Overview
This lecture explains the physiological process of human sexual reproduction, the four stages of the sexual response cycle, fertilization, and various contraception methods to prevent pregnancy.
Human Sexual Response Cycle
- The sexual response cycle consists of four phases: excitement, plateau, orgasm, and resolution.
- Excitement phase: parasympathetic reflexes cause genital arterioles to dilate and fill with blood.
- Plateau phase: muscle tension, heart rate, blood pressure, and breathing increase; male urethral sphincter contracts.
- Orgasm phase: rhythmic muscle contractions occur; males ejaculate sperm, females may experience uterine contractions that aid sperm movement.
- Resolution phase: body relaxes; males enter a refractory period, females may not require a refractory period and can have serial orgasms.
Fertilization Process
- Ovulation releases a secondary oocyte that remains viable for about 24 hours.
- Sperm can survive in the female reproductive tract for 3-5 days.
- Sperm must travel from the vaginal canal, through the cervix, uterus, and into the fallopian tube.
- Most sperm die or are blocked by acidic environments, cervical mucus, or white blood cells.
- Sperm must become "capacitated" to penetrate the oocyte.
- Female secretions loosen the sperm’s acrosome, releasing enzymes to penetrate the oocyte's corona radiata and zona pellucida.
- Sperm fuses with oocyte membrane, triggering oocyte to complete meiosis II, and blocks entry of other sperm.
- Male and female pronuclei merge to form a diploid zygote, which immediately begins mitosis.
Contraception Methods
- Sterilization (tubal ligation or vasectomy) permanently prevents gametes from meeting.
- Barrier methods (condoms, diaphragms, sponges, cervical caps) physically block sperm from reaching the egg.
- Hormonal methods (pills, shots, patches, rings) prevent ovulation and alter cervical mucus or endometrium.
- Intrauterine devices (IUDs) prevent zygote implantation in the uterus.
Key Terms & Definitions
- Human Sexual Response Cycle — four physiological phases during sexual activity: excitement, plateau, orgasm, resolution.
- Capacitation — process where sperm lose protective proteins to become capable of fertilizing an egg.
- Acrosome — cap-like structure on sperm head that releases enzymes to penetrate the egg.
- Zona Pellucida — glycoprotein layer surrounding the oocyte’s plasma membrane.
- Pronucleus — nucleus from egg or sperm before they fuse in fertilization.
- Zygote — fertilized egg cell resulting from fusion of gamete nuclei.
- Sterilization — permanent procedure to prevent gametes from meeting (tubal ligation/vasectomy).
- IUD (Intrauterine Device) — device inserted into uterus to prevent implantation.
Action Items / Next Steps
- Review the stages of the sexual response cycle and their physiological effects.
- Study how contraception methods disrupt fertilization or implantation.
- Prepare for next lecture on zygote implantation and development.