Overview
This lecture explains the physiological processes of human sexual intercourse leading to fertilization and describes various methods of contraception that prevent pregnancy.
Human Sexual Response Cycle
- Masters and Johnson identified four phases: excitement, plateau, orgasm, and resolution.
- Excitement phase involves sexual arousal and increased blood flow to genitalia via the parasympathetic nervous system.
- Plateau phase includes heightened muscle tension, heart rate, blood pressure, and breathing; male urethral sphincter contracts.
- Orgasm phase features rhythmic muscular contractions; males ejaculate sperm, and female contractions may help retain sperm.
- Resolution phase is when the body relaxes; males enter a refractory period, while females may not need one physiologically.
Sperm Journey and Fertilization
- During ovulation, an ovary releases a secondary oocyte that can be fertilized within about one day.
- Sperm deposited in the vagina face acidic environments and cervical mucus barriers; few succeed in reaching the oocyte.
- Sperm must undergo capacitation, losing protective proteins to prepare for fertilization.
- The sperm penetrates the corona radiata and zona pellucida layers of the oocyte using hydrolytic enzymes.
- Once a sperm fuses with the oocyte membrane, calcium ions trigger completion of meiosis and block other sperm entry.
- Male and female pronuclei form, merge, and their chromosomes combine to create a diploid zygote that begins mitosis.
Contraception Methods
- Sterilization (tubal ligation, vasectomy) permanently blocks gamete pathways.
- Barrier methods (condoms, diaphragms, sponges, cervical caps) physically prevent sperm from reaching an egg.
- Hormonal methods (pill, shots, patches, rings) inhibit ovulation and alter the endometrium and cervical mucus.
- Intrauterine devices (IUDs) prevent zygote implantation in the uterus.
- Successful gestation requires zygote implantation in the uterus soon after fertilization.
Key Terms & Definitions
- Oocyte — an immature egg cell released during ovulation.
- Capacitation — changes that make sperm capable of fertilizing an oocyte.
- Acrosome — the cap on a sperm's head containing enzymes for penetrating the oocyte.
- Zona pellucida — glycoprotein layer surrounding the oocyte's plasma membrane.
- Pronucleus — haploid nucleus from each gamete before fusion.
- Zygote — a fertilized egg cell with combined genetic material from both parents.
- Sterilization — permanent surgical contraception method.
- Hormonal contraception — methods using synthetic hormones to prevent ovulation.
Action Items / Next Steps
- Prepare for next lecture on implantation and gestation.
- Review the four phases of the sexual response cycle and methods of contraception.