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BASSEY CHRISTIE EDDY
MEDICINE AND SURGERY
13/MHS01/037
PHS 204: ENDOCRINOLOGY AND REPRODUCTIVE
PHYSIOLOGY ASSIGNMENT
PHYSIOLOGY OF ERECTION
An understanding of the basic physiology of erection (that is, how an erection works) will
allow patients to understand not only the causes of erectile dysfunction (ED) but will also lay the
foundation for the understanding of ED treatments. 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 (fantasy, smell, etc).
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.
A multistep process is necessary to obtain an erection. The first step is tumescence. The two
major physiological events in tumescence are arterial and arteriolar vasodilatation, which causes
increased blood flow to the lacunar spaces and simultaneous relaxation of the sinusoidal smooth
muscle, allowing distension. The second step is veno-occlusion that causes rigidity of the penis.
The increase in blood flow actually raises pressure in the male organ and compresses the
subtunical venular plexus between the tunica albuginea and the peripheral sinusoids, thus
reducing the venous outflow. The tunica is stretched to its capacity, and the emissary veins
between the inner circular and the outer longitudinal layers are occluded, with a decrease in the
venous outflow to a minimum. At this point there is an increase in partial pressure of oxygen (to
about 90 mmHg) and in intracavernous pressure (around 100mm Hg) that brings the penis to its
erect state. When ischiocavernous muscles tense, the pressure in the corpora cavernosa rises to
several hundreds of millimeters of mercury and causes the rigiderection phase.
The physiology of male erection depends upon the degree of sexual stimulation he is receiving.
Erection is caused by parasympathetic impulses from the sacral cord (S2, S3, S4) to the penis.
These same motor nerves innervate the ischiocavernosus and bulbospongiosus muscles. The
result is that penile arterioles dilate and the penile venules constrict. This puts high-pressure
blood flow into the corpus spongiosum and the corpora cavernosa. The penis becomes erect it is
said to be in a state of TUMESCENCE, i.e., a condition of being swollen, or a swelling. The
scrotum also begins to elevate as the penis becomes erect
.
PHYSIOLOGY OF COITUS
Although the interplay between the local, spinal and central levels is essential to the development
of the normal sexual response both in man and women, in humans the brain (central)
mechanisms are more important than in other animals.
For instance, sexual excitation can be aroused by virtue of central mechanisms alone. By
hearing, seeing or even smelling, so-called erotic stimuli, which are mostly learned and of
cultural origin, may evoke sexual excitation through the sensory systems, limbic system,
hypothalamus and autonomous nervous system.
How the Nervous System Works During Sexual Intercourse ?
The many physiological, behavioral and psychological reactions which accompany the sexual
intercourse are mediated by a complex mechanism, involving:
autonomic and somatic nervous systems;
peripheral circulatory system at the level of the genitalia;
spinal cord and peripheral nerves of the lower abdomen;
central nervous system;
endocrine system (sexual hormones).
All these systems interact among themselves in a complex fashion.
No specific "sex area" has been located in the human brain, but the parts of the brain thought to
be most concerned with sexual response are the hypothalamus and the limbic system.The
physiology of arousal in the male is elicited in numerous ways. The neurological mechanisms are
what we have an interest in for this course. Sensory organs detect anal stimulation, skin
stimulation, perineal stimulation as well as friction on the glans penis. All of these sensations are
transmitted via the pudendal nerve to the sacral plexus. In addition, an inflamed/irritated urethra,
bladder, prostate, seminal vesicles, testes and/or vas deferens drives transmissions to the sacral
plexus. Likewise, the male's sexual drive causes sexual organs to overfill causing increased
secretions and vasocongestion.
The physiology of arousal in the female is elicited in numerous ways. The neurological
mechanisms are what we have an interest in for this course. Sensory organs detect anal
stimulation, groin stimulation, perineal stimulation as well as friction on the glans clitoris and
labia or in the labial groove. All of these sensations are transmitted via the pudendal nerve to the
sacral plexus. In addition, an inflamed/irritated urethra or bladder drives transmissions to the
sacral plexus. Likewise, the female's sexual drive causes sexual organs to overfill causing
increased secretions and vasocongestion. Furthermore, emotional states, learned behaviors and
blood levels of estrogens, progesterone and corticoids govern the female's sexual drive. These
signals are also transmitted to the sacral plexus.