ABSTRACT
The onset and maintenance of penile tumescence is initiated by central psychogenic or peripheral reflexogenic sensory stimuli (or both), as described in greater detail in the following chapters. Both central and peripheral pathways stimulate sacral parasympathetic efferent nerve fibers, which ultimately cause the relaxation of the vascular smooth muscle in the resistance arterial bed and the trabeculae of the corpora cavernosa. These events result in a higher rate of blood flow into the penis, and greatly increase the compliance of the cavernosal bodies, enabling expansion and accommodation of the increased blood volume within the cavernosal lacunae. The increased blood flow also potentiates local vasodilation through endothelial shear stress-induced responses. During this early filling phase, the hemodynamic environment within the erectile tissues transitions into an arterial system with respect to blood flow and oxygen tension. However, blood pressure remains low.
