Download physiology_of_coitus_and_erection_1

NAME: ADEMOLA ADESOLA TITITLOPE
Matric no: 13/mhs03/002
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
Coitus can be defined as the physical union of male and female genitalia
accompanied by rhythmic movements usually leading to the ejaculation of
semen from the penis into the female reproductive tract.
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
end- organ 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 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 general can 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.
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 inter- course is being performed can initiate the male act,
culminating in ejaculation. 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
Erection (as discussed in question 1)
Lubrication
Ejaculation
LUBRICATION
During sexual stimulation, apart from causing erection, the parasympathetic
nerves also stimulate the bulbo-urethral glands and the glands of Littre to
secrete mucus. The mucus flows out through the urethra during intercourse to
aid lubrication of coitus. The female sexual organs rather than the male supply
most of the lubrication of coitus. Adequate lubrication is essential to a
successful sexual act because intercourse undertaken when there is inadequate
lubrication causes pain and may in fact inhibit rather than stimulate sexual
sensations. Some foreplay before the attempt at penetration into the female
genital organ usually ensures adequate lubrication.
EJACULATION
Ejaculation is the climax of the male sexual act. Stimulation of the penis during
sexual intercourse results in afferent impulses that arise from the touch
receptors in the glans penis and are transmitted to the spinal cord via the
internal pudendal nerves.
Ejaculation per se is a two stage reflex whose centres lie in the upper lumbar
segment of the spinal cord (L1 and L2) as well as the sacral segment of the
spinal cord (S1 and S2). When sexual stimulation becomes sufficiently intense,
the lumbar centres begin to discharge rhythmic sympathetic impulses that
leave the cord at L1 and L2 and are transmitted to the genitals via the
hypogastric plexus.
These impulses initiate emission, which is the first stage of ejaculation. During
emission, the smooth muscle of the epididymis, vas deferens, seminal vesicles
and prostatic capsule contract so the sperm and glandular secretions are
expelled into the internal urethra. During ejaculation proper, semen is expelled
from the urethra by contraction of the bulbo-cavernous muscle, which in turn
compresses the bulbus urethrae under the influence of the rhythmic impulses
that now arise in the sacral region (S1 and S2) of the cord. This entire period of
emission and ejaculation is called male orgasm. At its termination, the male
sexual excitement disappears almost entirely within 1 or 2 minutes and
erection ceases a process called resolution. Although the cerebrum is of major
importance in the male sexual act, it does not appear to be of critical
importance in its accomplishment. Hence, decerebrate animals and humans
with spinal cord transection above lumbar region can exhibit erection and
ejaculation following direct genital stimulation.
APPLIED PHYSIOLOGY OF ERECTION AND COITUS
i. Erectile dysfunction
ii. Priapism
References
Guyton and Hall. Textbook of Medical Physiology. 12ed. Elsevier Saunders
Oyebola O.D. Essential Physiology for Students of Medicine, Dentistry,
Pharmacy, and Related Disciplines. Vol1
http://www.ncbi.nlm.nih.gov/pubmed/12796638