i) Receptive fields.
The results show that primary afferent neurons have already established receptive fields in the hindlimb shortly after birth, and that these change little during subsequent development. This happens despite the postnatal period being the main time period over which mature sensory endings are formed in the periphery
(Payne et al, 1991; Ide, 192%).
The units have clearly delineated low threshold receptive fields at P3 in both hairy and glabrous skin. The size of these appears to be relatively stable. The only change observed was a decrease from P3 to PIO in mean receptive field size of units on thigh skin, caused by the appearance of a number of units with very small receptive fields (less than 0.5mm^). It is unclear why such a change did not occur in ankle skin where there are a number of units in this size range at P 3 . The small receptive fields at PIO in thigh skin were not associated with any particular type of hair, so it is unlikely that the different distribution of hair types in the two regions is the main factor.
The mechanical properties of the skin are likely to influence receptive field size. Thigh skin units may be differently affected by changes to these.
The von Frey thresholds of cells also did not change significantly during development. Again the same distribution of thresholds was found at P3 as at later ages.
Generally speaking, this data suggests that many of the receptive field characteristics are intrinsic to the sensory neurons, and not determined by their peripheral environment. Whilst innervating hairs enables the units to fire with great sensitivity to movements of individual hairs, the units still respond to skin deformation in a similar sized area of skin at all ages. Despite the likely changes in the physical environment and mechanical forces on cutaneous terminals during development the sensitivity of the endings to mechanical deformation appears to be fixed at birth and changes little afterwards.
ii) Firing properties.
Developmental changes were found in the firing properties of units to skin stimuli. Of these most marked was a general increase in the peak frequency of response of units and an increase in the number of spikes fired per stimuli from PIO to P20 in the hairy skin.
The population of units in hairy skin from PIO onwards fire more action potentials, whilst those in glabrous skin fire approximately the same. This change occurs over the time period that hair follicle innervation is maturing and is likely to be related to this (Payne et al, 1991)• Innervating hair follicles may help to amplify responses to stimuli. It is also possible
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that hair follicles as a target have specific effects on the firing properties of neurons that come into contact with them. It may result from a more general effect due to developing changes in the mechanical properties of hairy skin.
The peak frequency of firing of neurons increases during development for all skin areas. The increase takes place earlier in hairy skin than in glabrous skin. In the latter mainly from PIO to P20. Again this appear? in all areas to take place mainly as a generalized shift in the population towards higher frequencies of firing.
Conduction velocities of units similarly are shifted to higher values over the period studied; mainly due to the increasing degree of myelination of axons (Vejsada et al, 1 9 8 Ç) .
2.0.2. Comparison of different skin areas. i) Receptive fields.
Receptive fields are slightly but consistently larger on the thigh than on the ankle and on the foot than on the toes. This has previously been found for receptive fields on thigh and ankle in the adult (Lynn and Carpenter, 1982). At P3 the mean for all hairy skin units is significantly larger than that for all glabrous skin units. The difference at later ages is still there but is less prominent. The difference in sensitivity of endings in different skin areas is established at an early stage of development.
ii) Firing properties.
Mean maximum number of spikes fired is slightly higher at all ages on the thigh than on the ankle. There is no clear pattern for glabrous skin. Only at P2 0 is there a large
difference between hairy and glabrous skin.
The mean peak firing frequency is always higher in the order thigh>ankle>glabrous foot>toes. This difference is established at P 3 . The mean conduction velocity increases in a similar fashion for units in all areas. There is a suggestion of a rostrocaudal gradient in the hairy skin. The mean peak firing frequency reflects the general proximo-distal gradient in maturation of the limb. The mean for lateral thigh units is higher at P3 and PIO than that for ankle, before becoming slightly less at P20. Similarly, at PIO the mean for glabrous foot is higher than glabrous toe, but at P20 that for the toes is higher.
These differences in properties of units in different skin areas may be imposed on sensory neurons by the target. This could be due to the different mechanical properties of the skin in the different areas or it supplying different amounts of trophic factors to sensory neurons. Alternatively it could be due to the different skin areas being innervated by intrinsically different populations of cells. It is difficult to see how the latter possibility could be true since L5 DRG neurons innervate the proximal limb as well as the toes. Nevertheless some matching of afferents to skin areas cannot be ruled out.
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Section 2. The Postnatal Development of Skin Innervation.