The two pathways of arousal, psychogenic (from mental stimuli, at T10-L2 spinal levels) and reflexogenic (from physical stimuli, at S2-S4 spinal levels).  Loosely based on a diagram in <cite class="book" style="font-style:normal" id="harv">Elliott, S. (29 October 2012) "Sexuality and fertility after spinal cord injury" in Fehlings, M.G.; Vaccaro, A.R.; Maxwell B. , ed.   <i> <a rel="nofollow" class="external text" href="https://books.google.com/books?id=aQC-sQOCzhQC">Essentials of Spinal Cord Injury: Basic Research to Clinical Practice</a></i>, Thieme, p. 143  <small><a href="https://en.wikipedia.org/wiki/International_Standard_Book_Number" class="extiw" title="en:International Standard Book Number">ISBN</a>:  <a href="//commons.wikimedia.org/wiki/Special:BookSources/978-1-60406-727-9" title="Special:BookSources/978-1-60406-727-9">978-1-60406-727-9</a>. </small></cite>
<a href="//commons.wikimedia.org/wiki/File:Male_anatomy_blank.svg" title="File:Male anatomy blank.svg">File:Male anatomy blank.svg</a> from <a rel="nofollow" class="external text" href="http://www.luckymojo.com/faqs/altsex/penis.html">Alt.sex</a> and  <a href="//commons.wikimedia.org/wiki/File:Brain_stem_normal_human.svg" title="File:Brain stem normal human.svg">File:Brain stem normal human.svg</a> by Patrick J. Lynch, medical illustrator
CC BY 2.5
modified <a href="//commons.wikimedia.org/wiki/File:Male_anatomy_blank.svg" title="File:Male anatomy blank.svg">File:Male anatomy blank.svg</a> and <a href="//commons.wikimedia.org/wiki/File:Brain_stem_normal_human.svg" title="File:Brain stem normal human.svg">File:Brain stem normal human.svg</a>