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Sex: Evolutionary, Hormonal, and Neural Bases

12 Sex: Evolutionary, Hormonal, and Neural Bases

Sexual Behavior: · Reproductive Behavior Can Be Divided into Four Stages

· The Neural Circuitry of the Brain Regulates Reproductive Behavior

· Pheromones Guide Reproductive Behavior in Many Species

12 Sex: Evolutionary, Hormonal, and Neural Bases

Sexual Behavior: · The Hallmark of Human Sexual Behavior Is Diversity An causal study by the use of animal to investigate the relationship of Brain & Behavior

· Many Vertebrates Depend on Their Parents for Survival

12 Sex: Evolutionary, Hormonal, and Neural Bases

Sexual Differentiation:

· The Sex of an Individual is Determined Early in Life · How Should We Define Gender ­ by Genes, Gonads, Genitals, or the Brain? · Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

12 Sex: Evolutionary, Hormonal, and Neural Bases

Sexual Differentiation: · Social Influences Affect Sexual Differentiation of the Nervous System

· Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

12 Reproductive Behavior Can Be Divided into Four Stages

Four stages of reproductive behavior: 1. Sexual attraction

2. Appetitive behavior 3. Copulation 4. Postcopulatory behavior

Figure 12.1 Stages of Reproductive Behavior (Part 1)

Figure 12.1 Stages of Reproductive Behavior (Part 2)

12 Reproductive Behavior Can Be Divided into Four Stages

Sexual attraction brings males and females together ­ may be synchronized with:

· Physiological readiness to reproduce, indicated by odors reflecting estrogen levels in females · Learned associations, such as appearances

12 Reproductive Behavior Can Be Divided into Four Stages

Appetitive behaviors establish, maintain, or promote sexual interaction.

- appetitive () vs. consumptive () A proceptive female may approach males or perform "ear-wiggling." Male behaviors include staying near the female, sniffing, singing, and nest-building.

12 Reproductive Behavior Can Be Divided into Four Stages

Copulation involves one or more intromissions in which the male penis is inserted into the female vagina.

- appetitive vs. consumptive Following stimulation the male ejaculates sperm-bearing semen into the female.

12 Reproductive Behavior Can Be Divided into Four Stages

After first copulation a refractory phase follows ­ length varies with species.

The Coolidge effect refers to the faster resumption of mating behavior, with a different partner. If a female is willing to copulate, she is sexually receptive, or in estrus.

12 Reproductive Behavior Can Be Divided into Four Stages

Postcopulatory behavior varies across species.

In a copulatory lock, occurring in dogs and some mice, the penis swells temporarily and cannot be withdrawn from the female. Postcopulatory behavior also includes extensive parenting of offspring.

12 Reproductive Behavior Can Be Divided into Four Stages

In rats, females ovulate, or release eggs, every 4­5 days. She displays proceptive behavior and adopts a posture called lordosis, allowing intromission.

Pair bonds form between animals who live together before and long after copulation. (oxytocin)

12 Reproductive Behavior Can Be Divided into Four Stages

Hormones are important in mating behaviors.

A castrated male loses interest in mating as testosterone is no longer produced. Behavior is restored with testosterone treatment ­ the activational effect ­ hormones briefly activate behavior.

Figure 12.3 Androgens Permit Male Copulatory Behavior

12 Reproductive Behavior Can Be Divided into Four Stages

Estrogens produced at the beginning of the ovulatory cycle are important for female proceptive behavior.

A female without ovaries will respond to a combination of estrogen and progesterone treatments.

Figure 12.4 The Ovulatory Cycle of Rats

12 The Neural Circuitry of the Brain Regulates Reproductive Behavior

In female rats, the ventromedial hypothalamus (VMH) is crucial to the lordosis response through steroid actions.

Estrogen increases dendritic trees of neurons in the VMH.

Estrogen also stimulates progesterone receptors, which in turn contribute to lordosis through protein production.

12 The Neural Circuitry of the Brain Regulates Reproductive Behavior

The VMH sends axons to the periaqueductal gray in the midbrain, which projects to the medullary reticular formation. This in turn projects to the spinal cord via the reticulospinal tract. As the male mounts, sensory information via the spinal cord evokes lordosis.

Figure 12.5 Neural Circuits for Reproduction in Rodents (Part 1)

12 The Neural Circuitry of the Brain Regulates Reproductive Behavior

In male rats, the medial preoptic area (mPOA) coordinates male copulatory behavior.

The mPOA sends axons to the ventral midbrain, then to the basal ganglia to coordinate mounting. Axons also project through brainstem nuclei to the spinal cord.

12 The Neural Circuitry of the Brain Regulates Reproductive Behavior

One nucleus, the paragigantocellular nucleus (PGN) has fibers that normally inhibit the erection response ­ mPOA signals counteract the inhibition.

The vomeronasal organ (VMO) detects chemicals called pheromones, which activate male arousal. VMO information is sent to the medial amygdala, and in turn to the mPOA.

Figure 12.5 Neural Circuits for Reproduction in Rodents (Part 2)

12 Pheromones Guide Reproductive Behavior in Many Species

Hormones can signal readiness within an animal or provide information between animals. Examples: F prostaglandin in goldfish, pheromones in urine of mice and prairie voles Musth is a period of increased activity in male elephants ­ sexual status is marked by different pheromones.

Figure 12.6 Prairie Voles

12 The Hallmark of Human Sexual Behavior Is Diversity

Humans differ from other species in reproductive behaviors.

Humans can report subjective reactions, such as orgasm. The four phases of response are excitement, plateau, orgasm, and resolution ­ during the excitement phase, the phallus becomes erect.

Figure 12.7 Adult Human Reproductive Anatomy (Part 1)

Figure 12.7 Adult Human Reproductive Anatomy (Part 2)

12 The Hallmark of Human Sexual Behavior Is Diversity

Men and women differ in sexual response:

· Women have a greater variety of responses, with three typical patterns · Men, but not women, tend to have an absolute refractory phase following orgasm

Figure 12.8 Human Sexual Response Cycles (Part 1)

Figure 12.8 Human Sexual Response Cycles (Part 2)

12 The Hallmark of Human Sexual Behavior Is Diversity

In double-blind tests, neither researcher nor subject knows which treatment is received.

A low dose of androgens can stimulate interest and activity in males, and in some cases, in women.

12 The Hallmark of Human Sexual Behavior Is Diversity

Pheromones may affect human reproductive behaviors - menstrual cycles of women living together may synchronize.

The major histocompatibility complex (MHC) is a group of immune-related genes with many alleles.

Women prefer the smell of men with MHCs not too similar to their own.

12 Many Vertebrates Depend on Their Parents for Survival

Precocial animals are born with welldeveloped sensory and motor systems.

Altricial animals are less developed at birth. Both males and females may be involved in care of the young.

Many Vertebrates Depend on Their Parents for Survival

Rat mothers show four behaviors: nest building, crouching over pups, retrieving pups, and nursing. During pregnancy, exposure to hormones will prepare her brain to display these maternal behaviors immediately after giving birth.

Many Vertebrates Depend on Their Parents for Survival

A parabiotic preparation shows the effects of maternal hormones. A nonpregnant female exposed to the circulating hormones of a pregnant rat will display the same maternal behaviors.

* Estrogens, progesterone, prolactin are required for facilitating the performance of maternal behavior

Neural substrates of maternal behavior (both + and -)

· Double dissociation of mPOA & PAG shown by lesion study

mPOAX decrease of licking pups but no effect of nursing PAGX decrease nursing but not effect of licking

· There is also an inhibitory neural control of maternal behavior, especially in the virgin female.

Virgin female smells pups and show disliking of aversive response. somewhere located in olfactory bulb to mAMG/VMH (proof by lesion study & olfactory masking)

12 The Sex of an Individual Is Determined Early in Life

Sex determination is the early developmental event that decides if an individual will be male or female.

If the sperm that enters the egg has an Y chromosome, the offspring is male ­ if an X chromosome, the offspring is female.

12 The Sex of an Individual Is Determined Early in Life

The early indifferent gonads begin to change into ovaries or testes in the first month.

SRY gene ­ sex-determining region on the Y chromosome ­ is responsible for the development of testes Without an SRY gene, an ovary forms.

12 The Sex of an Individual Is Determined Early in Life

Hormones secreted by gonads,mainly from the testes, direct sexual differentiation.

Embryos have early tissues for both male and female structures. The wolffian ducts and the mü llerian ducts connect the gonads to the body wall.

Figure 12.11 Sexual Differentiation in Humans (Part 1)

12 The Sex of an Individual Is Determined Early in Life

In females, the müllerian ducts develop into the fallopian tubes, the uterus, and vagina ­ only part of the wolffian ducts remains.

In males, the wolffian ducts develop into the epididymis, vas deferens, and seminal vesicles, and the müllerian ducts shrink.

Figure 12.11 Sexual Differentiation in Humans (Part 2)

Figure 12.11 Sexual Differentiation in Humans (Part 3)

12 The Sex of an Individual Is Determined Early in Life

Two hormones from the testes make the system masculine:

· Testosterone ­ promotes the development of the wolffian system · Anti-mü llerian hormone (AMH) ­ induces the regression of the müllerian system

12 The Sex of an Individual Is Determined Early in Life

Testosterone also masculinizes other structures:

Other tissues form the prostate gland, scrotum, and penis. 5-reductase is an enzyme that converts testosterone into the more powerful dihydrotestosterone (DHT), necessary to form genitalia.

12 The Sex of an Individual Is Determined Early in Life

In Turner's syndrome, a person only has one sex chromosome ­ a single X.

The individual develops as a female ­ without the SRY gene, no masculinizing effects take place.

Figure 12.12 Sexual Differentiation and Gender Identity (Part 1)

Figure 12.12 Sexual Differentiation and Gender Identity (Part 2)

Figure 12.12 Sexual Differentiation and Gender Identity (Part 3)

12 The Sex of an Individual is Determined Early in Life

Congenital adrenal hyperplasia (CAH) is the result of female exposure to androgens before birth. Androgens are produced instead of corticosteroids and the newborn has an intersex appearance. No testes are present and ovaries are normal, despite appearance of external genitalia.

Figure 12.13 An Intersex Phenotype

12 The Sex of an Individual Is Determined Early in Life

Androgen insensitivity syndrome (AIS) occurs in XY people whose androgen receptors do not respond to testosterone. Testes remain internal and they develop mainly as women.

(X )

12 How Should We Define Gender ­ by Genes, Gonads, Genitals, or the Brain?

There is a consistent pattern among chromosomes, gonads, external genitalia, or internal structures ­ each is either masculine or feminine, although exceptions like AIS individuals can make it hard to judge sex.

Behavior is more difficult to define as masculine or feminine.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

The same hormones that masculinize the developing genitalia also masculinize the brain.

This organizational effect permanently alters behavior, in contrast to a temporary activational effect.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

Steroids have an organizational effect only when present during a sensitive period in early development.

Depending on the species and the behavior, this period may be before birth or just afterwards, in the neonatal period.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

The organizational hypothesis explains sexual differentiation:

A single steroid signal ­ androgen ­ masculinizes the body, the brain, and behavior.

In rats, many behaviors can be explained: if exposed to androgens early in life, they behave as males; if not, then as females.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

However, after exposing young female rats to estrogen ­ specifically estradiol, a female hormone ­ they failed to show lordosis as adults.

This raised the question of why female fetuses were not masculinized by circulating maternal estrogen.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

It was explained by the process of aromatization ­ the enzyme aromatase converts testosterone to estradiol.

The aromatization hypothesis says that testosterone enters the brain and is converted there to estrogens ­ these estrogens are what masculinize the developing rodent brain.

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

The estrogen in developing females does not enter the brain.

It is kept in the blood stream by binding with -fetoprotein, preventing aromatization and masculinization. -fetoprotein does not bind testosterone.

Figure 12.15 The Aromatization Hypothesis

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

Sexual dimorphism refers to sex differences in behavior or in structure.

In birds, the syrinx is a special organ that controls song. It is five times larger in males (complex songs) than in females (simple songs).

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

In rats, a nucleus in the preoptic area (POA) is larger in males than in females.

The nucleus is known as the sexually dimorphic nucleus of the POA (SDN-POA) ­ lesions in this area disrupt ovulatory and copulatory behaviors.

Figure 12.16 A Sex Difference in the Hypothalamus

Figure 12.17 Sexually Dimorphic Nucleus of the Preoptic Area (SDN-POA)

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

In rats, the spinal nucleus of the bulbocavernosus (SNB) is larger in males. Motoneurons in this nucleus innervate bulbocavernosus (BC) muscles that surround the base of the penis. Before birth, there is an almost equal number of these neurons and their target muscles in both sexes.

Figure 12.18 Sexual Differentiation of the Spinal Nucleus of the Bulbocavernosus (SNB) (Part 2)

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

Androgens act on the BC muscles in males to prevent their breakdown.

The innervating SNB motoneurons also survive, perhaps due to ciliary neurotrophic factor (CNTF). If the CNTF receptor is knocked out in mice, SNB neurons die.

Figure 12.18 Sexual Differentiation of the Spinal Nucleus of the Bulbocavernosus (SNB) (Part 1)

12 Gonadal Hormones Direct Sexual Differentiation of the Brain and Behavior

In nonrodent mammals, the BC motoneurons are found in Onuf's nucleus in the spinal cord.

Most female mammals retain BC muscles into adulthood ­ the BC surrounds the vagina.

The BC is larger in men than in women and men have more Onuf's motoneurons.

12 Social Influences Affect Social Differentiation of the Nervous System

Environmental and social influences can modulate masculinization.

A mother rat cares for males differently than females, helping to masculinize the developing nervous system.

Maternal attention has a different effect on females ­ will predispose them to being more or less attentive mothers.

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

A genetic mutation of the enzyme 5reductase results in incomplete masculinization of the genitalia.

Babies are usually raised as females. At puberty, masculinization progresses ­ known in the Dominican Republic as guevedoces, or "eggs (testes) at 12 (years)."

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

Two explanations:

People raised as girls who later behave as boys may have brains masculinized by prenatal testosterone, that proceed with development at puberty.

Or, early hormones have no effect ­ the Dominican culture recognizes that people can start out as girls and become boys, providing a social influence on gender development.

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

Two classes of possible influence on sexual orientation:

· Society's instruction on how one should behave when grown up · The biological factor ­ so strong in animal models ­ varying levels of fetal androgen direct future behavior

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

Brain structures may differ in homosexuals.

LeVay examined the INAH-3 nucleus of the POA. The nucleus was larger in men than in women, and larger in heterosexual men than in homosexual men.

It is not clear if the size difference is a result, or a cause.

Figure 12.20 Interstitial Nuclei of the Anterior Hypothalamus (Part 1)

Figure 12.20 Interstitial Nuclei of the Anterior Hypothalamus (Part 2)

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

Fetal androgen exposure markers include: otoacoustic emissions, finger length patterns, eyeblinks, and skeletal features.

Measures of these indicate that lesbians as a group were exposed to slightly more androgens than heterosexual women were.

12 Do Early Gonadal Hormones Masculinize Human Behaviors in Adulthood?

Fetal androgen exposure is not as well understood in homosexual men.

One influence on the probability of homosexual behavior is the number of brothers born before. Research indicates that sexual orientation in both sexes is determined early in life.

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Biological Psychology, 5e

80 pages

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