Sexual Physiology

Sexual physiology and development begin in early childhood and represent a complex process of events that lead to the final development of sexual orientation and behavior. Sexual behavior and interactions include several changes that are quite different between males and females.

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Introduction

There are 3 ways to determine a person’s biologic sex:

  • Genetic sex: determined by presence of sex chromosomes (XX = female, XY = male)
  • Gonadal sex: determined by presence of reproductive structures (ovaries in females, testes in males) 
  • Phenotypic sex: Human gonads produce different hormones that determine external phenotype.

Terms

  • Gender identity: 
    • Represents individual’s inner sense of maleness or femaleness
    • Usually established by age 3
  • Gender role or behavior:
    • Represents behaviors exhibited by or expected from an individual
    • Can be congruent or incongruent with individual’s gender identity 
  • Sexual orientation:
    • Individual’s pattern of physical and emotional arousal
    • An individual assessment of one’s sexual orientation is termed sexual identity. 
    • Examples:
      • Homosexuality: attraction to those with same gender identity
      • Heterosexuality: attraction to those with opposite gender identity
      • Bisexuality: attraction to either gender identity
      • Asexuality: attraction to neither gender identity
    • Coming out: defined as individual acknowledging their sexual orientation 
  • Gender dysphoria:
    • Emotional discomfort arising from misalignment between gender identity and sexual identity 
    • Seen in boys more frequently than in girls

Behaviors during sexual development

Appropriate:

  • During early childhood years:
    • Showing interest in exploring one’s own or other’s genitals
    • Undressing self or others
    • Masturbatory movements
    • Asking reproductive or sexual questions
    • Increased interest in sex words or play
    • Masturbation is considered a normal behavior at all ages and occurs equally in both genders (it is pathological only when it interferes with normal functioning).
  • Exploring human sexuality is normal, especially during teenage years, with different- or same-sex partners.

Abnormal:

  • Repeated insertion of objects into vagina or anus
  • Inappropriate sex play (simulating genital–genital, oral–genital, or anal–genital sexual intercourse)
  • Use of force in sex play
  • Age-inappropriate sexual knowledge

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Tanner Stages

The Tanner stages is a scale that measures physical/sexual development in children, adolescents, and adults. It involves genitalia in males, breast in females, and pubic hair in both.

Table: Tanner stages in females
Tanner 1 Tanner 2 Tanner 3 Tanner 4 Tanner 5
Age Prepubertal 8–11.5 years 11.5–13 years 12–15 years > 15 years
Pubic hair Villus hair only Sparse hair along the labia Coarse and curly hair covers the pubis. Adult hair that does not spread to the thigh Adult hair reaching the thigh
Breasts Elevation of papilla only Breast buds are palpable (1st sign of puberty in females) and areolae are enlarged. Breast tissue grows with no contour or separation. Breasts enlarge and areolae form secondary mound on the breast. Adult breast contours are present. Only the papilla is raised.
Other observations Adrenarche and ovarian growth Clitoral enlargement, labial pigmentation, growth of uterus Axillary hair, acne Menarche and development of menses Adult genitalia
Table: Tanner stages in males
Tanner 1 Tanner 2 Tanner 3 Tanner 4 Tanner 5
Age Prepubertal 8–11.5 years 11.5–13 years 12–15 years > 15 years
Pubic hair Villus hair only Sparse hair at base of the penis Coarse and curly hair appears over the pubis. Adult-quality hair in the pubic area, sparing the thigh Adult-quality hair in the pubic area, reaching the thigh
Genitalia
  • Testes < 2 cm
  • No growth of the penis
  • Testes: 2.5–3.2 cm
  • Thinning and reddening of the scrotum
  • Testes: 3.3–4 cm
  • Increase in length of the penis
  • Testes: 4.1–4.5 cm
  • Growth of the penis, darkening of the scrotum
  • Testes > 4.5 cm
  • Adult genital size
Other observations Adrenarche Decrease in body fat Gynecomastia, breaking of voice, increased muscle mass Axillary hair, voice change, acne Facial hair, increase in muscle mass

Sexual Response Cycle

Sexual dysfunctions (in males or females) arise from problems involving any stage of the sexual response cycle.

Table: Stages of the sexual response cycle
Stage Changes in females Changes in males Changes in both
Desire
  • Motivation or interest in sexual activity
  • Expressed by sexual fantasies
Excitement/arousal
  • Begins with either fantasy or physical contact
  • Vaginal lubrication
  • Clitoral erection
  • Labial swelling
  • Elevation of the uterus in the pelvis (tenting)
  • Begins with either fantasy or physical contact
  • Erection and testicular enlargement
  • Flushing
  • Nipple erection
  • Hemodynamic changes (increased respiration, pulse, and BP)
Orgasm Vaginal and uterine contractions
  • Tightening of the scrotal sac
  • Secretion of a few drops of seminal fluid
  • Ejaculation
  • Facial grimacing
  • Release of tension
  • Slight clouding of consciousness
  • Involuntary anal sphincter contractions
  • Acute increase in BP and pulse
Resolution Women experience little or no refractory period. Men have a refractory period lasting minutes to hours during which they cannot re-experience orgasm.
  • Muscles relax.
  • Cardiovascular state returns to baseline.
  • Sexual organs return to normal baseline.

Normal Sexual Changes with Aging

Healthy sexual behavior and desire persist into elderly adulthood.

Pharmacotherapy exists to facilitate sexual behavior:

  • Sildenafil: to assist erection in men
  • Hormonal creams/pills: to counteract vaginal atrophy in women
Table: Sexual changes with aging
Changes in females Changes in males Changes in both
  • Vaginal dryness and thinning (due to decrease in estrogen)
  • Decreased sensitivity in sexual organs (nipple, clitoral, vulva)
  • Slower erection
  • Weaker erection
  • Decrease in intensity of ejaculation
  • Longer refractory period
  • More stimulation needed
Normal or slight decrease in the desire for sexual interest and activity

Clinical Relevance

  • Gender dysphoria: also known as gender identity disorder. Individuals with gender dysphoria have difficulties reconciling the incongruence between their expressed gender and assigned gender (continuous inner conflict between gender identity and sexual identity). The 1st signs of cross-gender behaviors begin around age 3, the time when gender identity is established.
  • Female sexual dysfunction: disorder in any part of the female sexual response cycle. This group of disorders includes desire disorders, arousal disorders, orgasmic disorders, and pain disorders. The dysfunction might result from stresses and interpersonal conflicts, as well as from physical illness or medication/substance use. These disorders cause significant distress to the patient.
  • Male sexual dysfunction: disorder in any part of the male sexual response cycle. This group of disorders includes desire disorders, arousal disorders, and orgasmic disorders. The dysfunction may be caused by psychological disorders, endocrine diseases, neurogenic dysfunction, chronic medical illness, or medication/substance abuse.

References

  1. Blitzstein S., Kaufman M., Ganti L. (2016). Sexual dysfunctions and paraphilic disorders. Chapter 16 of First Aid for the Psychiatry Clerkship, 4th ed. McGraw-Hill Education/Medical, p. 172.
  2. Sadock, B. J., Sadock, V. A., & Ruiz, P. (2014). Human sexuality and sexual dysfunctions. Chapter 17 of Kaplan and Sadock’s Synopsis of Psychiatry: Behavioral Sciences/Clinical Psychiatry, 11th ed. Philadelphia: Lippincott Williams and Wilkins, pp. 564–574.
  3. Forcier, M. (2020). Adolescent sexuality. UpToDate. Retrieved June 22, 2021, from https://www.uptodate.com/contents/adolescent-sexuality

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