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

Abstract

SUMMARY1. 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,sm 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 psm 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,um, and their centres were about ,m apart; these values are consistent with those predicted above. We conclude that the experimentally determined sensitive areas represent the receptive fields (at threshold) of individual E. COOPER AND J. DIAMOND 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 005 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.

The effects of nerve section and of colchicine treatment on the density of mechanosensory nerve endings in salamander skin.

“…In the 6-day animal, however, changes in the distribution of the critical stimulus were clearly apparent; there were fewer low-threshold spots, many more in the higher stimulus range (thereby completely changing the shape of the histogram from its usual skewed distribution), and a significant increase in non-responsive spots. By day 7 about 95 % of the touch receptor population was not detectable with the largest stimuli we were able to apply with our mechanical stimulator (Cooper & Diamond, 1977).…”

“…In the preceding paper (Cooper & Diamond, 1977) we analysed the distributions of sensitivities of these mechanosensory endings to obtain a measure of their density in the skin. In the present report we use this information to examine the effects of colchicine on the mechanoreceptor function and distribution, both of the treated nerves and of the adjacent nerves supplying the skin of the salamander hind limb.…”

“…The (Diamond et al 1976). In the animals we now use the fields of 15 and 17 usually do share a common frontier, and field extension 'only rarely can be exploited; & Diamond, 1977) in a region of skin shared by the 15th and the 16th nerve was investigated after the 16th nerve on one side was treated with colchicine. There was no loss in the population of receptors feeding into the untreated 16th nerve, compared to the control 16th nerve (column A).…”

“…Our criterion for this rejection was based on the responses of the first fifteen points of the random survey made on the treated side; if ten or more of these were in the high-threshold 731 E. COOPER, J. DIAMOND AND C. TURNER range (requiring a stimulus of 2 5 ,tm/msec or greater) as compared to the normal side, in the skin of which only about three would fall into this category, the experiments were discontinued. In the remaining twelve animals both the experimental and the contralateral control nerve fields were investigated by the routine random survey technique (Cooper & Diamond, 1977). Fig.…”

Mathematical analysis of competition between sensory ganglion cells for neurotrophic factor in the skin

“…But it introduces a new one: when competition modifies the competitive capacity, the fittest axon becomes so strong that it eliminates all the others. This competitive exclusion appears indeed to occur in certain situations, such as the elimination of polyneuronal innervation of muscle fibers during development [49,50] or the mosaic organization of skin innervation by rapidly adapting terminals in salamanders [51], but not in others such as the overlapping arrangement of axonal fields in the skin of frogs [52]. Moreover, this earlier model predicted that competitive exclusion would still occur even with very high levels of trophic factor production, implying that the total number of axons innervating a region would not be increased, but experiments involving the overexpression of neurotrophins in the skin have shown that innervation density does increase in the skin in general [46], and in touch domes in particular [19].…”