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PHYSIOLOGY OF ERECTION
INTRODUCTION
The peripheral pharmacology of local mechanisms of penile erection is known today thanks to
recent advance in the study of the regulation of erectile tissue smooth muscle tone. 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. Nitric oxide is the main proerectile
neuromediator. Noradrenaline, released by sympathetic fibers, contracts penile smooth muscle
fibers and is antierectile. Recent progress in the peripheral pharmacology of penile erection allows
new perspectives in the treatment of erectile dysfunction. The spinal cord represents a major site for
the neural regulation of penile erection. The latter occurs in response to stimuli from peripheral or
supraspinal origin. Different neural structures in the brainstem (nucleus paragigantocellularis), pons
and hypothalamus (nucleus paraventricularis) send projections to the thoracolumbar sympathetic
and lumbosacral parasympathetic nuclei at the origin of proerectile peripheral pathways. Serotonin
and oxytocin are candidates as neuromediators involved in the supraspinal control of penile
erection. Studying the central command of penile erection allows an approach to the
pathophysiology of psychogenic erectile dysfunction.
Penile Erection—Role of the Parasympathetic Nerves.
An erection (clinically: penile erection or penile tumescence) is a physiological phenomenon in which
the penis becomes firmer, engorged and enlarged. Penile erection is the result of a complex
interaction of psychological, neural, vascular and endocrine factors, and is often associated with
sexual arousal or sexual attraction, although erections can also be spontaneous. The shape, angle
and direction of an erection varies considerably in humans.
Physiologically, erection is triggered by the parasympathetic division of the autonomic nervous
system (ANS), causing nitric oxide (a vasodilator) levels to rise in the trabecular arteries and smooth
muscle of the penis. The arteries dilate causing the corpora cavernosa of the penis (and to a lesser
extent the corpora spongiosum) to fill with blood; simultaneously the ischiocavernosus and
bulbospongiosus muscles compress the veins of the corpora cavernosa restricting the egress and
circulation of this blood. Erection subsides when parasympathetic activity reduces to baseline.
As an autonomic nervous system response, an erection may result from a variety of stimuli, including
sexual stimulation and sexual arousal, and is therefore not entirely under conscious control.
Erections during sleep or upon waking up are known as nocturnal penile tumescence (NPT). Absence
of nocturnal erection is commonly used to distinguish between physical and psychological causes of
erectile dysfunction and impotence.
A penis which is partly, but not fully, erect is sometimes known as a semi-erection (clinically: partial
tumescence); a penis which is not erect is typically referred to as being flaccid, or soft
Penile erection is the first effect of male sexual stimulation, and the degree of erection is
proportional to the degree of stimulation, whether psychic or physical. Erection is caused by
parasympathetic impulses that pass from the sacral portion of the spinal cord through the pelvic
nerves to the penis. These parasympathetic nerve fibers, in contrast to most other parasympathetic
fibers, are believed to release nitric oxide and/or vasoactive intestinal peptide in addition to
acetylcholine. The nitric oxide especially relaxes the arteries of the penis, as well as relaxes the
trabecular meshwork of smooth muscle fibers in the erectile tissue of the corpora cavernosa and
corpus spongiosum in the shaft of the penis, This erectile tissue consists of large cavernous
sinusoids, which are normally relatively empty of blood but become dilated tremendously when
arterial blood flows rapidly into them under pressure while the venous outflow is partially occluded.
Also, the erectile bodies, especially the two corpora cavernosa, are surrounded by strong fibrous
coats; therefore, high pressure within the sinusoids causes ballooning of the erectile tissue to such
an extent that the penis becomes hard and elongated. This is the phenomenon of erection.
MECHANISM OF ERECTION
There are two major mechanisms involved in the physiology of penile erection which are;
1, Increased blood inflow
2, decreased blood outflow
The firm and enlarged state of the penis is called an erection. There are four factors that lead to the
erection of the male genitals; neural, endocrine, vascular and physiological. The complex interaction
between these four factors causes the penis to erect. When there is a disruption in any of the
functions of these four factors, erection dysfunction or impotence may occur. In order to prevent
any event of disruption, it is necessary that a male knows the mechanics of erection.
The physiological explanation for the mechanics of erection is simple. There are only two
mechanisms that are involved in erection; increased blood inflow and decreased blood outflow.
Initially, erection is triggered upon the functioning of physiological stimuli. It starts with the libido
function which is directly happening in the cerebral cortex. This is the sexual urge that makes a man
feel the need for sex. When the libido arises, an impulse is sent to the spinal center which is then
sent to the penis nerves.
The blood inflow mechanism of erection starts right after the impulse is sent to the penis nerves.
When the penis nerve endings receive the impulse, arterioles dilate as the smooth muscles relax.
Because of this relaxation that occurs in the muscles, the spaces in the corpora nervosa, the
chambers in the penis, are filled in. When these spaces are completely filled in, the space is no
longer enough to accommodate the blood flow, thus expansion follows.
The blood outflow mechanism of erection starts upon copulation. Copulation is also called sexual
intercourse. This takes place when the penis finally enters the vagina. This act continues until the
ejaculation and orgasm is attained. Ejaculation and orgasm is reached by the continuous friction
between the vaginal mucosa and penis glans. The friction is reinforced by the stimulation of
psychogenic factors to discharge sperm from the sympathetic pathway to the seminal pathway.
When the sperm reaches the seminal pathway, it is led to the posterior urethra by the blood flow.
The brain sends impulses again to the muscles and this signifies the end of the copulation because
by the time the impulses were received by the penis nerves, orgasm is reached. After orgasm and
ejaculation are reached, the discharge resumes and once again, the expanded muscles go back to
their normal size as they contract and the blood that comes from the sinusoidal spaces is flowed out
of the muscles.
When the above mechanism of erection is disrupted at anytime of the process, orgasm and
ejaculation is reached. Continuous events of disruption may totally damage the nerves in the penis
and this will result to impotence or erectile dysfunction
PHYSIOLOGY OF COITUS(male sexual act)
Neuronal Stimulus for Performance of the Male Sexual Act
The most important source of sensory nerve signals for initiating the male sexual act is
the glans penis.The glans contains an especially sensitive sensory endorgan system that
transmits into the central nervous system that special modality of sensation called
sexual sensation.The slippery massaging action of intercourse on the glans stimulates
the sensory end-organs, and the abnormal infertile sperm, compared with a normal
sperm on the right. sexual signals in turn pass through the pudendal nerve, then
through the sacral plexus into the sacral portion of the spinal cord, and finally up the
cord to undefined areas of the brain. Impulses may also enter the spinal cord from areas
adjacent to the penis to aid in stimulating the sexual act. For instance, stimulation of the
anal epithelium, the scrotum, and perineal structures in generalcan send signals into the
cord that add to the sexual sensation. Sexual sensations can even originate in internal
structures, such as in areas of the urethra, bladder, prostate, seminal vesicles, testes,
and vas deferens. Indeed, one of the causes of “sexual drive” is filling of the sexual
organs with secretions. Mild infection and inflammation of these sexual organs
sometimes cause almost continual sexual desire, and some “aphrodisiac” drugs, such as
cantharidin, increase sexual desire by irritating the bladder and urethral mucosa,
inducing inflammation and vascular congestion.
Psychic Element of Male Sexual Stimulation.
Appropriate psychic stimuli can greatly enhance the ability of a person to perform the
sexual act. Simply thinking sexual thoughts or even dreaming that the act of intercourse
is being performed can initiate the male act, culminating in ejaculation. Indeed,
nocturnal emissions during dreams occur in many males during some stages of sexual
life, especially during the teens. Integration of the Male Sexual Act in the Spinal Cord.
Although psychic factors usually play an important part in the male sexual act and can
initiate or inhibit it, brain function is probably not necessary for its performance
because appropriate genital stimulation can cause ejaculation in some animals and
occasionally in humans after their spinal cords have been cut above
the lumbar region. The male sexual act results from inherent reflex mechanisms
integrated in the sacral and lumbar spinal cord, and these mechanisms can be initiated
by either psychic stimulation from the brain or actual sexual stimulation from the sex
organs, but usually it is a combination of both.
Stages of the Male Sexual Act
Penile Erection—Role of the Parasympathetic Nerves.
Penile erection is the first effect of male sexual stimulation, and the degree of erection
is proportional to the degree of stimulation, whether psychic or physical.Erection is
caused by parasympathetic impulses that pass from the sacral portion of the spinal cord
through the pelvic nerves to the penis. These parasympathetic nerve fibers, in contrast
to most other parasympatheticfibers, are believed to release nitric oxide
and/orvasoactive intestinal peptide in addition to acetylcholine. The nitric oxide
especially relaxes the arteries of the penis, as well as relaxes the trabecular meshwork
of smooth muscle fibers in the erectile tissue of the corpora cavernosa and corpus
spongiosum in the shaft of the penis. This erectile tissue consists of large cavernous
sinusoids, which are normally relatively empty of blood but become dilated
tremendously when arterial blood flows rapidly into them under pressure while the
venous outflow is partially occluded. Also, the erectile bodies, especially the two
corpora cavernosa, are surrounded by strong fibrous coats; therefore, highpressure
within the sinusoids causes ballooning of the erectile tissue to such an extent that the
penis becomes hard and elongated. This is the phenomenon of erection.
Lubrication, a Parasympathetic Function.
During sexual stimulation, the parasympathetic impulses, in addition to promoting
erection, cause the urethral glands and the bulbourethral glands to secrete mucus.This
mucus flows through the urethra during intercourse to aid in the lubrication during
coitus. However, most of the lubrication of coitus is provided by the female sexual
organs rather than by the male. Without satisfactory lubrication, the male sexual act is
seldom successful because unlubricated intercourse causes grating, painful sensations
that inhibit rather than excite sexual sensations.
Emission and Ejaculation—Function of the Sympathetic
Nerves. Emission and ejaculation are the culmination of the male sexual act. When the
sexual stimulus becomes extremely intense, the reflex centers of the spinal cord begin
to emit sympathetic impulses that leave the cord at T-12 to L2 and pass to the genital
organs through the hypogastric and pelvic sympathetic nerve plexuses to initiate
emission, the forerunner of ejaculation.
Emission begins with contraction of the vas deferens and the ampulla to cause
expulsion of sperm into the internal urethra. Then, contractions of the muscular coat of
the prostate gland followed by contraction of the seminal vesicles expel prostatic and
seminal fluid also into the urethra, forcing the sperm forward. All these fluids mix in the
internal urethra with mucus already secreted by the bulbourethral glands to form the
semen. The process to this point is emission. The filling of the internal urethra with
semen elicits sensory signals that are transmitted through the pudendal nerves to the
sacral regions of the cord, giving the feeling of sudden fullness in the internal Deep
penile fascia.
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