologist Dr. Eric Wespes:

The amount of smooth muscle in the penis is the essential factor that determines a man’s ability to achieve normal erections.

And another study:

Sexology 2005: "It has been shown that decreased smooth muscle content is associated with an impaired erection."

Clearly, smooth muscle is important for strong, firm erections. The not-so-crazy part? The amount of smooth muscle in a man’s penis appears to decrease as he ages. In one study published in the International Journal of Impotence Research, Dr. Wespes measured the percent of smooth muscle in men of different ages. In his study, the men under forty had an average of 46 percent smooth muscle in their penis; men between forty-one and sixty had 40 percent; and men older than sixty had 35 percent. (Wespes)

An erection and the role of unhealthy smooth muscle in the penis:
It appears that when you don't have enough smooth muscle, or possibly when the smooth muscle is too "fatigued" to relax all the way, you aren't going to get a hard erection. Let's go over the two main problems that can occur with the smooth muscle during the erection process.

The problematic erection process:

1. Something turns you on.
2. The smooth muscle relaxes and blood flows into the penis. Possible Problem 1: The smooth muscle can't relax all the way. This might occur from many things, such as poor health. In terms of PE, however, it probably occurs if your penis smooth muscle is too tired from overtraining.
3. When the smooth muscle is completely relaxed, it presses against the tunica and cuts off the veins that take blood out of your penis. Possible Problem 2: The smooth muscle is completely relaxed, but the tunica is stretched beyond the smooth muscles "reach." This essentially creates a "gap" between the smooth muscle and the tunica, making it hard to shut off the veins.
4. If problem 1 or 2 occurs, a rock-hard erection won't occur. Problem 1 has already been dealt with by sparkyx in his PI thread. This theory tackles the possibility of problem 2.

Problem 2, further explanation:
At first glance, problem 2 may be hard to grasp, but it's very simple once you think about it. First, let's consider the smooth muscle and the tunica as two separate things we're enlarging, with two separate size "limits." For instance, let's do an example where the tunica "limit" is larger than the smooth muscle "limit." In this instance, let's say you're a 50 year old man who has never exercised his penis before with only 40 percent amount of smooth muscle (the average in Dr. Wespes study).

Your erections aren't as hard as they once were, as the smooth muscle is having a harder time reaching the tunica - in an essence there is a "gap". This has also caused your penis size to be a little smaller than it was when you were younger.

The good news is that the limit of your tunica has most likely stayed the same over the years, so the smooth muscle is what's holding you back. How is this good news? It appears that PE exercises the smooth muscle. This could possibly play a role in why a lot of men get quick, abundant "newbie" gains along with a quick increase in hardness. There smooth muscle was the limit, and PE enlarged the smooth muscle (or created more smooth muscle). In turn, the smooth muscle volume is sufficent enough to press all the way against the tunica, and the "gap" is eliminated.

A key point here is that you don't have to be a 50 year old man to have low amount of smooth muscle. We estimate that any man, at any age can have under-developed smooth muscle. For example, a man who doesn't obtain a lot of erections via sex and masturbation probably has poorly developed smooth muscle. It just so happens that men who are 18 tend to have a lot more erections than men who are 58.

Improper Training/PE Can Cause a "Gap"
It's been reported by several PEers who aren't overtraining and are gaining, that for some reason, they can never obtain "rock hard erections." In this case, the problem is probably that they are enlarging the tunica more than the smooth muscle.

How is this possible? We believe that, for the most part, when we PE we are dealing with two major distinct tissue types, connective tissue (the tunica) and smooth muscle. Given their differing structure and composition, the growth process for each tissue type is not the same; meaning a different training strategy is needed. Smooth muscle cells, unlike any other muscle type, retain the ability to increase not only in size but in cell numbers, while connective tissue does not have this ability. (More on how to enlarge each tissue in Part 2.)

The smooth muscle being the limit
On the other end of the spectrum, you can have the smooth muscle "limit" larger than the tunica "limit." In this case:

• The smooth muscle is at or approaching the maximum the tunica can hold.
• So the smooth muscle is bulging into the tunica (think of it as pumping up a 15" inner tube inside a 14" tire).
• This in turn causes the penis to be extremely firm and hard when you have an erection, or a "rock-solid" erection.

So, in an essence, if your goal is increased hardness, you'll want your smooth muscle limit to be above your tunica limit. But if you're looking for more gains, and your smooth muscle limit is above your tunica limit, you'll need to focus your PE efforts on your tunica.

Resources:

• The Penis is a Muscle!
• Scwartz, E. J., et al. “Sildenafil Preserves Intracorporeal Smooth Muscle After Radical Retropubic Prostatectomy.” Clinical Urology. Volume 171(2), February 2004, pp. 771-774.
• Wespes, E. “Smooth Muscle Pathology and Erectile Dysfunction.” International Journal of Impotence Research. Vol. 14 (2002), pp. 17-21.
• Seidel, C.L., and N. W. Weisbrodt. Hypertrophic Response in Smooth Muscle. Boca Raton: CRC Press, 1987.
• Mechanical stretch stimulates growth of vascular smooth muscle cells via epiderm - H Iwasaki, S Eguchi, H Ueno, F Marumo, Y Hirata
• Vascular Smooth Muscle Growth: Autocrine Growth Mechanisms - Bradford C. Berk - Center for Cardiovascular Research, University of Rochester, Rochester, New York Division of Endocrinology and Metabolism, Second Department of Internal Med Physiological Reviews, Vol. 81, No. 3, July 2001, pp. 999-1030 Copyright ©2001 by the American Physiological Society
• Weapons of penile smooth muscle destruction: androgen deficiency promotes accumulation of adipocytes in the corpus cavernosum.Traish AM, Kim N. - Department of Biochemistry, Boston University School of Medicine, Boston, Massachusetts 02118, USA.