sexual arousal in men?
How does sexual arousal in
You may well equate sexual arousal with getting
an erection. But arousal involves a lot more of your body than just your penis:
in fact, almost everything is in on the act.
Your breathing gets faster and heavier, your
heart beats more quickly, your blood pressure goes up, your nipples become erect
and your skin flushes. Blood flow to your penis increases, giving you an
erection, your testicles swell up, and your scrotum tightens.
Why? Because these are the reactions of the part
of the nervous system that controls our sexual responses.
There's the sympathetic nervous system,
responsible for flight or fight reactions, and the parasympathetic nervous
system, responsible for bringing us down from the highs we reach under
stress. You can read a lot more about arousal, excitement and orgasm on a
website about delayed ejaculation which
you can find here - with the
arousal to be found here.
How does the penis
The penis is a miracle of
bio-engineering, with an ability to change size and shape, become rigid and
flaccid, discharge urine and semen, and give its owner and his sexual partner
immense pleasure. No other part of the body seems so cleverly designed and so
The penis's superb design starts on the outside
and goes inwards. The skin of the penis is elastic, thin, hairless and loose, so
that when the organ becomes erect, it has maximum expandability and maximum
The underlying tissue contains nerve receptors
unique to the penis, found nowhere else on the body, and one neurologist
suggests that the sensory input from the penis to the brain actually has the
capacity to enhance the brain's capacity to process nerve impulses.
The urethra runs from the tip of the penis
to the bladder, via the structures that provide sperm and semen, through a
spongy mass of tissue called the corpus spongiosum.
This lies below two other
similar structures called corpora cavernosa, and the whole lot is surrounded by
an elastic membrane called the tunica albuginea. This tissue is strong but not
flexible, and it is susceptible to injury during sex: a penile "fracture"
is actually a tear in the tunica albuginea.
There are few muscles in the penis, and
certainly no muscles of the type that responds to physical exercise. What muscle
there is consists of smooth muscle, small cells whose function is to relax or contract
around the blood vessels supplying the corpora cavernosa, thereby allowing more
- or less - blood into the penis and causing an erection, or the subsequent and
inevitable return to the flaccid state.
Ejaculation naturally occurs in the penis,
but orgasm is a function of the brain. We tend to think of them as inseparable
events, but in fact they are separate processes and can occur independently of
each other. As the penis is stimulated, whether through intercourse or
masturbation, the brain receives nerve impulses which trigger the experience of
At the same time, a reflex reaction running along nerve pathways from
penis to spinal cord and back again first causes the seminal vesicles and prostate to
contract, thereby forcing semen into the base of the penis, followed by strong
contractions of the bulbo-urethral muscle, which eject the seminal fluid
forcibly into the world outside.
There's a comprehensive description of the
mechanism of erection below, but basically what happens is this: normally, the
smooth muscles which surround the penile arteries are contracted, so blood flow
However, under erotic stimulation, these smooth
muscles relax and allow the arterioles (small arteries) supplying the spongy
tissues of the corpora to expand so that these tissues fill with blood.
same time, the expansion of the corpora will squash the veins which normally
release blood from the penis under the fibrous web of the tunica albuginea. This
means that blood can flow in in greater quantities than it can leave, and
naturally enough the organ becomes tumescent.
When erotic stimulation stops, the smooth
muscles contract once more, and so once again restrict the inflow of blood. As
the penis becomes detumescent, the veins are able to open fully and the blood
flows out of the corpora in increasing amounts. The erection goes down and the
penis returns to its usual flaccid size.
The mechanism of erection
penis receives oxygenated and nutrient-rich blood through vessels called
arteries; once the blood has supplied nutrients and oxygen to the tissues of
the penis, it flows out through vessels called veins.
An erection - also known
as tumescence - and subsequent return of the penis to its flaccid state - detumescence
- are both caused by changes in blood flow through the arteries and veins of the
penis. To understand how this is achieved, it is necessary to know
something of the basic anatomy of the normal penis.
As you can see in the
diagram below, the penis is made up of three chambers, the bottom one of which,
the corpus spongiosum, surrounds the urethra, the passage
through which semen and urine exit the body.
Above and to each side of the urethra
are the two corpora cavernosa (singular corpus cavernosum).
Each of these chambers is made up of many small sinusoids, or
tissues spaces into which the blood can flow; the sinusoids are surrounded
by smooth muscle tissue which controls the flow of blood into and out of
the sinusoids as it contracts and relaxes.
As you would expect there
is a network of arteries and veins which supply blood to, and remove it from,
the sinusoids: the larger arteries and veins in this network are known as
the sinusoidal arteries and the emissary veins respectively.
the corpora cavernosa is a fibrous layer of tissue called the tunica
albuginea. The emissary veins
are located between the corpora cavernosa and their respective tunica
word about smooth muscles
several types of muscles in the human body: smooth muscle, such as that
found in the penis, is a type which is not under voluntary or conscious
you cannot control the muscles inside the internal chambers of the penis
in the way, for example, that you can control the movement of your arm.
Furthermore, these smooth muscles are innervated with different nerve cells to
those of the voluntary muscular system.
When the penis is flaccid, the
smooth muscles which surround the
arteries supplying the corpora cavernosa
with blood are contracted; this constricts the arterial network, which
therefore is unable to pass blood into the corpora cavernosa.
do not close the vessels completely, however, for this would starve the
penis of blood; rather, they constrict the vessels just enough that the
penis remains flaccid. The cavernous
smooth muscle is also contracted with the same effect. By contrast, the
penile veins and venules (smaller veins) are unaffected, and blood flows
out of the penis quite easily.
Under the influence of sexual stimulation,
the smooth muscles of the penile arteries and corpora cavernosa relax.
This allows an increased amount of blood to flow into the sinusoidal
spaces, which begin to swell and press up against the surrounding tunica albuginea.
As the tissues swell, the venules and veins of the corpora cavernosa are
compressed so that their internal diameter is reduced and their ability to
transport blood away from the corpora cavernosa is significantly reduced.
At the same time, the major veins of the penis, the emissary veins, are
squeezed between the tunica and the walls of the corpora cavernosa, so
they too can transmit less blood away from the penis.
Since the blood is trapped in the
penis, it gradually becomes engorged ands stiff, in a state of erection,
At full erection, the blood pressure in the corpora cavernosa
is the same as the pressure of blood leaving the heart.
loss of erection is exactly the same process in reverse: the smooth muscles contract and the arteries
again become constricted. This reduces the inflow of blood into the
penis. At the same time
the smooth muscles of the sinusoids become constricted which
reduces the external pressure on the venules and the emissary veins and
allows the blood to flow out.
(ED) and its treatment
Estimates suggest up to 10% of
the male population suffers from erectile dysfunction. As men get older,
the possibility of ED becomes greater. In some males ED takes the form of a total
inability to achieve an erection. In others the erection may be
incomplete or last insufficiently long to achieve any or satisfactory
In the early 1990's,
the most widely used treatment for erection
problems was self-injection, directly into the corpora cavernosa,
of various drugs. This injection had to be done shortly before sexual
intercourse was expected to take place.
The drug most often used was
called papaverine, which relaxes
the smooth muscles of the penis. But it was an inconvenient and uncomfortable
treatment which found limited acceptance among men, not least because such
injections can cause scarring and damage to the lining of the corpora cavernosa.
As an alternative to
these injections, some men were taught to introduce drugs directly into
the urethra: this could produce side-effects including a burning sensation.
Other approaches which were tired around this time included
use of suction devices and prostheses, the use of glyceryl trinitrate
patches applied to the penis, and, most desperately of all, surgery to
try and increase arterial blood supply
to the penis.
In the type of
smooth muscle in the
penis, each cell relaxes when it receives a small dose of a "chemical messenger"
called nitric oxide or NO for short. Nitric oxide is produced or
released from at least two sources: the endothelium cells found around
the smooth muscles and
which service smooth muscle.
In both cases the nitric oxide is produced
by a reaction from a chemical called L-arginine. The chemical reaction
is catalyzed by an enzyme called nitric oxide synthase.
The nitric oxide
produced by these two methods enters the smooth muscle cells and
activates another enzyme (called guanylate cyclase) which converts
another chemical called guanosine triphosphate (GTP) to cyclic guanosine
cGMP in turn activates certain other intracellular
enzymes (called protein kinases) which cause or promote chemical
reactions which relax the smooth muscle. cGMP can itself be inactivated.
A group of enzymes, called phosphodiesterases or (PDEs), cause cGMP to
be turned into a chemical which is ineffective to relax muscle. This
ineffective chemical is known as 5-GMP. Thus turning cGMP into 5-GMP
has the effect of removing the agent which causes the muscle to relax.
are actually other, similar, reactions in the tissues of the penis, but they
follow the same principle as the reaction pathway outlined above: and the
crucial thing is that each pathway is inhibited by the action of a PDE enzyme
(of which there are five types, each controlling a different sequence of
chemical events in the tissues of the penis).
Obviously, since PDE enzymes
inactivate the compounds like cGMP which promote relaxation of the smooth muscles,
then any chemical which inhibits the action of the PDE enzymes will leave more
cGMP to relax the smooth muscles and therefore should be offer a way of
promoting the erectile capacity of the penis.
sum up, from the journal Science, Dec 1992:
year , scientists proved definitively that in males, NO translates
sexual excitement into potency by causing erections. Key pelvic nerves
get a message from the brain and make nitric oxide in response. NO
dilates blood vessels throughout the crucial areas of the penis, blood
rushes in, and the penis rises to the occasion."
NO and Viagra
is a potent inhibitor of cyclic guanosine 3-
5-monophosphate phosphodiesterases (cGMP PDEs). It blocks the ability of the phosphodiesterase
enzyme (PDE) to turn cGMP into 5-GMP. The result is that cGMP levels
remain high and it is this high level of cGMP which is
responsible for maintence of the erection in the penis.
relaxation of the corpus cavernosum tissue and consequent penile
erection is mediated by elevation of cGMP levels, by virtue of the PDE inhibitory profile of
causes a rise in cGMP levels in
the corpus cavernosum because it prevents some of the cGMP from being
destroyed. It also causes the relaxation of the muscle in that tissue
which leads to erection of the penis.