Expressed in its simplest terms, in the body, certain smooth muscle cells (those muscles we cannot control, as opposed to skeletal muscles) respond to chemical signals released by different nerves. I have included them in a note on the next page, but I had to reach back to my chemistry doctoral studies to write that section and you will quickly see why I expect most readers to skip those paragraphs. The exact nerve-chemical processes are very complex. Blood flow into the penis is controlled by three neurotransmitter systems: adrenergic nerve fibers; cholinergic nerve fibers; and nonadrenergic-noncholoinergic fibers that release nitric oxide (NO). As noted above, during an erection the penis is transformed from a venous to an arterial organ. Sensory receptors on the glans penis and the penile skin lead to sensory nerves that converge to form the primary dorsal nerve of the penis; this becomes the pudendal nerve which courses up to the sacral segments S2, 3,4. Somatic nerves control sensory and motor functions of the body.

As we shall see, complex chemical interactions are involved in this process, which is where Cialis comes into play. In other words, parasympathetic autonomic nerves get it up; sympathetic autonomic nerves let it down and keep it down . This thoracolumbar sympathetic pathway controls detumescence and orgasm. The sympathetic nerves meanwhile originate from the eleventh and twelfth thoracic levels of the spinal segments, as well as the first and second lumbar spinal segments. The sacral parasympathetic input initiates erections. The parasympathetic nerve fibers originate from the sacral spinal cord, at levels 2, 3 and 4 (S2-4).

There are two types: parasympathetic and sympathetic. Autonomic nerves are not controlled by the individual and are "automatic" in their timing and function. The brain then controls the penis through two kinds of nerves, autonomic and somatic. Sensory factors such as audiovisual stimulation also have input through these same brain centers. Recent research has identified specific spots in the brain as the integration centers for sexual drive and sexual arousal,' perhaps triggered by psychological factors such as erotic fantasies or expectations. An erection (tumescence) is a neurovascular event, meaning that both the nervous and the circulatory systems are involved. The flow of blood in is strong, the flow of blood out is weak, and the result is an erection.