Men's private parts actually have three "erection forms"

When a man's cerebral cortex is excited by hearing, vision, smell, thinking, imagination, etc. related to sexual content, and the stimulation is transmitted through the thoracic and lumbar erection center of the spinal cord and acts on the corpus cavernosum of the penis, it will cause the arteries to dilate and a large amount of blood to flow into the corpus cavernosum of the penis; when the veins contract and the blood flow out of the corpus cavernosum decreases, the blood will be fully retained in the rich blood vessels and sinusoids in the corpus cavernosum, causing the penis to quickly produce a psychological erection. On the contrary, when the arteries contract and the blood flowing into the corpus cavernosum of the penis decreases, and the veins dilate and the abundant blood retained in the corpus cavernosum quickly flows back, the penis becomes flaccid again.

When the external genitalia is subjected to local stimulation such as direct touch, friction from walking, or internal stimulation of the rectum and bladder, it can cause a reflex erection. This is achieved by stimulating the low-level erection center in the sacral cord of the spinal cord and arousing its sexual excitement. Penile erection is a natural instinct, and male fetuses can have an erection in the womb. After a little boy is born, he may also have an erection when he urinates. Sometimes when he lies down to pee, the urine can spray straight onto an adult's face like a spring, often bringing unexpected laughter and joy to the parents who are changing diapers. However, the erection at this time does not have any "sexual" meaning. Any stimulation of the genitals themselves can cause a reflex erection, which is regulated by the pudendal nerve and sacral erection center.

These two kinds of psychological and physiological stimuli can act independently, or they can influence each other and be involved in sexual activities together. For example, when spinal cord injury occurs above the thoracic and lumbar region, the patient can only have an erection when the genitals are physically stimulated, and the ability to have a psychological erection disappears. When motor neuron damage occurs below the thoracolumbar region, stimulation of the genitals cannot cause an erection, but 24% to 57% of patients still experience psychological erections, that is, psychological stimulation can be transmitted through the thoracolumbar pathway. When spinal cord injury occurs at L2 to S1, both psychological and reflex erections exist, but the two cannot be coordinated, and ejaculation climax cannot be achieved through genital stimulation.

In addition to psychological erections and reflex erections, normal men also have a nocturnal erection. Men's sleep is always in the alternation of rapid eye movement sleep and slow eye movement sleep, and they also experience the physiological process of erection-flaccidity-erection-flaccidity. This is medically called nocturnal penile erection, which is a normal physiological process in healthy men.

Generally speaking, men will have 4 to 6 erections every night, each lasting 20 to 40 minutes, and a total erection of up to two and a half hours. This is because the brain always suppresses sexual responses during the day, otherwise men will inevitably encounter embarrassing situations from time to time. However, after falling asleep, the brain's inhibitory function disappears and the penis will spontaneously have an erection. This is completely normal and there is no need to worry.

The duration, frequency, and hardness of nocturnal erections decrease with age, and are strongest from adolescence to around 20 years old. Whether it is a psychological or reflex erection, the penis will quickly return to normal after the external stimulation is removed. Nocturnal erections are no exception. The penis will automatically become flaccid after being erect for a period of time, because if the blood is allowed to remain in the corpus cavernosum for a long time, the blood will be deprived of oxygen and a large amount of metabolic waste will accumulate, which is harmful to the tissues and may even cause necrosis in severe cases. There is a disease called abnormal penile erection, in which the patient's penis becomes continuously swollen, hard, painful, and bluish-gray in color for more than 4 to 6 hours. This is a urological emergency and requires emergency treatment in the hospital as soon as possible. Therefore, the penis does not remain erect all night long, but must have periods of relaxation and contraction. Morning erection is a clinically meaningful physiological phenomenon.

Some doctors have studied this phenomenon specifically and found that when a man is ill, his morning erections will weaken or disappear; and after his body recovers, his morning erections will also resume. Therefore, morning erections can be used as a reference indicator of men's health status. The presence or absence of morning erection can also be used as a reference indicator to judge the sexual function status of men.