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Welcome back to Just Answer. This is actually several questions.
1. do any nerves/veins run though the the corpus spongiosum? The bulbocavernous artery and veins and their smaller branches run through the corpus spongiosum. There are also small perineal nerve branches from the pudendal nerve located at the posterior margin of the urogenital diaphragm
2. does the corpus spongiosum end at the prostate? No, it doesn't. The spongiosum ends at the penis bulb which is connected to the urogenital diaphragm. The urogenital diaphragm forms the floor of the pelvic cavity. This is below the level of the prostate.
3. is the corpus spongiosum surrounded by the external sphincter musle? No it isn't. The external sphincter muscle is only at the tip of the penis (the urinary meatus). The bulbospongiosus muscle invests both the bulb of the urethra and distal corpus spongiosum. The muscle is supplied by a deep branch of the perineal nerve and helps to empty the final few drops of urine and to ejaculate semen.
4/5. do these nerves/veins enlarge accidently from urethral play and cause nocturnal emissions due to the sphincter muscle? Nerves do not enlarge. The blood vessels enlarge as the penis erects from pleasurable sensations or the anticipation of pleasurable sensations.
This has nothing to do with noctural emissions which are particularly common in the teens and occur to some men throughout their lives. Noctural emissions are involuntary ejaculations that occur when the man is asleep. No one knows exactly what causes them, but it does not appear to be related to frequency of masturbation (or other sex play).
Nocturnal emissions are not urine, but semen, so a full ladder isn't going to enter into the picture other than perhaps pressure on the prostate. The bulb of the penis is within the body where you cannot see it, and the "top of the corpus spongiosum" also cannot be seen because it is within the penis itself. Do you mean the glans penis which has the urethral meatus (opening) at the end of it? Any vein you could see on the outside of the corpus spongiosum would lie on the underside of the penis.
If you think you may have damaged your urethra or its sphincter, you would need to see a urologist.
I am not unsure of this and have worked in urology and taught anatomy. The corpus spongiosum is not on the top of the penis; it is on the bottom side and the urethra runs through it. The penis is composed of three types of erectile tissue. The two sections of corpus cavernosum are on top and the corpus spongiosum containing the urethra fits into a groove underneath and between the corpora cavernosa.
The vein that runs along the top of the penis is called the superficial dorsal vein. It provides blood to the skin and outside of the penis. The deep dorsal vein runs beneath it. Below is a cross-sectional diagram looking at the penis from its head (glans) towards the body. As you can see, urethral play would have nothing to do with an enlargement of the superficial or deep dorsal veins.