The flexion reflex threshold has been used as a measure of sensation in a group of premature infants born at 27-32 weeks postmenstrual age. The threshold in an area of local tissue damage created by routine heel lances was half the threshold on the intact heel on the other side. This indicated a hypersensitivity to tissue damage analogous to tenderness or hyperalgesia reported in adults. In a double-blind study, treatment of the damaged area with the topical anaesthetic cream, EMLA, was found to reverse this hypersensitivity or in other words increase the flexion reflex threshold. Treatment with placebo had no effect. The results show that the newborn infant central nervous system is capable of mounting a chronic pain response to local injury which can be reduced by local anaesthetic.
The sensory innervation of the hair follicles of the furry skin of the rat hindlimb has been investigated by using the Winkelmann silver technique to stain peripheral axons and their terminals. This technique was found to stain only large- and medium-sized dorsal root ganglion cells and all laminae of the dorsal horn except lamina II, and therefore it is likely that, while A beta and A delta afferent fibres in the skin are stained, C fibres are not. Small vellus hairs were the commonest type of hair on the hindlimb particularly above the ankle. Many were not innervated. Those that were had lanceolate terminals arranged as palisades parallel to the hair shaft with circumferential presumptive Ruffini piloneural complexes and free nerve endings external to this. Circumferential innervation patterns without palisades were not uncommon but palisades without circumferential fibres were rare. Guard hairs which varied considerably in size were the next commonest hair type. Considerably more of these were innervated, by three-to-15 afferents forming both palisades of ten-to-30 lanceolate terminals and circumferential terminals. Both the innervated vellus and guard hairs had an associated vertical fibre projecting toward the epidermis. Tylotrichs, the largest hairs on the hindlimb, were rare (1-2%) and were only found above the ankle, but all were densely innervated by many axons. A prominent single nerve contributed to an annular complex by forming a bilaminar arrangement of lanceolate and circumferential terminals within the outer connective tissue sheath. Each tylotrich had an associated Merkel cell-neurite complex (haarscheiben). Differences in the distribution, innervation density, and phase in the growth cycle of the different hair types were found for skin from different regions of the hindlimb, which, together with the extent of the polyneuronal innervation of most follicles, has important implications for the processing by the somatosensory system of the afferent input generated by brushing hairs.
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