Ejaculation is a biological process involving the expulsion of semen from the male reproductive tract. This reflex action plays a fundamental role in natural reproduction, facilitating the delivery of sperm for potential fertilization. It represents the final stage of male sexual stimulation.
The Ejaculatory Process
Ejaculation unfolds in two distinct phases, known as emission and expulsion. The initial emission phase involves the movement of sperm and seminal fluids into the prostatic urethra. During this stage, sperm travel from the epididymis and testes to the prostate, where they combine with secretions from various glands to form semen.
Muscular contractions around structures like the epididymis and vas deferens propel sperm towards the urethra. Fluids from the seminal vesicles and prostate gland are also added, mixing to create the complete seminal fluid. This mixture then collects at the base of the urethra, preparing for the next stage.
The subsequent expulsion phase involves the forceful ejection of semen from the body through the urethra. This occurs due to rhythmic, involuntary contractions of pelvic floor muscles, such as the bulbocavernosus muscle. These contractions happen at regular intervals, around every 0.8 seconds.
During expulsion, the bladder neck contracts to prevent semen from flowing backward into the bladder, while the external urethral sphincter relaxes. Once this phase begins, it becomes an involuntary reflex that cannot be voluntarily stopped.
Composition of Semen
Semen is a complex fluid, comprising more than just sperm, with its various components originating from different glands within the male reproductive system. Spermatozoa, the reproductive cells, constitute a small percentage, 1% to 5% of the total semen volume. These sperm cells are produced in the testes and mature within the epididymis.
A significant portion of semen, approximately 65% to 75%, comes from the seminal vesicles. This fluid is rich in fructose, which serves as the primary energy source for the sperm cells. The seminal vesicle fluid also contains amino acids, citrate, and prostaglandins.
The prostate gland contributes about 20% to 30% of the semen volume. Its secretions include proteolytic enzymes, such as prostate-specific antigen (PSA), which help the semen liquefy after ejaculation. Prostate fluid also provides citric acid, cholesterol, and zinc, with zinc helping to stabilize the sperm’s DNA. A small amount of fluid from the bulbourethral glands, less than 1%, provides lubrication for the urethra.
Neurological Control of Ejaculation
The process of ejaculation is regulated by the autonomic nervous system, functioning primarily as an involuntary reflex. Both the sympathetic and parasympathetic nervous systems play roles in coordinating this event. The sympathetic nervous system, originating from the thoracic and lumbar spinal cord segments, is responsible for triggering the emission phase. It causes the smooth muscle contractions that move sperm and glandular fluids into the urethra.
A specialized group of nerve cells in the lumbar spinal cord, known as spinal ejaculation generators, coordinate these actions. These cells receive sensory input from sexual stimulation and integrate signals to orchestrate the expulsion phase. The rhythmic muscle contractions during expulsion are controlled by a spinal reflex involving the pudendal nerve.