Professor Dave here, let’s talk about sex.
In the anatomy and physiology course, we learned all about hormones, and the elaborate organization
of their deployment that is the endocrine system.
If that sounds unfamiliar to you, definitely check out this tutorial first, as much of
that knowledge will be assumed here, although we will briefly reiterate some aspects to
fit a biopsychological context.
Even with what we know about hormones, it still may seem incredible that the secretion
of a few molecules from a handful of glands can be responsible for something as intense
as the sex drive, but sexual behavior can be understood scientifically just like any
other type of human behavior, so let’s go in for a closer look.
As we said, hormones will be the key players in this realm, and these hormones orchestrate
a wide variety of things.
First, from conception through to adulthood, they guide the development of our primary
and secondary sexual characteristics, as well as behavioral characteristics, which together
distinguish us as either male or female.
And once mature, these same hormones regulate the sex drive and reproductive behavior as well.
All of this activity is orchestrated by the neuroendocrine system.
Let’s quickly recall some of the glands and associated structures that will be most
relevant to this discussion, those being the pineal, hypothalamus, pituitary, thyroid,
parathyroid, thymus, and adrenal glands, all of which secrete hormones.
Of course there are several organs that are relevant to this system as well, most notably
the gonads, which are the testes for a male and ovaries for a female, as well as the pancreas
and other organs that secrete hormones in addition to performing their other functions.
We can also recall that hormones are of several classes, those being amino acid derivatives,
peptides and proteins, and steroids, and the last of these categories is the one that all
the sex hormones belong to.
These are released into the bloodstream to reach their targets, often slipping right
through the membrane due to their size and structure to find receptors within the cell.
They interact with these receptors to elicit a specific physiological response related
to body development or behavior, typically in some manner related to gene expression.
As we may recall, the sex hormones can be divided into androgens, like testosterone,
and estrogens, like estradiol.
It is true that the male gonads release more androgens than estrogens, and the opposite
is true for the female gonads, but all men and women have both in their bodies, so it
is overly reductive to refer to them as male and female hormones.
Release of sex hormones by the gonads is regulated by the anterior pituitary gland, which releases
other hormones like gonadotropin, which travels through the bloodstream to the gonads to stimulate them.
The levels of these hormones are relatively stable for males, whereas in females they
fluctuate in a cycle lasting around 28 days, which we call the menstrual cycle.
The anterior pituitary, in turn, is controlled by the hypothalamus, again through the release
of other hormones, which in this case travel through the hypothalamopituitary portal system.
For example, gonadotropin-releasing hormone is released from the hypothalamus, which stimulates
release of gonadotropin in the anterior pituitary, which then travels to the gonads to stimulate
release of sex hormones.
So hormones can be released as a response to other hormones, but also due to other nonhormonal
chemicals, psychosocial inputs, or even direct neural regulation.
The exception to this last method is the anterior pituitary, which has no neural input.
In general, hormone release due to neural signals tends to be a response to some experience,
such as arousal, or more subtle environmental factors.
So we’ve now outlined a few things about hormones, but how exactly are they able to
guide the development of the human body throughout childhood and early adulthood?
Humans exhibit sexual dimorphism, which means that similarly to many other animal species,
males and females have very different physical features.
As we may recall from the biology series, this starts at fertilization.
When a zygote is produced, it gets 23 chromosomes from each gamete, one set from the egg, and
one set from the sperm.
One of the chromosomes in each set is a sex chromosome, which will be either an X or a
Y chromosome.
The egg always has an X, and the sperm has either an X or a Y.
If the resulting zygote has two Xs, the fetus will develop into a female.
If an X and a Y, the fetus will begin to develop into a female, but at a certain point this
process will be overruled due to the chromosomal difference and it will develop into a male.
A six week old fetus possesses gonads that are essentially identical regardless of the
sex, but they will develop into either ovaries or testes depending on the genetic instructions.
On the Y chromosome, there is a gene that, when expressed, produces the Sry protein,
which influences the development of the gonad such that it will develop into a testis.
In the absence of this protein, development results in an ovary.
All the features of the external genitalia develop in similar ways, from identical precursors
that differentiate due to specific interactions involving the differing products of gene expression,
as well as the presence or absence of hormones like testosterone.
A similar process occurs again during puberty, when the secondary sexual characteristics
develop.
Growth hormone, gonadotropic hormone, and adrenocorticotropic hormone cause the gonads
and adrenal cortex to release other hormones which initiate the development of these characteristics,
as well as the maturation of the genitals.
In this way, while testosterone is not an exclusively male hormone, it is responsible
for masculinization, and while estrogen is not exclusively female, it is responsible
for feminization.
Obviously, the differences in the physical characteristics of males and females are very
clearly seen, but much more subtle is the sexual differentiation of the brain.
Those who place political correctness over scientific fact may not like it, but the male
and female brain are quite different in a variety of ways.
There are differences in regional sizes, such as with the sexually dimorphic nucleus of
the hypothalamus.
There are differences in numbers of neural and glial cells, number and types of synapses,
and more.
Of course there is plenty of overlap, as we are all humans, but it is undeniable that
these differences exist, and it seems clear that these differences develop at different
times and by different mechanisms.
When analyzing sexual behavior, cultural factors must be taken into account, but we are also
beginning to understand mechanisms in the brain that inform our sexuality.
These tend to be relegated to the cortex, hypothalamus, amygdala, and ventral striatum.
The cortex mediates the more complex and abstract aspects of any sexual experience, including
the variety of emotions that can be felt during the act.
We already mentioned that the hypothalamus is home to structural differences between
males and females, given the differing size of the sexually dimorphic nucleus, which influences
male and female sexual behavior, and it is also involved in hormone release as we have
already discussed.
The amygdala, in a general sense, is responsible for our experience of emotions and social
salience detection, so in this context, it would seem to be involved in mate selection.
And the ventral striatum is involved in reward, such as the anticipation of sexual activity,
and other rewarding stimuli.
Obviously there is much more to talk about here, as we haven’t discussed sexual orientation
and gender identity, but we will have to come back to this a bit later.
For now, let’s move on to some other topics.