Download [1:24pm, 12/06/2015] Nwando: THE PHYSIOLOGY OF COITUS

[1:24pm, 12/06/2015] Nwando: THE PHYSIOLOGY OF COITUS Simultaneous recordings of the
heart rate on the human female and male before, during and after coitus showed a marked
increase in heart rate accompanying the orgasm. The response of the male was more uniform
than that of the female. A striking parallelism between the response of the male and the female
in a given test was observed in several of the experiments. The simultaneous determinations of
the pulmonary ventilation showed a marked increase in the rate and minute volume in many of
the tests which suggested that hyperventilation occurred during the erotic arousal and
especially during the orgasm. Sexual intercourse, also called Coitus, or Copulation,
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. In some vertebrates, such as fish, eggs are laid outside of the body and fertilized
externally.
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. These events may be identified as occuring
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.
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. 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.
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). 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 pre-excitement 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.In some animals, such as sheep and migratory
birds, copulation occurs only seasonally. During the breeding season, hormones are produced
in both the male and female species that prepare their reproductive systems for copulation. In
the nonbreeding seasons, the hormone levels drop so that the animals are not capable of
fertilized breeding and do not have the desire to engage in copulation.
[1:25pm, 12/06/2015] Nwando: PHYSIOLOGY OF ERECTION
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 [Dean and Lue, 2005].
The level of contractility of the corporal myocytes is the critical determinant of the erectile
phenomenon. It is the result of a complex scenario made by the contemporaneous action of
neurotransmitters, neuromodulators, and hormones.
Corporal Smooth Muscle Relaxation and Erection
After a visual or tactile stimulus, nonadrenergic, noncholinergic nitrergic neurons and
endothelial cells release nitric oxide (NO), which is an unstable, gaseous, and transient mediator
synthesized by endothelial nitric oxide synthase (eNOS) and by neural nitrous oxide synthase
(nNOS). NO passes through smooth muscle cell (SMC) membrane and interacts with soluble
intracellular cytoplasmic guanylate cyclase (GC). GC activity is important to amplify NO signal
through the conversion of guanosine monophosphate (GMP) to cyclic GMP (cGMP). After that,
cGMP interacts with protein kinase, which phosphorylates several cellular proteins (K+
channels, myosin binding protein, Ca2+ channels, and pumps). Phosphodiesterase 5 (PDE5)
downregulates cGMP levels through the enzymatic degradation in GMP. The NO effect on
corporal smooth muscle could also be obtained by other neurotransmitters/neuromodulators,
such as vasoactive intestinal polypeptide (VIP), calcitonin generelated peptide, and
prostaglandins E1 and 2 (PGE1 and PGE2), which increase cyclic nucleotide and reduce Ca2+
intracellular levels.
Corporal Smooth Muscle Contraction and Detumescence
Detumescence of the penis is obtained through activation of the α1 adrenoreceptor or
endothelin a (ETa)/endothelin b (ETb) receptors (that bind principally ET1 but also ET2 and ET3)
and activation of protein Gq, that causes activation of phospholipase C, that cleaves PIP2 in
inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 causes the release of intracellular
calcium through interaction with the IP3 receptor, and calcium together with DAG activates
protein kinase C (PKC). However, activation of the α1-adrenoreceptor or ETa receptor subtype
causes activation of Rho A/Rho kinase and triggers cycling of myosin crossbridges along actin
filaments and the development of force through inhibition of myosin light chain phosphatase
[Bivalacqua et al. 2004]. The signaling downstream effect is increased corporal SMC tone
obtained, thanks to increased intracellular levels of calcium and increased calcium sensitization.
GAP Junctions
The supply of autonomic fibers to smooth muscle is limited; actually there is not a one-to-one
ratio between nervous fibers and SMCs. It was first postulated and then demonstrated that
coordinated responses to obtain penis erection/detumescence are due to GAP junctions
(connexin 43) among corporal myocytes. Sexual stimulation triggers release of
neurotransmitters from the cavernous nerve terminals. This results in relaxation of these
smooth muscles and the following events:
Dilatation of the arterioles and arteries by increased blood flow in both the diastolic and the
systolic phases
Trapping of the incoming blood by the expanding sinusoids
Compression of the subtunical venular plexuses between the tunica albuginea and the
peripheral sinusoids, reducing the venous outflow
Stretching of the tunica to its capacity, which occludes the emissary veins between the inner
circular and the outer longitudinal layers and further decreases the venous outflow to a
minimum
An increase in PO2 (to about 90 mmHg) and intracavernous pressure (around 100 mm Hg),
which raises the penis from the dependent position to the erect state (the full-erection phase)
A further pressure increase (to several hundred millimeters of mercury) with contraction of the
ischiocavernosus muscles (rigid-erection phase)
The angle of the erect penis is determined by its size and its attachment to the puboischial rami
(the crura) and the anterior surface of the pubic bone (the suspensory and funiform ligaments).
In men with a long heavy penis or a loose suspensory ligament, the angle usually will not be
greater than 90 degrees, even with full rigidity.
Three phases of detumescence have been reported in an animal study. The first entails a
transient intracorporeal pressure increase, indicating the beginning of smooth muscle
contraction against a closed venous system. The second phase shows a slow pressure decrease,
suggesting a slow reopening of the venous channels with resumption of the basal level of
arterial flow. The third phase shows a fast pressure decrease with fully restored venous outflow
capacity.
Erection thus involves sinusoidal relaxation, arterial dilatation, and venous compression. The
importance of smooth muscle relaxation has been demonstrated in animal and human studies.