Penile erection is generally a process in which the male penis is stimulated, the blood vessels are dilated, and blood rapidly fills the cavernous body of the penis, ultimately resulting in the penis propping up, hardening and lengthening. Its erection is generally categorized into three kinds: 1. Reflex erection: reflex erection caused by rapid blood filling of the corpus cavernosum when the male external genitals and surrounding tissues are directly stimulated, or when receiving stimulation from the internal peripheral organs, such as the rectum or the bladder, etc. It is achieved by stimulating the low-level erection centers of the sacral medulla oblongata in the spinal cord followed by the provocation of sexual arousal. 2. Psychogenic erection: When the male brain is stimulated by sexual stimuli, such as vision, touch, smell, and sexual fantasy, it can cause cortical excitation and thus penile erection, which is synergistic with reflexive erection. 3. Nocturnal erections: Normal men experience an average of three or more erections per night during rapid eye movement sleep, the reason for which is not clear. When urinary urgency, the bladder fills with urine, which also stimulates the urethra and penis, resulting in a reflex erection, which is normal.