>>Dr. Ketchum: The purpose of this video lecture is to cover the sexual response in males.
So just to remind you, these are the lectures that you’ve had so far: The functional anatomy
of the male reproductive system. We’ve discussed the hormone regulation of the male reproductive
system; we’ve discussed sperm and their development, and now we’re on the last lecture
for the male reproductive system, the sexual response in males. So the sexual response
in males is controlled by the autonomic nervous system. It includes the spinal reflexes, and
there are three different phases that are involved in the sexual response. There’s
the erection phase, which is followed by the emission phase, which is then followed by ejaculation.
The erection phase is regulated by the parasympathetic nervous system, whereas the
emission phase is regulated by the sympathetic nervous system as well as the ejaculation phase.
What we’re going to be doing is going through each of these phases. So we’re going
to begin with the erection phase. So there are two different stimuli that lead to an erection.
So the stimuli here are listed at the top: mechanical stimulation of receptors in the
penis as well as increased activity in descending neural pathways from the brain. So these are
the stimuli. These stimuli are going to act on inner neurons of the spinal cord. What
the spinal cord does then is it’s—it’s going to increase activity in the parasympathetic
neurons to the penis. And so once again, since the parasympathetic activity increases, sympathetic
activity decreases. Parasympathetic neurons to the penis can relax the arterioles.
They're going to relax the smooth muscle of the arterioles that supply blood to the erectile tissue of
the penis. So what that’s going to do is that’s going to cause vasodilation of the
arterioles that are supplying blood to this erectile tissue. So if you vasodilate, we know
that that’s going to increase the blood flow to this erectile tissue. Therefore, the blood
volume and the blood pressure are going to go up. So that’s going to lead to an erection.
Now, increased blood volume and blood pressure is also going to compress the veins in this
erectile tissue. So if you compress the veins, will you be able to remove the blood from
this tissue, or is the blood going to accumulate in this tissue? So the blood is actually going
to end up accumulating in this erectile tissue. So compression in the veins causes accumulation
of the blood in the erectile tissue. And so because you accumulate the blood there’s
less blood leaving the erectile tissue, and this starts a positive feedback loop. That’s
what causes erection.
Now there’s one other thing that I want to mention about the parasympathetic neurons
and their stimulation of the penis. We said that the parasympathetic nervous system is
going to relax the smooth muscle of the arterioles that supply this erectile tissue. The parasympathetic
nervous system is also going to cause the release of nitric oxide. And because you have
this release of nitric oxide, this is also abbreviated NO, this is a potent vasodilator.
So you have yet one more mechanism by which the arterioles supplying blood to the erectile
tissue vasodilate. So not only is it directly parasympathetic stimulation, but it’s also
the release of nitric oxide that will cause this.
So what I would like you to do is to read about impotence and erectile dysfunction,
and one of those treatments is Viagra. There’s another treatment called Cialis that you may
have heard about on television as well. What these various treatments do is they increase
blood flow to the erectile tissue, and they have different mechanisms by doing so.
So I’d like you to think about how these drugs may increase blood flow to the erectile tissue.
The next phase then is the emission phase. As the response to mechanical stimulation
from erection continues, okay, your neural activity is actually going to shift from parasympathetic
to a sympathetic control. So now we have the sympathetic nervous system taking over. The
sympathetic nervous system is going to cause contractions of the epididymis, the vas deferens,
and the ejaculatory duct. So what this means then is that we’re moving sperm. Okay,
we're going to move the sperm. So the sperm's going to be leaving the seminiferous tubules and
it’s going to be traveling into the urethra. Now the sympathetic nervous system also causes
secretions from the seminal vesicles and the prostate. So think back to the previous lecture
about what those secretions are from these two locations. So the end result of emission
is you’re moving the sperm—or semen rather, semen is a combination of sperm plus the fluid—you’re
moving the semen into the urethra. That’s what emission does; it’s the movement of
semen into the urethra using the sympathetic nervous system, which causes contractions
of the epididymis, the vas deferens, and the ejaculatory duct, and also causes secretion
from the seminal vesicles and the prostate. This phase, then, is followed by ejaculation.
The mechanical stimulation is still occurring, and this is still mediated by the sympathetic
nervous system. Remember with emission, the result of emission was moving the semen into
the urethra. Once that happens, this is going to stimulate ejaculation.
Okay, and what we mean by stimulating ejaculation is that it stimulates contractions of the
smooth muscle in the urethra and the skeletal muscle at the base of the penis. This also
stimulates closure of the urethral sphincter so that you do not have urine and semen mixing.
You don’t have that happening because there’s the closure of the urethral sphincter. So the
end result then of ejaculation is to move the semen from the urethra to the outside
of the body. One thing I want you to think about: so we said the sympathetic nervous system
here is stimulating closure of the ure—urethral sphincter. I want you to go back and look
at micturition, which is urination, right? And I want you to look at what causes micturition
and I want you to compare that to ejaculation, and I want you to think about how or another
reason why there is no urine mixed with semen. So that concludes this part of the video.