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Autonomic Nervous System Influences: The Role of the Sympathetic Nervous System in Female Sexual Arousal
- Indiana University Press
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Autonomic Nervous System In®uences The Role of the Sympathetic Nervous System in Female Sexual Arousal cindy m. meston and andrea bradford Anatomy of the Sympathetic Nervous System in the Female Genitalia The autonomic nervous system provides most of the innervation to the internal genital organs and is essential to the sexual response. It has generally been presumed that parasympathetic activity is responsible for achieving sexual arousal through localized vasocongestion, resulting in genital swelling and lubrication, while orgasm is mediated through a sympathetic response. However, the interaction of these two systems is complex and remains poorly understood. Innervation of the genitalia in human females runs primarily through a common network of converging autonomic and sensory ¤bers known as the pelvic plexus. In some nonhuman mammals, it has been observed that sympathetic and parasympathetic ¤bers may dually innervate a few postsynaptic neurons in the pelvic plexus. In addition, sympathetic and parasympathetic neurons in the pelvic plexus may sometimes communicate laterally (Dail, 1996). However, the degree to which different nerve types in the plexus might interact with one another remains speculative. The signi¤cance of such interactions is likewise unknown . Anatomical studies have indicated that the sympathetic nervous system ’s (SNS) contribution to the pelvic plexus originates from multiple sources. The superior hypogastric plexus gives rise to two sympathetic nerves that run bilaterally into the left and right pelvic plexuses (Donker, 1986; Maas, DeRuiter, Kenter, & Trimbos, 1999). Other inputs stem directly from sympathetic chain ganglia along the thoracolumbar spinal cord (Donker). In addition to these routes, genital tissues may receive innervation from so-called “short” adrenergic ¤bers that arise from localized ganglia . Ownman, Rosengren, and Sjoberg (1967) found that such ganglia were particularly abundant in the human vagina. Interestingly, estrogen and other sex steroids may signi¤cantly in®uence sympathetic innervation in the pelvic organs (e.g., Zoubina & Smith, 2001). Consistent with histochemical studies of human genital tissues (e.g., Ownman, Rosengren, & Sjoberg, 1967), it is generally accepted that norepinephrine is the dominant neurotransmitter of the SNS. Adrenergic nerve 66 ¤bers from the pelvic plexus target both vascular and nonvascular smooth muscle in most if not all female genital organs. The study of adrenoceptors in the genital tract is therefore extremely important to the understanding of the role of the SNS in physiological sexual arousal. However, this approach is not comprehensive. Studies of adrenoceptors alone cannot address the effects of nonadrenergic-noncholinergic (NANC) neurotransmitters , such as neuropeptide Y and galanin, which are often colocalized within sympathetic nerve ¤bers. The functional signi¤cance of these neuropeptides is only beginning to be understood (for review, see Argiolas, 1999). Attempts have been made to characterize the distribution of adrenoceptors in genital tissue. Anatomical studies have suggested that mammalian vaginal, cervical, uterine, and clitoral tissues contain both alpha1 and alpha2 adrenoceptors. Beta adrenoceptors are also present in some genital tissues, particularly the uterus, although they have received considerably less attention in studies of sexual function. In humans, both alpha1 and alpha2 adrenoceptors appear to regulate smooth muscle tone in vaginal and clitoral tissue (Min et al., 2001; Traish, Moreland, Huang, Kim, & Goldstein , 2000). Traditionally, it has been thought that alpha1 adrenoceptors are located postsynaptically and regulate smooth muscle contractility, while alpha2 adrenoceptors are located presynaptically and serve an autoregulatory function to inhibit release of norepinephrine and other neurotransmitters (e.g., Iversen, Iversen, & Saper, 2000). However, it is known that the alpha2 receptor subtype is found both pre- and postsynaptically. A recent study indicates that activation of postsynaptic alpha2 adrenoceptors in male corpus cavernosum induces smooth muscle contraction (Gupta et al., 1998). Therefore, alpha2 adrenoceptors appear to serve opposite ends, depending on their location within the synapse. This somewhat paradoxical conclusion must be approached with caution. Understanding how adrenergic mechanisms in®uence female sexual arousal is limited by current knowledge of the distribution of adrenergic receptors on the nerves serving the genitalia. Animal Models of Sympathetic Nervous System Activity and Sexual Arousal Pharmacological Manipulation of Sexual Behavior Using pharmacological treatments, a number of studies have demonstrated the in®uence of adrenergic transmission in the regulation of sexual behavior in females. Most studies of this nature have used ovariectomized animals treated with standardized doses of estradiol and progesterone. This strategy serves two purposes: ¤rst, to elicit sexually receptive behavior from a sexually unreceptive baseline, and second, to control for the potential in®uence of unequal sex hormone levels on adrenergic transmission. Animal models of female sexual behavior may assess several different meaAutonomic Nervous System In®uences l 67 [54.211...