Copied and pasted from
this thread at Thunder's... Beware, lots of science and big words ahead.
Appendix by Dr. Owens: Physiological cellular events during arousal/erections:
The smooth muscle cells in segments of the penis (corpora cavernosa) can either 1) relax, thereby allowing increased blood flow to the penis, or tumescence, or they can 2) contract, resulting in detumescence, or the flaccid state of the penis. Both events may be initiated in different ways, and it is ultimately the degree of contraction or relaxation of these smooth muscle cells that determines whether the penis is flaccid or erect.
Nerves to the penis release the neurotransmitter noradrenaline (a catecholamine), causing stimulation of alpha-receptors on the surface of adjacent smooth muscle cells. This leads to smooth muscle cell contraction, allowing only limited blood flow into the penis, which is now in the flaccid state, or undergoes detumescence if it was previously erect. Catecholamines can also be released during stressful situations associated with sexual activity such as fear of failure, performance anxiety, anger, shame, or embarrassment (Goldstein, 2000). Or they may be released in painful situations such as genital pain from infection of the prostate, epididymitis, and Peyronie’s disease, or non-genital pain such as headache (Goldstein, 2000). In either case, smooth muscle cells are stimulated to contract, resulting in reduced blood flow to the penis and detumescence.
To complicate matters further, different types of alpha-receptors (alpha-1 and alpha-2-subtypes) are located not only on the smooth muscle cells, but also directly adjacent on the nerve-buds, from where catecholamines and other neurotransmitters are released. From this location alpha-receptors modify how much of the transmitter substance is released from the nerves, and then reaches the smooth muscle cells (Crenshaw and Goldberg, 1996).
By blocking alpha-receptors on the smooth muscle cells with drugs like phentolamine (Regitine, Vasomax, mentioned on page 136), contraction of the smooth muscle cells can be prevented or at least reduced, allowing an erection to persist longer. This is why the drug phentolamine is effective in the treatment of ED and is currently being tested for possible FDA-approval as such a treatment option (Goldstein, 2000). The drug is already used as a blood pressure lowering medication for people with pheochromocytoma, a condition where excess catecholamines are produced in the body.
Alpha-2-receptors are blocked by Yohimbine, an alkaloid obtained from the bark of the African tree, Coryanthe yohimbe (Crenshaw and Goldberg, 1996; Morales, 2000). For over a century Yohimbine has been used as an aphrodisiac, and only recently has it become clear that Yohimbine has central effects (in the brain), increasing sexual arousal, as well as peripheral effects, blocking the catecholamine induced contractility in the smooth muscle of the penis (Bancroft, 2000). Alpha-2 receptors are located throughout the body on blood vessels and in the intestines as well, which explains some of the side effects (indigestion, nausea, dizziness, headache) experienced following the intake of Yohimbine (Crenshaw and Goldberg, 1996).
There are other ways smooth muscle cells in the penis can relax, resulting in an erection: During sexual arousal penile nerves release nitric oxide (NO), which activates the smooth muscle cells to release the messenger cyclic guanosine monophosphate (cGMP) inside the cells. It is this messenger signal which tells the contractile proteins inside the smooth muscle cells to stop contracting and to relax. The more of the messenger cGMP that is available inside the cells, the more they relax, and the more blood is channeled to the penis through widened pass ways, causing tumescence and erection.
Viagra enhances erections by increasing cGMP levels in erectile tissue smooth muscle cells. It increases the amount of intracellular cGMP by inhibiting its breakdown through the enzyme type 5 phosphodiesterase (PDE 5). Viagra only works in men who are sexually aroused (take a Viagra, mow the lawn, and nothing happens) and who produce some cGMP on their own. The drug can enhance a minor erection by making sure that the cGMP produced stays around in the cells for a while longer, instead of being removed by PDE 5. For men taking heart medication such as nitrates, Viagra can fatally potentiate the medicine effect and the cGMP released upon medicine intake may be increased to dangerous levels, causing a potentially deadly drop in blood pressure. It is important to emphasize that this only applies to situations where men take Viagra while simultaneously being on nitrates, and that Viagra otherwise is a safe drug to use. It is of utmost importance that Viagra is prescribed by a doctor and is not shared or taken without a doctor’s consent.
References:
Bancroft, J. (2000) Effects of alpha-2 blockade on sexual response: experimental studies with Delequamine (RS15385); International Journal of Impotence Research; Vol. 12, Suppl. 1, S64-S69
Crenshaw, T. L. & Goldberg, J. P. (1996) Sexual Pharmacology. Drugs That Affect Sexual Function. W. W. Norton & Company. New York. London; ISBN: 0-393-70144-1
Goldstein, I. (2000) Oral phentolamine: an alpha-1, alpha-2 adrenergic antagonist for the treatment of erectile dysfunction; International Journal of Impotence Research; Vol. 12, Suppl. 1, S75-S80
Morales, A. (2000) Yohimbine in erectile dysfunction: the facts; International Journal of Impotence Research; Vol. 12, Suppl. 1, S70-S74
Muller, J.E. et al. (1996) Triggering myocardial infarction by sexual activity. Low absolute risk and prevention by regular physical exertion; Journal of the American Medical Association; 275, 1405-1409
EJHS Book Review
----------------------------------------------------------
Anatomy of the Human Penis: The Relationship of the Architecture Between Skeletal and Smooth Muscles
GENG-LONG HSU*,{dagger}, CHENG-HSING HSIEH*, HSIEN-SHENG WEN*, WEN-LONG HSU*, CHIH-HSIUNG WU*, TSORNG-HARN FONG{ddagger}, SHYH-CHYAN CHEN{dagger} AND GUO-FANG TSENG{dagger}
From the * Microsurgical Potency Reconstruction Center, Taiwan Adventist Hospital, Taipei Medical University Hospital, the {ddagger} Department of Anatomy, College of Medicine, Taipei Medical University, and the {dagger} Department of Anatomy and Cell Biology, College of Medicine, National Taiwan University, Taipei, Taiwan, Republic of China.
Correspondence to: Dr Geng-Long Hsu, Microsurgical Potency Reconstruction Center, Taiwan Adventist Hospital, 424 Pa-Te Rd, Sec 2, Taipei 10558, Taiwan, Republic of China (e-mail:
[email protected]).
To investigate the anatomy of the ischiocavernosus muscle, bulbospongiosus muscle, and tunica albuginea and to determine their relationships to smooth muscle, which is a key element of penile sinusoids, we performed cadaveric dissection and histologic examinations of 35 adult human male cadavers. The tunica of the corpora cavernosa is a bilayered structure that can be divided into an inner circular layer and an outer longitudinal layer. The outer longitudinal layer is an incomplete coat that is absent between the 5-o'clock and 7-o'clock positions where 2 triangular ligamentous structures form. These structures, termed the ventral thickening, are a continuation of the anterior fibers of the left and right bulbospongiosus muscles. On the dorsal aspect, between the 1-o'clock and 11-o'clock positions, is a region called the dorsal thickening, a radiating aspect of the bilateral ischiocavernosus muscles. In the corpora cavernosa, skeletal muscle contains and supports smooth muscle, which is an essential element in the sinusoids. This relationship plays an important part in the blood vessels' ability to supply the blood to meet the requirements for erection, whereas in the corpus spongiosum, skeletal muscle partially entraps the smooth muscle to allow ejaculation when erect. In the glans penis, however, the distal ligament, a continuation of the outer longitudinal layer of the tunica, is arranged centrally and acts as a trunk of the glans penis. Without this strong ligament, the glans would be too weak to bear the buckling pressure generated during coitus. A significant difference exists in the thickness of the dorsal thickening, the ventral thickening, and the distal ligament between the potent and impotent groups (P <= .01). Together, the anatomic relationships between skeletal muscle and smooth muscle within the human penis explain many physiologic phenomena, such as erection, ejaculation, the intracavernous pressure surge during ejaculation, and the pull-back force against the glans penis during anal constriction. This improvement in the modeling of the anatomic-physiologic relationship between these structures has clinical implications for penile surgeries.
Key words: Corpora cavernosa, corpus spongiosum, bulbospongiosus muscle, ischiocavernosus muscle, distal ligament, tunica albuginea
http://www.andrologyjournal.org/cgi...stract/25/3/426