A quantitative study of the mechanosensory innervation of the salmander skin

We have investigated by physiological means the characteristics and organization of the mechanoreceptors in the skin of the salamander hind limb. A controlled mechanical stimulator with a tip diameter of a few microns was used to activate single mechanoreceptors, and afferent impulses were recorded from whole spinal nerves. 2. The mechanoreceptors of the skin are rapidly adapting. When random spots on the skin were tested, the stimulus strength needed to evoke an impulse varied from one location to another. The histogram of percentage occurrence of these critical stimuli in a given skin has the same form whether the sampled spots are 50 mum apart and are all included in a small area, or whether the spots are sampled as much as 1 mm apart across the entire limb. The histogram is skewed, with the highest-percentage occurrence in the low-threshold range. 3. One interpretation of the findings is that there is a single population of mechanoreceptors, of uniform threshold, spaced far enough apart for a stimulator of small diameter to be situated between them. By locating a low-threshold spot, and then determining the increase in stimulus required to excite it at a known distance away, it was possible to estimate the relationship between the strength of the stimulus and the receptive field size of such a population of receptors; we were then able to infer the average receptor spacing and receptive field size. The values are approximately 250 and 75 mum respectively. 4. From appropriate maps of detailed systematic surveys of the skin sensitivity to touch it was possible to define discrete low-threshold areas. These areas had approximate radii in the range 50-75 mum, and their centres were about 200-250 mum apart; these values are consistant with those predicted above. We conclude that the experimentally determined sensitive areas represent the receptive fields (at threshold) of individual mechanosensory endings. There are approximately twenty to thirty of them per square millimetre. 5. Receptive fields of single mechanosensory axons (i.e. of sensory 'units') were measured by an occlusion technique using two prodders. These fields vary from 0-05 to 2-5 mm2, and are generally organized as discrete areas with only a little overlap among them. Individual axons supply from five to seventy-five mechanoreceptors. 6. The findings provide a basis for quantitative studies of plasticity involving the mechanosensory system of the salamander skin.