Erection mecanisms

Erection : how it works

An understanding of the basic physiology of erection (that is to say how an erection works) will allow patients to understand not only the causes of erectile dysfunction (ED) but will also lay the foundation for the understanding of erectile dysfunction treatments.

Male Anatomy

penis anatomy1. The penis is composed of three cylinders: two corpora cavernosum and one corpora spongiosum that surrounds the urethra.

2. The corpora spongiosum regulates the diameter of the urethra to pass urine or semen.

3. The corpora cavernosum are sponges that contract themselves to prevent blood from entering the penis is flaccid while, or relax to let the arterial blood, the penis then swells (tumescence) then stiffens.

4. Corpora cavernosum are made ofmuscle fibers and connective tissue that form the frame and vascular reservoirs.

5. These three cylindrical bodies are surrounded by a fibroelastic named “tunica albuginea”.


Erection and loss of erection are related primarily to blood flow events regulated by the relaxation and contraction, respectively, of the smooth muscle in the penile arteries and the erectile bodies themselves. Erection is a hydraulic event, regulated by hormones and nerves, which allow increased blood flow into and storage of blood within the erectile bodies leading to an increased pressure and the development of rigidity (hardness). Penile erection is triggered by one of two main mechanisms: direct stimulation of the genitalia or through stimuli coming from the brain (fantasy, smell, etc).

Sexual stimulation

  • Upon stimulation, chemicals are released in the brain that cause signals to pass down the spinal cord and outward through special nerves (erectile nerves in the neuro-vascular bandle) into the penis. These nerves release another chemical (Nitric Oxide) that causes the aforementioned smooth muscle to relax and blood rushes into the erectile bodies, causing erection.
  • Anxiety or fear can prevent the brain signals from reaching the level required to induce erection. Medical conditions can block the erection arteries or cause scarring of the spongy erection tissue and prevent proper blood flow or trapping of blood and, therefore, limit the erection. Thus, the erection mechanism is much like a tire; a firm tire is dependent upon a hose that can deliver air in adequate amounts in a speedy fashion and a valve mechanism that holds the air in place. In the penis the hose is represented by the erection arteries, which rapidly carry blood into the erectile bodies and the valve mechanism, while complicated in its structure, ensures that the blood is trapped inside the erectile bodies until ejaculation occurs or the sexual stimulus has passed.

Response to sexual stimulation

  • An erection occurs when the corpora cavernosum, that run the length of the penis, become engorged with venous blood.
  • This may result from any of various physiological stimuli, also known as sexual stimulation and sexual arousal.
  • The corpus spongiosum is a single tubular structure located just below the corpora cavernosa, which contains the urethra, through which urine and semen pass during urination and ejaculation respectively. This may also become slightly engorged with blood, but less so than the corpora cavernosa.

Autonomic control

  • In the presence of mechanical stimulation, erection is initiated by the parasympathetic division of the autonomic nervous system (ANS) with minimal input from the central nervous system.
  • Upon stimulation, some nerves branches (derived from sacral plexus) release chemical substances, which, in turn causes release of nitric oxide from endothelial cells in the trabecular arteries.
  • Nitric oxide diffuses to the smooth muscle of the arteries, acting as a vasodilating agent.
  • The arteries dilate, filling the corpora cavernosum and in a certain way the corpora spongiosum with blood. Some muscles at the base of the penis also compress the veins of the corpora cavernosa, limiting the venous drainage of blood. Erection subsides when stimulation is discontinued;
  • Baseline stimulation from the sympathetic division of the ANS causes constriction of the penile arteries, forcing blood out of the erectile tissue.
  • After ejaculation or cessation of stimulation, erection usually subsides, but the time taken may vary depending on the length and thickness of the penis.

Voluntary and involuntary control

The cerebral cortex can initiate erection in the absence of direct mechanical stimulation (in response to visual, auditory, olfactory, imagined, or tactile stimuli) acting through erectile centers in the lumbar and sacral regions of the spinal cord. The cortex may suppress erection, even in the presence of mechanical stimulation, as may other psychological, emotional, and environmental factors.

Nocturnal erection

The penis erects during sleep or on waking up. Such an erection is medically known as nocturnal penile tumescence (informally: morning wood or morning glory). This type of erections are conserved in most of psychological erectile dysfunction. The erection is a reflex phenomenon responding to a stimulus or spontaneously (nocturnal erection).

Different kind of stimulation

Stimulations that will allow the erection can be:

  1. psychological: called desire
  2. sensory: (watch a seductive body, listen to a voice, smell perfume, caress the body of his partner …)
  3. physical: touching, caressing, stimulating the penis or other erogenous zones naturally leads to an erection.