Figure 11. Activation of the circular muscle during peristalsis in the distal colon.

During filling there is an activation of low threshold ascending excitatory and descending inhibitory nervous pathways. At threshold there is a transient oral contraction and anal relaxation of the circular muscle, the oral contraction forcing the bolus into the accommodating segment. The anal relaxation gives way to contraction resulting from activation of descending excitatory nervous pathways which aids in driving the bolus down the intestine.