An erection is not controlled by one system alone but by a coordinated effort involving the autonomic nervous system (ANS). This system, which manages the body’s involuntary actions, has two main divisions with often opposing effects. Understanding the distinct role each branch plays is necessary to appreciate how an erection is achieved, maintained, and subsides.
The Autonomic Nervous System’s Two Branches
The autonomic nervous system functions automatically, regulating processes like heart rate and digestion without conscious thought. It is divided into two principal branches: the parasympathetic nervous system (PNS) and the sympathetic nervous system (SNS). These divisions work in balance to help the body respond to its environment.
The parasympathetic nervous system is described as the “rest and digest” system. It conserves energy by slowing the heart rate and increasing intestinal and gland activity. When the body is in a calm state, the PNS is dominant, managing baseline functions and restoring the body after exertion.
In contrast, the sympathetic nervous system is responsible for the “fight or flight” response. When the body perceives stress, the SNS prepares it for intense physical activity. It increases heart rate, elevates blood pressure, and redirects blood flow toward the muscles.
The Parasympathetic Role in Achieving an Erection
An erection is initiated and sustained by the parasympathetic nervous system. During sexual arousal, the brain sends signals down the spinal cord to nerves in the pelvic region. These “rest and digest” signals must be dominant, which is why relaxation is conducive to the process.
These parasympathetic signals trigger the release of nitric oxide (NO) in the penis. Nitric oxide is a vasodilator that relaxes the smooth muscles lining the arteries. This relaxation widens the arteries, causing a substantial increase in blood flow into the two erectile chambers, the corpora cavernosa.
As these chambers fill with blood, they expand and become rigid. This expansion also compresses the veins that drain blood from the penis. This process, known as veno-occlusion, traps blood within the corpora cavernosa and sustains the erection’s firmness.
The Sympathetic Role in Ejaculation and Detumescence
While the parasympathetic system initiates the erection, the sympathetic nervous system completes the sexual response. As stimulation peaks, control shifts to sympathetic dominance, which is necessary for emission and ejaculation. This system stimulates the muscular contractions that propel semen into the urethra.
Following ejaculation or if arousal ceases, the sympathetic nervous system returns the penis to a flaccid state, a process called detumescence. It releases neurotransmitters, like norepinephrine, which cause the penile arteries to contract. This vasoconstriction reduces blood flow, allowing the trapped blood to be released.
This system’s role also explains why stress or anxiety can prevent an erection. When the sympathetic “fight or flight” system is highly active, it can override the parasympathetic signals needed for vasodilation. This makes it physiologically difficult to achieve the relaxed state required for an erection.
The Coordinated Balance of Both Systems
Erectile function is a clear example of the interplay between the two branches of the autonomic nervous system. It is not a matter of one system being solely responsible, but a sequential and balanced handover of control. These two systems must work together to execute this complex physiological event.
A simple way to remember their distinct roles is the mnemonic “Point and Shoot.” The “Point” phase, achieving the erection, is governed by the Parasympathetic nervous system. The “Shoot” phase, involving ejaculation, is controlled by the Sympathetic nervous system.
This balance is sensitive to disruption, and many forms of erectile dysfunction are rooted in an imbalance between these systems. Neurological damage, for example, can interfere with the signaling pathways. A smooth transition between the two is the basis of normal sexual function.