Sympathetically induced changes in the responses of guard hair and type II receptors in the cat.

1. Sympathetic effects on the activity of single guard hair receptors and slowly adapting, type II mechanoreceptors were studied in anaesthetized cats. 2. Spontaneous activity in type II receptors and mechanically evoked activity in both type II and guard hair receptors was recorded both without and with electrical stimulation of the lumbar sympathetic trunk. 3. Almost all guard hair receptors were desensitized by sympathetic stimulation, the mean effect being an increase in threshold to 2.2 times the unconditioned value. 4. One third of the more slowly adapting guard hair units showed sympathetically induced spontaneous activity. 5. These sympathetic effects on guard hair receptors appear not to be related to pilomotor responses or to changes in central arterial pressure. 6. Individual type II receptors were found to be excited by skin stretch along one of two axes: either parallel or perpendicular to the long axis of the leg. Intermediate orientations were not observed. 7. 'Perpendicular' units showed marked increases in resting discharge but no change in mechanical sensitivity during sympathetic stimulation. 'Parallel' units were unaffected. 8. The sympathetic effect appeared not to be related to changes in blood pressure or skin tension. 9. It is suggested that the location and orientation of stretch sensitivity of these receptors make them well suited for detection of limb movements; however, no satisfactory functional explanation was found for the pronounced sympathetic facilitation of only one subset of the receptors.