Figure 5 - Comparison of mechanosensitive properties of vagal and spinal afferent fibers innervating the esophagus of opossum (Didelphis virginiana).

a: Stimulus-response functions of three classes of afferent fibers to graded distention of the esophagus. The low-threshold mechanosensitive (LTM-vagus, open circles) fibers in the vagus nerve encode stimuli in the physiologic range and exhibit a saturation of firing at greater intensity (40 mmHg). However, in the spinal nerve, there are two distinct classes of mechanosensitive afferent fibers. A proportion of low threshold intensity-encoding fibers (LTM-spinal, open triangle) respond to innocuous to noxious intensity of stimuli, whereas some fibers have high thresholds (40 mmHg) for response to esophageal distention (HTM-spinal, open square). Like LTM-spinal fibers, HT-spinal fibers exhibit increasing response to incrementing pressure. (Source: Sengupta et al.,⁵⁹ with permission from the American Physiological Society.) b: Shows the frequency distribution of thresholds for response of vagal and spinal mechanosensitive afferent fibers. The majority of vagal afferent fibers exhibit response to distention <8 mmHg, whereas spinal afferents have two distinct populations. A greater number of fibers have low thresholds for response, and a relatively small number of fibers have high threshold for response.