Sexual Behaviour Notes

Sexual Behaviour

Menstrual Cycle vs. Estrous Cycle

• The menstrual cycle and estrous cycle are reproductive cycles in female animals, but they differ in key physiological aspects.
• Key differences include shedding of the uterine lining, hormonal regulation, and reproductive behavior.

Menstrual Cycle

• The menstrual cycle is the reproductive cycle of female primates, including humans, apes, and some monkey species.
• It is characterized by the monthly preparation of the uterus for potential pregnancy, followed by menstruation if fertilization does not occur.
• Key phases of the menstrual cycle:
  • Menstrual Phase: The endometrium (uterine lining) sheds, resulting in menstruation (bleeding). This marks the beginning of a new cycle.
  • Follicular Phase: The pituitary gland releases follicle-stimulating hormone (FSH), which stimulates follicle growth in the ovaries. The maturing follicles produce estrogen, which helps rebuild the endometrial lining.
  • Ovulation: A surge in luteinizing hormone (LH) triggers the release of an egg (ovum) from the ovary, making fertilization possible.
  • Luteal Phase: The ruptured follicle forms the corpus luteum, which secretes progesterone to maintain the uterine lining. If fertilization does not occur, the corpus luteum degenerates, hormone levels drop, and menstruation begins again.
• The entire cycle lasts about 28 days, though variations exist among individuals.

Estrous Cycle

• The estrous cycle is the reproductive cycle of non-primate mammals, such as rodents, cats, dogs, cows, and horses.
• Unlike the menstrual cycle, the uterine lining is reabsorbed rather than shed, meaning there is no menstruation.
• Key phases of the estrous cycle:
  • Proestrus: The ovaries begin developing follicles, and estrogen levels rise, preparing the female for mating.
  • Estrus: Often called "heat," this is when the female is sexually receptive, and ovulation occurs. Unlike primates, mammals with an estrous cycle only mate during this period.
  • Metestrus/Diestrus: If fertilization occurs, pregnancy begins; if not, the corpus luteum forms and later regresses, leading back to proestrus.
  • Anestrus: A period of reproductive inactivity that can vary depending on the species.

Feature Comparison

Human Sexual Behaviour

Sexual Activity and the Menstrual Cycle

• Men initiate sexual activity at a relatively constant rate throughout the woman's menstrual cycle.
• Women exhibit a peak in sexual initiation around ovulation, which coincides with the highest levels of estradiol (a form of estrogen).
• Just before and during the luteinizing hormone (LH) surge, which triggers ovulation, women report increased sexual desire, more frequent sexual fantasies, and a higher likelihood of initiating sexual activity.
• Fluctuations in ovarian hormones (estradiol and progesterone) affect female sexual interest.

Testosterone and Male Sexual Behavior

• Testosterone has an activational effect on male sexual behavior, enhancing libido and sexual motivation.
• Sexual activity itself can influence testosterone levels; anticipation of sex, sexual thoughts, and actual engagement in intercourse can increase testosterone production.

Neural Activity and Sexual Arousal

• Cortical activation during sexual arousal involves multiple brain regions, including:
  • Occipitotemporal and inferotemporal cortex (visual processing of sexual stimuli)
  • Parietal cortex (sensory integration)
  • Orbitofrontal and medial prefrontal cortex (decision-making, social and emotional processing)
  • Insular and cingulate cortex (emotion and autonomic regulation)
  • Premotor cortex (motor preparation and coordination)
• The visual cortex is highly active during sexual arousal.
• Orgasm is associated with suppression of the prefrontal cortex, which governs rational thought and self-control.

Health Benefits of Orgasm

• Research indicates that orgasms have several health benefits, including improvements in:
  • Headaches and other pain
  • Heart health
  • Menstrual cramps
  • Self-confidence
  • Sleep
  • Stress

Brain Disorders and Sexual Behavior

• Temporal lobe dysfunction has been linked to altered sexual behavior.
  • Focal epilepsy in the temporal lobe is often associated with a lack of sexual interest.
  • Other abnormalities in this region may lead to unusual or inappropriate sexual behaviors.
• The ventral striatum is activated by sexually provocative images.
• In rats, this region is active during the anticipation and experience of sex, as well as other pleasurable activities.

Brain Trauma and Sexual Behavior

• Brain trauma typically reduces libido, but in rare cases, it can lead to hypersexuality
• Examples show how frontal lobe and temporal lobe damage alters sexual behavior due to impaired self-control and damaged emotion/behavior regulation areas.
• Therapy and medication can help manage these conditions.

Neural Control of Male Sexual Behavior

• Erection and ejaculation are primarily controlled by neural circuits in the spinal cord.
• Motor neurons in the lumbar spinal cord coordinate pelvic movements necessary for copulation.
• The medial preoptic area (MPA) of the hypothalamus is the most crucial brain region for male sexual behavior.
  • Electrical stimulation of the MPA triggers copulatory behavior in males.
  • Destruction of the MPA abolishes sexual behavior.
  • During sexual activity, the MPA neurons show increased firing rates.
  • Mating increases glutamate release in the MPA.
• The MPA receives chemosensory input from the vomeronasal organ (VNO) and somatosensory information from the genitals.
• Androgens (e.g., testosterone) exert activational effects on MPA neurons.
• The MPA connects to spinal cord motor neurons through the periaqueductal gray matter (PAG) and nucleus paragigantocellularis (nPGi).
• The amygdala can inhibit erection and ejaculation when a male experiences stress or fear.

Neural Control of Female Sexual Behavior

• The ventromedial nucleus (VMN) of the hypothalamus is the most critical region for female sexual behavior.
  • Electrical stimulation of the VMN facilitates sexual behavior.
  • Lesions in the VMN eliminate sexual behavior.
• The VMN receives chemosensory information from the vomeronasal system and somatosensory input from the genitals.
• During orgasm, several brain regions show increased activity, including:
  • The junction between the midbrain and diencephalon
  • The lateral putamen
  • The cerebellum

The Genetics of Sex

• Biological sex is determined by sex chromosomes inherited from a sperm and egg.
  • $XX$ is a biological female
  • $XY$ is biologically male
• The egg will only provide an X chromosome.
• The sperm can provide either an X or Y chromosome.

Sex Chromosome Abnormalities

• Turner Syndrome (XO)
  • Occurs when a child has only one X chromosome.
  • Individuals have typical female external genitalia but abnormal ovarian development.
  • Ovaries do not produce eggs or sufficient hormones, leading to infertility.
  • Treatment includes growth hormone, plastic surgery, and hormone replacement therapy.
• Klinefelter Syndrome (XXY)
  • Most common sex chromosome abnormality.
  • Individuals are phenotypically male but have reduced fertility.
  • Hormone therapy is needed to support the development of secondary male characteristics.
• Jacob Syndrome (XYY)
  • Phenotypically male with subtle physical and behavioral differences.
  • Tend to be taller and leaner.
  • Higher risk of eye, elbow, and chest abnormalities.
  • Fertile, but slightly more likely to produce abnormal sperm.

Sexual Dimorphism in the Brain

• Certain brain structures differ in size and volume between males and females.
• Sexually dimorphic areas include:
  • Interstitial nuclei of the anterior hypothalamus (INAH)
  • Thalamus
  • White matter in cerebral hemispheres
  • Synaptic density

Sex Differences in Cognition

• Males show a slight advantage in visuospatial tasks.
• Females show a slight advantage in verbal tasks.
• These differences are influenced by current levels of testosterone and estrogen.
• Increased estradiol levels correlate with greater activation in language centers of the brain.
• Increased testosterone levels correlate with enhanced activation in spatial processing regions.

Sexual Orientation

• Defined by a stable pattern of attraction toward a particular sex.
• Prevalence:
  • 1. 8% of adult males and 1.5% of adult females identify as gay or lesbian.
  • 3% of adult males and 0.9% of adult females identify as bisexual.
• Sexual orientation is distinct from sexual behavior and sexual fantasy.

Elements of Physical Attractiveness

• Facial and bodily symmetry are generally perceived as more attractive.
• Evolutionary Preferences in Attractiveness
  • Males prefer youthful female features.
  • Female preferences for male features vary depending on the type of relationship:
    • Masculine features are considered sexually desirable.
    • Less masculine features are preferred for long-term relationships.

Romantic Love and Sexual Desire

• Vasopressin and oxytocin play key roles in social bonding and attachment.
• Oxytocin is released during:
  • Hugging, cuddling, and sexual intercourse
  • Childbirth and breastfeeding

Sexual Dysfunctions and Their Treatments

Common Female Sexual Dysfunctions

• Reduced sexual desire (low libido)
• Orgasm problems

Common Male Sexual Dysfunction

• Erectile dysfunction (ED)

Physiological Mechanism of Erections

• Parasympathetic neurons release acetylcholine (ACh) and nitric oxide (NO) into the spongy tissue of the penis.
• Nitric oxide (NO) causes blood vessels to relax and expand, allowing increased blood flow, resulting in an erection.

Treatment for Erectile Dysfunction

• Medications like Viagra (sildenafil), Cialis (tadalafil), and Levitra (vardenafil) enhance the effects of nitric oxide.
• Psychological therapies, hormone therapy, and lifestyle changes can also help treat sexual dysfunctions in both men and women.