Download PHYSIOLOGY OF ERECTION INTRODUCTION An erection

PHYSIOLOGY OF ERECTION
INTRODUCTION
An erection (clinically: penile erection or penile tumescence) is a
physiological phenomenon in which the penis becomes firmer, engorged and
enlarged. The penis is an organ with paired erection chambers (corpora
cavernosa), which are filled with spongy erectile tissue (corporal sinusoids)
composed predominantly of muscle. Erection and loss of erection are related
primarily to blood flow events regulated by the relaxation and contraction,
respectively, of the smooth muscle in the penile arteries and the erectile bodies
themselves. Erection is a hydraulic event, regulated by hormones and nerves,
which allow increased blood flow into and storage of blood within the erectile
bodies leading to an increase in pressure and the development of rigidity
(hardness). Penile erection is triggered by one of two main mechanisms: direct
stimulation of the genitalia or through stimuli coming from the brain. Erections
may also be spontaneous.
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.
Upon stimulation, chemicals are released in the brain that cause signals to pass
down the spinal cord and outward through special nerves (nervi erigentes) into
the penis. These nerves release another chemical (Nitric Oxide) that causes the
aforementioned smooth muscle to relax and blood rushes into the erectile bodies,
causing erection. Anxiety or fear can prevent the brain signals from reaching the
level required to induce erection. Medical conditions can block the erection
arteries or cause scarring of the spongy erection tissue and prevent proper blood
flow or trapping of blood and, therefore, limit the erection. Thus, the erection
mechanism is much like a tire; a firm tire is dependent upon a hose that can
deliver air in adequate amounts in a speedy fashion and a valve mechanism that
holds the air in place. In the penis the hose is represented by the erection arteries,
which rapidly carry blood into the erectile bodies and the valve mechanism, while
complicated in its structure, ensures that the blood is trapped inside the erectile
bodies until ejaculation occurs or the sexual stimulus has passed.
PHYSIOLOGY
An erection occurs when two tubular structures, called the corpora cavernosa,
that run the length of the penis, become engorged with venous blood. This may
result from any of various physiological stimuli, also known as sexual stimulation
and sexual arousal. The corpus spongiosum is a single tubular structure located
just below the corpora cavernosa, which contains the urethra, through which
urine and semen pass during urination and ejaculation respectively. This may also
become slightly engorged with blood, but less so than the corpora cavernosa.
Autonomic Control;
In the presence of mechanical stimulation, erection is initiated by the
parasympathetic division of the autonomic nervous system (ANS) with minimal
input from the central nervous system. Parasympathetic branches extend from
the sacral plexus into the arteries supplying the erectile tissue; upon stimulation,
these nerve branches release acetylcholine, which, in turn causes release of nitric
oxide from endothelial cells in the trabecular arteries. Nitric oxide diffuses to the
smooth muscle of the arteries (called trabecular smooth muscle), acting as a
vasodilating agent. The arteries dilate, filling the corpora spongiosum and
cavernosa with blood. The ischiocavernosus and bulbospongiosus muscles also
compress the veins of the corpora cavernosa, limiting the venous drainage of
blood. Erection subsides when parasympathetic stimulation is discontinued;
baseline stimulation from the sympathetic division of the ANS causes constriction
of the penile arteries, forcing blood out of the erectile tissue.
After ejaculation or cessation of stimulation, erection usually subsides, but the
time taken may vary depending on the length and thickness of the penis.
An erection can either be voluntary or involuntary as even in absence of direct
mechanical stimulation, the cerebral cortex can initiate it through erectile centers
in the lumbar and sacral regions of the spinal cord. The cortex may suppress
erection, even in the presence of mechanical stimulation, as may other
psychological, emotional, and environmental factors.
An erection is a common indicator of sexual arousal and is required for a male to
effect vaginal penetration and sexual intercourse. The scrotum may, but not
always, become tightened during erection. Generally, the foreskin automatically
and gradually retracts, exposing the glans, though some men may have to
manually retract their foreskin.
After reaching puberty, erections occur much more frequently. Male erections are
common for children and infants, and even occur before birth.
Spontaneous or random erections
Spontaneous erection, also known as involuntary, random or unwanted erection,
is commonplace and a normal part of male physiology. Socially, such erections
can be embarrassing if they happen in public or when undesired. Erections can
occur spontaneously at any time of day, and if clothed may cause a bulge which (if
required) can be disguised or hidden by wearing close-fitting underwear, a long
shirt, or baggier clothes.
APPLIED PHYSIOLOGY
Erectile Dysfunction;
Erectile dysfunction (also known as ED or "(male) impotence") is a sexual
dysfunction characterized by the inability to develop and/or maintain an erection.
The study of erectile dysfunction within medicine is known as andrology, a subfield within urology.
Erectile dysfunction can occur due to both physiological and psychological
reasons, most of which are amenable to treatment. Common physiological
reasons include diabetes, kidney disease, chronic alcoholism, multiple sclerosis,
atherosclerosis, vascular disease, and neurologic disease which collectively
account for about 70 percent of ED cases. Some drugs used to treat other
conditions, such as lithium and paroxetine, may cause erectile dysfunction.
Erectile dysfunction, tied closely as it is to cultural notions of potency, success and
masculinity, can have devastating psychological consequences including feelings
of shame, loss or inadequacy; there is a strong culture of silence and inability to
discuss the matter. In fact, around one in ten men will experience recurring
impotence problems at some point in their lives.
Priapism
Priapism is a medical condition which could possibly be painful, and is a prolonged
erection at least four hours long, which does not return to its flaccid state, despite
the absence of both physical and psychological stimulation.
PHYSIOLOGY OF COITUS
INTRODUCTION
Coitus/sexual intercourse or copulation is a reproductive act in which the male
reproductive organ (in humans and other higher animals) enters the female
reproductive tract. If the reproductive act is complete, sperm cells are passed
from the male body into the female, in the process fertilizing the female egg and
forming a new organism.
To accomplish internal copulation certain body and organic adaptations are
necessary. In the human male, the penis serves both excretory and reproductive
functions. During intercourse, the blood flow is temporarily increased and
trapped in the penis so that it becomes enlarged and elevated, a condition known
as erection. Erection changes the normally soft and flaccid organ to one of greater
size and rigidity to permit easier penetration into the reproductive tract of the
female. Sexual intercourse both culminates and terminates in orgasm, a process
in which the male expels semen—containing sperm cells, which may unite with
and fertilize the female egg, and a seminal plasma that contains cell nutrients,
water, salts, and metabolites—into the female’s vaginal canal. The male’s ability
to produce and secrete semen, as well as to function sexually, is dependent on
the androgen hormones, which circulate in the male’s body. In the female
reproductive system, an external opening leads to the vagina, which in turn
communicates with the uterus (or womb), a thick-walled, pear-shaped organ
where the sperm fertilizes the egg and where the fetus develops. In human beings,
a pattern of physiological events occurs during sexual arousal and intercourse.
STAGES
These events may be identified as occurring in a sequence of four stages:
excitement, plateau, orgasm, and resolution. The basic pattern is similar in both
sexes, regardless of the specific sexual stimulus.
Excitement Stage:
In the excitement stage, the body prepares for sexual activity by tensing muscles
and increasing heart rate. In the male, blood flows into the penis, causing it to
become erect; in the female, the vaginal walls become moist, the inner part of the
vagina becomes wider, and the clitoris enlarges.
Plateau Stage:
In the plateau stage, breathing becomes more rapid and the muscles continue to
tense. The glans at the head of the penis swells and the testes enlarge in the male;
in the female, the outer vagina contracts and the clitoris retracts.
Orgasm:
At orgasm the neuromuscular tension built up in the preceding stages is released
in a few seconds. In the woman, the vagina begins a series of regular contractions;
in the man, the penis also contracts rhythmically to expel the sperm and semen
(ejaculation).
Resolution Stage:
The succeeding resolution stage brings a gradual return to the resting state that
may take several hours. In the male, the penis shrinks back to its normal size; in
the female, the vagina and other genital structures also return to their preexcitement condition. The resolution stage in men contains a refractory period of
several minutes to a few hours, during which the man is incapable of further
sexual arousal. Women have no such refractory period and can quickly become
aroused again from any point in the resolution stage.