Chapter 4 Summary

  1. The male external genitalia comprise the penis and the scrotum. A man’s penis contains three erectile structures and encloses the urethra. Its erotic sensitivity is highest on the glans and frenulum. The foreskin, which covers the glans, is removed in the operation of circumcision. Circumcision offers some health advantages, but the prevalence of circumcision in the United States is decreasing. Health problems affecting the penis include inflammation of the glans (balanitis), inability to retract the foreskin (phimosis), entrapment of the foreskin behind the glans (paraphimosis), pathological curvature of the penis (Peyronie’s disease), and penile cancer.
  2. The scrotum contains the testicles and has muscular and vascular mechanisms for maintaining them below the regular body temperature.
  3. A man’s internal reproductive structures include six paired structures—the testes, epididymides, vasa deferentia, seminal vesicles, ejaculatory ducts, and bulbourethral glands—as well as two unpaired midline structures, which are the prostate gland and urethra.
  4. The testis contains seminiferous tubules, in which sperm are produced; the tubules are lined by Sertoli cells, which nurture the developing sperm and secrete peptide hormones. Between the seminiferous tubules lie Leydig (interstitial) cells, which secrete steroid and peptide hormones. The most important health problem affecting the testes is testicular cancer. This can affect young men, but it is one of the most curable cancers. The epididymis is the location where sperm mature and become more concentrated. The vasa deferentia store sperm and transport them to the urethra. Sperm constitute only about 1% of the volume of the ejaculate.
  5. The prostate gland and seminal vesicles add the noncellular portion of semen (seminal plasma), which consists of water, salts, fructose, buffers, semenogelin, enzymes, and antioxidants. The bulbourethral glands produce a slippery liquid (“pre-cum”) that may be discharged from the urethra in small amounts before ejaculation.
  6. The prostate gland can be affected by inflammation (prostatitis), age-related enlargement (benign prostatic hyperplasia), and prostate cancer. It is possible to screen for prostate cancer by regular digital rectal examination and by a blood test (PSA test). The treatments for prostate cancer (prostatectomy, hormone therapy) often have a serious impact on a man’s sex life, and the question of whether and how to treat early-stage prostate cancer is controversial.
  7. Sexual functions are regulated by the nervous and endocrine systems. Erection of the penis and clitoris involves a spinal reflex that begins with stimulation of nerve endings in the genital skin. Inputs from the brain powerfully modulate these reflexes. Erection involves the filling of vascular spaces (sinusoids) in the erectile tissue: This process is controlled by the balance of activity in the sympathetic and parasympathetic innervation of the tissue. A key molecule, the neurotransmitter nitric oxide, is released by parasympathetic nerve terminals and promotes erection.
  8. Emission is the loading of the various glandular components of semen into the urethra. It is followed quickly by ejaculation, in which muscular contractions squeeze the urethra and eject the semen in a sequence of pulses from the urethra. Orgasm is the subjective pleasurable experience that usually accompanies ejaculation, along with the physiological events that underlie it.
  9. Most societies prohibit public exposure of the genitals, apparently to regulate sexual arousal and minimize sexual conflicts. There is considerable variation in attitudes toward nudity, however.