Sexual Responses in the Human Spinal Cord
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Altered sexual function is one of the most devastating consequences of spinal cord trauma (SCT). Despite this fact, current knowledge of the neural circuitry regulating sexual response in the spinal cord (SC) in healthy humans is remarkably incomplete. In order to better understand the changes that occur to sexual responses following SCT, we must elucidate the neural transmission of sexual function in healthy humans. Functional magnetic resonance imaging (fMRI) techniques to map neuronal function have been adapted for the SC and can now reveal this neural circuitry. We mapped, with spinal fMRI, neuronal activity in the lower thoracic, lumbar and sacral SC in healthy men (n = 10) and women (n = 9) that occurs in response to intermittent audiovisual stimulation (AVS), intermittent genital self-stimulation (GSS) and the combination of the former and latter, applied continuously and simultaneously until orgasm (AVGSS). MR images revealed predominantly increased signal intensity changes (ΔS+) in the autonomic preganglionic nuclei of the lower thoracic, lumbar and sacral SC in women and mostly decreased signal intensity changes (ΔS-) in comparable regions in men. In functional MR images, ΔS+ are related to increased neuronal input while ΔS- are associated with diminished neuronal input to a particular region. Linear regression analyses uncovered a greater number of inverse correlations between SC ΔS and scores of sexual function in women than in men indicating greater descending modulation of SC circuits regulating sexual responses in women than in men. Taken together, our results demonstrate that spinal fMRI is an effective and sensitive technique that can reveal signal intensity changes in the lower thoracic, lumbar and sacral SC associated with AVS, GSS and AVGSS in healthy men and women.