The diagram below on the basic mechanisms of erection and flaccidity of the penis also shows the blood supply and innervation of the penis. The mechanisms of erection and flaccidity are shown in the upper and lower inserts, respectively.
Penile erection is a complex physiologic process that occurs through a coordinated cascade of neurologic, vascular, and humoral events. In the flaccid penis, a balance exists between blood flow in and out of the erectile bodies.
ErectionWith sexual arousal through imaginative, visual, auditory, tactile, olfactory, and other erotic stimuli, nitric oxide (NO) is released by nonadrenergic, noncholinergic (NANC) neurons. On arousal, parasympathetic activity triggers a series of events starting with the release of nitric oxide and ending with increased levels of the intracellular mediator cyclic guanosine monophosphate (cGMP). Non-adrenergic, non-cholinergic nerves and vascular endothelium release nitric oxide in response to sexual arousal, which activates cytoplasmic guanylate cyclase, converting GTP into cGMP. Cyclic AMP (cAMP) and cyclic GMP (cGMP), the intracellular second messengers mediating smooth-muscle relaxation, activate their specific protein kinases, which phosphorylate certain proteins to cause opening of potassium channels, closing of calcium channels, and sequestration of intracellular calcium by the endoplasmic reticulum.
It is well established that NO and cGMP are the most important transmitters for onset and maintenance of erection. Detumescence and return to the Flaccid stateDetumescence.After ejaculation or cessation of erotic stimuli, sympathetic tonic discharge resumes, resulting in contraction of the smooth muscles around sinusoids and arterioles.
During the return to the flaccid state, cyclic GMP is hydrolyzed to GMP by phosphodiesterase type 5. Molecular Mechanism of Penile Smooth-Muscle Contraction.Norepinephrine from sympathetic nerve endings, and endothelins and prostaglandin F2 from the endothelium, activate receptors on smooth-muscle cells to initiate the cascade of reactions that results in elevation of intracellular calcium concentrations and smooth-muscle contraction.
Trabecular muscle tone is controlled and penile blood vessel smooth muscle tone may be influenced by three neuroeffector systems. Nonadrenergic-noncholinergic (NANC) nerves, which control blood vessel and corporal smooth muscle relaxation. To sum upIn the flaccid state, the smooth muscle cells of the penile arteries and the corpora cavernosa are in a state of tone (contraction). The mechanisms of erection and detumescence are much more complex than that described above with many other factors and secondary messengers playing a role.
The pudendal nerve is a sensory, autonomic, and motor nerve that carries signals to and from the genitals, anal area, and urethra. The pelvic region and route of the pudendal nerve are so three dimensional they are hard to visualize. To the non-medical person (the layman), the pelvic area is hard to understand due to its 3D nature and complexity. The above image of the pelvis was just bones and the body was drawn in, so you could see where the pelvis is. The pudendal canal, also called Alcock's Canal, is the second most common location of pudendal nerve entrapment, after the ischial spine. This shows the route the pelvic and pudendal nerves take as they feed off of the sacral plexus. Information found on this website is offered for support and educational purposes and should not replace professional medical advice. This site complies with the HONcode standard for trustworthy health information: verify here.
The peripheral nervous system is divided into two major parts: the somatic nervous system and the autonomic nervous system. The somatic nervous system consists of peripheral nerve fibers that send sensory information to the central nervous system AND motor nerve fibers that project to skeletal muscle. The cell body is located in either the brain or spinal cord and projects directly to a skeletal muscle.
The autonomic nervous system is divided into three parts: the sympathetic nervous system, the parasympathetic nervous system and the enteric nervous system. Loving Animals Since i was eating animals so had to learn how to feed animals(call it paying it forward >;) Tuskegee U. This rat model of cavernous nerve crush injury mimics neural damages associated with radical prostatectomy in human.

Identically to human, the rat model of cavernous nerve crush injury displays ED (figure 1). Briefly, bilateral cavernous nerve crush is performed in rats under anaesthesia usually 3-4 weeks before experimentation (figure 2). During erection, relaxation of the trabecular smooth muscle and vasodilatation of the arterioles results in a severalfold increase in blood flow, which expands the sinusoidal spaces to lengthen and enlarge the penis.
Originally termed endothelial-derived relaxing factor, NO is known to be the most important physiologically occurring vasoactive molecule in the entire cardiovascular system.
The increased levels of cGMP alter transmembrane calcium ion flux, resulting in cavernosal smooth muscle relaxation, dilatation of cavernosal and helicine arteries and engorgement of lacunar spaces.
Finally, cGMP is metabolized to GMP via phosphodiesterase, of which four isoforms (types 2, 3, 4, and 5) have been identified in human penile tissue.
Other phosphodiesterases are also found in the corpus cavernosum, but they do not appear to have an important role in erection. Protein kinase C is a regulatory component of the calcium-independent, sustained phase of agonist-induced contractile responses. Relaxation of the smooth muscle (arterial and cavernosal) causes increased inflow of blood into the lacunar spaces of the corpora cavernosa.
After ejaculation or cessation of the erotic stimuli, the smooth muscle surrounding the arteries and the lacunar spaces contracts. Placing them side by side and identifying the key items allows you to more easily grasp the route of the pudendal nerve, along with the potential entrapment locations on the route. This image does an excellent job of showing bone structure, nerve paths, and ligaments in a 3D manner. Since we can no longer see where we sit down, the features of interest are mostly hidden from view.
Imagine you have just followed Alice in Wonderland into her magical, miniature world and are now two inches tall.
Can you see how the chair pushes on the all that tissue, which in turn mashes those nerves against the bones or ligaments they run over?
If you have any further questions, just ask our intrepid guides, who can be found in the group discussion areas. Note thebundle of pudendal nerves, veins, and arteries running together through the pudendal canal. Superficial dissection allows showing many more nerves, as well as the pudendal canal.
Drawing illustrates pudendal nerve arising from sacral nerve roots S2–S4, exiting pelvis to enter gluteal region through lower part of greater sciatic foramen and reentering pelvis through lesser sciatic foramen. It gives you a three dimensional look of what the pelvis looks like, the particular muscle groups within the pelvis and where some of the larger nerves are within the pelvis, including the pudendal nerve. The autonomic nervous system controls smooth muscle of the viscera (internal organs) and glands.
The enteric nervous system is a meshwork of nerve fibers that innervate the viscera (gastrointestinal tract, pancreas, gall bladder).
The expansion of the sinusoids compresses the subtunical venular plexus against the tunica albuginea.
This also applies to corpus cavernosum function, where local smooth muscle relaxation, and in turn erection, is mediated predominantly by NO release. The expanding lacunar spaces compress the subtunical venous plexus against the tunica albuginea, decreasing cavernosal venous outflow, increasing intracavernosal pressure, with resulting penile rigidity.
Sildenafil inhibits the action of phosphodiesterase (PDE) type 5, thus increasing the intracellular concentration of cGMP.
Phosphodiesterase type 5 (PDE 5) is the predominant isoform in human corporal smooth muscle.
Roughly 45 percent of the cavernosal body is made up of smooth muscle.The common mechanism of these agents may be via regulation of smooth muscle calcium. The arterial pressure expands the relaxed trabecular walls, thus expanding the tunica albuginea with subsequent elongation and compression of the draining venules. The inflow of blood is reduced and the venous drainage of the corporeal spaces is opened, returning the penis to the flaccid state.

The pudendal nerve comes down from sacral nerve roots 2,3, and 4, runs underneath the piriformis muscle, goes between the (SS)sacrospinous and (ST)sacrotuberous ligaments at the ischial spine, travels through alcock's canal between the obturator internus and levator ani muscles, and divides into 3 branches. The most common point of entrapment is at the ischial spine, where the pudendal nerve runs under the sacrospinal ligament. When you stop and think about what you're looking it, what it does, and how well it does it, it's really an engineering marvel. This image also gives amazing detail on all those tiny yellow nerves in the anal and perineum regions.
Pudendal nerve gives rise to inferior rectal nerve, perineal nerve, and dorsal nerve of penis or clitoris.
Note location of falciform process of sacrotuberous ligament, which is possible site for pudendal nerve entrapment. Note how the nerve travels under the sacrospinous ligament near the ischial spine and then branches out.
This is a great place to start your journey in identifying major structures within the pelvic region. In addition, stretching of the tunica compresses the emissary veins, thus reducing the outflow of blood to a minimum.
Venous outflow drops as the expanding cavernosal spaces compress the venous plexus and the larger veins passing through the tunica albuginea. The rapid filling of the cavernosal spaces compresses venules resulting in decreased venous outflow, a process often referred to as the corporeal veno-occlusive mechanism.
Erection of the penis is thus a haemodynamic event under the control of the autonomic nervous system. With the right doctor and good preparation on your part, you can move swiftly through the diagnosis step. Imagine all your organ weight pushing down as you go about your everyday vertical existence. Pudendal nerve emerges from pelvis inferior relative to piriformis muscle and enters gluteal region medial relative to sciatic nerve, superficial relative to sacrospinous ligament, and deep relative to sacrotuberous ligament. This is a great job of showing the three dimensional route of the nerve. In the flaccid state, inflow through the constricted and tortuous helicine arteries is minimal, and there is free outflow via the subtunical venular plexus. Coordination of the neuronal activity from psychogenic stimuli occurs in the hypothalamus while reflexogenic erection involves a polysynaptic coordination in the sacral parasympathetic centres. That something else, for PNE, is bones or ligaments pushing for a long time at a pressure level far above what our poor little bodies were designed for.
You see a perfect cross section, exposing all those little things running up and down at the point of cross section.
The bicycle seat gets right up in there and pushes ever so hard on the pudendal nerve, who frequently is totally overwhelmed by the experience and has been known to go on strike! However, this only increases pressure elsewhere, resulting in an even higher chance of injury. Note the three branches of the pudendal nerve and how they serve different areas.
Notice that the somatic nervous system has only one neuron between the central nervous system and the target organ while the autonomic nervous system uses two neurons. The important concept in this image is the way the pelvis holds up the internal organs in the abdominal cavity, much like a bowl holds jello. Muscle, fat, and other soft tissue doesn't cause concentrated pressure: only harder things can do that.
The poor stressed out fellow has not yet reached the ischial spine, which it does only about two inches further down. For those who are susceptible, the result of all this pressure and nerve vulnerability is PN or PNE.

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