Interdependency of stress relaxation and afferent nerve discharge in rat small intestine.

To be able to characterize intestinal mechano-electrical transduction, i.e. the mechanoreceptor behaviour, quantitative nerve studies with controlled and quantified stimulus are needed. This study aimed to determine the relationship between mechanical stress relaxation and afferent discharge adaptation evoked by fast isovolumetric bag distensions in the rat jejunum.Multiunit afferent activity was recorded in vivo from jejunum afferents from five male Wistar rats. The jejunum was distended via a bag at a distension speed of 0.5 ml/s to volumes of 0.2, 0.25, 0.3 and 0.4 ml, respectively. The distension was stopped and the volume was kept constant for 2 min to induce stress relaxation. The pressure in the bag, the afferent discharge (spike rate) and the diameter of the segment during the relaxation time were recorded simultaneously.The afferent discharge responses to distension showed a pattern with a peak during the sudden loading followed by decreasing activity with time. At distension volumes of 0.2, 0.25, 0.3 and 0.4 ml, the afferent discharge declined faster and to a greater extent (94%, 91%,96% and 87%) than the stress decreased (55%, 45%, 59% and 56%) during stress relaxation (p<0.001). Both the stress and the afferent discharge during the constant volume distension were independent of the distension volumes (p>0.5). The stress and the afferent discharge during the distension can be described mathematically on the basis of the quasi-linear theory of viscoelasticity. The association between the stress and the afferent discharge during the constant volume distension is linear with the same slope under various distension volumes.Intestinal mechanoreceptors were sensitive to the stress stimulus and a linear association between the stress relaxation and afferent discharge adaptation was found. The quasi-linear theory of visco-elasticity can be transferred to analysis of mechanical stimulus evoked afferent discharge.