Anatomy of Penis and Mechanism of Erection
Anatomy of Penis – The basic construction
Mechanism of erection
| - Neurologic origin - Role of neurotransmitters - Sustenance of erection |
Anatomy of Penis – The basic construction
The penis is an organ that is responsible for excretion of urine and reproduction. It is positioned in the front and side of the pubic arch, anterior to the body. It remains in two conditions, flaccid and erect. In the flaccid condition it is cylindrical shape, but when erect, assumes the form of triangular prism with rounded angles. It is composed of a pair of corpora cavernosa penis and corpus cavernosum urethrae which is tucked between the two corpora cavernosa penis. Corpora cavernosa penis are laterally located and are made of spongy tissues that are composed of smooth muscles, fibrous tissues, spaces, veins, and arteries and covered by a membrane called tunica albuginea. Corpus cavernosum urethrae which is medially located, is covered by corpus spongiosum.
Corpora cavernosa penis are laterally (left and right side) located, lying on the dorsal (front) side and are made of spongy tissues that are composed of smooth muscles, fibrous tissues, spaces, veins, and arteries and are lined by an epithelium. The Corpora cavernosa penis is covered by a membrane called tunica albuginea and separated by connective tissue septa. It constitutes the major part of the penis, with the anterior (from head to end) three fourth being closely attached to one another but at their proximal (opposite) end, they diverge in the form of crura. The crura are attached to rami of the pubic arch. The distal part of corpora cavernosa ends a hemisphere like structure called as the glans penis.
Corpus cavernosum urethrae which is medially located, is covered by corpus spongiosum which lies on the underside of the penis. The urethra, which is the last part of the urinary tract, traverses the corpus spongiosum and ends at an opening called as the known as the meatus which marks the tip of the glans penis. It is serves the purposed of excretion of urine and is also a passage for the ejaculation of semen.
The spongiosum is composed of sinusoidal spaces of larger dimensions than those of the corpora cavernosa and with less smooth muscle.
Comparing to corpora cavernosa penis, the tunica albuginea surrounding the
spongiosum is fragile, but despite that spongiosum is capable of an erectile response.
Glans Penis
The distal part of the corpus spongiosum is an enlarged cone shaped structure called glans penis. In the flaccid (relaxed) state, the glans penis is partially or completely covered by the foreskin or prepuce, unless the penis is circumcised. During erection, the foreskin retracts to expose the glans.
The production of sperms takes place in the testes which are located below the penis. Epididymis acts as a store house for them. During ejaculation, vas deferens propels the sperms ahead. Seminal vesicles add fluid and along with vas deferens turn into ejaculatory ducts and join the urethra inside the prostrate gland. There are more secretions added by the prostrate and the bulbourethral glands before the semen is expelled through the penis.
Haemodynamics (blood supply)
1. Arterial (oxygenated) blood supply to the penis
Internal iliac artery provides blood to both the corpus cavernosum. The arterial blood supply systems are divided into two, the deeper and the superficial. The internal pudendal artery is a deeper arterial system that supplies blood to the erectile tissues. In the pelvis, it passes under the sacrospinous ligament, crosses over the sacrotuberous ligament, and gives off the perineal artery in Alcock’s canal, where it runs under the superficial transverse perineal muscle and the symphysis pubis. It passes through the pelvic floor adjacent to the inferior ramus of the ischium near the bulb of the urethra and gives off four branches which are the artery to the bulb of the penis, the urethral artery, the deep artery of the penis and the dorsal artery of the penis which play a key role in supplying blood. After this it reaches the corpus cavernosum to form one element of the paired dorsal arteries. The superficial system arises from the lateral inferior pudendal artery supplies blood to the tissues around the erectile organ.
2. Venous (Deoxygenated blood) drainage of the penis
The deoxygenated (venous) blood recedes from the penis in three planes: superficial, intermediate and deep.
Lymphatic drainage (removal of unwanted elements)
Lymph’s are clear fluids that pass through a network of lymph vessels and along the way remove infection, cellular debris and toxins from every part of the body. The lymph fluids finally drain into the liver for disposal and get broken down in the gall bladder. From most of the penis, lymph drains into the superficial inguinal lymph nodes and the vessels from the glans penis drain into the deep inguinal lymph nodes.
Neuroanatomy – Involvement of the nerves in erection
The neuroanatomy of the penis place a significant role in getting the penis erect as it passes stimulus to the peripheral nerves involved. For penile erection to take place, there are three sets of peripheral nerves involved.
- Parasympathetic nerves from the second to fourth sacral (S2–S4) segments
- Sympathetic nerves from the tenth thoracic to the second lumbar (T10–L2) thoracolumbar outflow
- Somatic fibers through the pudendal nerves
The dorsal nerve of the penis is one of the two terminal branches of the pudendal nerve which arises in the pudendal canal and passes anteriorly into the deep perineal space. It then runs to the dorsum of the penis where it passes lateral to the arteries. It supplies both the skin and glans penis. The penis is richly innervated with sensory nerve endings, especially the glans and is innervated with sensory nerve endings, especially the glans. The cavernous nerves from the inferior hypogastric plexus pass through the urogenital diaphragm to reach the penis. The skin covering the root of the penis is supplied by the ilioinguinal nerve and the posterior scrotal branches of the perineal nerve.
Anatomy/ Mechanism of erection
Erection is assumed to be a part of sexual arousal, though in a healthy meal, it can occur otherwise also. Besides its primary function of represent sexual arousal, it can also occur due to friction, retraction of the prepuce or due to pressure from a filled urinary bladder. In healthy males, erection occurs several times per night during the REM phases of sleep. There are some who also experience morning erections. These are called an “unprovoked erection” and take place 2 to 3 times in a week. It signifies healthy functioning of the erection pathways. Erections are also observed in infant boys, and in utero. The physiological process of erection is initiated in the brain and involves the nervous and vascular systems.
Neurologic origin
The control of the smooth muscle tissue of the corpora cavernosa is done by the autonomic system which is controlled by reflex centers in the cord or cortical centers that are accessible to multiple (visual or sensory) stimuli. This results in relaxation of the corpora cavernosa. The perineal muscles are controlled by the somatic system. The vessel engorgement resulting in the swelling due to a reduction in the alpha-sympathetic tonus of the cavernous tissue permitting influx of arterial blood, and also due to decreased venous flow from compression of the subalbugineal venous network against the tunica albuginea of the corpus cavernosurn. The rigidity is maintained due to an increase in intracavernous arterial pressure which is simultaneous with contraction of the perineal muscles (ischiocavernosus) under the somatic control of the pudendal nerve.
Neurotransmitters such as epinephrine, acetylcholine and nitric oxide are some of the chemicals for initiating this relaxation. When there is stimulation from the brain it sends impulses to the nerves to release nitric oxide. Nitric oxide is released from nonadrenergic noncholinergic (NANC) nerves and endothelial cells. This neurotransmitter stimulates the guanylate cyclase enzyme system in penile smooth muscle. This
result in increase in the level of cyclic guanosine monophosphate (cGMP) .The cGMP causes the smooth muscles of the corpora cavernosa to relax and dilates the blood vessels of the penis which allows rapid inflow of blood.At this point, the two corpora cavernosa and corpus spongiosum can hold seven times their normal amount of blood. The incoming blood fills the corpora cavernosa, making the penis expand due to engorgement of vessels.
Sustenance of erection
The function of tunica albuginea is to keep the blood trapped in the corpora cavernosa which sustains the erection. Reversal in erection occurs when cGMP levels in the corpora cavernosa fall, causing the smooth muscles of the corpora cavernosa to contract, stopping the inflow of blood and opening veins that drain blood away from the penis. The falling of the levels of the cGMP in the corpora cavernosa is attributed to the enzyme phosphodiesterase type 5 (PDE5) that destroys it with the help of the nerves from the sympathetic nervous system. Any defect in this complex flow of events can result in erectile dysfunction.
- Erectile dysfunction as a result of abnormality in the system are illustrated below-
- Cerebral vascular accidents such as multiple sclerosis, Parkinson’s disease, and spinal cord injury can cause neurogenic erectile dysfunction.
- Erectile dysfunction due to neurovascular abnormalities due to vascular disease and diabetes.
- Diseases such as peyronie’s disease, traumatic perineal and penile injuries can harm the neurovascular and anatomical structures which can produce erectile dysfunction.
Random Posts
Filed under: ED Guide by admin on Mon, June 28th, 2010
June 28, 2010