Ejaculation is the process of semen expulsion from the urethra in men. Orgasm is the sensation of pleasure, relaxation, and/or satisfaction that accompanies sexual climax in men and women. It is believed that these pleasurable sensations are derived from central nervous system processing of afferent information from the pudendal nerve.1 It is important to state that orgasm and ejaculation are distinct physiological processes; although it is typical for both to occur simultaneously in men this is not a physiological necessity.2
The processes of ejaculation is under control of the nervous system. Ejaculation typically follows a period of sexual stimulation, which may be both tactile from stimulation of the penis and/or other body regions and psychogenic from emotional, mental, and sensory stimulation that accompany sexual activity. The precise triggers for orgasm are incompletely understood but appear to vary widely from person to person. There is great heterogeneity in the amount and type of stimulation required to stimulate orgasmic/ejaculatory response; this is thought to be related to biological, situational, intrapersonal, genetic, neurochemical, and emotional factors.1,3,4
It is important to appreciate that much of what is known about the neuronal and hormonal regulation of ejaculation has been gleaned from animal studies;5,6 studies on the neurophysiology of orgasm/ejaculation in humans are difficult for logistical and ethical reasons.
Ejaculation is divided into two distinct events:7
- Emission: During emission sperm from the vas deferens are deposited in the posterior urethra. Simultaneously there is deposition of seminal fluid from the prostate and seminal vesicles. The bladder neck closes tightly to prevent retrograde flow of semen during the next phase of ejaculation. Most events in seminal emission are under the control of the sympathetic nervous system T10-L2 nerve roots extending to the pelvic plexus and from there to the hypogatric nerves.1 Hence, they are not under voluntary control and may not produce distinct somatic sensations although most men will experience a sensation of "ejaculatory inevitability" during this phase.
- Ejection: During ejection forceful contractions of the bulbospongiosus and ischiocavernous muscles in coordination with relaxation of the external urethral sphincter leads to expulsion of semen in an antegrade fashion from the urethra.1,8 Tight coaptation of the bladder neck is essential in preventing retrograde ejaculation. This phase of ejaculation (other than bladder neck contraction) is mediated by the somatic nervous system from S2-4 nerve roots and is hence associated with distinct somatic sensation and (in most men) orgasm.
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