How do erection start?

HOW is an erection formed?
There are at least TWO possible ways to interpret the question. Is the question:
(#1)What physiology makes a male get an erection OR it could be

#1: During and after puberty, when the male brain is sexually stimulated or if the bladder is full overnight, an erection will develop. The brain with help from the Prostate stimulates (Vasil stimulation) tiny blood vessels & valves in the penis and it fills with blood increasing the volume and causing the penis to become erect the same way a floppy balloon becomes rigid/erect when filled with air. For the Erectile Dysfunction set, this Vasil stimulation from the brain doesn't work so well anymore & they take a drug that does it so they can get and keep an erection.

If an erection lasts more than a few (4) hours or becomes painfully erect that may be a condition called a Priapism, which is basically blood trapped in the penis with no "off switch". This can cause damage to the penis & damage to the circulatory system & should be treated.