Penile Anatomy and Erectile Physiology

The penis is the male organ of copulation and the conduit for the urethra. It is built around three columns of erectile tissue, and erection is a neurovascular event driven largely by nitric-oxide-mediated relaxation of cavernosal smooth muscle. This topic covers penile anatomy and the physiology of erection, emission, and ejaculation.

Definition

Penile erection is a neurovascular event in which neural signals trigger nitric-oxide-mediated relaxation of arterial and trabecular smooth muscle in the corpora cavernosa, increasing arterial inflow and trapping blood against the tunica albuginea (the veno-occlusive mechanism) so that the penis becomes rigid.

Scope

The entry covers the gross and microscopic anatomy of the penis (paired corpora cavernosa, corpus spongiosum, tunica albuginea, glans, and the penile urethra), its blood supply and innervation, and the neurovascular and biochemical mechanisms of erection, detumescence, emission, and ejaculation. It is a reference description of normal anatomy and physiology and does not address erectile or other penile disorders or their treatment.

Core questions

• How is the penis structurally organised, and what is erectile tissue?
• What blood supply and innervation does the penis receive?
• What neural and biochemical events produce an erection?
• How do the veno-occlusive mechanism and detumescence work?
• How are emission and ejaculation coordinated?

Key concepts

• Corpora cavernosa and corpus spongiosum
• Tunica albuginea
• Cavernosal smooth muscle
• Nitric oxide and cyclic GMP signalling
• Veno-occlusive mechanism
• Autonomic and somatic penile innervation
• Emission, ejaculation, and detumescence

Mechanisms

The penis contains two dorsal corpora cavernosa and a ventral corpus spongiosum that surrounds the urethra and expands distally to form the glans, all enclosed by the dense tunica albuginea. With sexual stimulation, parasympathetic and nonadrenergic-noncholinergic nerves, together with endothelium, release nitric oxide, which raises cyclic GMP in cavernosal smooth muscle and causes relaxation. Arterial inflow increases and the expanding sinusoids compress the subtunical venules against the tunica albuginea, reducing venous outflow (the veno-occlusive or corporo-veno-occlusive mechanism) so that the corpora become engorged and rigid. Detumescence follows when sympathetic tone and phosphodiesterase activity restore smooth-muscle contraction. Emission delivers sperm and accessory-gland secretions into the posterior urethra under sympathetic control, and rhythmic contraction of the bulbospongiosus muscle produces ejaculation.

Clinical relevance

Penile anatomy and the neurovascular physiology of erection underlie the clinical understanding of erectile and ejaculatory function and the anatomy of pelvic and penile surgery. This entry describes normal structure and function for educational orientation and is not a basis for diagnosing or treating erectile dysfunction or other penile disorders.

Evidence & guidelines

The physiology described follows established reviews of penile erection, including the role of nitric oxide as a mediator, alongside standard anatomical references. As a normal-physiology topic it is not governed by disease-specific guidelines.

History

The neurovascular model of erection was consolidated in the late twentieth century, when the central role of nitric oxide and cyclic GMP in cavernosal smooth-muscle relaxation was established and integrated with the veno-occlusive mechanism, as summarised in reviews by Andersson and Wagner and by Dean and Lue.

Key figures

• Karl-Erik Andersson
• Tom F. Lue
• Antonio Argiolas

Seminal works

• andersson-1995
• argiolas-1994
• dean-2005

Frequently asked questions

What causes a penile erection?

Neural and endothelial release of nitric oxide raises cyclic GMP in cavernosal smooth muscle, relaxing it; arterial inflow rises and the engorged sinusoids compress venules against the tunica albuginea, trapping blood and producing rigidity.

What is the veno-occlusive mechanism?

As the cavernosal sinusoids fill and expand, they compress the small subtunical veins against the tunica albuginea, reducing venous outflow so that blood is retained and the erection is maintained.