From the perspective of sociobiology, natural selection favors genes (including those that code for neurocircuitry and neurochemicals) that increase chances for gene duplication. One could say that the biological aim of living creatures is to organize in such a way as to duplicate their genes. In mammalian societies, care of the young is of upmost importance. But mammals, depending on their environment, organize things differently.
Jaak Panksepp points out in Affective Neuroscience: The Foundations of Human and Animal Emotions (1998) that "among our brethren great apes" there is no single plan for family structure." He writes: "Whereas gibbons appear to mate for life with a single partner, gorillas prefer a harem-type family structure, orangutans tend to be social isolates, with the sexes coming together mostly for copulatory purposes, while chimpanzees are quite social and promiscuous, sharing partners rather indiscriminately." The photograph above right is of two gibbons from Highland Farm Gibbons Sanctuary (image links to source).
Regardless of family matters, certain things must get done to duplicate those genes. When my husband was a young lad, his friend told him that there was something inside a man that had to get inside a woman to make a baby. "So how does it get there," my very analytical husband-to-be asked with disbelief, "fly through the air?"
Sex differences in brain anatomy:
From his neuroscientist point of view, Panksepp points out that due to the "branching of control factors for brain and body organization, it is quite possible for a male-type body to contain a female-type brain, and for a female-type body to contain a male-type brain." But before we delve into issues of gender and sexuality, we will first discuss brain anatomy and circuitry related to optimizing reproduction. While they share some neurochemicals such as oxytocin, Panksepp explains that specific brain circuits and chemistries that are distinct for males and females mediate sexual urges that foster reproduction.
When fetal steroids masculinize the XY rat's brain, a specific area of the pre-optic area (POA) is enlarged compared to females. (We will discuss more specifics about how the mammalian brain is masculinized a little later in this section.) Panksepp explains that this enlarged, masculinized area is called the sexually dimorphic nuclei of the preoptic area (SDN-POA). In this case, the term "dimorphic" means a structure that occurs in two different forms—the female form and the male form. Compared to the more robust preoptic area in males, Panksepp explains that in females, "many neurons in this part of the brain die during fetal development for lack of testosterone, or more precisely its product estrogen, which is a powerful growth factor for these neurons."
When fetal steroids masculinize the XY human's brain, a specific area of the brain is also enlarged compared to females, although the size difference is not as great as that found in rats. This enlarged brain area is called the interstitial nuclei of anterior hypothalamus (INAH). In the image below (links to source) from Neuroscience, Purves et al., editors, Sineuar Associates, Inc., the large vivid pink area represents the anterior hypothalamus.
The enlarged pre-optic area (POA) of the male rat brain is an important part of what here we will call MATING neurocircuitry. Panksepp writes: "Following lesions of the POA, male rats that have had abundant sexual experience will seek access to receptive females, even though they do not attempt to copulate with them. In other words, their social memories, situated perhaps in the cingulate cortex, amygdala, and nearby areas of the temporal lobes, are still capable of motivating social approach, although sexual engagement is no longer initiated." Later, he adds: "Castrated male rats that have lost their sexual ardor can be reinvigorated simply by placing testosterone directly into the POA."
In Part 1 of CorticalBrain.com, we discuss Paul MacLean's triune brain concept. MacLean also played a role in delineating MATING neurocircuitry. Panksepp writes: "Paul MacLean mapped out the monkey brain for sites from which genital arousal (erections) could be evoked by localized ESB [electrical brain stimulation]. He discovered a broad swath of tissue, in higher limbic areas, where sexual response could be elicited. They included, prominently, areas such as the septal area, bed nucleus of the stria terminalis [BNST], and preoptic areas, all of which converge through the anterior hypothalamus into the medial forebrain bundle of the lateral hypothalamus."
There are other differences between the female brain and the masculinized male brain. For example, growth in the masculinized corpus callosum, which connects the two cerebral hemispheres, is reduced in males when compared to females. Panksepp points out that androgen (testosterone is the primary androgen) and estrogen receptors are concentrated in certain brain areas, "down to the lower reaches of the spinal cord, where both male and female sexual reflexes are controlled." Panksepp reports that in the lower spinal cord, the nucleus of the bulbocavernosus is distinctly larger in males than in females.
As we discuss above, specialized neurochemicals combine with specialized neurocircuitry and, as Panksepp puts it, "can trigger complex and coordinated sequences of sexual behavior." He writes: "If one places a small, naturally occurring, nine-amino acid peptide called vasotocin into the brains of male frogs and lizards, they begin to exhibit courting sounds and sexual behaviors. Given the opportunity, males treated with vasotocin mount and clasp females and copulate." The picture to the right (links to source) of two green anoles mating naturally is courtesy of winott(at)snakesandfrogs.com.
In mammals, Panksepp explains that two neurochemicals very similar to the reptilian vasotocin play a role in sexual behavior. These two mammalian neurochemicals are vasopressin and oxytocin; they "assume key roles in controlling certain aspects of sexual behaviors" and each "differs from vasotocin [the reptilian neurochemical] by only one amino acid."
Panksepp emphasizes that vasopressin and oxytocin are not strictly male and female neurochemicals. Both play a role in the reproductive and parental behavior of both males and females. Vasopressin is more abundant in the male brain and has a primary effect on male sexual and social behavior. Vasopressin mediates "many aspects of male sexual persistence (including courtship, territorial marking, and intermale aggression)." Panksepp elaborates on the male rat sex act:
the general male strategy (facilitated by testosterone) is to exhibit fairly persistent searching for numerous sexual interactions … followed by the emission of vigorous … 50 KHz vocalizations … which, if the female does not object, culminate in … copulatory behavior. … the male mounts the female from the rear, palpating her flanks with his forepaws to arouse an arched-backed, rump-raised receptive posture called lordosis. Whereupon, the male rat exhibits sets of rapid thrusting movements called intromissions, which, if well guided, lead to entry of the penis into the vagina. After a series of intromissions, the male ejaculates, which is accompanied by a "deep thrust," and then he pushes off, often falling over in the process. He then attends to personal matters, with intense grooming of his genital area, with a shift to 22 KHz … vocalizations.
Panksepp points out that, in the female brain, vasopressin energizes "some of the more aggressive aspects of maternal behavior (i.e., protecting the young from harm)."
Oxytocin is more abundant in the female brain. Panksepp writes: "Animal research indicates that both brain opioid and oxytocin circuits are activated by various pleasurable pro-social activities, such as grooming, play, and sexual interchange." Oxytocin mediates "female social and sexual responsivity (especially the tendency of female rodents when mounted to exhibit lordosis…)," writes Panksepp. He explains that sensitization of female sexual eagerness transpires in the ventromedial nucleus of the hypothalamus and that damage to this area impairs responsivity. Panksepp writes:
The sex hormones that prepare the body for fertilization also dramatically change neurochemical sensitivities in this part of the brain. … Hormone priming (just like normal estrus) leads to a proliferation of oxytocin receptors in the medial hypothalamus, as well as an expansion of the dendritic fields, which physically expand, reaching out toward the incoming oxytocinergic nerve terminals arising from more rostral neurons. This completes a circuit that sensitizes the lordosis reflex of the spinal cord (and presumably prepares the female psychologically to interact seductively with males).
Regarding the female rat's role in the sex act, Panksepp writes: "The most evident behaviors in the rat are repeatedly running toward and away from the male, or past him in a hopping, darting fashion with the head wiggling and many 50 KHz vocalizations."
In the male brain, oxytocin sustains "some of the gentler aspects of male behavior (e.g., the tendency of fathers to be nonaggressive and supportive toward their offspring)." Oxytocin "also appears to help mediate the behavioral inhibition, or 'refractory period,' that follows orgasm in males."
Gender determinants—the role of testosterone:
In Affective Neuroscience, Panksepp writes: "One is typically born either genetically female (with the XX pattern of sex chromosomes) or genetically male (with the XY pattern)." These sets of chromosomes are not, however, the ultimate determination of gender. Panksepp says that "masculinization results from the organizational effects of fetal testosterone, which, in humans, occur during the second trimester of pregnancy."
So what prompts fetal testosterone? "What the Y chromosome provides for the male is testis determining factor (TDF)," explains Panksepp, "which ultimately induces the male gonadal system to manufacture testosterone. The XX pattern allows things to progress in the ongoing feminine manner, unless some external source of testosterone (or, more accurately, one of its metabolites) intervenes." The photograph to the left is of an X chromosome and a Y chromosome and was taken with an electron microscope (image links to source). This photograph is part of "Life in the Universe" coursework. Nicholas M. Short, Sr. NASA, developed the coursework.
After fetal testosterone has been synthesized from cholesterol, via many steps that include the intermediates progesterone and dihydroepiandrosterone, it can be biochemically modified in two distinct ways to imprint maleness onto the XY fetus. Panksepp explains that the "timing and intensity" of these processes determine how the XY fetus's brain and body development proceeds. The two processes to which Panksepp refers are clarified below:
To organize the male body, the enzyme 5α-reductase assists in converting testosterone into the steroid dihydrotestosterone (DHT)
To organize the male brain, the enzyme aromatase assists in converting testosterone into the steroid estrogen
These processes are confusing since most people associate estrogen production strictly with being female when in reality it is estrogen that organizes the male brain. Panksepp explains that "the XX sex chromosome pattern informs the female body to manufacture proteins such as the steroid-binding factor alpha-fetoprotein… ." This chemical "protects the female fetus from being masculinized by the generally high levels of maternal estrogens. If there is not enough of this fail-safe factor, or it the maternal levels of estrogens are so high that they saturate the available alpha-fetoprotein, the female will proceed toward a male pattern of development—sometimes in both body and mind, sometimes in one but not the other, depending on the hormonal details that have transpired."
The effects of maternal stress on gender:
Regarding development of the XY fetus and the two modifications to testosterone that imprint maleness mentioned above, Panksepp notes that the "products of testosterone metabolism are critical ingredients that dictate whether a genetic male will continue along the male path in terms of body and brain development, both before and after puberty." He explains that "homosexuality and bisexuality are promoted if 'errors' occur in the various control points of these biochemical processes … ." Regarding such errors, Panksepp writes: "It has been repeatedly shown in animal models that maternal stress can hinder the normal process of brain masculinization by desynchronizing the underlying physiological processes… ."
In a normal litter from unstressed rat mothers, approximately eighty percent of the male offspring will exhibit male-typical, sex-seeking behavior while twenty percent remain asexual. Panksepp emphasizes that stress changes these ratios. "When a pregnant rat is exposed to any of a variety of stressors during the last trimester (third week) of the three-week gestation period," only about 20 percent of male offspring will exhibit male-typical, sex-seeking behavior while sixty percent are either bisexual or homosexual. The bisexual XY rats exhibit male behavior with a highly receptive female and female behavior in response to a sex-seeking male. The homosexual XY rats exhibit lordosis when a sexually aroused male mounts them. As in unstressed litters, the remaining twenty percent of rats born to stressed rat moms are asexual. Panksepp does point out, however, that environment has some effect on sexuality. He writes: "The male offspring of stressed mothers exhibit more 'normal' sexual behavior if they are housed continuously during adulthood with sexually experienced females."
The role of genetics and timing in determining gender:
In addition to the effects of stress on gender development, Panksepp provides an example of how a genetically induced neurochemical deficiency can affect gender, at least during childhood. Some babies born in the Dominican Republic appear female at birth, although close inspection would reveal some enlargement of what seems to be a clitoris. Panksepp explains that in the womb, these XY children do secrete testosterone at the usual time and since they have normal aromatase activity, their testosterone is converted to estrogen. Accordingly, their brains are fully organized as male. Because they are genetically deficient in 5α-reductase, however, testosterone is not converted to DHT so their bodies do not appear male at birth. When such XY children "enter puberty and begin to secrete testosterone," writes Panksepp, "they develop male-typical bodies—with an increase of body hair, deepening of the voice, enlargement of the penis, and finally, the descent of the testes. Male-typical sexual urges also begin to emerge. Thus, the boys' pubescent erotic desires come to be directed toward females, even though they were reared as girls throughout childhood!" These boys are called guevedoces, which means "penis at 12," notes Panksepp.
"The hormones secreted at the onset of puberty," observes Panksepp, activate "the latent male or female sexual proclivities that have remained comparatively dormant within brain circuits since infancy." He writes: "Thus, the brain substrates for sexuality that are organized by these early hormonal experiences help determine what type of gender identities, erotic desires, and sex behaviors individuals will exhibit at puberty, when the elevations in hypothalamic gonadotrophic hormones and gonadal sex steroids begin to 'activate' sexual tendencies."
Mariateresa Molo et al., in "Characteristics of Brain Activity in Patients With Gender Identity Disorder," provides support for the idea that an XY fetus can end up with a female-type brain and that an XX fetus can end up with a male-type brain. The researchers found bioelectrical similarities in the brains of female controls and male-to-female transsexuals. Likewise, they found similarities in the brains of male controls and female-to-male transsexuals.
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