Sex and the Brain: Reproductive Behavior and Sexual Differentiation

Vocabulary practice flashcards covering reproductive behavior stages, neural circuitry, human sexual response, sex determination and differentiation pathways, and neuroendocrine mechanisms of sexual dimorphism.

Sexual attraction

The first stage of reproductive behavior that brings males and females together, which may be synchronized with physiological readiness to reproduce and learned associations such as appearance.

Appetitive behaviors

Species-specific behaviors that establish, maintain, or promote sexual interaction, such as ear-wiggling in female rats or staying near and sniffing the female in male rats.

Proceptive

A behavioral state describing a female actively displaying appetitive behaviors (such as ear-wiggling or approaching males) to invite copulation.

Copulation

The mating act (coitus) consisting of one or more intromissions in which the male inserts his penis into the female vagina and ejaculates sperm-bearing semen.

Lordosis

A stereotypic receptive posture adopted by female rats in which the back is arched and the hindquarters are elevated, allowing male intromission.

Coolidge effect

The phenomenon characterized by a faster resumption of mating behavior and shortened refractory period when an animal is presented with a novel or different mating partner.

Refractory phase

A temporary period of sexual inhibition following copulation or orgasm during which an individual is unable to copulate or orgasm again.

Copulatory lock

A postcopulatory phenomenon in dogs and some mice where the penis temporarily swells and cannot be withdrawn from the female vagina immediately after mating.

Parthenogenesis

A form of asexual reproduction ("virgin birth") in which an unfertilized ovum develops into an embryo, occurring occasionally in certain vertebrates such as Komodo dragons, pit vipers, and turkeys.

Activational effect

A temporary and reversible influence of a hormone that switches a behavior on only while that hormone is present in the bloodstream (e.g., testosterone restoring copulatory drive in castrated adult males).

Ventromedial hypothalamus (VMH)

A hypothalamic region in female rats crucial for mediating the lordosis response through steroid actions; estrogen expands its dendritic branches and induces progesterone receptor synthesis.

Medial preoptic area (mPOA)

A hypothalamic region in male rats that coordinates copulatory behavior (mounting) by integrating hormonal and sensory inputs and projecting to the ventral midbrain, basal ganglia, and spinal cord.

Paragigantocellular nucleus (PGN)

A brainstem nucleus with descending fibers that normally inhibit spinal erection reflexes; signals from the mPOA counteract this inhibition during mating.

Pheromones

Chemical signals released into the external environment that transmit information and trigger behavioral or physiological responses between individuals of the same species.

Vomeronasal organ (VNO)

A specialized collection of sensory receptor cells located near the olfactory epithelium that detects pheromones and projects to the accessory olfactory bulb.

Bruce effect

The termination of pregnancy in a female rodent induced by exposure to urine pheromones from a new, unfamiliar male.

Masters and Johnson four-phase model

The human sexual response cycle consisting of four sequential physiological phases: excitement, plateau, orgasm, and resolution.

Altricial animals

Animals (such as rats and humans) that are born in an undeveloped, helpless state and require extensive parental care to survive.

Precocial animals

Animals (such as ducks and horses) that are born in a relatively developed physical state and can survive with minimal parental care.

Parabiotic preparation

An experimental surgical procedure where two animals are joined together to share a single common circulatory blood supply, demonstrating the hormonal control of maternal behaviors.

Anteroventral periventricular nucleus (AVPV)

A nucleus in the hypothalamus that regulates maternal behavior in rats and is sexually dimorphic, being larger in females than in males.

Sex determination

The early developmental process that decides whether a fetus will develop male or female gonads, determined by whether the fertilizing sperm provides an $X$ or a $Y$ chromosome.

SRY gene

The sex-determining region of the $Y$ chromosome responsible for directing indifferent gonads to develop into testes by producing the SRY protein.

Sexual differentiation

The process through which individuals develop distinct male or female bodily structures, neural circuits, and behaviors under the direction of gonadal hormones.

Anti-Müllerian hormone (AMH)

A peptide hormone secreted by embryonic testes that causes regression of the Müllerian ducts during male development.

Wolffian ducts

Embryonic internal ducts that develop into the epididymis, vas deferens, and seminal vesicles in the presence of testosterone.

Müllerian ducts

Embryonic internal ducts that develop into the oviducts, uterus, and inner vagina in the absence of anti-Müllerian hormone ($\text{AMH}$).

$5\alpha$-reductase

An enzyme that converts testosterone into the potent androgen dihydrotestosterone ($\text{DHT}$), which directs the development of the scrotum and penis from external genital skin.

Turner's syndrome

A genetic condition in which an individual has only one $X$ chromosome ($XO$) and develops as female due to the absence of the $SRY$ gene and a lack of masculinizing hormones.

Congenital adrenal hyperplasia (CAH)

A genetic condition where the adrenal glands overproduce androgens prenatally, causing intersex external genitalia in $XX$ individuals despite having normal ovaries.

Androgen insensitivity syndrome (AIS)

A condition in $XY$ individuals whose androgen receptors fail to respond to testosterone, causing testes to remain internal and the external phenotype and gender identity to develop typically as female.

Guevedoces

Individuals with a genetic $5\alpha$-reductase deficiency who have an $XY$ genotype and are raised as girls due to feminized external genitalia at birth, but undergo physical virilization into men at puberty.

Organizational effect

A permanent structural modification of the nervous system and behavior caused by steroid hormones acting during an early sensitive developmental window.

Sensitive period

A critical time frame during early development (neonatal/prenatal, and later puberty) in which steroid hormones can permanently organize brain structures and future behaviors.

Aromatization hypothesis

The hypothesis proposing that testicular testosterone enters rodent brain neurons and is converted into estrogens by the enzyme aromatase to masculinize the nervous system.

$\alpha$-fetoprotein ($\alpha\text{FP}$)

A plasma protein found in fetal rodents that binds circulating maternal estrogens to prevent them from entering the brain and inadvertantly masculinizing developing female fetuses.

Sexually dimorphic nucleus of the POA (SDN-POA)

A nucleus in the rat preoptic area that is markedly larger in males than in females due to neonatal exposure to aromatized androgens; lesions disrupt male copulatory and female ovulatory behaviors.

INAH-3 (interstitial nucleus of the anterior hypothalamus 3)

A sexually dimorphic human hypothalamic nucleus identified by Simon LeVay that is larger in heterosexual men than in women and homosexual men.

$2\text{D}:4\text{D}$ ratio

The ratio between index finger ($2\text{D}$) length and ring finger ($4\text{D}$) length; an anatomical marker of prenatal androgen exposure that is significantly lower in lesbians and men compared to heterosexual women.

Cloacal exstrophy

A rare birth condition in $XY$ individuals born with functional testes and normal testosterone levels but without a penis; historically raised as females, though approximately half eventually identify as male.