71. Physiology of Copulation

Introduction

• Human sexual activity depends on precise interactions between the nervous system, reproductive organs, and psychological factors. This chapter explains the physiological basis of erection, ejaculation, orgasm, and the mechanisms that contribute to healthy sexual function.

• Copulation is the physiological process of sexual intercourse that enables the transfer of sperm from the male to the female reproductive tract.
• Human sexual responses involve coordinated neural, vascular, hormonal, and psychological mechanisms.
• Understanding these mechanisms is essential for diagnosing and managing sexual dysfunctions, including erectile dysfunction.

Male Sexual Act

• The principal events of the male sexual act are penile erection, vaginal penetration, and ejaculation.
• These events are coordinated by reflex mechanisms integrated within the spinal cord.
• The male reproductive tract receives both sympathetic and parasympathetic autonomic innervation.
• The penis also receives somatic sensory and motor innervation.
• Sexual function depends primarily on neural mechanisms but is influenced by hormonal, vascular, biochemical, and psychological factors.
• Normal sexual performance requires coordinated interactions among the nervous system, endocrine system, and reproductive organs.

Erection

Blood supply:

• The penis receives its arterial supply mainly from branches of the internal pudendal artery.
• The deep arteries of the penis supply the erectile tissues, while venous drainage occurs primarily through the dorsal veins.

Mechanism

• Penile erection is mediated predominantly by the parasympathetic nervous system.
• Parasympathetic fibers arise from the sacral spinal cord segments and travel through the pelvic splanchnic nerves.
• These nerves release neurotransmitters that promote relaxation of penile smooth muscle.
• Nitric oxide is the principal mediator responsible for erection.
• Nonadrenergic, noncholinergic nerve fibers are an important source of nitric oxide in erectile tissue.
• Nitric oxide activates guanylyl cyclase within smooth muscle cells. This increases intracellular cyclic guanosine monophosphate concentration.
• Cyclic guanosine monophosphate produces smooth muscle relaxation, vasodilation, and increased penile blood flow.
• Vasoactive intestinal peptide and acetylcholine may also contribute to erectile responses.
• Phosphodiesterase type 5 inhibitors, such as sildenafil, tadalafil, and vardenafil, enhance erection by preventing degradation of cyclic guanosine monophosphate.
• These drugs are widely used in the treatment of erectile dysfunction.

Impotency

• Erectile dysfunction is the persistent inability to achieve or maintain an erection sufficient for satisfactory sexual activity.
• Temporary erectile dysfunction may occur during stress, anxiety, fatigue, or acute illness.
• Chronic erectile dysfunction is commonly associated with vascular disease, diabetes mellitus, neurological disorders, hormonal abnormalities, or certain medications.
• Evaluation should focus on identifying the underlying cause.
• Phosphodiesterase type 5 inhibitors, such as sildenafil, are commonly used for treatment.

Ejaculation

• Ejaculation is the expulsion of semen from the male reproductive tract during orgasm. It consists of two sequential phases: emission and expulsion.

Emission

• Emission is the movement of sperm and glandular secretions into the posterior urethra.
• Sperm are stored primarily in the epididymis and distal vas deferens before ejaculation.
• During emission, smooth muscle contraction of the vas deferens, seminal vesicles, and prostate propels their contents into the urethra. This phase is mediated mainly by the sympathetic nervous system.
• Sympathetic fibers involved in emission travel through the hypogastric nerves.
• Contraction of the internal urethral sphincter helps prevent retrograde flow of semen into the urinary bladder.

Ejaculation

• Expulsion is the forceful propulsion of semen from the urethra to the exterior.
• This phase occurs through rhythmic contraction of the bulbospongiosus and other pelvic floor muscles.
• These muscles are skeletal muscles supplied by the pudendal nerve.
• The reflex is integrated within the lumbosacral segments of the spinal cord.
• Sensory stimulation from the genital organs activates the ejaculatory reflex pathway.
• Ejaculatory expulsion is usually accompanied by orgasm and pleasurable sensations.

Neural Regulation

• Ejaculation depends on coordinated sympathetic, parasympathetic, and somatic neural activity.
• Sympathetic pathways are particularly important for emission, whereas somatic motor pathways mediate muscular contractions during expulsion.
• The precise neurotransmitters involved in central ejaculatory control remain incompletely understood.
• Disorders affecting autonomic nerves, such as diabetic autonomic neuropathy, may impair ejaculation and can lead to retrograde ejaculation.

Premature Ejaculation

• Premature ejaculation is a common sexual dysfunction characterized by ejaculation occurring earlier than desired during sexual activity. It may be lifelong or acquired and can cause significant personal distress and relationship difficulties.
• A commonly used diagnostic criterion is ejaculation occurring within approximately one minute of vaginal penetration in lifelong cases.
• Affected individuals often have limited control over ejaculation and experience frustration or avoidance of sexual intimacy.

Causes and Treatment

• Premature ejaculation results from a combination of psychological, neurobiological, and relationship-related factors.
• Anxiety, performance concerns, emotional stress, and interpersonal conflicts may contribute to the condition.
• Biological factors such as altered serotonin signaling, prostatitis, and certain endocrine disorders may also be involved.
• Evaluation should include assessment of psychological and medical causes.
• Treatment is individualized and often combines behavioral and medical approaches.
• Counseling and sexual education can improve confidence and reduce performance anxiety.
• Behavioral techniques, including the stop–start and squeeze methods, may help delay ejaculation.
• Selective serotonin reuptake inhibitors and topical anesthetic agents are commonly used when required.
• Stress reduction, regular physical activity, and relaxation practices may improve overall sexual well-being.
• Erection is mediated predominantly by parasympathetic activity, whereas ejaculation depends mainly on sympathetic activation.
• Understanding this autonomic balance may help patients develop better control over ejaculation.

Female Sexual Act

• Female sexual responses involve coordinated neural, vascular, muscular, endocrine, and psychological mechanisms.
• During sexual arousal, blood flow increases to the external genitalia, vagina, breasts, and other reproductive structures.
• Increased vascular engorgement causes enlargement of the clitoris and erection of the nipples.
• The clitoris contains abundant sensory nerve endings and plays a major role in sexual arousal and pleasure.
• Sensory stimulation of the clitoris, nipples, and other erogenous areas enhances sexual excitement.
• Emotional, visual, auditory, olfactory, and tactile stimuli can all contribute to sexual arousal.
• Increased blood flow to the vaginal wall promotes transudation of fluid through the vaginal epithelium. This process contributes to vaginal lubrication during sexual activity.
• Autonomic nerves release vasoactive mediators, including vasoactive intestinal peptide, which facilitate vasodilation and lubrication.
• The greater vestibular glands also increase mucus secretion and further lubricate the vaginal opening.
• Adequate lubrication reduces friction and facilitates comfortable sexual intercourse

Vaginal Changes

• Vaginal secretion increases significantly during sexual excitement.
• Vaginal lubrication results from both plasma transudation and glandular secretions.
• Tactile stimulation of the clitoris, labia, breasts, and surrounding genital structures enhances these responses.
• Psychological arousal can also initiate vaginal lubrication even in the absence of direct physical stimulation.
• Sexual arousal is associated with increased pelvic blood flow and heightened sensitivity of genital tissues.

Orgasm

• Orgasm is the peak phase of the sexual response cycle and is characterized by intense sexual pleasure and physiological arousal.
• In females, orgasm is commonly associated with rhythmic contractions of the pelvic floor muscles and the lower vagina.
• Although these contractions may facilitate sperm transport, orgasm is not essential for fertilization or conception.
• Orgasm is accompanied by marked activation of the autonomic nervous system.
• Heart rate may increase substantially, sometimes reaching 150 beats per minute.
• Arterial blood pressure and respiratory rate increase transiently.
• Facial flushing and sweating may occur because of increased sympathetic activity.
• Skeletal muscle tension and involuntary muscular contractions are commonly observed.
• These responses are mediated by complex neural and neuroendocrine mechanisms involving sympathetic activation and catecholamine release.
• Transient hormonal changes involving the pituitary and other endocrine glands may also occur during orgasm.

Fate of Sperms in Female Genital Tract

• During ejaculation, approximately 200–300 million spermatozoa are deposited in the female reproductive tract.
• Only a small proportion of sperm successfully traverse the cervix, uterus, and uterine tubes.
• Sperm transport is aided by their own motility, uterine contractions, and ciliary activity within the female reproductive tract.
• The cervical mucus around ovulation provides a favorable environment for sperm survival and migration.
• Viable sperm may remain within the female genital tract for up to 3–5 days under optimal conditions.
• Although millions of sperm are ejaculated, only a few hundred usually reach the site of fertilization in the ampulla of the uterine tube.
• Before fertilization, sperm undergo capacitation, a series of functional changes that occur within the female reproductive tract.
• Capacitation enables sperm to bind to and penetrate the ovum. This is followed by the acrosomal reaction, during which enzymes released from the acrosome help the sperm penetrate the corona radiata and zona pellucida.
• Normally, only one sperm fertilizes the ovum.
• After fertilization, mechanisms are activated that prevent entry of additional sperm.
• The vast majority of spermatozoa degenerate and are removed by phagocytic cells within the female reproductive tract.