A quantitative study of the mechanosensory innervation of the salmander skin.

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Abstract
The characteristics and organization of the mechanoreceptors in the skin of the salamander [Ambystoma tigrinum] hind limb were investigated by physiological methods. 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. The mechanoreceptors of the skin were 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 had the same form whether the sampled spots were 50 .mu.m apart and were all included in a small area, or whether the spots were sampled as much as 1 mm apart across the entire limb. The histogram was skewed, with the highest percentage occurrence in the low-threshold range. There may be 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. The average receptor spacing and receptive field size were then inferred. The values were approximately 250 and 75 .mu.m, respectively. 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 .mu.m, and their centers were about 200-250 .mu.m apart; these values were consistent with those predicted above. The experimentally determined sensitive areas apparently represent the receptive fields (at threshold) of individual mechanosensory endings. There were approximately 20-30/mm2. Receptive fields of single mechanosensory axons (i.e., of sensory units) were measured by an occlusion technique using 2 prodders. These fields varied from 0.05-2.5 mm2, and were generally organized as discrete areas with only a little overlap among them. Individual axons supplied from 5-75 mechanoreceptors. The findings provide a basis for quantitative studies of plasticity involving the mechanosensory system of the salamander skin.