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Single unit activity was presented and analyzed using impulses/second except where instantaneous frequency was used. Data is presented as mean?��?SEM unless otherwise stated; n refers to the number of single units and N to the number of animals. Statistical comparison was carried out using Student��s t-test, one-way anova and non-parametric tests as appropriate; P? http://www.selleckchem.com/products/Temsirolimus.html of the guinea pig is a visible, thickened band of circular muscle. In the guinea pig, two neurovascular bundles lie on the serosal surface of the distal rectum which both insert into the muscle at the point where the circular musculature begins to thicken. Therefore, our working definition of internal anal sphincter in the guinea pig was all smooth circular muscle distal to the neurovascular bundle insertions. Nerve branches from the neurovascular bundles that entered the oral verge of the internal anal sphincter were not investigated in the current study. The origin of these nerve branches is not known. The other neural pathway was via the pelvic ganglia which gave rise to multiple small nerve branches (rectal nerves) that innervated the rectum and the internal anal sphincter. We investigated small rectal nerve bundles that directly entered the musculature of the internal anal sphincter, which could be directly traced back to the pelvic ganglia. Recordings of muscle force made by the http://en.wikipedia.org/wiki/I%CE%BAB%CE%B1 transducer on the tissue stretcher were primarily a reflection of internal anal sphincter tone with occasional small contributions from remaining distal rectal tissue. Resting tone (15?mN?��?0.07, N?=?7) was constant after the preload (20?mN) was applied. Contractions http://www.selleckchem.com/products/DMXAA(ASA404).html were rarely seen, but were more prevalent in preparations with attached rectal tissue which shows intermittent irregular spontaneous contractions in similar preparations.5,11 Afferent activity was recorded from 20 to 40?��m diameter rectal nerve branches directly entering the musculature of the internal anal sphincter. Despite their small size, most branches contained numerous stretch-sensitive afferents. Only those branches with afferents that could be reliably discriminated as single units were used in these studies. Of 11 stretch-sensitive units from six preparations, 10 were silent under basal conditions, the other unit had irregular discharge of 0.3?Hz unrelated to contractile activity. Stretch-sensitive afferents to the IAS were not activated by contractile activity originating form attached rectal tissue. Afferents responded robustly to circumferential stretches of 10?s duration, in a slowly adapting manner. The internal anal sphincter was systematically stretched with 1�C4?mm stretches of 10?s duration at 40?s intervals. Over this range, muscle force generated in the tissue in response to stretch was linear (N?=?6).