Original article Sacral nerve stimulation increases activation of the primary somatosensory cortex by anal canal stimulation in an experimental model K. M. Griffin 1 , M. Pickering 2 , C. O’Herlihy 3 , P. R. O’Connell 1,4 and J. F. X. Jones 1 1 Health Sciences Centre, School of Medicine and Medical Sciences, 2 Conway Institute, and 3 Department of Obstetrics and Gynaecology, University College Dublin, and 4 Surgical Professorial Unit, St Vincent’s University Hospital, Dublin, Ireland Correspondence to: Dr J. F. X. Jones, Room C226, Health Sciences Centre, School of Medicine and Medical Sciences, University College Dublin, Belfield, Dublin 4, Ireland (e-mail: james.jones@ucd.ie) Background: Sacral and posterior tibial nerve stimulation may be used to treat faecal incontinence; however, the mechanism of action is unknown. The aim of this study was to establish whether sensory activation of the cerebral cortex by anal canal stimulation was increased by peripheral neuromodulation. Methods: A multielectrode array was positioned over the right primary somatosensory cortex of anaes- thetized rats. A brief burst of electrical stimulation was applied to either the left sacral root or the left posterior tibial nerve, and evoked potentials from anal canal stimulation were signal-averaged at intervals over 1 h. At the end of the experiment, the cerebral cortex was removed and probed for polysialylated neural cell adhesion molecule (PSA-NCAM). Results: Sacral nerve root and posterior tibial nerve stimulation significantly increased the peak ampli- tude of primary cortical evoked potentials by 54·0 and 45·1 per cent respectively. This change persisted throughout the period of observation. The density of PSA-NCAM-positive cells in the somatosen- sory cortex underlying the electrode array was increased by approximately 50 per cent in the sacral nerve-stimulated group. Conclusion: Brief sacral neuromodulation induces profound changes in anal canal representation on the primary somatosensory cortex, providing a plausible hypothesis concerning the mechanism of action of neuromodulation in the treatment of faecal incontinence. Surgical relevance Neuromodulation is an important new treatment in the man- agement of pelvic floor disorders, including faecal incontinence. The mechanisms of action are unclear and better understanding would facilitate patient selection and technical developments. Sensory activation of the sensory cerebral cortex by anal stimulation is increased by a brief burst of peripheral neuromodulation in an experimental model. The effects persist during the period of observation and are associated with up- regulation of PSA-NCAM in the underlying cortex, indicative of neuroplasticity. Peripheral neuromodulation has direct effects on the sensory cerebral cortex and measurement may provide an objective means of patient selection. Intermittent may be as effective as continuous neuromodulation, with implications for device programming and design. Presented to a meeting of the Physiology Society, Dublin, Ireland, July 2009, and Neuroscience Ireland, Dublin, Ireland, September 2010, and published in abstract form as Proc Physiol Soc 2009; 15: C48 and Ir J Med Sci 2010; 179(Suppl 7): S284 Paper accepted 24 February 2011 Published online 17 May 2011 in Wiley Online Library (www.bjs.co.uk). DOI: 10.1002/bjs.7536  2011 British Journal of Surgery Society Ltd British Journal of Surgery 2011; 98: 1160–1169 Published by John Wiley & Sons Ltd