31.10
男性の性的反応: 勃起と射精
性的刺激は、身体的な接触や視覚または聴覚による刺激など、さまざまな形で現れます。これが起こると、脊髄の仙骨部分の副交感神経反射が活性化されます。この反射は一酸化窒素 (NO) の放出を刺激し、陰茎の細動脈を拡張して、勃起組織 (陰核海綿体と尿道海綿体) への血流を増加させます。
勃起組織を満たす血液…
性的刺激に反応して、副交感神経ニューロンは一酸化窒素またはNOの局所放出を刺激し、陰茎の細動脈を拡張します。これにより、勃起組織(海綿体と海綿体)への血流が増加します。
勃起組織に充填された血液は、血液が陰茎から出るのを防ぐために静脈を圧迫します。
その結果、ペニスが充血して硬くなり、勃起につながります。
性的興奮の臨界点では、腰部の交感神経線維によって調整された脊髄反射が射精を引き起こします。
この反射は、精巣上体、輸精管、精嚢、および前立腺の協調収縮を引き起こし、精液を前立腺尿道に押し込みます。
精液が陰茎に入った後、骨盤底の球海綿筋や坐骨海綿筋などの筋肉が収縮します。
これらの管、腺、筋肉の収縮は、射精と呼ばれるプロセスで尿道から外部に精液を強制的に排出します。
View the full transcript and gain access to JoVE Core videos
Q1: How does nitric oxide cause an erection?
Sexual stimulation activates parasympathetic neurons that release nitric oxide in the penis. Nitric oxide dilates the arterioles, increasing blood flow into the erectile tissues—the corpora cavernosa and corpus spongiosum. The blood filling these tissues compresses the veins, preventing blood from leaving and causing the penis to become engorged and rigid.
Q2: What role do the accessory glands play in ejaculation?
During ejaculation, a sympathetic reflex triggers coordinated contractions of the accessory glands of the male reproductive system, including the seminal vesicles and prostate gland. These contractions propel semen into the prostatic urethra in a process called emission, which precedes the forceful expulsion of semen from the body.
Q3: What is the difference between emission and ejaculation?
Emission occurs when sympathetic fibers in the lumbar spine trigger contractions of the epididymis, vasa deferens, seminal vesicles, and prostate, propelling semen into the prostatic urethra. Ejaculation follows when pelvic floor muscles like the bulbospongiosus and ischiocavernosus contract, forcefully expelling semen from the urethra to the exterior.
Q4: Which spinal nerves control the male sexual response?
Parasympathetic fibers in the sacral portion of the spinal cord initiate erection by stimulating nitric oxide release. Sympathetic fibers located in the lumbar region, typically at T12-L2, coordinate the reflex that triggers ejaculation through contractions of reproductive ducts and glands.
Q5: How do pelvic floor muscles contribute to ejaculation?
During ejaculation, pelvic floor muscles including the bulbospongiosus and ischiocavernosus contract forcefully around the urethra. These muscular contractions expel semen from the penis to the exterior, completing the ejaculatory process after semen has entered from the accessory ducts of the male reproductive system.
Q6: What prevents blood from leaving the penis during an erection?
As blood fills the erectile tissues—the corpora cavernosa and corpus spongiosum—it exerts pressure that compresses the veins in the penis. This compression prevents venous outflow, trapping blood within the erectile tissues and maintaining the rigid, engorged state characteristic of an erection.
Q7: What types of sexual stimuli trigger the parasympathetic reflex?
Sexual stimulation can take various forms, including physical touch and visual or auditory cues. Any of these stimuli activate the parasympathetic reflex in the sacral portion of the spinal cord, which initiates nitric oxide release and the cascade of physiological events leading to erection.