The nervous system is divided into three components: the central nervous system, which encompasses the brain, brain stem, and the spinal cord, the peripheral nervous system, which includes the sensory receptors and effector muscles and organs in the body, and the autonomic nervous system which is part of both the peripheral and central nervous system and controls visceral and largely unconscious functions (Barker & Barasi, 2005). The sexual response activates all these systems.
(a) The central nervous system, (b) The peripheral nervous system
(a) The autonomic nervous system controls the smooth muscles and secretions in all the male reproductive organs (Dail, 1993). This includes the parasympathetic and sympathetic systems discussed later. It is part of the peripheral nervous system but also considered by many to also be part of the cerntral nervous system. For the purposes of this essay, it will be included in the section with the peripheral nervous system.
This essay will explore the time course of penile arousal and erection, beginning at the supraspinal level, that is, the brain component of the central nervous system, and following it down to the post- ganglionic mechanisms of erection in the penis via the actions of the peripheral and autonomic nervous systems. It should be noted that this is not the only route that erection can occur through: reflex erections can occur from penile stimulation through a one-step afferent pathway that bypasses the autonomic and most cerebral systems, but this will not be discussed presently. However, most erections are a combination of these two pathways (Steer, 2000), so a later consideration of this second pathway may become prudent.
Central nervous system
The normal psychogenic pathway from the brain to the penis is poorly understood, but appear to involve mainly the hypothalamic regions, which integrates input from various limbic and cortical regions. Several hypothalamic areas have been shown to modulate erections in rats, including medial preoptic (MPOA) regions (MacLean, 1996), while Barker and Barasi (2005)’s findings suggest the ventromedial area is the main control centre for the sympathetic nervous system and the lateral hypothalamic area being the main centre for parasympathetic activation. Steer (2000) reviews lesion, electrostimulation, and tracing data and identifies two pathways from the hypothalamus to sacral autonomic centres. The first goes from the dorsomedial hypothalamus through dorsal and central grey matter, travels to the locus coeruleus, and then projects to the mesencephalic reticular formation, where descending efferent input is sent through the dorsal columns of the spine down to the thoracolumbar and sacral autonomic nuclei. The other pathway arises from the dorsomedial and ventromedial hypothalamus, travels through the ventral and central tegmental regions, the pontine reticular formation, and the substantia nigra, and finally through the ventrolateral area of the pons and...